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Ventral stream visual processing and the autism spectrum

Emma Grinter BA (Hons)

School of Psychology University of Western Australia

This thesis is presented in fulfilment of the degree of Doctor of Philosophy, and in partial fulfilment of the requirements for the Master of Psychology (Clinical Psychology) degree,

University of Western Australia

2009

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ABSTRACT

The Weak Central Coherence (WCC; Frith & Happé, 1994) theory of autism

spectrum disorders (ASDs) posits that individuals with an ASD have difficulty integrating

local information to form a coherent global percept. Recent studies have provided evidence

for a profile of visual ability in the dorsal visual pathway in ASDs that is consistent with

WCC. The over-arching aim of the research reported in this thesis was to determine

whether a similar profile of ability extends to the ventral visual pathway for both

individuals with an ASD and individuals with high levels of autistic traits within the

general population. A literature review, a pilot study and four experimental studies

addressed this major aim.

A key aim of the literature review was to evaluate Braddick, Atkinson and Wattam-

Bell�s (2003) suggestion that many developmental disorders share an impairment of the

dorsal visual stream. Studies assessing local and global processing in the dorsal and ventral

visual pathways for five developmental conditions were reviewed. The potential for ASDs

to be distinguished from other developmental disorders on the basis of a unique impairment

in global processing, rather than sharing a deficit restricted to the dorsal stream, was

acknowledged. Several issues concerning the psychophysical assessment of visual

perception in children were also raised. Additionally, it was identified that the use of some

of the more commonly employed stimuli to assess functioning in the ventral visual stream

may limit the capacity of researchers to draw conclusions concerning global processing

abilities in this pathway.

Following this, the stimuli employed in the research reported in the thesis were

carefully selected based on the issues raised in the literature review. Glass (1969) patterns

were used to assess ventral stream global processing; they consist of randomly distributed

dot dipoles, a proportion of which conform to a global structure achieved by aligning the

dots within pairs along imaginary contours. A Global Dot Motion (GDM) task was used as

a measure of global processing in the dorsal stream. In this task, a proportion of dots move

in a coherent direction and the remaining noise dots move in random directions. Concentric

structure was used in both the Glass patterns and the GDM stimuli. Finally, radial

frequency (RF) patterns were also used to assess functioning involving the ventral pathway.

These are closed contour shapes created by deforming a circle by sinusoidally varying the

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radius as a function of polar angle and manipulating the number of cycles of deformation

(the RF) within the shape (Wilkinson, Wilson, & Habak, 1998). High frequency RF shapes

(e.g. RF10 and 24) are processed locally, whereas low RF patterns (e.g. RF3 and 5) evoke

global pooling.

The pilot study addressed one of the issues raised in the literature review regarding

the merit of using the method of constant stimuli (MOCS) rather than staircase procedures

to estimate psychophysical thresholds. This study compared the thresholds of four adult

observers using both a staircase procedure and a MOCS method for Glass patterns, GDM

stimuli and RF patterns. The results indicated that the majority of thresholds obtained using

the MOCS method did not differ significantly from those obtained using the staircase

procedure, making MOCS preferable for use with children as it is more robust to early

mistakes or inattentiveness.

The purpose of Study 1 was to determine whether the visuospatial characteristics

seen in ASDs extend to individuals in the general population who score high on self-rated

measures of autistic-like behavioural traits as measured by the Autism-spectrum Quotient

(AQ; Baron-Cohen, Wheelwright, Skinner, Martin, & Clubley, 2001). The study included

two experiments. Experiment 1 compared the performance of students with low (N = 20)

and high (N = 19) AQ scores on the Embedded Figures Test (EFT; Witkin, Oltman, Raskin,

& Karp, 1971) and the Block Design subscale of the Wechsler Adult Intelligence Scale

(WAIS; Wechsler, 1997). Individuals scoring high in autistic-like traits showed superior

capabilities on both tasks. Experiment 2 evaluated the influence of intelligence on these

group differences in EFT performance. Twenty low and 15 high AQ scorers completed the

EFT and four subscales of the WAIS. The advantage of the high AQ group compared to the

low AQ group on the EFT was replicated, and was found to be independent of IQ. These

results provide evidence for the extension of cognitive autistic-like traits into the general

population, and highlight the value of using this population to enhance our understanding

of visual performance in ASDs.

Study 2 investigated, firstly, whether individuals scoring high on the AQ display a

similar profile of EFT and GDM performance to that seen in ASDs. Secondly, this study

examined whether differences in EFT performance are related to enhanced local or reduced

global processing in the ventral visual stream. A GDM task assessed global processing in

the dorsal visual pathway, and Glass pattern stimuli were used to examine global

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processing in the ventral visual pathway. Lower-level processing in the ventral stream was

assessed using a pulsed-pedestal task that assessed the contrast sensitivity of the

parvocellular (P cell) input, the predominant input to the ventral stream. The results

indicated that high AQ students (N = 26) were faster on the EFT, and had poorer global

motion and global form thresholds than those scoring low on the AQ (N = 29). However,

the two groups did not differ on the pulsed-pedestal task assessing lower-level input into

the ventral stream. These results suggest that individuals with high levels of autistic-like

traits have difficulties with global integration in both visual pathways, consistent with

WCC.

Finally, the integrity of the ventral cortical pathway in ASDs was assessed in two

separate studies. Study 3 used Glass patterns to examine global ventral stream processing,

and an orientation discrimination task to assess local ventral stream processing. The

orientation discrimination task consisted of two dot pairs exactly matching the

characteristics of the dipoles in the Glass pattern task; one pair was oriented vertically and

the other at a positive or negative angle from vertical. The EFT was also included. Thirty-

three children with an ASD and a large typically developing (TD) group (N = 117)

participated. While the ASD group exhibited the characteristic enhanced ability to detect

embedded figures, they had equivalent global processing thresholds on the Glass pattern

task, and experienced more difficulty in local discrimination of vertical orientation than the

TD children. Importantly, it was the mean threshold for the TD group that was

unexpectedly high on the global processing task, and possible reasons for the poor

performance of this group are discussed.

The fourth and final study used RF patterns to assess local and global form

perception. The TD children (N = 126) and children with an ASD (N = 34) discriminated

between zero amplitude modulation (circles) and modulated RF patterns processed either

locally (RF24) or globally (RF3). The results of Study 4 indicated that, compared to TD

children, children with an ASD required greater shape deformation to identify RF3 shapes,

consistent with a difficulty in global processing in the ventral stream. No group difference

was observed for RF24 shapes, suggesting that local ventral stream processing of this

nature is intact in the ASD group. These outcomes support the position that a deficit in

global visual processing is present in the ventral pathway in ASDs, which is consistent with

the notion of WCC. Importantly, the thresholds for the TD group were similar to those

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reported in the literature for adult observers on both tasks, suggesting that the difficulties

present in Study 3 were avoided with the RF stimuli.

Overall, the findings indicate that individuals with an ASD and those in the general

population scoring high in mild, autistic-like traits typically perform poorly on visual tasks

requiring global processing. This suggests a profile of visual-perceptual abilities more

consistent with WCC than theories positing enhanced local abilities without global

processing deficits. Similar to recent imaging and electrophysiological studies, these

findings encourage future research to revisit the concept of WCC in ASDs, as well as to

consider using high AQ populations to inform our understanding of the mechanisms

underlying perceptual and cognitive functioning on the autism spectrum.

References

Baron-Cohen, S., Wheelwright, S., Skinner, R., Martin, J., & Clubley, E. (2001). The

Autism-Spectrum Quotient (AQ): Evidence from Asperger Syndrome/high-

functioning Autism, males and females, scientists and mathematicians. Journal of

Autism and Developmental Disorders, 31, 5-17.

Braddick, O., Atkinson, J., & Wattam-Bell, J. (2003). Normal and anomalous development

of visual motion processing: motion coherence and 'dorsal-stream vulnerability'.

Neuropsychologia, 41, 1769-1784.

Frith, U., & Happé, F. (1994). Autism: Beyond "theory of mind". Cognition, 50, 115-132.

Glass, L. (1969). Moire effect from random dots. Nature, 223, 578-580.

Wechsler, D. (1997). Wechsler Adult Intelligence Scale (3rd ed.). San Antonio: The

Psychological Corporation.

Wilkinson, F., Wilson, H., & Habak, C. (1998). Detection and recognition of radial

frequency patterns. Vision Research, 38, 3555-3568.

Witkin, H. A., Oltman, P. K., Raskin, E., & Karp, S. S. (1971). A manual for the Embedded

Figures Tests. Palo Alto, CA: Consulting Psychologists Press.

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TABLE OF CONTENTS

ABSTRACT ................................................................................................................................. iii

ACKNOWLEDGEMENTS ........................................................................................................... x

STATEMENT OF CANDIDATE�S CONTRIBUTION ................................................................ xi

MANUSCRIPTS AND PUBLICATIONS GENERATED FROM THIS THESIS ......................... xii

CHAPTER 1. Introduction: Autism, the broader spectrum and central coherence .................. 1

Autism and the broader spectrum ................................................................................................... 2

Identifying the cause(s) of autism ................................................................................................... 5

The present thesis ......................................................................................................................... 17

References ................................................................................................................................... 22

CHAPTER 2. Vision in developmental disorders: Is there a dorsal stream deficit? ............... 37

Abstract ....................................................................................................................................... 38

Introduction ................................................................................................................................. 39

The human visual system ............................................................................................................. 40

Vision in the developmental disorders .......................................................................................... 48

Evaluating the dorsal stream hypothesis of developmental disorders ............................................. 62

Methodological considerations and future directions .................................................................... 64

Summary and conclusions ............................................................................................................ 68

References ................................................................................................................................... 70

CHAPTER 3. Pilot Study: A comparison of threshold estimates for two psychophysical presentation methods. ................................................................................................................ 87

Introduction ................................................................................................................................. 88

Method ........................................................................................................................................ 90

Results ......................................................................................................................................... 94

Discussion ................................................................................................................................... 96

References ................................................................................................................................... 98

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CHAPTER 4. Visuospatial analysis and self-rated autistic-like traits .................................... 101

Abstract ...................................................................................................................................... 102

Introduction ................................................................................................................................ 103

Study 1 ....................................................................................................................................... 105

Study 2 ....................................................................................................................................... 109

General Discussion ..................................................................................................................... 113

References .................................................................................................................................. 116

CHAPTER 5. Global visual processing and self-rated autistic-like traits .............................. 121

Abstract ...................................................................................................................................... 122

Introduction ................................................................................................................................ 123

Methods ..................................................................................................................................... 128

Results ........................................................................................................................................ 134

Discussion .................................................................................................................................. 139

References .................................................................................................................................. 145

CHAPTER 6. Local and global orientation discrimination in autism spectrum disorders and the relationship to detection of embedded figures ................................................................... 155

Abstract ...................................................................................................................................... 156

Introduction ................................................................................................................................ 157

Method ....................................................................................................................................... 162

Results ........................................................................................................................................ 168

Discussion .................................................................................................................................. 173

References .................................................................................................................................. 181

CHAPTER 7. Perception of shapes targeting local and global processes in autism spectrum disorders .................................................................................................................................... 193

Abstract ...................................................................................................................................... 194

Introduction ................................................................................................................................ 195

Method ....................................................................................................................................... 197

Results ........................................................................................................................................ 201

Discussion .................................................................................................................................. 203

References .................................................................................................................................. 208

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CHAPTER 8. General Discussion ........................................................................................... 212

Summary of Findings ................................................................................................................. 213

WCC as a unified explanation of ASD symptomatology ............................................................. 221

Implications for the design of ASD studies ................................................................................. 224

Implications for the Broader Autism Phenotype (BAP)............................................................... 226

Summary and Conclusions ......................................................................................................... 230

References ................................................................................................................................. 232

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ACKNOWLEDGEMENTS

First and foremost, I offer my sincerest gratitude to my supervisors. I am thankful to

Murray Maybery, who first introduced me into the world of autism research as an

undergraduate and has supported me throughout the past 4.5 years with his patience,

knowledge and thoughtful advice. I would also like to express my appreciation to David

Badcock, whose abiding commitment to scholarship has been an inspiring example and I

thank him for his supervision and mentorship. They both gave generously of their time and

expertise, and I am deeply grateful to them for the trust and support that they gave me to

work with them on this research project.

Thanks must also go to the numerous individuals also shared their expertise with me

along the way. In particular, Jude Cullity for her hard work on computer programming, and

taking the time to explain it to me; Jo Badcock for helping to put things in perspective, as

well as her keen attention to detail; and Alana Maley-Berg for sharing her knowledge of

autism. And hugely, to Liz Pellicano, who has been an inspiring source of brilliance.

Several people helped with data collection, and I would like to thank them

wholeheartedly - Rachelle Fox, Lynsey Harborow, Kelly Scaramozzino. This research

could not have been carried out without the help of Newman primary and high schools. The

time the children and staff devoted to this project was remarkable. Special thanks must also

go to the children with autism and their families who so willingly participated in this

research. Not only did they allow us into their homes, but they shared their life experiences

and exceptional courage and for this I have an enormous amount of gratitude. I truly hope

that this small bit of research will play a role in understanding this complex disorder.

On a more personal note, I would like to acknowledge my family who were not only

willing experimental guinea-pigs, but have always supported and encouraged me to do my

best in all matters of life; my friends Kirsty, Ella and Sian, who celebrated every small

success and reminded me to have fun; and Karina, Elizabeth and Pia with whom I shared

not only an office, but every step of the way.

And finally, no one has helped me more in writing this thesis than my wonderful

husband Brett. Through each stage of preparation, he shared the burdens, anxieties and

pleasures of this research, and helped with computer programming when it was beyond me.

His love, support and unlimited faith in me got me to the end. Thank you.

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STATEMENT OF CANDIDATE�S CONTRIBUTION

The research presented in this thesis was conducted as part of a larger study

assessing the visual abilities of children with autism. I contributed substantially to the

design and implementation of the experiments in consultation with other members of the

research group. I prepared the manuscripts for publication, and all of the work involved

assistance from both of my supervisors (Murray Maybery and David Badcock). The

programming of the experimental protocols was conducted by Judith Cullity and the second

experiment in Chapter 4 was conducted by Pia Van Beek. The data for the experiments

reported in Chapters 6 and 7 was collected, in addition to the data for the larger study, by

me, Lynsey Harborow, Rachel Fox and Kelly Scaramozzino. Additional guidance in the

preparation of the manuscripts in Chapters 5 and 7 was provided by the other co-authors

(Elizabeth Pellicano and Johanna Badcock). The co-authors on each manuscript have

provided approval for these pieces of work to be included in this thesis.

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MANUSCRIPTS AND PUBLICATIONS GENERATED FROM THIS THESIS

Chapter 2

Grinter, E. J., Maybery, M. T., & Badcock, D. R. (2009). Vision in the developmental

disorders: Is there a dorsal stream deficit? Manuscript under review, Brain

Research Bulletin1.

Chapter 4

Grinter, E. J., Van Beek, P. L., Maybery, M. T., & Badcock, D. R. (2009). Visuospatial

analysis and self-rated autistic-like traits. Journal of Autism & Developmental

Disorders, 39, 670-677.

Chapter 5

Grinter, E. J., Maybery, M. T., Van Beek, P. L., Pellicano, E., Badcock, J. C., & Badcock,

D. R. (in press). Global visual processing and self-rated autistic-like traits. Journal

of Autism and Developmental Disorders.

Chapter 7

Grinter, E. J., Maybery, M. T., Pellicano, E., Badcock, J. C., & Badcock, D. R. (Accepted

for Publication). Perception of shapes targeting local and global processes in

autism spectrum disorders. Journal of Child Psychology and Psychiatry.

1 Brain Research Bulletin has a specific structure and referencing format for review papers, but for consistency of presentation across the chapters in this thesis, the format is altered in the manuscript presented in Chapter 2.

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CHAPTER 1

Introduction

Autism, the broader spectrum and central coherence

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Autism and the broader spectrum

Autism is a pervasive developmental disorder defined in the DSM-IV classification

system (American Psychiatric Association, 2000) by impairments in reciprocal social

interaction and in communication, and by the presence of repetitive behaviours or

stereotyped interests. The social interaction difficulties include limited eye-contact,

difficulty interpreting mental states, failure to develop peer relations appropriate to

developmental level and lack of social or emotional reciprocity. Core communication

deficiencies include either a delay or total lack of expressive language, as well as marked

impairment in non-verbal behaviour. In cases where language is present, individuals often

have difficulty initiating and sustaining conversation, grammatical structures may be

immature, pitch, intonation, rate or rhythm may be abnormal, and there may be

disturbances in the pragmatic use of language. Restricted, repetitive and stereotyped

behaviours may present as an encompassing preoccupation with one or more interest or

activity, inflexible adherence to non-functional routines or rituals, stereotyped and

repetitive motor mannerisms such as hand flapping, or a persistent preoccupation with

movement or the parts of objects. For a child to be diagnosed with autism, delayed or

abnormal symptoms must be observed in each of the three domains (a total of at least six

symptoms must be identified, with at least two in social interaction and one each in

communication and restricted, repetitive and stereotyped patterns of behaviour and

interests), with impairment in at least one of the three domains identified as present before

three years of age (American Psychiatric Association, 2000).

The prevalence of autism has been estimated to be approximately 1 in every 500

people (Geschwind, 2009), with a male-female ratio of 4.3:1 (Fombonne, 2003). The

disorder is highly heterogeneous and thus the term Autism Spectrum Disorder (ASD; Wing,

1996) has been frequently used to describe the different variants. Two subtypes of autism

are generally referred to in the literature, although this distinction is not made in the DSM-

IV (Rinehart, Bradshaw, Brereton, & Tonge, 2002). Low-functioning autism is defined by

the presence of intellectual disability (i.e. IQ less than 70), whereas high-functioning autism

is characterised by relatively intact cognitive functions (i.e. IQ greater than 70) and

comprises approximately 25% of those diagnosed with the disorder (Folstein, 1999). In

addition, two variants of autism have been identified that exhibit some, but not all of the

characteristic autism symptoms. For Asperger�s syndrome (AS) to be diagnosed an

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individual must meet two of the three criteria for autism, the exception being the absence of

�delay or deviance in early language development� (American Psychiatric Association,

2000, p.74). Pervasive Developmental Disorder Not Otherwise Specified (PDDNOS) also

involves a severe and pervasive impairment in the development of reciprocal social

interaction, and while there is also some impairment in communication or the presence of

stereotyped behaviours, interests, or activities, these symptoms are not of sufficient number

or severity to warrant a diagnosis of autism or AS (American Psychiatric Association,

2000).

Recent discussion has conceptualised ASDs as reflecting developmental difficulties

lying at the extreme end of a continuum (Happé, Ronald, & Plomin, 2006; Mandy & Skuse,

2008) with AS and PDDNOS falling between autism on one end and typical development

on the other (Baron-Cohen & Robertson, 1995; Baron-Cohen, Wheelwright, Skinner,

Martin, & Clubley, 2001; Frith, 1989; Wing, 1988). Evidence for the fact that this

continuum extends to the general population comes predominantly from studies of the

families of autistic probands. Studies comparing relatives of individuals with an ASD to

family members of control groups have described a number of subtle personality, social and

language features for the ASD families that are milder but qualitatively similar to autistic

traits (see Bailey, Palferman, Heavey, & Le Couteur, 1998, for a review). Key

characteristics include rigid personality with obsessive traits (Lainhart et al., 2002; Piven et

al., 1997b), socially reticent or aloof dispositions (Bolton et al., 1994; Murphy et al., 2000),

reports of fewer and less reciprocal friendships (Lainhart et al., 2002; Piven, Palmer,

Jacobi, Childress, & Arndt, 1997a), and abnormal language characteristics including

impaired pragmatic language use (Bishop et al., 2004; Landa et al., 1992; Le Couteur et al.,

1996; Pickles et al., 2000; Piven et al., 1997a; Whitehouse, Coon, Miller, Sainsbury &

Bishop, in press). Though typically very subtle in expression, these features closely parallel

the three core symptom domains of autism.

Several instruments have been developed that detect variation in mild autistic

symptomatology within the general population as well as successfully discriminating

individuals with an ASD from unaffected individuals. These measures include the Social

Responsiveness Scale, (Constantino, Przybeck, Friesen, & Todd, 2000), the Autism-

Spectrum Quotient (AQ, Baron-Cohen et al., 2001), the Childhood Asperger Syndrome

Test (Scott, Baron-Cohen, Bolton, & Brayne, 2002) and the Autism Spectrum Screening

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Questionnaire (Ehlers & Gillberg, 1993). The validity of these instruments is shown by

their capacity to differentiate autism (Baron-Cohen et al., 2001; Constantino et al., 2003) or

AS (Woodbury-Smith, Robinson, Wheelwright, & Baron-Cohen, 2005) from control

samples. For example, using the AQ, Baron-Cohen et al. (2001) were able to correctly

classify 79.3% of AS and high functioning autistic individuals using a cut off score of 32

(2% of typically developing controls were falsely classified using this criterion), and

Woodbury-Smith et al. (2005) correctly classified 83% of patients suspected of having AS

as either meeting or not meeting the diagnostic criteria for the condition, using a cut off

score of 26. Continuity of the behavioural traits associated with ASDs, with no evident

boundary between typical development and the clinical condition, has been found in the

general population (not limited to relatives of individuals with an ASD) using these

instruments (Baron-Cohen, Hoekstra, Knickmeyer, & Wheelwright, 2006; Baron-Cohen et

al., 2001; Constantino, Przybeck, Friesen, & Todd, 2000; Constantino & Todd, 2003, 2005;

Hoekstra, Bartels, Verweij, & Boomsma, 2007; Posserud, Lundervold, & Gillberg, 2006;

Williams et al., 2005; Woodbury-Smith et al., 2005).

Studies using these measures have found milder forms of the behaviours

characteristic of the autism phenotype in some university students, with higher

concentrations in particular disciplines, and in some first degree relatives of individuals

with autism. For example, Austin (2005) found that university students in courses with

substantial mathematical content (e.g. physics, engineering) had higher AQ scores than

biological science and non-science students. Students with parents in scientific occupations

also scored higher on the AQ when compared to students with parents in non-scientific

occupations. Similar differences among undergraduates had been reported earlier by Baron-

Cohen et al (2001), however, Carroll and Yung (2006), using a much smaller sample found

no difference in AQ scores between science and humanities undergraduates. Jobe and

White (2007) found that students scoring high on the AQ were significantly more likely to

experience social skills deficits, loneliness and difficulties in interpersonal relationships

than students scoring low on the AQ (see also Kanne, Christ, & Reiersen, 2009).

Additionally, higher Social Responsiveness Scale scores have been found among siblings

of autistic probands from both multiple and single incidence families when compared to

siblings of children with psychopathology unrelated to ASDs (Constantino et al., 2006).

Thus, the notion that ASDs exist on a continuum is generally supported throughout the

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literature. Whereas autism involves severe impairment across all three of the symptom

domains referenced in its diagnosis, it appears that subtle manifestations of some or all of

these features are present as a Broader Autism Phenotype (BAP) in unaffected relatives,

and extend in a normal distribution to typically developing individuals with no autistic

family members.

Identifying the cause(s) of autism

Since it was first described, researchers have been trying to identify an underlying

cause that results in the triad of symptoms associated with autism. Evidence from twin and

family studies points to an unspecified biological dysfunction largely influenced by genetic

factors (see Rutter, 2000, for a review). While promising candidates have been identified,

no genetic markers for autism have currently been confirmed (Glessner et al., 2009;

Lauritsen & Ewald, 2001; Szatmari et al., 2007; Wang et al., 2009). Thus, researchers have

sought cognitive explanations in an attempt to identify a single underlying feature that may

emerge from multiple biological aetiologies and that accounts for the various behavioural

manifestations of autism (Frith, Morton, & Leslie, 1991; Morton & Frith, 1995, 2001). For

a cognitive trait to be considered necessary and sufficient to cause the development of

autism, it must ideally satisfy several criteria: firstly, the proposed characteristic must be

universal in that it is present in all individuals with autism; secondly, the characteristic must

be unique to autism and not present in other developmental disorders, or its presence in

combination with other specific factors must be unique; and thirdly, it is expected that the

incidence and severity of the cognitive characteristic should be directly related to the

behavioural symptoms in each of the three domains (Bailey, Phillips, & Rutter, 1996;

Pellicano, 2005). Additionally, the purported cognitive characteristic must also be present

to some degree in those who exhibit the BAP if the explanatory power of the feature is to

be related to the entire spectrum of autistic disorders (Bailey & Parr, 2003; Bailey et al.,

1996). However, while numerous hypotheses regarding the nature of the cognitive

abnormality in autism have been advanced (Hobson, 1989; Klin & Volkmar, 1993;

Minshew, Goldstein, & Siegal, 1997; Rutter, 1968), in addition to the proposal of a

multiple deficits account (Bailey & Parr, 2003; Bailey et al., 1996; Happé & Ronald, 2008;

Pellicano, Maybery, Durkin, & Maley, 2006), three cognitive theories specifying a core

underlying deficit have dominated the literature over the past three decades: the Theory of

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Mind (ToM) hypothesis, the Executive Dysfunction hypothesis, and Weak Central

Coherence (WCC) theory.

Theory of Mind

Theory of mind refers to the ability to impute mental states to the self and to others

(Premack & Woodruff, 1978). The ability to make inferences about what other people

believe to be the case in a given situation allows one to predict their behaviour (Dennett,

1978). The most widely used test of ToM capability is the �false belief� task. In a classic

version of this task, the participant watches a sequence of events involving two characters,

Sally and Ann. Sally places her ball in a basket and leaves the room. Ann moves the ball to

her own basket. When Sally returns to the room, the participant is then asked where Sally

will look for her ball. The correct answer �in her basket� is given based on the participant�s

ability to infer the mental state of Sally. Children are generally capable of answering

correctly by the time they reach the age of 4 to 5 years (Astington, Harris, & Olson, 1988;

Wimmer & Perner, 1983). Baron-Cohen et al. (1985) initially reported that a large

proportion of individuals with autism answer that Sally will look in Ann�s box. This failure

to represent Sally�s belief has been taken as evidence of impaired ToM in autism. Under the

ToM hypothesis, individuals with autism are severely delayed in the ability to ascribe

beliefs and desires to others (Baron-Cohen et al., 1985). Research has demonstrated that

many children with autism fail tasks that require an understanding of other minds (see

Baron-Cohen, Tager-Flusberg, & Cohen, 2000, for a review), resulting in the proposal of a

primary cognitive deficit in the ability to �mindread� as a single explanation for autism.

The ToM account of autism has provided a means for understanding, recognising

and addressing the social and communicative difficulties of this disorder (Baron-Cohen,

Tager-Flusberg, & Cohen, 1993). However, it is increasingly being noted that a deficit in

�mindreading� alone is not sufficient to cause autistic symptomatology (Frith & Happé,

1994; Klin, Volkmar, & Sparrow, 1992). Some researchers have reported that between 15

and 55 percent of children with autism pass first-order (usually false belief) ToM tasks

(Happé & Frith, 1996) and some children with autism also pass more advanced second-

order ToM tasks that involve inferring someone�s mental state concerning another�s mental

state (Tager-Flusberg & Sullivan, 1994). Thus, the deficit is not universal, in that

substantial numbers of children with ASDs develop theory of mind capabilities, albeit

delayed compared to typically developing children (Baron-Cohen et al., 1985; Bowler,

7

1992; Perner, Frith, Leslie, & Leekham, 1989). In addition, the ToM deficit is not specific

to ASDs in that other clinical groups, such as individuals with intellectual handicap, deaf

children and children with William�s syndrome, have also been shown to perform poorly on

theory of mind tasks (Peterson & Siegal, 1995; Porter, Coltheart, & Langdon, 2008;

Sullivan & Tager-Flusberg, 1999; Yirmiya, Erel, Shaked, & Solomonica-Levi, 1998).

Finally, while it might explain some of the social impairments seen in ASDs, a deficit in

ToM has difficulty explaining the non-social symptoms, particularly the repetitive

behaviours and stereotyped interests that characterise the conditions.

Executive functioning

Executive function (EF) is a broadly defined cognitive construct originally used to

describe the deficits found in patients with frontal lobe lesions. It refers to the many

cognitive capabilities associated with the frontal cortex that are required to prepare for and

execute complex behaviour, such as planning, working memory, impulse control,

inhibition, shifting set and self-monitoring (Hill, 2004; Ozonoff, 2001). Executive

dysfunction has been found with some consistency across different ages and ability levels

for people with an ASD when compared with appropriate controls (see Hill, 2004 and

Pennington & Ozonoff, 1996, for reviews). At a clinical level, the proposal that problems of

executive functioning contribute to the symptoms of ASDs appears plausible: the features

used to diagnose ASDs include an encompassing preoccupation or unusual interest that is

abnormal in intensity, inflexible adherence to non-functional routines, stereotyped body

movements, and preoccupation with parts or sensory qualities of objects. Additionally,

executive functioning impairments are thought to contribute to the development of the

social difficulties seen in ASDs (Hill, 2004).

One strength of the executive function theory is the potential for identification of a

specific pattern of executive dysfunction that distinguishes ASDs from other disorders.

While there is currently evidence pointing towards particular difficulties with planning

(Guerts, Verté, Oosterlaan, Roeyers, & Sergeant, 2004; Ozonoff, Pennington, & Rogers,

1991), flexibility (Ozonoff & Strayer, 2001, but see Happé, Booth, Charlton & Hughes,

2006) and with set-shifting (Hughes, Russell, & Robbins, 1994), a unique profile for ASDs

has yet to be identified (see also Happé et al., 2006). Additionally, executive functioning

deficits have also been documented in a variety of other conditions, including ADHD,

schizophrenia, obsessive-compulsive disorder, and Tourette syndrome (see Pennington &

8

Ozonoff, 1996; Sergant, Geurts, & Oosterlaan, 2002, for a review). Thus, the problem of

discriminant validity affects the EF hypothesis in that there is the possibility of executive

dysfunction occurring as a general consequence of developmental disorders. Finally, and

similar to the ToM hypothesis, executive dysfunction is not a universal feature of autism

(Pellicano et al., 2006; Rajendran & Mitchell, 2007). Ozonoff et al. (1991) used a very

liberal criterion to define an EF deficit and found and that 96 percent of an ASD group

performed less well on EF tasks than the control group mean (see also Ozonoff, Rogers, &

Pennington, 1991), whereas Pellicano et al. (2006) used a more stringent criterion and

found that these difficulties were only present in 50 percent of their sample. Ozonoff (2001)

highlighted that most executive tasks suffer from measurement imprecision in that they tap

more than one operation simultaneously, or they have low reliability (Rabbitt, 1997), thus

the universality of executive dysfunction in ASDs cannot yet be ruled out (Hill, 2004).

Therefore, while one of the strengths of the executive dysfunction theory of ASDs is that it

can account for many of the non-social elements of the conditions, its largest problem is

that the concept of EF is multifaceted, making it difficult to delinate and therefore to create

tests that measure only one aspect of the construct (Rajendran & Mitchell, 2007).

Weak central coherence

Some individuals with an ASD display abilities in areas such as art, music,

calculation, memory or jigsaw puzzles that are often not simply at mental age level, but

exceed what would be expected by someone of their chronological age (Happé, 1999).

Therefore, in addition to the classic triad of impairments, individuals with ASDs can often

present with �islets of ability� and an uneven profile on IQ tests, usually characterised by a

deficit on the Comprehension subtest and a marked peak on the Block Design subtest of the

Wechsler Intelligence Scales (Dawson, Soulières, Gernsbacher, & Mottron, 2007; Ehlers et

al., 1997; Happé, 1994). Both the ToM and EF theories of ASDs are deficit accounts and as

such they both have one further problem in that they have difficulty explaining why some

functions are not only spared but also, on occasion, superior in people with ASDs.

Motivated by the strong belief that both the assets and the deficits of ASDs sprout from a

single cause at the cognitive level, Frith (1989) proposed that ASDs are characterised by a

specific imbalance in integration of information at different levels. A characteristic of

normal information processing appears to be the tendency to draw together information to

construct higher-level meaning in context, or �central coherence�. Frith suggested that this

9

universal feature of human information processing is disturbed in ASDs, and that a lack of

central coherence could explain very parsimoniously the assets and deficits unique to the

disorders. So, individuals with an ASD were hypothesised to show �weak central

coherence� (WCC) defined by a processing bias for featural and local information, and

relative failure to extract gist, or �see the big picture� in everyday life (Happé & Frith,

2006). On the basis of this theory, Frith predicted that individuals with ASDs would be

relatively good at tasks where attention to local information � relatively piece-meal

processing � is advantageous, but poor at tasks requiring the recognition of global meaning.

This account is intuitively appealing in that, consistent with the requirements for a

diagnosis, children with an ASD often show a preoccupation with the details or parts of

objects, and may notice small changes in the environment that lead to distress. While Frith

(1989) originally suggested that a weakness in central coherence could by itself account for

theory of mind impairment, this notion was later reviewed such that each were

conceptualised as two rather different cognitive characteristics that underlie ASDs (Frith &

Happé, 1994). Thus, this theory does not assume that WCC plays a primary role in the

causation of the social abnormalities seen in ASDs, although research demonstrating that

individuals with an ASD experience difficulties combining information across eye gaze,

facial expression and face identification illustrate the plausibility that a bias towards details

may contribute to social-communication difficulties associated with the conditions.

Interest in the WCC account of ASDs has grown rapidly since the early work by

Frith (1989) and Happé (1999; Frith & Happé, 1994). In that time, and in response to

empirical findings, the coherence account has been modified from Frith�s original idea in

three important ways. Firstly, the original suggestion of a core deficit in global processing

has changed from a primary problem to a more secondary outcome, possibly arising as a

relative deficit in comparison to more dominant local or detail-focused processing (but see

Happé & Booth, 2008). Second, the idea of a core deficit has given way to the suggestion

of a processing bias or cognitive style that can be overcome in tasks with explicit demands

for global processing (Happé & Frith, 2006). Last, the capacity of WCC to explain the triad

of impairments has been reconsidered (see above), with recognition that weak coherence

may be one aspect of cognition in ASDs that occurs alongside deficits in social cognition

rather than causing or explaining these difficulties (Happé & Frith, 2006). Because WCC

theory forms the basis of the current thesis, the next three sections will outline the research

10

devoted to appraising WCC in ASDs, achieved through assessing global and local

processing. In persons with an ASD, WCC has been shown to manifest at several different

levels (see Happé, 1999, for a review), but the most substantial amount of support has been

established at the visual-cognitive level.

Verbal-semantic abilities

Over four decades ago, Hermelin and O�Connor (1967) showed that control

participants recalled sentences and semantically related lists of words far better than

unconnected or randomly arranged word strings, and that this advantage was greatly

diminished in people with autism (see also Tager-Flusberg, 1991). This suggests that

individuals with autism do not benefit from the presence of meaningful information in

memory tests.

Frith and Snowling (1983) used homographs (words with one spelling but two

meanings and pronunciations) to determine whether persons with autism could use sentence

context to derive meaning and determine the correct pronunciation of the homograph (e.g.

�In her eye there was a big tear�; �In her dress there was a big tear�). Children with autism

chose the more frequent pronunciation, regardless of context, for four of the five

homographs assessed. They were less proficient at using the sentence context to

disambiguate pronunciation of the homograph when compared to children with dyslexia,

who in turn performed more poorly than typically developing children. Given that general

intellectual impairment may have contributed to these results, Happé (1997) used better-

matched groups and found positioning the homograph at the end of the sentence in order to

provide maximal benefit of the preceding context led to improved accuracy in the control

group, but did not facilitate performance in the children with autism (see also Jolliffe &

Baron-Cohen, 1999; Lopez & Leekam, 2003).

Finally, Jolliffe and Baron-Cohen (2000) explored linguistic processing in ASDs

using a global integration test, whereby participants were required to rearrange sentences in

accordance with a theme in order to tell the most coherent story. While individuals with an

ASD performed equivalent to an IQ- matched control group in the condition containing

temporal cues (stories referred to the time of day or year in order to assist sentence

rearrangement), they were significantly poorer on the task that relied solely on integrating

information within the context of a theme. Performance on this latter task was also

significantly worse compared to performance on the temporal cued task within the ASD

11

group. The authors suggested that the ASD group had greater difficulty with the sentences

that required continually establishing relationships between the different pieces of

information and the context in which they were presented, because they were less able to

interpret and be sensitive to contextual information.

Visual-cognitive abilities

Evidence for WCC in ASDs also appears in research demonstrating a preserved or

perhaps even superior capacity to engage in detailed visuospatial analysis in individuals

with the condition. Multiple studies have shown individuals with an ASD either matched

(Kaland, Mortensen, & Smith, 2007; Ropar & Mitchell, 2001) or outperformed (Morgan,

Maybery, & Durkin, 2003; Pellicano et al., 2006; Shah & Frith, 1993) comparison groups

on the Block Design subtest of the Wechsler Intelligence Scales or the similar Pattern

Construction subtest of the Differential Ability Scales. Successful performance on the

Block Design task requires �first the breaking up of each design into logical units, and

second a reasoned manipulation of blocks to reconstruct the original design from separate

parts� (Kohs, 1923). Importantly, it is the requirement of segmenting the Gestalt into

constituent parts that may favour individuals with WCC (Shah & Frith, 1993).

Similarly, there are also multiple reports that, relative to controls, participants with

an ASD show equivalent (Brian & Bryson, 1996; Kaland et al., 2007; Lee et al., 2007;

Ozonoff et al., 1991; Ropar & Mitchell, 2001) or advanced (de Jonge, Kemner, & van

Engeland, 2006; Edgin & Pennington, 2005; Jarrold, Gilchrist, & Bender, 2005; Jolliffe &

Baron-Cohen, 1997; Morgan et al., 2003; Pellicano, Gibson, Maybery, Durkin, & Badcock,

2005; Pellicano et al., 2006; Shah & Frith, 1983) capabilities on the Embedded Figures Test

(EFT). Each item of the EFT requires locating a previously seen simple figure within a

larger complex figure (Witkin, Oltman, Raskin, & Karp, 1971). Thus, similar to the Block

Design task, the key feature of the EFT is that a complex figure can be segmented or

include smaller constituent parts. WCC in someone with an ASD may result in failure to

perceive the gestalt of the complex figure, allowing the individual to easily perceive the

design in terms of its constituent parts and to thus quickly identify the embedded figure.

Alternatively, an individual with intact central coherence may first need to overcome the

tendency to perceive the test stimulus as a global form in order to focus on the separate

parts to achieve successful performance.

12

WCC has also been demonstrated on drawing tasks. Children with autism tend to

begin drawing with a detail rather than the structural global components of a figure, to more

generally draw in a piecemeal fashion and to create drawings in which the overall

configuration is violated (Booth, Charlton, Hughes, & Happé, 2003; Fien, Lucci, &

Waterhouse, 1990; Mottron & Belleville, 1993, 1995; Prior & Hoffmann, 1990). Starting

with local features and being apparently unperturbed by the global configuration is

arguably why individuals with autism are able to produce more accurate copies of

geometrically impossible figures than typically developing persons (Mottron, Belleville, &

Menard, 1999).

According to WCC, individuals with an ASD should be relatively less susceptible to

many visual illusions as they should be less able to integrate the target stimuli with the

(misleading) contextual elements that give rise to the illusions. Happé (1996) asked

participants to make judgements about visual illusions by presenting them with Titchener

circles, Muller-Lyer figures, Kanizsa triangles, the Ponzo, Poggendorf and Hering illusions,

and control figures. In each task, the critical information was to judge whether two

components of the illusion were the same or different size, for example, whether the shafts

on the two Muller-Lyer figures were of the same length. As expected, typically developing

individuals were susceptible to the illusions and judged, for example, that two lines of

physically identical length were different. In contrast, individuals with autism were more

able to make accurate judgments of the critical stimulus components, which was interpreted

in terms of them being less influenced by the distorting context. Comparable results were

reported by Bölte, Holtmann, Poustka, Scheurich and Schmidt (2007) using the same

illusions and response method. In contrast, Ropar and Mitchell (1999, 2001) administered

similar tasks but for which observers were required to adjust the size of a comparison

stimulus to match a second stimulus presented in an illusion-causing context, in an attempt

to overcome possible response biases in the procedure used by Happé (1996). Ropar and

Mitchell were unable to replicate Happé�s findings in two different samples of children

with autism, leading them to conclude that central coherence was intact with respect to

visual processing in autism. Importantly, recent research has demonstrated immunity to

perceptual illusions when a motor component is involved in the response (e.g. Dewar &

Carey, 2006), which may account for Ropar and Mitchell�s results given that they used a

matching response methodology.

13

Navon (1977) hierarchical stimuli have also been used to assess WCC in ASDs. In

the Navon task, stimuli consist of two levels: a global configuration and the local

constituent elements (e.g. a large H made of little Ss). Research has shown that typically

developing individuals respond more quickly and accurately to global forms than to local

forms, which is an effect referred to as a global advantage (Kimchi, 1992; Navon, 1977)1.

When typical participants are presented with a sequence of hierarchical stimuli and are

asked to name either a global letter or a local letter, in cases where there is incongruity

between the two levels, then naming is slower for the local than for the global letter. When

global advantage and global-to-local interference effects occur together, this is referred to

as global precedence (see Kimchi, 1992, for a review). According to WCC theory, one

would expect that individuals with an ASD would not display these precedence effects and

possibly would process the local level faster (but see Badcock, Whitworth, Badcock &

Lovegrove, 1990)2. While under some circumstances individuals with an ASD show faster

and more accurate responding to the local level relative to comparison individuals (Mottron

& Belleville, 1993; Plaisted, Swettenham, & Rees, 1999; Wang, Mottron, Peng,

Berthiaume, & Dawson, 2007), those with an ASD are able to respond to the global level of

hierarchical stimuli with similar efficiency to comparison individuals (Mottron, Burack,

Iarocci, Belleville, & Enns, 2003; Mottron, Burack, Stauder, & Robaey, 1999; Ozonoff,

Strayer, McMahon, & Filloux, 1994; Plaisted et al., 1999; Rinehart, Bradshaw, Moss,

Brereton, & Tonge, 2000; Rondan & Deruelle, 2007; Wang et al., 2007). A similar pattern

of results has been found for hierarchical stimuli using geometrical patterns (e.g. squares

aligned to form a circle) in that ASD groups do not differ from control groups in number of

global choices made or reaction time to respond to global targets (Iarocci, Burack, Shore,

Mottron, & Enns, 2006, Experiment 2; Rondan & Deruelle, 2007). This outcome is

unaffected by the number and size of local elements, and the presentation duration

(Plaisted, Dobler, Bell, & Davis, 2006). These findings, particularly on the Navon tasks,

1 Various factors such as visual angle, retinal location, size ratio, stimulus duration and familiarity can change the global advantage (Lamb & Robertson, 1990; Robertson & Lamb, 1991) and thus must be taken into account during stimulus development. 2 Badcock, et al. (1990) demonstrated that removing the low spatial frequencies from the hierarchical stimuli (the clarity of both local and global images is essentially unaltered by this procedure) removed the global advantage in typical individuals. They argued that the faster transmission of low spatial frequency information from retina to cortex supported the effect normally obtained. Therefore, based on these results, local and global processing are not critical determinants of performance on this task and provided the early visual pathways in individuals with autism transmit information at the normal rate, then their will be no difference in performance. The task, therefore, might not directly address issues relevant to WCC theory.

14

have led authors to propose that global processing is spared in ASDs, and that perhaps

enhanced local processing occurs in the context of intact integrative mechanisms.

Alternatively, if one were to take the perspective of Badcock et al. (1990), then the ability

of the ASD groups to respond to the global level of Navon hierarchical stimuli implies that

the early stages of the visual pathways in individuals with ASDs process information at the

same rate as the early stages of the visual pathways in typically developing observers.

Superiority in local processing associated with ASDs has been demonstrated on

featural and conjunctive visual search tasks (Jarrold et al., 2005; O'Riordan & Plaisted,

2001; O'Riordan, Plaisted, Driver, & Baron-Cohen, 2001; Plaisted, O'Riordan, & Baron-

Cohen, 1998). However, conjunctive visual search tasks require perceptual integration of

the target features, thus these studies also provide evidence that individuals with autism

retain the capacity to combine information to create a new perceptual representation. Other

areas providing support for the notion of intact global processing in autism include

identifying whole letters versus letters made up of shapes and letters presented within a

relevant versus irrelevant context, and identifying objects versus silhouettes3 (Mottron et

al., 2003), configural and featural discrimination learning (Plaisted, Saksida, Alcántara, &

Weisblatt, 2003), and perception of musical chords (Heaton, 2003). In contrast, Jarrold and

Russell (1997) assessed children�s ability for canonical counting by asking children with

autism and children with moderate learning difficulties to count dot stimuli which were

either randomly arranged or in canonical formation (i.e., as in the dots on a dice). They

found that children with autism showed less benefit in counting speed when the dots were

arranged canonically relative to the children with learning difficulties. This was consistent

with WCC in that children with autism apparently used a local counting strategy (i.e., dot

by dot).

Visual-perceptual abilities

According to WCC theory, superior local processing should be accompanied by

poor global processing. The evidence from the visual-cognitive paradigm is relatively

consistent in that, compared to matched controls, individuals with an ASD demonstrate

good or even superior performance on tasks requiring local processing. However, as

outlined above, studies investigating visuospatial integration of information in individuals

3 Many of these tasks may have the same issues with the transmission of low spatial frequency information as the original Navon task and so require further investigation.

15

with an ASD have, on occasion, provided equivocal results with respect to global

processing abilities. Inspired by this data, others have instead suggested that ASDs are

characterised by intact global processing abilities and that preserved or superior

performance on visuospatial tasks may be due to enhanced local processing abilities that do

not impact on or compromise integration capabilities (Happé & Frith, 2006; Mottron,

Dawson, Souliéres, Hubert, & Burack, 2006; Plaisted et al., 2003; but see Happé & Booth,

2008). Research in the visual-perceptual sphere has provided an important means to

investigate these alternative positions and to investigate local and global processing in a

system for which these functions are relatively well understood. Further, utilizing the visual

system has allowed for a more precise delineation of the definition used for �global

processing� than has been possible for the tasks assessing more cognitive-based abilities.

For the latter tasks, it is often difficult to discern whether the researchers are looking to

assess global processing in terms of processing that involves integrating local elements into

a larger perceptual whole, or whether they are referring to processing a larger area of space.

In vision research, these two aspects of processing are clearly demarcated.

While Chapter 2 provides a detailed description of the visual system, the essential

information is provided here in order to explain how local and global processing is

conceptualized in the remainder of the current thesis. To summarise, in the primate visual

system, three types of cells relay visual information through the lateral geniculate nucleus:

the magnocellular (M), parvocellular (P) and koniocellular (K) 4 streams (Merigan &

Maunsell, 1993; Xu et al. 2001). While there is considerable intermixing of the M and P

signatures in the cortex, the P pathway predominantly feeds into the ventral stream and is

implicated in form perception (Beason-Held et al., 1998, Kourtzi & Kanwisher, 2000),

whereas the M pathway provides substantial input into the dorsal stream, and has an

important role in the processing of motion (see Culham, He, Dukelow & Verstraten, 2001,

for a review). Small cellular receptive fields ensure that the earlier stages of each pathway

perform more local processing, whereas larger receptive fields result in global processing

occurring in higher visual areas (Van Essen & Gallant, 1994). Thus, with respect to the

visual system, global processing is defined as a process that requires the accumulation of

4 The K pathway is currently thought to be concerned primarily with blue-yellow colour perception (Callaway, 2005; Sumner, Anderson, Sylvester, Haynes, & Rees, 2007), with longer conduction velocities and more diverse response properties than the M and P cell responses (Casagrande et al., 2007) and will not be considered further here.

16

information provided from the simple cells in V1 to form a coherent global percept in

higher cortical areas. Psychophysical research investigating both local and global

processing in the dorsal and ventral streams in ASDs is reviewed in detail in Chapter 2,

along with similar research for four other developmental disorders. However, the literature

on ASDs will be briefly summarised here to provide background to the aims of the research

reported in this thesis.

Numerous researchers have investigated sensitivity to visual stimuli targeting global

processing in the dorsal stream using tasks that require observers to detect coherence in

motion flow. Several studies report inpaired coherence thresholds for observers with an

ASD (Bertone, Mottron, Jelenic, & Faubert, 2003; Davis, Bockbrader, Murphy, Hetrick, &

O'Donnell, 2006; Milne et al., 2002; Pellicano et al., 2005; Spencer et al., 2000, but see Del

Viva, Igliozzi, Tancredi, & Brizzolara, 2006) or for subgroups within ASD samples (Milne

et al., 2006; Tsermentseli, O'Brien, & Spencer, 2008). However, these impairments appear

to occur in the presence of intact lower-level processing within the dorsal stream (Bertone,

Mottron, Jelenic, & Faubert, 2005; Pellicano et al., 2005) suggesting that only the higher

levels of this pathway are impaired in ASDs, inconsistent with Braddick, Atkinson and

Wattam-Bell�s (2003) suggestion of a more general dorsal stream impairment in

developmental disorders. An alternative explanation of these empirical findings is that

individuals with an ASD experience difficulties on visual tasks that require the integration

of local information to form a coherent percept, similar to the tenets of WCC theory. If this

were the case, then it could also be expected that individuals with an ASD would

demonstrate impaired abilities on tasks assessing higher-level, integrative functioning in the

ventral visual stream, in conjunction with intact (or perhaps even superior, according to

Plaisted et al., 1999) functioning at lower, more local levels. The literature in this instance

is less clear, with some researchers reporting impaired performance on tasks assessing

global ventral stream abilities in observers with autism (Bertone et al., 2005; Spencer &

O'Brien, 2006; Tsermentseli et al., 2008), while others do not report global ventral

processing impairment in either mixed ASD (Milne et al., 2006) or autism-only (Davis et

al., 2006; Del Viva et al., 2006; Spencer et al., 2000) samples relative to neurotypical

comparison groups. The only direct assessment of lower-level functioning in the ventral

visual stream revealed higher contrast sensitivity on an orientation discrimination task in

children with autism relative to an IQ and chronological age-matched comparison group

17

(Bertone et al., 2005). While the particulars of the methodological differences among the

above studies are discussed further in Chapter 2, it is suffice to say that a comprehensive

understanding of local and global ventral stream visual processing in ASDs remains

elusive.

The present thesis

WCC theory is an appealing explanation of the behaviours associated with ASDs in

that it has the capacity to explain both the notable capabilities and acute deficits that are

characteristic of the conditions in terms of a single underlying processing style � that of

attention to the parts or details of stimuli at the expense perceiving global, integrated

stimuli. However, recently it has become evident that perhaps an enhanced local processing

capability without concomitant difficulties in more complex, global processing is more

applicable to ASDs, a position termed Enhanced Perceptual Functioning (EPF) by Mottron

et al. (2006). In attempting to explain the superior performance of individuals with an ASD

on some of the tasks mentioned above, as well as explain the behavioural characteristics of

the condition, each of these theories proposes a profile of perceptual processing purportedly

unique to the autism spectrum. If this is the case, then one would expect that similar

patterns of atypical performance would be observed for the dorsal and ventral visual

pathways when assessing local and global processing in individuals with an ASD. Whilst

the results across a majority of the studies assessing the capabilities of the dorsal visual

stream in ASDs can be interpreted as evidence for disrupted global processing in this

pathway, the evidence relevant to a similar pattern of performance in the ventral stream is

less consistent. Essentially, many of these studies assessing ventral stream processing do

not examine both local and global abilities nor do they use stimuli with similar

characteristics at both the local and global levels (but see Bertone et al., 2005). Thus, the

principal aim of the present thesis was to further elucidate the capabilities of the ventral

visual stream of individuals on the autism spectrum, in order to examine the capacity of the

WCC and EPF theories to account for the processes of vision. In particular, there are

several issues that remain to be investigated: (1) whether the putative visual perceptual

style in ASDs is consistent with the notion of a general dorsal stream impairment, or

whether visual abilities in ASDs might be unique in relation to other developmental

disorders; (2) whether the visuospatial characteristics of ASDs extend to individuals in the

18

general population who score highly on self-rated measures of autistic-like behavioural

traits, and if so, whether WCC or EPF in the visual-perceptual domain is associated with

nonclinical autistic-like traits and (3) whether local processing is enhanced and/or global

processing is impaired in the ventral visual stream in individuals with an ASD. This thesis

provides a review of the literature and describes three studies that attempt to address these

issues.

The thesis begins with a research review that focuses on delineating the visual

perception literature relevant to assessing visual functioning at the local and global levels of

the dorsal and ventral visual streams in developmental conditions (see Chapter 2). Several

issues are identified in relation to the methods most commonly used to assess visual

functioning. In particular, the stimuli used so far to assess global processing in the ventral

visual stream may have tapped local contour processes in V1 and thus may represent local

processing as much as global processing in higher cortical regions. With this caveat in

mind, the remainder of the review focuses on those developmental disorders for which

early to mid-level visual ability has been assessed: developmental dyslexia, ASDs,

developmental dyspraxia, Williams syndrome and fragile X syndrome. These conditions

were considered together in order to establish whether a common profile of dorsal stream

anomalies applies to the developmental disorders, consistent with Braddick et al.�s (2003)

suggestion, or whether instead some of the conditions show certain anomalies not

expressed in the other disorders.

In order to assess ventral visual stream processing in children with an ASD we

changed the way in which psychophysical tasks were administered in accordance with one

of the issues identified in Chapter 2. This change was evaluated in adult observers in a pilot

study (see Chapter 3) which compared the method of constant stimuli (MOCS) to a

staircase method in order to estimate psychophysical thresholds. The MOCS is preferable

for use with children as it is less sensitive to mistakes or inattentiveness early on in the

experimental procedure (Spry, Johnson, McKendrick, & Turpin, 2003).

Before assessing the ventral visual stream capabilities associated with autistic-like

traits in the general population, it was necessary to establish that individuals with high

levels of these traits share some key visual-cognitive characteristics already identified for

ASDs proper. Therefore, Study 1 focused on examining the relationship between self-

reported autistic-like traits and two visual-cognitive abilities (see Chapter 4). We examined

19

group differences in visuospatial ability in university students scoring high and low in non-

clinical autistic-like traits measured by the AQ. The first experiment used the EFT and the

Block Design subscale of the Wechsler Adult Intelligence Scales III (WAIS-III) to assess

visuospatial analytic abilities. We broadened the scope of the research beyond that of the

only existing study of this nature by selecting samples with more extreme scores on the

AQ, collecting both error and reaction time EFT data, and including the Block Design task.

We reasoned that since individuals with the broader autism phenotype show milder

versions of both the behavioural and cognitive features of the condition without exhibiting

the clinical syndrome, then members of the general population also exhibiting these mild

behavioural traits should also share the cognitive characteristics. In the second experiment

reported in Chapter 4, we evaluated whether group differences in the EFT were

independent of intellectual capabilities. Consistent with research demonstrating superior

EFT performance in ASD groups compared to typically developing comparison groups, we

predicted that students scoring high in autistic-like traits would outperform those scoring

low in those traits when verbal and nonverbal ability are taken into account.

After establishing in Study 1 that individuals within the general population who

score high on autistic-like traits share similar visuospatial abilities to individuals with an

ASD, Study 2 posited that it might be possible to increase our understanding of the impact

of autism on visuospatial capabilities, such as assessed by the EFT, by examining visual-

perceptual capabilities in student populations scoring high versus low on the AQ. The aim

of Study 2 was therefore to establish whether individuals scoring high in self-rated autistic-

like traits (when assessed relative to those scoring low in such traits) exhibit a similar

pattern of visual ability to that seen in individuals with ASDs of superior EFT performance

but impairment on a task assessing global processing in the dorsal visual stream. This was

followed by an examination of local and global ventral visual stream processing (see

Chapter 5). We predicted that students scoring high on the AQ would exhibit faster times to

locate embedded figures in addition to higher thresholds on a global dot motion task

(assessing global dorsal stream processing) when compared to students scoring low on the

AQ. We further expected that if WCC contributes at least in part to EFT performance in

individuals high in autistic-like traits, then this group should have higher thresholds on a

task assessing global ventral stream processing. In contrast, if EPF alone contributes to EFT

performance, then it was expected that the group scoring high in autistic-like traits would

20

have superior thresholds on a low-level ventral visual stream task in conjunction with intact

thresholds on tasks measuring global processing for either the dorsal or ventral streams.

The final two studies examined local and global ventral visual stream processing in

a large sample of typically developing children and in a sample of children with an ASD. In

order to maintain continuity with Study 2, in Study 3 we employed the same measure of

ventral stream global processing as in the earlier study, Glass (1969) patterns, and also used

the children�s EFT (see Chapter 6). Glass patterns require integration of local orientation

signals. Thus, an additional task was developed to assess the orientation discrimination

capabilities of the neurons in V1. Study 3 therefore used this simple orientation

discrimination task and the Glass pattern task to assess local and global ventral stream

processing, respectively. Study 4 also assessed local and global visual functioning in the

ventral stream but did so using two forms of radial frequency (RF) pattern (Wilkinson,

Wilson, & Habak, 1998, see Chapter 2 for a summary), thereby minimising task differences

in assessing the two levels of functioning (see Chapter 7). This study is the first to assess

visual functioning in ASDs using RF patterns. Consistent with the literature, we expected

the children with an ASD to exhibit superior performance on the EFT. As with Study 2, we

reasoned that if ASDs are characterised by WCC, then the children with an ASD would

display elevated thresholds on the Glass pattern task and the RF task that also assessed

visual integration in higher cortical regions, and equivalent thresholds on the orientation

discrimination task and the RF task that also assessed local ventral stream abilities.

Conversely, if ELP best typifies ASDs, then the ASD group should have superior

thresholds on the local processing tasks, and at least equivalent performance on the global

processing measures, relative to the control group. In both studies, a novel statistical

approach was used to conduct the group comparisons. This technique involves regressing

each experimental variable (such as age, gender and IQ) onto the relevant psychometric

variables for a large and diverse sample of typically developing children. The regression

function is then used to generate expected scores for the children with an ASD, against

which their actual scores are compared. This approach is advocated as being more sensitive

than traditional matched-group comparisons or analysis of covariance (Brock, Jarrold,

Farran, Laws, & Riby, 2007). In Study 4, the relationships of EFT performance to

performances on the visual-perceptual tasks were also explored.

21

The results from the four empirical studies are then summarised in the General

Discussion and the implications for the WCC and ELP theories are discussed. It is here that

we endorse Happé and Booth�s (2008) idea that Frith�s original notion of WCC in ASDs

deserves a renewal of interest, and perhaps using psychophysical tasks designed to test

integrative capabilities in addition to local processing. The relationship of the findings from

the empirical studies reported in the thesis to those found using imaging studies is also

considered. Finally, broader issues concerning the contribution that individuals in the

general population scoring high in autistic-like traits can make to our understanding of

ASDs are addressed. Methodological limitations of the studies and suggestions for future

research are also discussed.

22

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37

CHAPTER 2.

Vision in developmental disorders: Is there a dorsal

stream deficit?

Emma J. Grinter, Murray T. Maybery,

and David R. Badcock

38

Abstract

The main aim of this review is to evaluate the proposal that several developmental

disorders affecting vision share an impairment of the dorsal visual stream. First, the current

definitions and measurement approaches used to assess differences in both local and global

functioning within the visual system are considered. Next, studies assessing local and

global processing in the dorsal and ventral visual pathways are reviewed for five

developmental conditions for which early to mid level visual abilities have been assessed:

developmental dyslexia, autism spectrum disorders, developmental dyspraxia, Williams

syndrome and Fragile X syndrome. The reviewed evidence is broadly consistent with the

idea that the dorsal visual stream is vulnerable in developmental disorders. However, the

potential for a unique profile of visual abilities that distinguish some of the conditions is

posited, given that for some of these disorders ventral stream deficits have also been found.

We conclude with ideas regarding future directions for the study of visual perception in

children with developmental disorders using psychophysical measures.

39

Introduction In 2003 Braddick, Atkinson and Wattam-Bell suggested that the dorsal visual

stream is vulnerable during development. Supporting this claim was a body of evidence

indicating that a variety of developmental disorders show anomalies in the detection of

motion coherence in a field of dots, a function attributed to processing within the dorsal

visual pathway. Often this anomalous motion perception was associated with normal

performance on tasks requiring detection of coherent structure in stationary patterns, a

capability attributed to processing in the ventral stream of the cortical visual system. These

authors therefore posited that abnormalities in dorsal stream functioning are characteristic

of developmental disorders. This conclusion was based predominantly on the

psychophysical studies that measured coherence thresholds for global motion as an index of

dorsal-stream functioning. However, as will be explained below, there are multiple stages

within both of these cortical pathways and it is unlikely that a single task could capture

processing at every level of either stream. While studies assessing the ventral pathway in

Williams syndrome were outlined by Braddick et al., fewer studies examining multiple

levels within the dorsal or ventral visual streams had been conducted for the other

developmental disorders included in their argument. Much research assessing the visual

capabilities of different levels within both visual pathways for several developmental

disorders has occurred since then, and there have been advances in the way the visual

system is conceptualised and measured. Accordingly, it is now pertinent to re-evaluate

whether it is the case that developmental disorders can be characterised by a general

vulnerability in the dorsal visual stream.

The aim of this review is to consider five developmental conditions for which early

to mid level visual abilities have been investigated - developmental dyslexia,

developmental dyspraxia, Williams syndrome, Fragile X syndrome, and autism spectrum

disorders (ASDs) - to evaluate whether the pattern of performance on visual tasks is

restricted to impairment in dorsal stream functioning. While there are studies of the

anatomical development of the visual system (e.g. Livingstone, Rosen, Drislane, &

Galaburda, 1991), patterns of saccadic eye movements (e.g. Kemner, Verbaten, Cuperus,

Camfferman, & van Engeland, 1998), and the involvement of the cerebellum (e.g. Takarae,

Minshew, Luna, & Sweeney, 2004), in visual perception (all of which involve the dorsal

stream), the focus of this review is on psychophysical measurements of visual functioning

in developmental disorders. The studies investigating visual functioning in this manner

40

frequently use similar methods across different disorders. Therefore, following an update

on recent theories regarding the pathways in the visual system and an outline of the visual

paradigms most commonly used in investigations of early to mid level visual abilities, this

review will summarise critical findings associated with each developmental disorder. We

consider what contribution this research makes towards our understanding of these

paediatric conditions in addition to evaluating whether performance on visual tasks is

consistent with impairment in the dorsal stream. The advantage of considering the

developmental disorders together is that we can evaluate whether this purported profile of

anomalous dorsal stream processing is common to several disorders, or whether instead

some of the conditions show certain visual anomalies not expressed in the other disorders.

The human visual system

Structure of the visual system

In the largest visual pathway of the primate visual system, information is

transmitted from the retina to the lateral geniculate nucleus (LGN) and then on to the

primary visual cortex (V1) via three distinct sub-pathways: the magnocellular (M),

parvocellular (P) and koniocellular (K) streams1 (Casagrande, Yazar, Jones, & Ding, 2007;

Merigan & Maunsell, 1993). These sub-pathways account for the majority of the input to

V1, although anatomical and physiological evidence shows other pathways containing

fewer fibres exist (Kaplan, 2004). The segregation of sub-pathways is very obvious within

the LGN, which is composed of six prominent layers, the lower two consisting of large cell

bodies known as the M (or magno) cells, and the upper four consisting of smaller cell

bodies known as the P (or Parvo) cells, with the K (or Konio) cells interlaminar to each of

these six main layers. These cells differ in their physiology as well as their anatomy

(Kaplan, 2004; Maunsell et al., 1999).

The M cell population has relatively large receptive fields, is not systematically

selective for colour, and has lower spatial resolution, higher temporal resolution and faster

conduction speeds than the P cell population (although the populations do have

1 The koniocellular pathway is currently thought to be concerned primarily with blue-yellow colour perception and to have slower conduction velocities and more diverse response properties than the M and P cell responses (Sumner, Anderson, Sylvester, Haynes, & Rees, 2007). Since it has not been a focus in research on developmental disorders, the koniocellular pathway will not be considered further.

41

considerable overlap on many of these dimensions (see Kaplan, 2004; Merigan &

Maunsell, 1993). The relative specialisation of function in the M and P cells led to the

suggestion of specialised neural pathways (Breitmeyer & Ganz, 1976; DeYoe & Van

Essen, 1988; Livingstone & Hubel, 1988). In extra-striate cortical regions, the M cells

provide the predominant input to the dorsal stream leading to the dorsolateral occipital

cortex (Maunsell, 1987) and regions of the posterior parietal lobe (Goodale & Westwood,

2004). This pathway responds well to rapidly changing stimuli such as flicker and motion

(Livingstone & Hubel, 1987; Zeki, 1978). Studies of primate physiology, lesions in humans

and neuroimaging have identified an important role for the dorsal pathway in the

processing of motion (Culham, He, Dukelow, & Verstraten, 2001). The P cells provide the

predominant input to the ventral visual stream leading to inferotemporal areas of the

temporal lobe (Goodale & Westwood, 2004). This pathway is optimised for encoding

information about shape and colour, and responds to slower moving or stationary stimuli

(Ungerlieder & Mishkin, 1982). Evidence from neuropsychology suggests the ventral

stream is implicated in form perception (Beason-Held et al., 1998). Currently, it is

understood that the idea that motion processing relies exclusively on the dorsal stream, and

form processing relies exclusively on the ventral stream, is too simplistic (Ross, Badcock,

& Hayes, 2000) and that the cortical pathways show appreciable cross-talk (Braddick,

O'Brien, Wattam-Bell, Atkinson, & Turner, 2000; Merigan & Maunsell, 1993; Tanskanen,

Saarinen, & Parkkonen, 2008).

Cortical projections from the visual pathways proceed in a hierarchical manner,

from lower to higher cortical areas (Livingstone & Hubel, 1988; Maunsell & Newsome,

1987). At the earliest stage of visual perception the neurons in the primary visual cortex

(V1) extract information about the orientation, curvature, and spatial and temporal

frequency of stimuli from small regions in the retinal image (i.e. predominantly local

processing, Dobbins, Zucker, & Cynader, 1987; Hubel & Wiesel, 1968). Higher visual

areas combine the information from V1 to extract more global aspects of images. With

respect to the ventral stream, it has been argued that V2 comprises an intermediate stage of

angle processing by combining orientation information from filters in V1 (Hedge & Van

Essen, 2000), and by detecting implied and second-order contours (von der Heydt,

Peterhans, & Baumgartner, 1984). V4 then encodes more complex object features than

edge orientation, such as complex curved shapes (Pasupathy & Connor, 2002). Thus, V4

42

has been argued to have an important role in global form perception (Wilson & Wilkinson,

1998). In the dorsal stream, direction sensitivity arises in V1 in primates (Hubel, 1982), and

the integration of information received from these cells, occurring in V3 and V5, results in

preferential activation in response to fronto-parallel motion (Braddick et al., 2001). Higher

up in the dorsal pathway, V6 cells are characterised by their preference for different types

of pattern structures revealed only through large field motion such as rotation, and radial

expansion or contraction of the retinal image (Fattori, Pitzalis, & Galletti, in press; Pitzalis

et al., 2006). Thus these areas have a central role in global motion perception.

While it was initially proposed that cortical projections proceed only in a

feedforward manner, this idea has been revised following the discovery of an extensive

network of feedforward and feedback interconnections (Van Essen & Maunsell, 1983; Zeki

& Shipp, 1988). The fact that conduction is faster for the larger M cells than for the smaller

P cells allows for multiple cortical interactions via feedback or (in instances of masking) by

M activity interfering with slower P activity at various levels of the visual system

(Breitmeyer, Levi, & Harwerth, 1981; Harwerth & Levi, 1978; Williamson, Kaufman, &

Brenner, 1978, but see Lennie, 1993). Thus, a �magnocellular speed advantage� has been

reported in the primate (Maunsell et al., 1999) and human (Klistorner, Crewther, &

Crewther, 1997) literature. Several theories of visual processing have been proposed (see

Bullier, 2001; Laycock, Crewther, & Crewther, 2007; Zeki & Shipp, 1988) in which this

magnocellular advantage allows for the possibility of information carried by the M cells

modulating the response to the later arrival of information carried by the P cells. The

magnocellular advantage is thought to be very important in normal vision as it involves the

initiation of attention mechanisms in the parietal cortex, allowing for a fast and automatic

initial global analysis of a visual scene (Saalmann, Pigarev, & Vidyasagar, 2007).

It is the magnocellular pathway that feeds to the dorsal cortical stream that has been

of particular interest to researchers investigating developmental disorders and that underlies

Braddick et al.�s (2003) dorsal stream vulnerability hypothesis. Specifically, it has been

proposed that the larger M cells are more at risk early in the disease process than the P

cells, since neurons with larger cell bodies and axon diameters are more susceptible to

damage (Quigley, Dunkelberger, & Green, 1988). Additionally, magnocellular pathway

loss might be more readily detected because there are far fewer M cells than P cells

(approximately 80% of the retinal ganglion cell population is P cells, 8-10% M cells and 5-

43

10% K cells Dacey & Petersen, 1992). Thus even if neurons were lost proportionally across

all cell types, the sparser M cell system may demonstrate more readily detectable functional

loss (McKendrick, Badcock, & Morgan, 2004). These factors are consistent with Braddick

et al.�s (2003) suggestion that measures of dorsal stream function may be more likely to

show impairment or that the stream is �more vulnerable in development� (p. 1779) and

therefore when a paediatric disorder is present, the likelihood of the magnocellular pathway

exhibiting an abnormality may be increased, even if all cell types are affected. It is not yet

clear whether this �magnocellular disadvantage� does indeed manifest across multiple

developmental disorders. Therefore, investigating the evidence for dorsal stream

impairment in these conditions forms the basis of the current review.

Much of the research investigating vision in the developmental disorders retains the

conceptualisation of specialised but linked dorsal and ventral pathways, processing

information hierarchically. Importantly, in order to determine the specificity of the

purported magnocellular/ dorsal pathway deficit in developmental disorders, the integrity of

both the dorsal and ventral streams at both early and later visual processing stages must be

assessed. In order to summarise the studies that have assessed functioning of this nature in

the developmental disorders, we first describe a selection of ways in which different levels

of both pathways are assessed psychophysically. This summary is by no means exhaustive.

We focus on the methods most commonly used thus far to assess visual abilities in the

developmental disorders. With respect to psychophysical studies, it is important to note that

the whole visual pathway from retina to motor response is assessed. However, it is assumed

that critical aspects of particular tasks are performed at specific points along the pathway

and that failure on those aspects can identify the locus of a particular psychophysical effect

(Teller, 1980). Information gained from electrophysiological and imaging studies provides

an important addendum to the psychophysical literature regarding physiological events or

anatomical loci, and where relevant is included in this review.

Methodology used to assess visual functioning

Early Visual Processing

As outlined above, there are a variety of functions performed by the neurons in V1,

and it is impossible to measure all these functions simultaneously. Thus, the most common

psychophysical methods determine the minimally detectable presence of one stimulus

44

attribute at a time. Predominantly, it is a contrast threshold that has been measured.

Research has determined that, when presented at appropriate temporal frequencies, gratings

with very low spatial frequencies, well below the peak of the contrast sensitivity function,

can be used to assess M cell functioning, whereas those well above the peak may be used to

address P cell performance (Legge, 1978; Skottun, 2000). One reason for using extreme

values is that the peak of the contrast sensitivity function varies with display size and mean

luminance (Kelly & Burbeck, 1984; McCann & Hall, 1980; McCann, Savoy, & Hall,

1978). Contrast sensitivity tasks assessing the parvocellular system generally employ high

spatial frequency gratings with a low temporal frequency, whereas tasks assessing the

magnocellular system typically use low spatial frequency gratings, or Gaussian blobs, with

a high temporal frequency. For grating stimuli turned on and off gradually, M cells give

little or no response at any spatial frequency (Kelly & Burbeck, 1984) whereas P cells

respond at various intensities depending on the spatial frequency and contrast (Hicks, Lee,

& Vidyasagar, 1983). Thus, the spatial and temporal characteristics of stimuli assessing the

contrast sensitivity of M and P cells must be chosen carefully.

In a typical population, infants� contrast sensitivity is poor compared to that of

adults; newborns can see stripes only if the spatial frequency is less than 1.0 cycles per

degree and at high contrast, whereas adults can see spatial frequencies almost 40 times that

amount (see Maurer & Lewis, 2001a, 2001b, for reviews). Contrast sensitivity improves

during early development, but takes approximately 7 years to reach adult levels (Ellemberg,

Lewis, Liu, & Maurer, 1999).

Global Processing

Global Dot Motion (GDM) stimuli provide a sensitive measure of dorsal stream

capability (Newsome & Paré, 1988) in that they assess global processing predominantly

associated with areas V3a and V5 (Braddick et al., 2001; Britten, 2004). In one common

form of GDM stimulus, a proportion of dots on a computer monitor move coherently and

the remaining (noise) dots move in random directions at the same speed. Steps are taken to

prevent observers detecting the signal motion direction by tracking the trajectory of a single

dot. For instance, the lifetimes of single dots can be limited, with each disappearing dot

replaced by a new dot at a different location (Newsome & Paré, 1988), or the dot can

continue throughout the lifetime of the display, but be assigned to signal or noise directions

at random for each frame transition (Edwards & Badcock, 1994). The ability to perceive

45

global coherent motion therefore depends on successful detection and integration of local

motion signals over both space and time (Burr, Morrone, & Vaina, 1998; Dickinson,

Broderick, & Badcock, 2009; Snowden & Braddick, 1989). The smallest proportion of dots

that have to move coherently for the observer to perceive coherent global flow gives the

threshold for coherent motion detection.

There are two ways in which global processing in the ventral stream has been

assessed in the literature examining developmental disorders. The first presents a coherent

form signal defined by short, high contrast line segments that are oriented according to a

geometric rule (e.g. vertical, concentric), with all other line elements randomly oriented

(see Figure 1a). The smallest proportion of lines that have to be coherently oriented for the

observer to discriminate a field containing the pattern from one that does not gives the

threshold for coherent form detection. In V1, the response of orientation-tuned columns can

be facilitated by long range connections to other columns preferring the same orientations

in adjacent parts of the visual field (Loffler, 2008). Recent investigations suggest that

detectability of contours created by line segments can be enhanced in a similar way as that

seen by the facilitation of long-range connections in V1 (Field & Hayes, 2004; Li &

Gilbert, 2002). For this reason, global-form detection tasks that can be completed by

detecting extended contours may well allow a grouping contribution from V1 and should

therefore be avoided if the aim is to investigate global processing in V4. Instead, Glass

patterns (Glass, 1969) provide a useful alternative as they specifically target high-level

integrative processing in the ventral stream (Tse et al., 2002; Wilson & Wilkinson, 1998).

Glass patterns consist of randomly distributed dot dipoles, a proportion of which conform

to a global structure, which is achieved by aligning the dots within pairs along contours of

the desired global structure (such as concentric or parallel; see Figure 1b). These stimuli

minimise facilitation by long range connections between orientation-tuned columns in V1

because random dispersion of the dot dipoles means there is no systematic alignment of any

dot dipole with neighbouring dipoles, resulting in very few contours longer than a dot pair.

The nature of the noise in the stimulus display (randomly oriented dipoles) means that long

range facilitation processes are less likely to link signal contours as selectively. In Glass

patterns, an observer must combine the information from within multiple pairs of dots to

perceive the overall structure.

46

With respect to global processing in typical populations, for the dorsal stream Gunn

et al. (Gunn et al., 2002) reported that motion coherence thresholds for a task where

observers were required to locate a target strip in which the direction of motion was

opposite to the rest of the display does not reach adult levels until approximately 10-11

years. Conversely, Parrish, Giaschi, Boden and Dougherty (2005) concluded that the

perception of coherent global motion reaches adults levels at approximately 6 years. Parrish

et al. (2005) also reported that performance on motion-defined form tasks improved up

until approximately age 7 years, whereas performance on texture-defined form tasks

continued to improve up to the oldest age group they assessed (11-12 year olds), suggesting

that global abilities in the ventral pathway develop later than those in the dorsal pathway. In

other work on ventral stream global processing, Lewis et al. (2004) reported that thresholds

for parallel and concentric Glass patterns were immature at 6 years of age, but were adult-

like by 9 years of age (see also Porporino, Shore, Iarocci, & Burack, 2004), for an example

of global form processing developing until 8 years of age using a non-psychophysical

stimulus). Thus, most forms of global processing appear to mature prior to adolescence.

Figure 1. (a) Example of a coherent line segment stimulus, taken from Milne et al. 2006,

and (b) example of a 100% coherent concentric Glass pattern

First- and Second-order Processing

Our visual world contains both luminance- (first order) and contrast- (second order)

defined information (Schofield, 2000). Separate mechanisms for processing first- and

47

second-order stimuli, both stationary and moving, have been demonstrated (see Clifford &

Vaina, 1999, for a review). Frequently, first-order motion and form stimuli are luminance-

modulated noise patterns created by adding grey-scale noise to sinusoidal luminance

modulation (e.g. a vertical sinusoid for translational motion; see Figure 2a). Second-order

motion and form stimuli are often texture-modulated noise patterns produced by

multiplying rather than summing modulating sine waves with the grey-scale noise (see

Figure 2b). Critically, mean luminance level varies across space for first-order stimuli, and

is therefore detectable by linear spatial operators. Second-order stimuli vary in contrast and

not local mean luminance and are therefore intended to be invisible to the linear spatial

operators operating at the signal frequency, such as those found in early vision (Badcock &

Derrington, 1985). Bertone and colleagues (Bertone, Mottron, Jelenic, & Faubert, 2003,

2005) refer to these first- and second-order stimuli as �simple� and �complex� stimuli,

respectively, since the first-order stimuli are purported to be processed by linearly-summing

output of simple cells in V1, whereas additional neural processing is required before

second-order stimuli are perceived, and this processing occurs further along in the visual

streams (Wilson, Ferrera, & Yo, 1992).

However, it is unclear whether the dynamic stimuli such as those described by

Bertone et al. (2003) are able to cleanly differentiate between simple and complex

processing. For the dynamic second-order stimuli, it is possible that an observer can select

one bar within the image and track its direction across space (known as �attentive

tracking�) rather than integrate information across the multiple elements of the display

(Derrington, Allen, & Delicato, 2004). Derrington et al. reported that this is most likely to

occur at or near the contrast threshold, and thus provides a third mechanism (over and

above the first- and second-order systems) by which these stimuli may be perceived. If

attentive tracking can be used to perceive second-order stimuli, this may subvert the ability

of such tasks to assess dependence on integrative capabilities at more complex levels.

While stimuli are available that avoid this potential problem (see Badcock, Clifford, &

Khuu, 2005; Badcock & Khuu, 2001; Edwards & Badcock, 1995, for examples of first- and

second-order Glass pattern and GDM stimuli), these have not yet been applied to the study

of developmental disorders.

In investigating typical development, Lewis et al. (2007) reported that first- and

second-order perception of static stimuli was equivalent for 5 year-olds compared to adults.

48

These findings are consistent with those of Bertone et al. (2008) who also reported no age

differences in thresholds for first- and second-order static stimuli. Ellemberg et al. (2004)

reported that for both first- and second-order motion stimuli, thresholds for 5 year-olds

were higher than those for adults, but this was more pronounced for the second-order than

the first-order stimuli. Bertone et al. (2008) found a similar pattern of results in their 5-6

year old age group, but reported that second-order motion perception reached adult levels

earlier (7-8 years) than first-order motion perception (9-10 years) when assessing older

children. These findings suggest that first- and second-order processing of form stimuli

appears to mature earlier than first- and second-order processing of motion stimuli.

Figure 2. (a) Example of a static first-order stimulus, and (b) example of a static second-

order stimulus, taken from Bertone, Hanck, Cornish, and Faubert (2008)

Vision in the developmental disorders

Braddick and colleagues (2003) present a body of evidence suggesting that

functioning within the ventral visual stream matures earlier than dorsal stream functioning.

They suggest that the later development of the dorsal stream provides a greater opportunity

for anomalous development to impair functioning within this pathway. Thus, when a

developmental disorder is present, the dorsal stream may be more susceptible to

impairment. They suggest that this vulnerability is not specific to one particular condition,

but rather is characteristic of many developmental disorders. Accordingly, they postulate

qualitatively similar impairments in the dorsal stream across these conditions. For the

psychophysical tasks of interest in this review, some of the summaries above are consistent

with the suggestion that the ventral visual pathway develops earlier than the dorsal stream

49

in typical development, as evidenced by performance on first- and second-order static tasks

reaching adult levels earlier than is the case for equivalent dynamic tasks (Bertone et al.,

2008; Lewis et al., 2007). However, the evidence reviewed suggests that for global

processing tasks the distinction may be more equivocal, with some studies reporting adult-

like performance at similar ages for GDM tasks (Gunn et al., 2002) and Glass pattern tasks

(Lewis et al., 2004), and one study even reporting that sensitivity to global form develops

later than sensitivity to coherent motion (Gordon & McCullough, 1999; Parrish et al.,

2005). Thus, the assertion that dorsal stream functioning develops later than the ventral

stream, making individuals with a developmental disorder more susceptible on tasks

designed to assess the dorsal pathway requires further assessment. However, the purpose of

this section is to test more generally the claim that the dorsal stream is especially at risk in

the presence of a developmental disorder by reviewing the literature for those conditions in

which visual performance has been assessed using the psychophysical methods outlined

above. For each disorder, we first consider how impairment in the dorsal stream may relate

to the symptomatology of the condition, and then outline the results of studies that have

examined visual abilities in those affected by the disorder.

Dyslexia

Developmental dyslexia is a specific disability in which individuals do not acquire

proficient reading skills, despite sufficient cognitive abilities and education (WHO, 2005).

Because reading is primarily a visual task requiring the integration of information from

successive fixations (Badcock & Lovegrove, 1981), it is possible that some of the reading

difficulties seen in dyslexia are the result of anomalies in processing visual information. In

particular, initial proposals suggested a role for the magnocellular system in reading that

involved the suppression of the parvocellular system during saccades (Breitmeyer, 1993).

In light of more recent evidence suggesting that it is the magnocellular system rather than

the parvocellular system that is the target of suppression during saccades (Anand &

Bridgeman, 1995), other hypotheses have been explored concerning the role of the

magnocellular system in reading problems in dyslexia. For example, Vidyasagar (1999)

argued that, when reading, sequential scanning of individual letters during fixation periods

is necessary for effective letter identification. Since the large receptive fields of the ventral

stream areas involved in object recognition do not code well for location, feedback from the

dorsal stream could feed the location of the letters of each word in a temporal sequence to

50

the ventral stream (Vidyasagar, 2004, 2005). According to Vidyasagar, when learning to

read, this attentional gating has to be trained to move sequentially across lines of text.

Purportedly, difficulties in this process can happen even with small lesions affecting the M

cells in critical parts of the visual field, preventing effective attentional spotlighting over

the letters during each fixation.

Much research has focused on determining whether dyslexic readers do indeed

show impairment in the magnocellular pathway evidenced by reduced contrast sensitivity.

This has already been the subject of an extensive review, and a detailed evaluation is

beyond the scope of this paper; hence a brief summary is provided. In his review, Skottun

(2000, see also Skottun & Skoyles, 2006, 2007) reported that, of the 22 studies which

investigated spatial contrast sensitivity in dyslexia, four found impairments at low spatial

frequencies, suggesting a problem in M cell functioning (see also Sperling, Lu, Manis, &

Seidenberg, 2003), eleven studies found evidence of deficits of a nature incompatible with

a deficiency in the magnocellular system, and seven studies were inconclusive (see also

Williams, Stuart, Castles, & McAnally, 2003). Similarly, of the seven studies investigating

temporal contrast sensitivity, only two provided evidence consistent with an M cell deficit

in dyslexia, while the other five were inconclusive. Skottun suggested that most of the

research reviewed did not adequately distinguish between M and P cell functioning in that

many studies did not involve spatial frequencies below the peak of the contrast sensitivity

function. He thus concluded that further research needs to be conducted to establish

whether the popular theory of a magnocellular deficit in dyslexia can be supported.

However, contrast sensitivity is only one property of the neurons in V1. Not included in

Skottun�s review were those studies that demonstrate greater visible persistence at low

spatial frequencies in individuals with dyslexia when compared to control groups (Badcock

& Lovegrove, 1981; Slaghuis & Ryan, 1999; Slaghuis & Lovegrove, 1984), which has also

been explained in terms of a magnocellular pathway deficit (Lovegrove, Martin, &

Slaghuis, 1986). While the notion of a magnocellular deficit explaining the reading

difficulties in dyslexia has been very popular, it appears that the evidence from measures of

contrast sensitivity is currently unable to support the claim of a simple and consistent link

between the two (Skottun, 2000). However, attempts have been made to explain why some

studies find differences whereas others do not, based on subtle differences in task

properties. For example, with respect to the attentional gating hypothesis, Vidyasagar

51

(2004, 2005) suggested that the small lesions purportedly affecting the M cells might not

always be detectable with the usual tests of M cell functioning, which may explain why

some investigators (e.g. Amitay, Ben-Yehudah, Banai, & Ahissar, 2002; Skottun, 2000) do

not agree that there is a specific M cell impairment in dyslexia. This issue may be

compounded by the fact that a uniform definition of dyslexia has not been used when

selecting participants (Hogben, 1996).

Also not included in Skottun�s review were studies investigating high-level

processing in the dorsal stream. While there are reports of equivalent performance

(Tsermentseli, O'Brien, & Spencer, 2008; White et al., 2006), the majority of studies

indicate that children with dyslexia are less sensitive than age- and IQ-matched controls to

coherent motion stimuli (Cornelissen, Richardson, Mason, Fowler, & Stein, 1995; Hansen,

Stein, Orde, Winter, & Talcott, 2001; Pellicano & Gibson, 2008; Raymond & Sorenson,

1998; Slaghuis & Ryan, 1999; Talcott, Hansen, Assoku, & Stein, 1998). Finally, all studies

assessing higher-level processing in the ventral stream have found intact abilities when

comparing individuals with dyslexia to matched controls (Hansen et al., 2001; Tsermentseli

et al., 2008; White et al., 2006). Both Hansen et al. (2001) and White et al. (2006) used line

segment stimuli, whereas Tsermentseli et al. (2008) used Glass pattern stimuli. These

studies are in agreement despite using different methodologies and the concern (outlined

above) regarding the ability of line segment stimuli to tap global processes. Thus, were it to

be the case that a magnocellular deficit affects contrast sensitivity and global motion

processing in dyslexia, it does not appear that the underlying causes impact on the ventral

pathway.

Overall, given that the ventral stream appears to be intact at both the earlier and

later stages of visual functioning in dyslexia, it would appear that any visual deficits in this

condition have the potential to be restricted to the dorsal stream, consistent with Braddick

et al.�s (2003) dorsal stream vulnerability hypothesis. Given the varied and conflicting

results on contrast sensitivity measures, it remains to be clarified as to whether the

reasonably consistent impairments in global motion processing in dyslexia are accompanied

by deficits at the earlier levels of the dorsal stream. Considering the properties of the cells

in V1 other than contrast sensitivity (such as direction selectivity or speed of processing)

will be important in making this distinction. It has also been argued that future research

assessing visual abilities would be facilitated by adopting an agreed and consistent

52

definition of the diagnostic criteria for dyslexia (Hogben, 1996), and examination of the

profiles of subgroups within this population (Borsting et al., 1996).

Autism Spectrum Disorders

Individuals with an ASD exhibit delays in language development, social and

communication difficulties and repetitive, stereotypic behaviours and interests (American

Psychiatric Association, 2000). In this condition, anomalous visual abilities may impact on

the perception of faces and body gestures essential for social communication (e.g. Deruelle,

Rondan, Gepner, & Tardif, 2004; Pellicano, Jeffery, Burr, & Rhodes, 2007). However,

while not required for a diagnosis, it is the commonly reported motor functioning deficits in

ASDs (see Rinehart, Bradshaw, Brereton, & Tonge, 2001, for a review) that are most likely

linked to specific difficulties in dorsal stream perception. The dorsal pathway has an

important role in conveying information about the spatial relations between objects (see

Milner & Goodale, 2008, for a review) and about their motion, and thus is purported to be

involved in position coding and visually guided actions. Therefore, it is this pathway that is

likely to be implicated in the abnormalities in co-ordination, gait, balance and posture that

are frequently observed in children with an ASD. The potential for these anomalies to arise

from visual deficits is highlighted by evidence that children with autism have a very weak

postural reactivity to visually perceived environmental motion (Gepner, Mestre, Masson, &

de Schonen, 1995). One possible explanation posited to account for these results was that

children with autism have a deficit in the perception of motion and therefore experience

less need to adjust their posture in response to environmental motion when compared to

typically developing children. Thus, many studies have focused on determining whether

children with an ASD exhibit a specific motion processing deficit, consistent with dorsal

stream impairment.

Several researchers have reported higher motion coherence thresholds in individuals

with high functioning autism compared to matched control groups on GDM tasks (Davis,

Bockbrader, Murphy, Hetrick, & O'Donnell, 20062; Milne et al., 2002a; Pellicano, Gibson,

Maybery, Durkin,

2 Davis et al. (2006) administered two GDM tasks, the first requiring children to identify the direction of motion and the second requiring identification of whether two stimuli were moving in the same or different directions. Short and long presentation-duration versions of these tasks were administered. Children with autism showed a deficit in identifying the direction of motion in the long presentation condition only.

53

& Badcock, 2005; Spencer & O'Brien, 2006; Spencer et al., 2000; Tsermentseli et al., 2008,

but see de Jonge et al., 20073). As an alternative to GDM stimuli in assessing global motion

processing, Vandenbroucke, Scholte, van Engeland, Lamme and Kemner (2008) employed

plaid stimuli that can be perceived as a coherently moving pattern when integrated or,

alternatively, as two transparent gratings sliding over each other. The proportion of time the

plaid was seen as coherent rather than sliding did not differ for an ASD group compared to

age- or IQ-matched control groups, suggesting no evidence of impaired global motion

perception in ASDs with this task. However, the transition between transparency and

coherence in plaids is a gradual one and the decision point between one percept and the

next is open to subjective interpretation. Thus, this can result in variability in responses

which may mask any group differences that may exist.

With respect to lower-level dorsal stream functioning, several studies have reported

intact flicker contrast sensitivity thresholds in individuals with high functioning autism

when compared to age- and non-verbal IQ-matched controls (Bertone et al., 2005; Davis et

al., 2006; Pellicano et al., 2005). This suggests that the visual difficulties experienced by

individuals with autism are not a function of deficient M cell contrast sensitivity. Rather,

the favoured interpretation has been that impaired global motion thresholds in the presence

of intact flicker contrast sensitivity thresholds is indicative of impairment in global

processing at the higher levels of the dorsal cortical stream (Bertone et al., 2005; Pellicano

et al., 2005).

Regarding lower-level ventral stream processing, Davis et al. (Davis et al., 2006)

and Sanchez-Marin and Padilla-Medina (Sanchez-Marin & Padilla-Medina, 2008) reported

that ASD groups, relative to controls, had lower contrast sensitivity thresholds (or better

performance) for the detection of high spatial frequency gratings. However, de Jonge et al.

(2007) found no significant difference in ability to perceive orientation between a group

with ASD compared to an age- and IQ-matched control group for high spatial frequency

gratings. When higher-level functioning in the ventral pathway has been assessed,

individuals with an ASD have been found to exhibit comparable performance on coherence

thresholds for global structure in line segment tasks when compared to matched control

3 In this study, the ASD and control groups did not differ in motion coherence thresholds for a GDM task, however, the task had an unlimited stimulus presentation time, and the magnitude of the steps sizes was large (5%) compared to studies which have found a difference in coherence thresholds (e.g. Pellicano et al., 2005). These features may have limited the task�s sensitivity to subtle differences between the two groups.

54

groups (Blake, Turner, Smoski, Pozdol, & Stone, 2003; Milne et al., 2006; Spencer et al.,

2000). In contrast, impaired Glass pattern thresholds consistent with anomalous global

processing in the ventral stream have been found in sub-groups of individuals with autism,

but not the whole mixed ASD samples, when compared to control groups (Spencer &

O'Brien, 2006; Tsermentseli et al., 2008)4. Thus, while it appears that difficulties in higher-

level global grouping in the dorsal cortical stream may be able to account for the elevated

global motion thresholds in ASDs, it is currently unclear whether there is a comparable

impairment in higher-level global processing in the ventral pathway. If it is the case that the

contour detection tasks evoked by line segment stimuli can be processed by the cellular

networks in V1 (Field & Hayes, 2004; Li & Gilbert, 2002; Loffler, 2008), then, with the

exception of Vandenbroucke et al. (2008), the results outlined above appear to be consistent

with the notion that individuals with autism are unimpaired on tasks requiring lower level

processing in the form pathway (with respect to both contour detection and contrast

sensitivity), but exhibit difficulties on form tasks relying more heavily on higher-level

integration, such as in detecting concentric Glass patterns.

Bertone et al. (2003) assessed visual performance in ASDs with first- and second-

order translating, radiating and rotating motion stimuli. The second-order stimuli are

considered more �complex� than the first-order stimuli as they require additional neural

processing. No significant group differences in direction discrimination were found with

first-order motion perception, but the autism group required higher modulation depths to

discriminate the direction of motion for all second-order patterns, relative to an age-

matched control group. To assess ventral stream processing, Bertone et al. (2005) used

first- and second-order form stimuli constructed in the same way as the motion stimuli in

Bertone et al. (2003). Their autism group performed better than age-matched controls on

the first-order form task (i.e. they required less modulation of contrast to determine whether

a grating was horizontal or vertical), but the autism group performed more poorly than

controls on the second-order form task. Bertone et al. (2005) suggested that these results

4 Vandenbroucke et al. (2008) recorded event-related potentials in response to figure-ground segregation of textured figures in order to examine the roles of feedforward, feedback and horizontal connections in visual processing in ASD. Horizontal connections are thought to play an important role in boundary detection and individuals with an ASD showed diminished cortical activity and had more difficulty on the figure-ground task that relied mainly on boundary detection. Vandenbroucke et al. therefore argued that deficient horizontal connections in low-level visual processing characterise ASDs. However, it is difficult to reconcile how impairment in early contour linkage, suggested by Vandenbroucke et al., in addition to impaired Glass pattern detection, can occur in the presence of intact perception in line segment tasks. Importantly, further study concerning the underpinnings of the line segment coherence task is required.

55

may reflect �atypical neural connectivity mediating the extraction of low-level orientation

information within the visual processing hierarchy in autism� (p. 2436).

Other visually-based abnormalities have also been demonstrated in individuals with

autism in the form of superior performance in detecting embedded figures and in

reproducing block designs relative to controls (see Grinter, Van Beek, Maybery, &

Badcock, 2009, for a review). Both these tasks require the ability to overcome the natural

tendency to initially perceive the gestalt in order to focus on individual stimulus elements.

In an attempt to account for both the strengths and weaknesses seen in ASDs, Weak Central

Coherence theory was proposed (Frith, 1989). Under this account, children with ASDs have

difficulty combining local information to create a coherent global percept, a consequence of

which can be their superior performance on tasks that require attention to details. The

central tenets of Weak Central Coherence theory are consistent with Bertone et al.�s (2005)

suggestion that individuals with an ASD have difficulty processing complex information

that requires the integration of information from multiple cortical regions. The findings

within the dorsal visual stream in ASDs are also consistent with these hypotheses in that

low-level processing appears to be intact, whereas individuals affected by these disorders

display reduced sensitivity in global processing. Taken together these findings do not

support an impairment specific to the dorsal visual system, but instead suggest a profile of

visual performance characterised by difficulties in integrating information at the higher

levels of both visual pathways.

To summarise, it does not appear that Braddick et al.�s (2003) suggestion of

impairment that is specific to the dorsal stream is characteristic of ASDs. Because sub-

cortical dorsal stream processing remains intact in this population, it seems that any

impairment in dorsal stream functioning in individuals with ASDs is restricted to the global

level. While there is also some evidence of anomalous global processing in the ventral

stream in autism, a comprehensive understanding of the capabilities of the ventral stream at

both local and global stages in autism remains elusive.

Dyspraxia

Clumsiness, lack of coordination and poor balance are some of the most noticeable

features of developmental dyspraxia (Motohide & Möbs, 1995). Visual information has an

important role in the planning and execution of coordinated movements (Jeannerod, 1996),

and thus it is possible that visual perceptual deficits also play an important role in

56

dyspraxia. Many of the developmental milestones that children with dyspraxia struggle

with, like catching a ball, jumping or tying shoelaces, are linked to visual perceptual

deficits such as reduced gain in pursuit eye movements (Langaas, Mon-Williams, Wann,

Pascal, & Thompson, 1998). However, it is difficult to account for the visuo-motor deficits

seen in dyspraxia without reference to the dorsal visual stream, given its role in conveying

information about the spatial relations between objects and about their motion (see

discussion in ASDs section above). Thus it is possible that impaired transmission of visual

information, particularly within the dorsal stream, is implicated in the lack of co-ordination,

poor balance and poor visuo-motor task performance seen in dyspraxia. Such deficits

would be expected to affect visual processes that require the coding of information about

the spatial positions of objects relative to the observer (Milner & Goodale, 2008).

In an attempt to establish whether children with dyspraxia do demonstrate a

disruption to the dorsal visual system, O'Brien, Spencer, Atkinson, Braddick, & Wattam-

Bell (2002) measured thresholds on a GDM task, and compared them to thresholds on a

line-segment contour detection task. Children with dyspraxia were impaired in the ability to

detect coherent line-segment structure, but global motion processing ability was unaffected

compared to an age and verbal mental-age matched control group. In another study,

Sigmundsson, Hansen and Talcott (2003) applied the same GDM and coherent line

segment measures as were employed by Hansen et al. (2001) to test whether impaired

visual function is characteristic of children with motor impairments. In contrast to O�Brien

et al. (2002), Sigmundsson et al.�s �dyspraxic� group was not formally diagnosed; instead, it

comprised the extreme 25% of scorers on the Movement ABC (Henderson & Sugden,

1992) test attending a regular classroom. Sigmundsson et al. (2003) reported that

developmental clumsiness was associated with difficulties in the detection of both global

visual motion and the coherent organisation of static line segments.

While O�Brien et al. (2002) suggested that the discrepancy in their results for tasks

assessing the two visual pathways indicates that children with dyspraxia have a specific

deficit in global processing in the ventral pathway, Sigmundsson et al. (2003) clearly

provide conflicting evidence. Of relevance here is the argument advanced earlier that the

coherent line stimuli used in these two studies are likely to also assess visual abilities

associated with the earlier stages in the ventral cortical pathway in addition to tapping

global form processing mechanisms. The data from the two studies using these tasks to

57

assess visual processing in dyspraxia indicate impairment at some level in the ventral

stream, but given the possibility that processing in V1 may contribute to contour detection,

the precise locus of the impairment remains unclear. Glass patterns would assist in

clarifying this issue, since, as noted earlier, they more specifically target high-level

integrative processing in the ventral stream. Furthermore, O�Brien et al (2002) matched

their samples for chronological age and verbal mental age, and excluded any child with a

comorbid diagnosis. Sigmundsson et al. (2003), on the other hand, did not take IQ into

account apart from noting that no child had any reported history of learning or reading

disability. The failure to match samples may have impacted on the differences reported in

the Sigmundsson et al. study.

A global motion processing deficit, when not associated with a deficit in early

visual processing, signifies disruption to the visual processes in the later stages of the dorsal

stream, and could be particularly central to the symptomatology of dyspraxia given the role

the dorsal pathway plays in visually guided movement (Edwards & Badcock, 1993;

Warren, Kay, Zosh, Duchon, & Sahuc, 2001; Whitney et al., 2007). However, both O�Brien

et al. (2002) and Sigmundsson et al. (2003) used only translational motion to assess global

motion perception. Translational motion can be encoded in V1 but is globally grouped in

MT/V3a (Amano, Edwards, Badcock & Nishida, 2009), whereas expansion/contraction and

concentric motion are thought to be processed in MST (Badcock & Khuu, 2001; Duffy &

Wurtz, 1991; Edwards & Badcock, 1993; Movshon, 1990). Directly relevant to dyspraxia is

that the optic flow that results from either self-movement or the movement of large objects

near the observer is captured by expansion/contraction GDM stimuli. These critical global

motion capabilities are yet to be assessed in individuals with dyspraxia. If, in clarifying the

conflicting findings presented by O�Brien et al. (2002) and Sigmundsson et al. (2003),

future research is unable to identify a global motion processing deficit for dyspraxia, this

may suggest that a non-visual deficit is central to the symptomatology of this disorder,

perhaps one arising from parieto-motor or cerebellar dysfunction (O'Brien et al., 2002).

Alternatively, if a dorsal stream deficit is found, the research must be able to additionally

account for the ventral stream difficulties established in the current papers. Whether the

dorsal stream is indirectly affecting the visual attention capabilities of the ventral pathway

is still to be determined. Assessment of lower-level capabilities would provide important

additional information regarding the integrity of both visual pathways in dyspraxia,

58

particularly in identifying whether any GDM deficit arises as a result of impaired early

input to the dorsal stream.

Therefore, to summarise, the assessment of visual capabilities in dyspraxia is

currently incomplete. Two studies have focused on global processing in both visual

streams. In assessing the ventral stream, both studies used stimuli that potentially rely on

local processing rather than global grouping in this pathway. Regardless, the deficits

reported suggest an anomaly in ventral visual stream processing in dyspraxia. Precisely

what this means for our understanding of the condition is unclear, since Sigmundsson et al.

(2003) also reported impairment in global motion processing for their �clumsy� group.

Thus, impairment extending to the dorsal stream may also be implicated in difficulties in

the coordination of space-based movements. Importantly, Sigmundsson et al. (2003)

introduced the possibility of studying a non-clinical sample with similar characteristics to

dyspraxia to inform our understanding of the condition proper. Finally, assessing the

perception of global motion for expanding and contracting stimuli would be beneficial

since these capabilities are most directly related to movement, but they are yet to be studied

in the dyspraxia population. Thus, it is yet to be clearly established whether deficits in the

dorsal stream are present, consistent with Braddick et al.�s (2003) hypothesis, and actually

contribute to the poor visuo-motor processing seen in dyspraxia.

William�s Syndrome

Individuals with Williams syndrome (WS; a congenital deficit resulting from a

deletion on chromosome 7q11.23) experience difficulties in spatial cognition as well as

delayed language and motor development (Bellugi, Lichtenberger, Jones, Lai, & St George,

2000; Mervis et al., 2000). Visuo-spatial ability (Bellugi et al., 2000) and motor function

(Hocking, Bradshaw, & Rinehart, 2008) are particularly affected in William�s syndrome

and neurobiological studies demonstrate atypical function and structure in posterior

parietal, posterior thalamic (encompassing the pulvinar region, which provides direct input

to the visual streams and MT; see Ellerman, Siegal, Strupp, Enbner, & Ugurbil, 1998, for a

review) and cerebellar regions that are important in performing space-based actions (Meyer

& Minshew, 2002; Mobbs et al., 2004; Reiss et al., 2004). Thus, it has been hypothesised

that the visuo-spatial impairments in WS stem from developmental problems within the

dorsal visual pathway (Eckert et al., 2006). Even though functional imaging (Eckert et al.,

2005; Ellerman et al., 1998) and post-mortem (Holinger, Sherman, McMenamin, Bellugi,

59

& Galaburda, 2002) studies have supported this hypothesis, there are relatively few

psychophysical studies measuring the capabilities of the two visual streams in WS.

Nakamura, Kaneoke, Watanabe and Kakigi (2002) outlined a case study in which a

boy with WS demonstrated global motion perception thresholds similar to those reported in

the literature for typically developing individuals. Reiss, Hoffman and Landau (2005)

examined three different types of motion processing ability in WS. They used biological

motion detection (animations of �lights� or dots attached to the joints of the body displayed

in brief video sequences, Johansson, 1973), GDM stimuli and a 2-D form-from-motion task

(discriminating which panel contained moving elements that formed a rectangular shape

within a noise background). Individuals with WS performed at normal levels on both the

biological motion and GDM tasks but had elevated thresholds on the form-from-motion

task. In addition to GDM and coherent line segment tasks, Atkinson et al. (1997) assessed

performance on a visuo-spatial manipulation task expected to tap additional functioning

subserved by the dorsal stream (Milner & Goodale, 1995). The task involved posting a card

into a slot of variable orientation. Children with WS were less accurate on this task than

controls, and demonstrated anomalies in posting behaviour not seen in any controls. In

addition, the children with WS had higher GDM thresholds, but intact thresholds on the

line segment task, compared to typically developing individuals. In further work, Atkinson

et al. (2003) again administered GDM and coherent line segment tasks, but this time to a

larger group of WS children. When comparing their performance on these tasks to the age

equivalent performance of typically developing children, Atkinson et al. (2003) found a

subgroup of WS children who were distinguished only in exhibiting high global motion

thresholds, and an additional subgroup of WS children who demonstrated high thresholds

for both global motion and line segment coherence. Given that similar patterns of

performance are often seen in younger typically developing children, the authors posited

that the difficulties seen in WS may be the result of immaturity in the visuo-spatial

processing system that is more predominant in the dorsal stream. Later, Atkinson et al.

(2005) followed these initial studies by examining global motion and form sensitivity in

adults with WS to clarify whether motion processing difficulties are a transient

developmental feature or a persistent aspect of cerebral organisation in WS. The WS adults

exhibited higher thresholds in both the global motion and the coherent line segment tasks

when compared to matched controls. There was substantial variability within the WS

60

group, with performance outside the normal range not being a feature of every WS

individual.

The studies assessing global motion perception in WS provide a reasonably

coherent profile of impaired GDM perception in this population, consistent with both the

symptoms of impaired spatial abilities, and with the outcomes of imaging and post-mortem

research concerning the dorsal stream in this condition. Regardless, it would appear that

visual anomalies in WS are not restricted to the dorsal stream, as proposed by Braddick et

al. (2003). While Atkinson and colleagues (2003; 2005) report a greater deficit on GDM

tasks than on line segment coherence tasks, a proportion of individuals with WS still

displayed decreased sensitivity to coherence on the ventral stream task. It will be important

for future research to determine whether the dorsal stream impairment stems from earlier

stages of this visual pathway, and how it impacts, if at all, on ventral�stream functioning. A

complete assessment of the local and global properties of the ventral stream, being mindful

of the concerns raised above with respect to the line segment stimuli, in addition to an

assessment of the local properties of the dorsal stream in WS, will assist in clarifying some

of these issues.

Fragile X Syndrome (FXS)

Fragile X syndrome (a disorder arising as a result of a trinucleotide repeat in the

FMR-1 gene) is associated with weaknesses in attentional control (Munir, Cornish, &

Wilding, 2000), linguistic processing (Belser & Sudhalter, 2001) and visuo-spatial

cognition (Cornish, Munir, & Cross, 1998, 1999). Decreases in the FMR1 protein product

result in neurons in the visual cortex having immature dendritic spines (Irwin et al., 2001;

2002). Kogan et al. (2004a; 2004b) hypothesised that the impaired performance on visuo-

motor tasks characterising the FXS phenotype may be the result of the magnocellular

neurons being more susceptible to the loss of FMR1 protein.

In the first of two studies, Kogan et al. (2004b) evaluated the possibility of a

perceptual dorsal stream deficit resulting from neurobiological changes in FXS by

comparing individuals with FXS to both chronological age and mental age matched control

groups on a variety of visual tasks. Global dorsal-stream processing was evaluated using a

GDM task identical to that used by Atkinson et al. (1997) and Spencer et al. (2000), which

required identifying the side of the screen containing a strip of dots moving coherently in a

translational pattern. Sensitivity to coherent form was assessed using line segment stimuli

61

that contained a target of concentrically aligned segments on one side. The functioning of

the M and P cells was assessed using flicker contrast sensitivity tasks, whereby low and

high spatial frequency stimuli, respectively, were modulated at both low and high temporal

frequencies. The FXS patients had reduced sensitivity to global motion, but equivalent

sensitivity to form stimuli, relative to both control groups. The individuals with FXS also

showed significantly reduced M cell contrast sensitivity when compared to the

chronological age matched group, but differed significantly from the mental age matched

group only for low spatial frequency stimuli modulated at a high temporal frequency. The

three groups displayed no significant differences in P cell contrast sensitivity. In their

second study, Kogan et al. (2004a) examined performance on first- and second-order

dynamic and static stimuli (as described by Bertone et al., 2003, 2005). The FXS group had

elevated contrast thresholds on both first- and second-order motion stimuli, for direction

discrimination, when compared to chronological age-matched controls and controls

matched for developmental age. The FXS group had significantly higher contrast thresholds

on the first-order static stimuli, for orientation discrimination, when compared to the age-

matched controls, but not relative to the developmental-matched controls. In addition, the

FXS group were less sensitive to second-order static orientation stimuli when contrasted

with both comparison groups.

Kogan et al. (2004a) state that their results reflect a �clear pervasive impairment of

motion perception in FXS� (p. 1638). This is consistent with Braddick et al.�s (2003) notion

of a general dorsal stream impairment arising in the presence of a developmental disorder,

particularly given that both local and global processing within this pathway have been

assessed using multiple techniques. However, this conjecture requires further examination

considering that less than half of Kogan et al.�s (2004a) participants were able to complete

the first- and second-order dynamic tasks. Alternatively, Kogan et al. (2004a) also posit that

the form processing deficit seen clearly for only second-order stimuli is evidence of �a

generalised cortical dysfunction in integrative mechanisms of early visual input regardless

of its source� (p. 1638). It is possible that visuo-motor performance in this population may

be related to impaired dorsal stream functioning as a result of the effects of decreases in the

FMR1 protein as suggested by Kogan et al. However, it is critical that we keep in mind that

the aforementioned studies have demonstrated a correlation between impaired dorsal stream

functioning and losses in the FMR1 protein, but the genetic abnormality may not be the

62

cause, given that other developmental disorders also demonstrate impaired dorsal stream

processing without this particular genetic anomaly having been identified.

Evaluating the dorsal stream hypothesis of developmental disorders

Braddick et al. (2003) argued that deficits in global motion processing in

developmental disorders, specifically WS, dyslexia, autism and hemiplegic children,

provide evidence for �an early vulnerability in the motion processing system of a very basic

nature� (p. 1779). Consistent with this notion, each of the five developmental disorders

considered above has a specific symptom profile that can potentially be related to

impairments in dorsal stream functioning. However, while the physiological properties of

the M cells may result in them being more at risk for damage during development, the

results of the present review suggest that, in some instances at least, the problem in

developmental disorders is not at the lower, M cell level but rather occurs further along in

the dorsal stream. For instance, contrast sensitivity of M cells appears to be unaffected in

ASDs (Bertone et al., 2005; Pellicano et al., 2005, although other properties of the M cells,

such as speed of processing are yet to be assessed), whereas perception of GDM is impaired

(Milne et al., 2002a; Pellicano et al., 2005; Spencer & O'Brien, 2006; Tsermentseli et al.,

2008). Similarly, the results from studies assessing M-cell contrast sensitivity in dyslexia

are inconclusive (Skottun, 2000), whereas studies investigating visible persistence provide

more evidence for an M-cell deficit in this condition (Badcock & Lovegrove, 1981;

Slaghuis & Ryan, 1999). Additionally, there is a reasonably consistent pattern of impaired

performance on tasks assessing integrative motion processing in the dorsal stream in

dyslexia (Cornelissen et al., 1995; Pellicano & Gibson, 2008; Talcott et al., 1998). If

impairment was to be found at the M cell level in any of the developmental disorders, then

subsequent difficulties in global motion processing may be expected to arise as a result of

flow-on effects from the earlier level. However, when global motion impairments occur

without a corresponding deficit in M cell capabilities (provided all the capabilities have

been assessed), then explanations other than a specific dorsal stream deficit must be

considered. In ASDs for example, it has been suggested that while the early stages of

cortical visual processing can extract local information adequately, difficulty is experienced

when local information is accumulated in higher cortical visual areas to form a global

63

percept (Bertone et al., 2005; Pellicano et al., 2005). While further research needs to be

conducted concerning this pattern in the ventral visual stream, it is clear that this

interpretation offers the possibility of ASDs exhibiting a profile that can be distinguished

from dorsal stream impairment alone.

This review also suggests that it is still too early to decisively state that dyspraxia

and WS have visual impairments arising from difficulties in the magnocellular pathway.

The symptoms of dyspraxia suggest an important role of the dorsal stream; however the

results of studies assessing GDM perception in this disorder are inconclusive. The lower

levels of the dorsal stream, as well as sensitivity to expanding and contracting global

motion, remain to be assessed for dyspraxia. Similarly, in WS, while GDM perception

appears to be impaired (Atkinson et al., 2005; Atkinson et al., 1997), no study has

evaluated whether the inputs from lower levels are also affected. However, the findings

from dyslexia and FXS are more congruent with the dorsal stream impairment hypothesis

(Braddick et al., 2003). Functioning in the ventral stream appears to be intact in these

conditions, and while further work needs to be done to clarify the results at the lower levels

of the dorsal stream in dyslexia, impairment is evident in the dorsal pathway in both

dyslexia and FXS.

Some caution is warranted in that the conclusions of this review are based on

comparisons of data across studies of the disorders investigated individually. While mostly

the same methodology and test environments have been used, not all experimental factors

have been controlled, which may have introduced some variability in results, necessitating

a degree of assumption when comparing outcomes across studies. However, there are

robust studies that have assessed two target groups simultaneously (e.g. ASD and dyslexia,

Pellicano & Gibson, 2008; Tsermentseli et al., 2008) and have reported patterns of results

consistent with the conclusions we draw in this review article.

Importantly, what this review highlights is that, while the dorsal stream does appear

to be affected in some way in each of these disorders, there is the potential for the

identification of unique patterns of abilities across the different levels of the visual

pathways in some of the disorders. If a unique profile of visual ability could be established

for any of the disorders, these tasks might allow for the earlier recognition of such disorders

when screening for problems early in childhood. However, while the research reviewed

suggests promising possibilities (in particular, see Bertone et al. (2005; Bertone & Faubert,

64

2006) for an attempt to characterise specific perceptual signatures as potential tools for

dissociating condition-specific aetiology in ASD and FXS), unique profiles of visual

processing for the developmental disorders are yet to be identified.

Methodological considerations and future directions

The summaries above highlight how information from a specialised area, vision

research, can be applied to clinical populations to advance understanding of the symptoms

and neurology central to these disorders (see also Green et al., 2009, for an example in

schizophrenia). Psychophysical measurement is a useful way of assessing brain functioning

in relation to vision because the processes invoked by such tasks are generally quite well

understood relative to other cognitive processes, both functionally and neuroanatomically.

Psychophysics allows us to see how local and global processing manifests in visual

abilities, and thus may act as an indication of how specific cortical areas process

information. However, as highlighted by the summary of recent developments in

conceptualising the visual system hierarchy, it is important that clinical researchers

incorporate the latest understanding of the processes assessed by certain tasks into the

designs of their studies to ensure maximum progress. Therefore, the aim of the following

sections is to briefly consider the methodological limitations of some of the studies

considered above, and make suggestions for future research in this regard.

Ventral stream stimuli

The need to accommodate recent developments in our understanding of the visual

system is particularly noticeable with respect to the line segment tasks designed to assess

ventral stream functioning. The evidence regarding whether the identification of contours

can be achieved by the neurons at stages earlier than V4 (Field & Hayes, 2004; Loffler,

2008) is of particular significance. Future research may consider employing radial

frequency (RF) patterns (Wilkinson, Wilson, & Habak, 1998) as an alternative to coherent

line segment tasks. RF patterns are closed contour shapes created by deforming a circle (see

Figure 3). The deformation is produced by sinusoidally varying the radius as a function of

polar angle, and the number of cycles of modulation corresponds to the RF number. For RF

patterns of high frequency, such as the RF24 pattern on the right of Figure 3, performance

65

for discriminating the whole shape from a circle is better than when only part of the closed

shape is deformed (Loffler, Wilson, & Wilkinson, 2003), but only by an amount that can be

explained by probability summation of the detection of independent local features. In

contrast, there is evidence that curvature and position information is pooled along the entire

circumference of the pattern for stimuli of low radial frequency, such as the RF3 pattern in

the middle of Figure 3 (Bell & Badcock, 2008; Bell, Badcock, Wilson, & Wilkinson, 2007;

Loffler et al., 2003), consistent with global signal integration in shapes with up to about

eight cycles of modulation (Loffler et al., 2003; Wilkinson et al., 1998). Given that

sensitivity to global versions of these shapes cannot be explained by local cues such as

contour orientation or local curvature, RF patterns are ideal stimuli to examine local and

global processing within the ventral visual stream, however, there is currently no equivalent

stimuli that assesses the dorsal stream in a similar manner to the RF patterns. This is an

important consideration, because if functionality in the ventral and dorsal visual streams is

to be compared, the two streams should ideally be assessed using stimuli that have similar

processing requirements at both the early and late levels in each pathway. While Glass

patterns and GDM stimuli achieve this requirement at the later, global processing stages in

the ventral and dorsal streams respectively, no study to date has compared performance on

these global tasks with performance at the earlier, local processing stages. One way to do

this may be to create stimuli designed to assess using dipole orientation discrimination and

dipole motion direction discrimination. One advantage of the dynamic and static first- and

second-order stimuli employed by Bertone and colleagues in atypical and typical

populations (Bertone et al., 2008; Bertone et al., 2003, 2005) is that, despite the

shortcomings outlined above, they represent the only paradigm that attempts to control for

processing requirements while assessing different levels in the two processing streams.

Figure 3. Examples of (a) a circle (b) an RF3 stimulus and (c) an RF24 stimulus

66

Contrast sensitivity as a measure of magnocellular functioning

With respect to the dorsal stream, if M cells are adversely affected in developmental

disorders, it is possible that measures of contrast sensitivity may not reveal these

differences. Any impairment may become more apparent further along the dorsal stream,

perhaps as a result of limited inputs for summation of movement direction information. The

research summarised in the section on the human visual system above points to the need for

future research to consider other properties of V1 that rely on input from M cells, such as

the precision of direction coding (Gur, Kagan, & Snodderly, 2005; Livingstone & Hubel,

1984), in addition to contrast sensitivity.

Additionally, as noted above, if any developmental disorder affects the M and P

cells in similar proportions, less impairment might be observed with P cell functioning than

for M cell functioning, given the relatively greater number of residual P cells (Dacey, 1993;

Dacey & Petersen, 1992). This leaves open the possibility of impairment in P cells as well

as in M cells, but where the former is not as apparent as the latter. The expression of a P

cell impairment may depend on the number of P cells required to perform a task properly.

An impairment that impacts both the M and P cells and flows through to affect higher

levels of the dorsal and ventral streams respectively may explain why the perception of

global form along with the perception of global motion appear to be disrupted in dyspraxia

and WS. Currently, P cell functioning in these two disorders has not been assessed, and it is

unclear whether the deficits on the coherent line segment task reflect difficulties in local or

global form processing.

Sample size and stimulus presentation methods

One key difference between vision research using experienced, neurotypical

observers and research with children with developmental disorders concerns the reliability

of the data collected. Vision research frequently uses designs in which smaller numbers of

observers are assessed on many repetitions of the same stimuli. In contrast, and common to

much clinical research, developmental researchers typically use larger samples with fewer

trial repetitions. This is practical in that it reduces the amount of time one particular child is

required to maintain attention on the task. The desire to use less time-consuming

procedures means that staircase methods are often chosen. However, these methods can be

67

susceptible to mistakes or inattentiveness early on in the staircase (Roach, Edwards, &

Hogben, 2004; Spry, Johnson, McKendrick, & Turpin, 2003), thus affecting the capacity of

the procedure to provide a reliable threshold estimate (Treutwein, 1995; Wichmann & Hill,

2001). While there are more time consuming staircase methods that minimise this problem

(Findlay, 1978), they are not commonly employed. The method of constant stimuli is more

robust in that it enables the entire psychometric function to be assessed, and while it also

takes longer than staircase methods, it may ensure more reliable estimates of the

individual�s threshold, since the specific values tested later in the sequence are not

dependent on early responses, and thus an attentional lapse affects only the specific trial on

which it occurs. Within the literature cited above, only Bertone et al. (2003, 2005), Kogan

et al. (2004a), and Nakamura et al. (2002) report using the method of constant stimuli.

Additionally, while developmental studies may aim to have larger sample sizes to

compensate for the fewer repetitions at each level of the psychometric function for

individual observers, many studies do not obtain that larger sample size, so the

generalizability of the results is unclear (e.g. Bertone et al., 2003; Davis et al., 2006; Del

Viva, Igliozzi, Tancredi, & Brizzolara, 2006; Sanchez-Marin & Padilla-Medina, 2008, all

used a sample size of 12 or less). Limited sample size can be particularly problematic in

disorders where subtyping may be pertinent (e.g. dyslexia, Hogben, 1996) and also where

variability in the phenotype is common (e.g. in WS and ASDs). The reporting of effect

sizes in the literature would assist in addressing this concern.

Variation in clinical samples

One other important issue concerns the fact that not all children within a clinical

group may show atypical functioning with reference to a control sample. For example,

Ramus (2003) noted that 37 out of 128 children with dyslexia across seven different

studies displayed elevated thresholds in tasks assessing dorsal stream functioning. Milne et

al. (2002b; 2006) reported that not all children with autism showed elevated global motion

thresholds, but rather that the difference in central tendency between the clinical and

control groups reflected a skewed distribution in the group with autism (see also Atkinson

et al., 2003; Roach et al., 2004). This suggests that researchers may be better advised to

select those participants who show reliable performance differences to typical observers

and investigate these individuals in detail. It is, of course, clear that if only a small

68

proportion of participants show the differences then those visual performance factors are

unlikely to be central to the developmental disorder. The reason why some children with

developmental disorders show differences in visual perception compared to typically

developing controls remains to be established. However, the variation within clinical

groups does appear to indicate that such abnormalities might not have a causal role in these

disorders, but rather may be an indication of wider neurological atypicalilties. It will be

important for future research to consider the distributions of scores from the psychophysical

tasks for the clinical groups. Perhaps differences within clinical groups on psychophysical

task performance could relate to symptom severity, so it may be useful for future studies to

collect data on symptomatology along with psychophysical data. Additionally, as briefly

noted earlier, performance on the visual tasks covered in this review typically does not

reached adult levels until the middle primary school years. Whether an impairment

identified for a disorder represents a developmental delay or an enduring deficit then

becomes an issue. This issue is informed by the use of developmental age-matched controls

as well as chronological age-matched controls, and also by the assessment of adult samples.

An ideal approach is to assess large samples varying in age that then enable the comparison

of developmental functions for individuals with the disorder and those of typical

development (Brock, Jarrold, Farran, Laws, & Riby, 2007).

Summary and conclusions

To conclude, the application of psychophysical research methods to evaluate vision

in developmental disorders offers the possibility of rigorously investigating the functional

capabilities of specific brain regions in a manner that adds to what can be revealed through

imaging and electrophysiological recording. Overall, the research has often been consistent

with the hypothesis that the dorsal stream is particularly susceptible to damage during

development (Braddick et al., 2003; Lovegrove, 1993, see McKendrick et al., 2004, for

alternative possibilities), with individuals with developmental disorders exhibiting

difficulty in visual tasks assessing this stream. However, several studies report ventral

stream abnormalities, either in conjunction with dorsal stream impairment, or in isolation.

Whether ventral stream abnormalities occur as the result of abnormal input from the dorsal

stream or whether they can be impacted differentially is still to be determined. Advances in

69

conceptualising the visual streams, the interconnectedness of the two, and the role they play

in directing visual attention, in conjunction with more sophisticated and accurate methods

of assessing visual functioning in developmental disorders, should assist with the resolution

of some of these issues in the future. Overall, it seems that ASD is the most promising

condition for demonstrating a unique profile of visual functioning that extends beyond the

original �dorsal stream impairment� hypothesis of developmental disorders. In particular,

impairment in developmental dyslexia and FXS appears to be restricted to the dorsal

stream, and the assessment of the two pathways in developmental dyspraxia and WS is

incomplete, whereas individuals with ASD appear to have difficulties in global grouping in

the later stages of the dorsal visual stream. It will be important to clarify whether this

difficulty in global processing extends to the ventral pathway in order to further elucidate

the nature of any neurological deficit, as indicated by visual processing atypicalilties,

associated with the disorder.

70

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87

CHAPTER 3.

Pilot Study

A comparison of threshold estimates for two

psychophysical presentation methods

88

Introduction

As outlined in Chapter 2, the selection of appropriate experimental procedures and

stimuli to assess critical aspects of visual functioning should be based on our current

understanding of the capabilities of the visual system. With this in mind, we designed

several tasks to assess visual functioning in both individuals scoring high in self-rated

autistic-like traits and children with autism spectrum disorders, with a specific focus on the

form processing pathway. This chapter will outline the tasks used to assess local and global

processing within the ventral visual stream, as well as one task to assess global motion

processing. Because the ultimate aim is to test children with these experimental stimuli, a

modification involving the use of the method of constant stimuli (MOCS) rather than a

staircase procedure was made to the tasks. This was expected to improve the reliability of

testing with children. In order to ensure that this change did not impact upon the thresholds

of adult observers, the effect of using MOCS versus a staircase procedure was assessed in a

pilot study reported in this chapter.

As indicated in Chapter 2, Glass (1969) patterns are preferable to stimuli composed

of line segments when assessing global processing in the ventral visual stream as Glass

patterns are less likely to tap the local contour integration capabilities of horizontal

connections in V1. As such, we employed Glass stimuli composed of dot pairs aligned in a

coherent concentric pattern to assess global form processing in the studies outlined in

Chapters 5 and 6. Alternative stimuli for assessing both local and global processing within

the ventral visual stream are radial frequency (RF) patterns (Bell & Badcock, 2008; Loffler,

2008). These were utilized in the study reported in Chapter 7. Global dot motion (GDM)

stimuli were used in the study reported in Chapter 5. GDM stimuli can be used to present

many global motion structures (e.g. concentric, fronto-parallel, radial; see Badcock &

Khuu, 2001; Edwards & Badcock, 1993). However, to be consistent with the concentric

nature of the Glass pattern task, a concentric version was pilot tested here and was chosen

for the study in Chapter 5.

89

Pilot Study: Staircase vs. Method of Constant Stimuli

Psychophysical experiments differ in the methods used to estimate the stimulus

difference that can be discriminated in some fixed percentage of presentations. For

example, in a staircase method that converges on the 79% correct performance level

(Levitt, 1971), the experiment usually begins with a stimulus that is easy to detect. The

level is decreased (i.e. the stimulus is made more difficult to detect) only after three

successive correct responses, and is increased (i.e. made easier to detect) with each

incorrect response. Reversals in the direction of these changes are expected as the

threshold is approached. The stimulus values at which reversals occur are averaged to

determine the 79% correct performance level. Alternatively, in the method of constant

stimuli the experimenter chooses a number of stimulus values (usually from five to nine)

which are likely to encompass the threshold value. This fixed set of stimuli is presented

multiple times in a random order, with the stimulus values occurring equally often overall.

The proportion of correct responses for each stimulus level is calculated and these data are

plotted in a psychometric function to which a smooth curve (Eq 3.2 below) is fitted, using

non-linear regression, from which the 79% correct threshold can be obtained (Ehrenstein &

Ehrenstein, 1999).

Each of the procedures has advantages and disadvantages. For example, staircase

methods typically require fewer trials and thus they reduce the amount of time an observer

is required to maintain attention on the task. However, the levels presented are serially

dependent on observer responses and thus these can often be susceptible to mistakes or

inattentiveness early on in the staircase (Roach, Edwards, & Hogben, 2004), thus affecting

the capacity of the procedure to provide a reliable threshold estimate (Treutwein, 1995;

Wichmann & Hill, 2001). While there are more time consuming staircase methods that

minimise this problem (Findlay, 1978) they are not commonly employed. MOCS, on the

other hand, is more robust in that it enables the entire psychometric function to be assessed

(a staircase procedure is limited to the values surrounding the threshold). While its major

drawback is that it is time consuming because it usually employs a larger number of trials,

it typically ensures more reliable estimates of the individual�s threshold (Ehrenstein &

Ehrenstein, 1999).

90

The aim of this study was to compare thresholds derived under the staircase and

MOCS procedures for our chosen tasks. Using MOCS for assessing the thresholds of

children would be preferable given that this methodology is likely to be less susceptible to

early errors or inattentiveness. As an initial step it was decided to determine whether the

MOCS procedure yielded similar thresholds to the staircase procedure in adult observers,

before using the MOCS methodology with children. Therefore, thresholds derived from the

two different presentation schedules were compared for the GDM, Glass pattern and RF

pattern tasks.

Method

Observers

Four observers were used, one of the authors (EG), and three other adults who were

naïve to the aims of the experiment. All had normal or corrected to normal vision. The ages

of the observers ranged from 24-47 years with a mean of 29 years. There were three

females and one male.

Apparatus

All stimuli were presented on an LG L1730SF 271mm x 340mm resistive touch

screen driven by a Sony Vaio laptop computer. The screen was 1024 x 768 pixels and had a

refresh rate of 75Hz. All stimuli were drawn using MATLAB 6.1 (Mathworks, 2002) and

displayed using the WinVis toolbox (Neurometrics Institute, 2004). Responses were

recorded by the software via touch screen input. Luminance calibration was performed

using an Optical photometer (Cambridge Research Systems) measuring luminances as a

function of look-up table value.

Stimuli

Global Dot Motion Stimuli

Each stimulus display consisted of 50 white dots with a luminance of 203 cd/m2,

presented on a grey background of 30 cd/m2. Each trial consisted of two stimulus displays

presented simultaneously, one containing a proportion of dots (signal) moving in a coherent

direction, while the remainder moved in random directions at the same speed (noise). The

other display contained only dots moving in random directions but at the same speed

91

(noise). From trial to trial, signal dots moved either clockwise or anticlockwise. The

direction of movement in the target display and the display containing the signal was

selected randomly for each trial.

There were two 6.48°-diameter circular apertures positioned side-by-side and

separated by 2.28°. Each dot measured 0.16° in diameter, resulting in a dot density of 0.66

dots/deg2. For each dot, the spatial step size was 0.27° resulting in a stimulus speed of

5.06o/s since a move occurred every 53.33 ms (once every 4 screen refreshes). Stimuli were

presented as eight-frame sequences, with a total duration of 426.66 ms. All dots lasted the

entire stimulus duration unless they moved outside the aperture, in which case they were

randomly repositioned inside the aperture. However, the subset of dots which moved in the

signal direction was randomly chosen for each frame transition. No overlap between the

dots was permitted. There was a 1s period between a response being registered and the

presentation of the next stimulus during which a blank screen was shown at the background

luminance.

Glass Patterns

The Glass pattern stimuli also contained two displays, both of which consisted of

dot dipoles randomly located within the display. The target display contained a variable

proportion of dot pairs arranged in a concentric pattern while the remaining dipoles were

randomly oriented. All of the dipoles were randomly oriented in the noise display. Like the

GDM task, the presentation duration was 426.66 ms and no overlap between dots or dots

pairs was permitted. The characteristics of the individual dots, the grey background, and

extent of the 50 dot-pair displays exactly matched the features of the GDM task. The

separation between the dots in a dipole also matched the dot step sizes in the GDM task.

Radial Frequency Patterns

The closed contour shapes in Loffler et al.�s (2003) study were created according to

the following equation:

r(è) = rmean(1 + Asin(ù è + ö )) (3.1)

where r and è (in radians) are the polar coordinates of the contour, rmean is its mean radius,

and A, ù and ö are the amplitude, radial frequency (RF), and phase of the pattern,

respectively. The frequency determines the number of lobes of the stimulus, or the number

of cycles of modulation in 360o. Frequencies of ù = 3, 5, 10, and 24 were used in this study

92

(see Figure 1). In each trial, one shape was a circle (A = 0) and the other was a deformed

pattern (A > 0). Whether the deformed pattern was presented on the left or right was

randomly determined, with equal probability for each choice. Patterns were always

presented with random phases (global rotations of the pattern), rendering it impossible for

the participants to predict the exact location of the lobes from trial to trial. The luminance

profile of a radial cross section approximated a fourth derivative of a Gaussian set at 99%

contrast and a peak spatial frequency of 8c/ o. The presentation duration was 200 ms and the

RF patterns had a mean radius of 1.5o with a centre to centre separation of 3.75o. For this

task, the threshold estimate was the amplitude of modulation required to consistently detect

distortion.

Figure 1. The graphs on the left show the cycles of modulation in 360o that comprise the

RF patterns, and the images on the right show examples of the corresponding RF patterns.

Procedure

Observers sat in a darkened room 0.75m from the screen. Viewing was binocular

and no feedback concerning the accuracy of the response was given. The tasks were

administered in a random order for each observer.

Staircase thresholds were established using a

correct performance level (three up, one down, Levitt, 1971). Eight reversals were

collected, with the threshold being taken as the mean of the last four reversals. The GDM

staircases started with a signal strength of 50%,

which was reduced after each of the first three reversals, resulting in a step size of one dot

for the last six reversals. Because Glass pattern thresholds are typically higher (Badcock,

Clifford, & Khuu, 2005;

began with a signal strength of 70%

The RF patterns began with a suprathreshold amplitude

step size of .008, which was halved after each of the first three reversals, resulting in a step

size of A = 0.001 for the last six reversals.

MOCS thresholds were obtained from the psychometric function by fitting the

equation:

where exp is the exponential function, ó is a scalar determining the slope of the

psychometric function, y is the number correct out of 20, and x is the amplitude or signal

level. This threshold term in Eq 3.2

consistent with the staircase procedure the 79% correct point

recorded and used for comparison

the thresholds reported in the literature for eac

procedure. The stimulus levels for each task are outlined in Table 1.

Observers sat in a darkened room 0.75m from the screen. Viewing was binocular

and no feedback concerning the accuracy of the response was given. The tasks were

administered in a random order for each observer.

Staircase thresholds were established using a procedure converging on the 79%

correct performance level (three up, one down, Levitt, 1971). Eight reversals were

collected, with the threshold being taken as the mean of the last four reversals. The GDM

staircases started with a signal strength of 50%, and with an initial step size of eight dots,

which was reduced after each of the first three reversals, resulting in a step size of one dot

for the last six reversals. Because Glass pattern thresholds are typically higher (Badcock,

Clifford, & Khuu, 2005; Wilson & Wilkinson, 1998) than GDM thresholds, the staircase

began with a signal strength of 70% but used the same step sizes as

The RF patterns began with a suprathreshold amplitude (A in Eq 3.1)

.008, which was halved after each of the first three reversals, resulting in a step

0.001 for the last six reversals.

MOCS thresholds were obtained from the psychometric function by fitting the

Y =

where exp is the exponential function, ó is a scalar determining the slope of the

psychometric function, y is the number correct out of 20, and x is the amplitude or signal

threshold term in Eq 3.2 equates to the 75% correct threshold,

consistent with the staircase procedure the 79% correct point

and used for comparison. There were seven stimulus levels chosen to encompass

the thresholds reported in the literature for each task, with 20 trials for each in the MOCS

procedure. The stimulus levels for each task are outlined in Table 1.

93

Observers sat in a darkened room 0.75m from the screen. Viewing was binocular

and no feedback concerning the accuracy of the response was given. The tasks were

procedure converging on the 79%

correct performance level (three up, one down, Levitt, 1971). Eight reversals were

collected, with the threshold being taken as the mean of the last four reversals. The GDM

and with an initial step size of eight dots,

which was reduced after each of the first three reversals, resulting in a step size of one dot

for the last six reversals. Because Glass pattern thresholds are typically higher (Badcock,

Wilson & Wilkinson, 1998) than GDM thresholds, the staircase

the same step sizes as for the GDM staircases.

(A in Eq 3.1) of .016, and initial

.008, which was halved after each of the first three reversals, resulting in a step

MOCS thresholds were obtained from the psychometric function by fitting the

(3.2)

where exp is the exponential function, ó is a scalar determining the slope of the

psychometric function, y is the number correct out of 20, and x is the amplitude or signal

orrect threshold, however, to be

on the fitted curve was

. There were seven stimulus levels chosen to encompass

h task, with 20 trials for each in the MOCS

procedure. The stimulus levels for each task are outlined in Table 1.

94

Table 1. MOCS levels for the GDM, Glass pattern, and RF pattern tasks.

MOCS level

Task 1 2 3 4 5 6 7

GDMC (%

coherent dots) 2 4 6 8 10 12 14

Glass (%

coherent dipoles)

8 10 12 14 16 18 20

RF3 (A) .0016 .0022 .0031 .0044 .0061 .0086 .012

RF5 (A) .0009 .0013 .0018 .0025 .0036 .005 .007

RF10 (A) .0005 .0007 .001 .0015 .002 .0029 .004

RF24 (A) .001 .0013 .0017 .0023 .0029 .0038 .005

Results

Results are summarized in Figure 1. For the GDM task, there was no significant

difference between thresholds for the MOCS procedure (M = 10.72, 95% CI = 6.14 to

15.30, coefficient of variation (CV) = 26.87) and the staircase procedure (M = 14.50, 95%

CI = 4.71 to 24.29, CV = 42.21), t(3) = 1.97, p = .15. For the Glass pattern task, the MOCS

procedure produced lower thresholds (M = 15.93, 95% CI = 10.27 to 21.60, CV = 22.34)

than the staircase procedure (M = 18.45, 95% CI = 12.12 to 24.78, CV = 21.57), t(3) = 4.01,

p < .05.

For the RF patterns, there was a main effect of RF, F (3, 9) = 23.85, p < .01, çp2 =

.89, with RF3 patterns producing significantly higher thresholds (M = .0071, 95% CI =

.0052 to .0089, CV = 31.04) than the other three tasks, and RF5 patterns producing

significantly higher thresholds (M = .0039, 95% CI = .0030 to .0049, CV = 28.99) than the

RF10 (M = .0021, 95% CI = .0016 to .0026, CV = 27.34) and RF24 (M = .0023, 95% CI =

.0021 to .0026, CV =13.62) tasks. There was no main effect of procedure for the RF pattern

tasks, F(1, 3) = .26, p = .65, çp2 = .08.

95

Figure 1. Thresholds on the staircase and MOCS procedures for the GDM, Glass pattern,

and RF pattern tasks. Confidence intervals for the MOCS thresholds are omitted for

consistency given that the staircase procedure doesn�t provide this information.

96

Discussion

The aim of this pilot study was to determine the effect of using a staircase procedure

versus a MOCS procedure for threshold estimation. In a small sample of adult observers,

the MOCS produced equivalent thresholds on the GDM task and the majority of RF tasks,

and slightly lower thresholds on Glass pattern tasks. Early mistakes in a staircase will bias

the estimate higher, consistent with the findings of higher coherence thresholds on the

staircase Glass pattern task. Despite this difference on the Glass pattern task, the thresholds

obtained fell within the ranges found in other studies using similar tasks. Specifically, the

thresholds reported in the literature for the perception of concentric Glass patterns range

from 11.6% (Wilson, Wilkinson, & Asaad, 1997) to approximately 20% (Badcock et al.,

2005; Wilson & Wilkinson, 1998) depending on stimulus parameters such as area

(Dickinson, Broderick & Badcock, 2009). The thresholds obtained under both stimulus

presentation schedules in this pilot study fall within this range of estimates. Concentric

GDM stimuli typically elicit thresholds of 5-8% in experienced observers (Badcock &

Khuu, 2001; Scase, Braddick, & Raymond, 1996). In the current study, the mean threshold

for concentric GDM was 12.61% with a range of 7.56 � 21.3%. While the average reported

here was elevated compared to those reported in the literature, this is to be expected with

naive, inexperienced observers (see Tsermentseli, O'Brien, & Spencer, 2008, for similar

thresholds (M = 14.9%) in naive adults). Finally, the mean amplitudes required to

discriminate RF24, RF10, RF5 and RF3 patterns were .0023, .0021, .0039, and .0071,

respectively (expressed as a Weber fraction, see Figure 1). It is important to observe that

individuals in this pilot study exhibited thresholds consistent with those reported by Bell et

al. (2007) for normal adult observers, which were approximately .003 and .007 for RF24

and RF3 patterns (see also Wilkinson, Wilson & Habak, figure 1).

Given that the MOCS procedures took only approximately 60 s longer to administer

than the staircase procedures and produced acceptable threshold estimates in adult

observers, the MOCS methodology will be employed in the studies assessing the visual

capabilities of children in Chapters 6 and 7. Where there were discrepancies, the MOCS

produced the lower threshold, consistent with the concern that staircase errors cause an

upward bias on thresholds. While this was a minimal effect in the pilot study, we will use

MOCS later to avoid such effects, especially since it could be a bigger concern in children.

This has the benefit of reducing the impact of mistakes or inattentiveness early on in the

97

task whilst maintaining an efficient method of threshold estimation (see also Simmons et

al., 2009).

98

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Levitt, H. (1971). Transformed up-down methods in psychoacoustics. Journal of the

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alternative hypothesis for psychophysical performance variability in dyslexia.

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Vision Research, 36, 2579-2586.

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(2009). Vision in autism spectrum disorders. Vision Research, 49, 2705-2739.

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Tsermentseli, S., O'Brien, J., & Spencer, J. (2008). Comparison of form and motion

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Wichmann, F. A., & Hill, J. (2001). The psychometric function: II. Bootstrap-based

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Wilkinson, F., Wilson, H., & Habak, C. (1998). Detection and recognition of radial

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Wilson, H., & Wilkinson, F. (1998). Detection of global structure in Glass patterns:

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101

Chapter 4.

Visuospatial analysis and self-rated autistic-like traits

Emma J. Grinter, Pia L. Van Beek,

Murray T. Maybery and David R. Badcock

Chapter 4 could not be included in the digital version of this thesis for copyright reasons. Please refer to the physical copy of the thesis, held in the University Library.

121

Chapter 5.

Global visual processing and self-rated autistic-like traits

Emma J. Grinter, Murray T. Maybery, Pia L. Van Beek, Elizabeth Pellicano,

Johanna C. Badcock and David R. Badcock

Chapter 5 could not be included in the digital version of this thesis for copyright reasons. Please refer to the physical copy of the thesis, held in the University Library.

155

CHAPTER 6.

Local and global orientation discrimination in autism spectrum disorders and the relationship to detection of

embedded figures

Emma J. Grinter, Murray T. Maybery, Elizabeth Pellicano,

Johanna C. Badcock, and David R. Badcock

156

Abstract

Individuals with an autism spectrum disorder (ASD) demonstrate an enhanced

ability to detect embedded figures that contrasts with the impairments associated with the

developmental condition. Ventral visual stream processing is associated with the perception

of static form, and thus is expected to play an important role in identifying hidden figures

on the Embedded Figures Test (EFT). Local and global processing in the ventral visual

stream was assessed for typically developing children and children with an ASD. The local

task was a simple orientation task that required identifying which of a pair of dots was tilted

from the vertical. The global task required identifying concentric structure in Glass

patterns. The EFT was also administered. Children with an ASD identified hidden figures

faster on the EFT but required a larger angle to be able to discriminate differences from

vertical when compared to the typically developing children. The groups did not differ in

ability to identify concentric structure in Glass patterns. These results suggest that local

ventral stream processing, as assessed by the orientation discrimination task, is impaired in

ASDs, whereas global ventral stream processing, as assessed by the Glass patterns, is

intact. The results are in contrast to those studies reporting either enhanced local form

processing or impaired global form processing in autism and do not assist in distinguishing

between two competing accounts of ability in ASDs, the Weak Central Coherence and

Enhanced Perceptual Functioning accounts.

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Introduction Autism spectrum disorders (ASDs) are diagnosed on the basis of impairments in

social and communication ability, language difficulties and restricted, repetitive and

stereotyped patterns of behaviour (American Psychiatric Association, 2000). Diagnosis is

based on behavioural criteria because no specific biological markers are known. However,

the severity of symptoms within the disorder varies greatly, as does the nature of symptoms

with respect to age, intellectual disabilities and language delay (Hus, Pickles, Cook, Risi, &

Lord, 2007; Scott, Baron-Cohen, Bolton, & Brayne, 2002). Thus, the endophenotype

associated with ASDs can be difficult to objectify, suggesting that perhaps the definition of

the condition requires inclusion of neurobiological or neurocognitive characteristics that

might be more closely linked to underlying brain abnormalities or genetic factors than the

behavioural characteristics (de Jonge et al., 2007; Hill & Frith, 2003). One prominent

theory of neurocognitive functioning in ASDs is the Weak Central Coherence (WCC, Frith,

1989; Happé, 1999) account. This theory suggests that individuals with an ASD

demonstrate a weakness in extracting overall meaning, resulting in a reduced awareness of

the global aspects of stimuli in conjunction with a relatively heightened awareness of the

details or parts of stimuli (Happé, Briskman, & Frith, 2001).

Research has shown that individuals with an ASD are biased towards processing the

details of stimuli as opposed to the global information when presented with a large letter

shape made up of smaller letters (Mottron, Belleville, & Menard, 1999; Plaisted,

Swettenham, & Rees, 1999; Rinehart, Bradshaw, Moss, Brereton, & Tonge, 2000), and

show superior performance on visual search tasks that require the ability to ignore the

gestalt in order to perceive the local elements of a complex stimulus (O'Riordan & Plaisted,

2001; O'Riordan, Plaisted, Driver, & Baron-Cohen, 2001; Plaisted, O'Riordan, & Baron-

Cohen, 1998; Plaisted, Saksida, Alcántara, & Weisblatt, 2003). In particular, individuals

with an ASD have been shown to exhibit either equivalent (Bölte, Holtmann, Poustka,

Scheurich, & Schmidt, 2007; Brian & Bryson, 1996; Chen, Lemonnier, Lazartigues, &

Planche, 2008, but only errors recorded; Kaland, Mortensen, & Smith, 2007; Minshew,

Williams, Gastgeb, & Bodner, 2008, child group only; Ozonoff, Pennington, & Rogers,

1991; Ropar & Mitchell, 2001, Asperger's group) or superior (Edgin & Pennington, 2005;

Jarrold, Gilchrist, & Bender, 2005; Jolliffe & Baron-Cohen, 1997; Morgan, Maybery, &

Durkin, 2003; Pellicano, 2007; Pellicano, Gibson, Maybery, Durkin, & Badcock, 2005;

Ropar & Mitchell, 2001, autism group; Shah & Frith, 1983) ability to detect hidden figures

158

relative to control individuals on versions of the Embedded Figures Test (EFT, Coates,

1972; Witkin, Oltman, Raskin, & Karp, 1971). According to WCC, the advantage on the

EFT is the result of a processing style in which individuals with an ASD quickly perceive

the individual parts of the image, whereas typically developing individuals must first

overcome the global percept in order to successfully perceive the local parts. However, in

response to findings demonstrating that global aspects of stimuli can, under some

circumstances, be processed in the typical manner by individuals with an ASD (Mottron,

Burack, Stauder, & Robaey, 1999; Plaisted et al., 1999), others have suggested that a

primary superiority in perceptual analysis could underlie biases in information processing

in ASDs (Mottron, Dawson, Souliéres, Hubert, & Burack, 2006; Plaisted et al., 2003). The

�Enhanced Perceptual Functioning� (EPF, Mottron et al., 2006) theory suggests that

processing in ASDs is abnormal such that the salience of local features is enhanced, but this

is not thought to be the result of impaired integration of information to form a coherent

whole. Thus, the EPF account suggests that enhanced performance on the EFT in ASDs is

the consequence of a local bias that develops as the result of superior low-level processes.

Because it is possible to assess visual abilities that are dependent primarily on local

or global processing, the assessment of the visual system provides a unique test for

distinguishing between the predictions of the WCC and EPF accounts of ASDs. This is

achievable because the visual system consists of two specialized but linked pathways1 that

process information in a predominantly hierarchical manner (Goodale & Westwood, 2004).

While the cortical pathways show appreciable anatomical cross-talk (Braddick, O'Brien,

Wattam-Bell, Atkinson, & Turner, 2000; Merigan & Maunsell, 1993; Tanskanen, Saarinen,

& Parkkonen, 2008), the ventral pathway is implicated in form perception (Beason-Held et

al., 1998; Kourtzi & Kanwisher, 2000), whereas the dorsal stream has an important role in

the processing of motion (for a review, see Culham, He, Dukelow, & Verstraten, 2001) and

attention (Laycock, Crewther, & Crewther, 2008; Laycock, Crewther, & Crewther, 2007).

Currently, it is understood that the idea that motion processing relies on the dorsal stream

only, and form processing relies on the ventral stream only, is too simplistic (Braddick et

al., 2000; Geisler, 1999). This is well illustrated by studies demonstrating that, despite the

EFT not involving motion perception, both the parietal and occipital lobes are activated

1 Konio-cellular pathways are thought to provide input to a third pathway, but this currently appears to be concerned primarily with blue-yellow colour perception (Callaway, 2005; Sumner, Anderson, Sylvester, Haynes, & Rees, 2007) and will not be considered further here.

159

during performance of the EFT (Manjaly et al., 2003; Ring et al., 1999). Nevertheless, it is

expected that form processing involving the pattern elements of the EFT would be more

central to EFT performance than motion processing capabilities, and it is for this reason

that the current study focuses on the integrity of the ventral visual stream in individuals

with an ASD.

With respect to the ventral stream, at the earliest cortical stages of visual perception

the neurons in the primary visual cortex (V1) extract information about the orientation and

spatial and temporal frequency of stimuli to provide a spatially limited signal for form

perception (DeValois & DeValois, 1988; Hubel & Wiesel, 1968). Because the receptive

fields are small, however, V1 information must be integrated to arrive at a global percept of

form. Area V4 represents an important intermediate level of the ventral visual pathway

which extends from V1 to inferior temporal cortex. While integration of information can

occur via lateral, long-range connections correlating the activity of distant receptive fields

within V1 (Gilbert, Das, Ito, Kapadia & Westheimer, 1996), it is in V4 that complex global

structure is extracted from the spatially localised orientation information encoded by V1

neurons (Gallant, Connor, Rakshit, Lewis, & Van Essen, 1996; Kobatake & Tanaka, 1994;

Wilson & Wilkinson, 1998). Therefore, both the local, V1 and global, V4 levels must be

assessed when attempting to determine a profile of neurocognitive visual ability specific to

ASDs.

To date, research assessing ventral stream processing in ASDs has been unable to

provide a neurocognitive profile that is consistent with either the ELP or WCC hypotheses.

Studies assessing the contrast sensitivity of the parvocellular pathways, which provide the

predominant input into the ventral visual stream (Breitmeyer & Ganz, 1976; DeYoe & Van

Essen, 1988; Livingstone & Hubel, 1988), have produced inconsistent results. Davis,

Bockbrader, Murphy, Hetrick and O�Donnell (2006) and Sanchez-Marin and Padilla-

Medina (2008) reported lower contrast sensitivity thresholds (or better performance) in

ASD groups for pattern detection, whereas Bertone, Mottron, Jelenic and Faubert (2005)

found that individuals with an ASD had equivalent parvocellular contrast sensitivity

thresholds compared to age and IQ matched controls. de Jonge et al. (2007) also report no

significant differences between ASD and age and IQ matched controls on the Vistech

contrast sensitivity charts, though they did not use a rigorous psychophysical task in their

assessment of contrast sensitivity. Finally, Bertone et al. also reported that their ASD group

160

displayed superior ability relative to controls on first-order luminance-defined patterns

designed to measure the minimally detectable contrast threshold required to identify

orientation. This task is thought to potentially assess functioning slightly further along the

ventral pathway than the contrast sensitivity tasks assessing pre-cortical functioning of the

cells that feed into the ventral stream because the inputs to layer 4 of V1 are broadly tuned

for spatial frequency but not orientation, whereas orientation tuning develops at layer 4

(Ferster & Miller, 2000).

Research assessing ventral stream global processing in autism is similarly

inconsistent. Milne et al. (2006) and Spencer et al. (2000) reported no differences in form

coherence thresholds when comparing children with autism and typically developing

children using a task that required detecting the presence of a global pattern revealed by

giving small line segments an orientation appropriate for the global pattern. Del Viva et al.

(2006) also report equivalent thresholds in children with autism compared to control groups

using similar stimuli in which contours are created by coherently aligned Gabor patches. In

contrast, Spencer and O�Brien (2006) and Tsermentseli, O�Brien and Spencer (2008)

assessed global processing in the ventral stream using a task that required detecting global

form in Glass (1969) patterns composed of aligned dot triplets as opposed to line segments.

These authors found no significant group differences on this task when an ASD group was

compared to a group of typically developing individuals. However, both studies reported

higher thresholds for a sub-group of children with autism when compared to the control

group, but the sub-group of children with Asperger�s disorder did not perform significantly

different from the control group. There are many differences between these studies which

might have an impact, but one possible factor is that, in contrast to the line segment stimuli,

there are very few contours longer than a dot triplet for the Glass patterns. It has been

demonstrated that horizontal connections in V1 exist between columns that prefer the same

orientation but different areas of the visual field, with strongest connections between

regions that could represent the extension of a continuous straight contour (see Loffler,

2008, for a review). This results in facilitation of a cell�s response when adjacent cells

tuned to the same orientation are firing (known as collinear facilitation). Thus, detection of

a target can be facilitated by neighbouring stimuli depending on spatial context, including

element position, separation and orientation. Li and Gilbert (2002, see also Field & Hayes,

2004) demonstrated that the spatial extent over which elements are perceptually combined

161

in contour detection matches the extent of the cortical long-range interactions in V1. These

findings suggest that contour detection using line segments oriented along a common axis

might be mediated by the same long-range interactions found in V1. Thus, while Glass

patterns specifically target high-level integrative processing in the ventral stream (Tse et al.,

2002; Wilson & Wilkinson, 1998), it is possible that the contours in line segment and

Gabor stimuli also invoke different contour integration processes involving lower-level

processing (see Pei et al., in press, for an example of VEP recordings of coherent Gabor

stimuli activating V1).

Finally, Bertone et al. (2005) also reported that an ASD group displayed impaired

ability to identify orientation for second-order contrast-defined patterns relative to a

comparison group. While the initial processing of second-order stimuli occurs higher in the

ventral visual stream (Smith, Greenlee, Singh, Kraemer, & Hennig, 1998) than the first-

order stimuli processed in V1, these simple oriented stimuli potentially do not utilise the

global, integrative processes employed further along in the ventral stream (V4, Tse et al.,

2002; Wilson & Wilkinson, 1998) for Glass pattern perception.

Importantly, in the research outlined above, only Bertone et al. (2005) use the same

group of children with an ASD to compare performance at two relatively low levels in the

ventral stream. Therefore, the aim of the present study was to assess both types of visual

functioning in the ventral stream using tasks that clearly target local and global processing

but for which differences between the stimuli are minimised. We used Glass patterns to

assess global ventral stream processing, and an orientation discrimination task to assess

local ventral stream processing. The orientation task consisted of two dot pairs, one of

which was aligned in a vertical orientation while the other was slightly tilted from vertical.

The characteristics of these dot pairs exactly matched those of the dot pairs comprising the

Glass pattern stimulus, thus minimising the differences between the local and global

stimuli. Smith, Bair and Movshon (2002) demonstrated that the oriented cells in V1

respond to the dot pairs in Glass patterns, thus the orientation task is an appropriate

assessment of early processing in the ventral stream. Both tasks were administered to the

typically developing and ASD children. In an attempt to replicate the advantage in

disembedding seen in the children with autism, the Children�s Embedded Figures Task

(Witkin et al., 1971) was also administered. The relationship between performance on local

and global ventral stream tasks and EFT performance were also examined in order to

162

determine whether particular strengths and weaknesses in ventral stream processing, as

outlined by the WCC and EPF theories, can explain superior EFT performance in ASDs.

We predicted, given previous results, that children with an ASD would display

faster times to identify the hidden figure on the EFT than the typically developing children.

If the WCC account better explains visual processing in ASDs, then, relative to a

neurotypical comparison group, individuals with an ASD should show elevated thresholds

on the Glass pattern task (i.e., poor global processing), which should be accompanied by

either equivalent or lower thresholds on the orientation discrimination task (i.e.,

intact/superior local processing). In this instance, it was also expected that faster times to

detect the hidden figure on the EFT would be associated with poorer global visual task

thresholds. If, however, EPF theory better explains visual processing in autism, then

individuals with an ASD should demonstrate better performance on the orientation

discrimination task (i.e., enhanced local processing), and no impairment on the Glass

pattern task (i.e., intact global processing) relative to typically-developing comparison

individuals. In this instance, faster EFT reaction times should be associated with superior

performance on the local visual processing task.

Method

Group Comparisons

Brock, Jarrold, Farran, Laws and Riby (2007; see also Jarrold & Brock, 2004)

demonstrated that when children with a developmental disorder and typically developing

children differ on characteristics such as verbal and non-verbal ability, problems can occur

when using methods such as �matched� control groups or analysis of covariance to control

for such variables. To avoid these confounds they suggest using a large, diverse group of

typically developing children and regressing each experimental variable onto the variables

that need controlling for. This regression function is then used to generate expected scores

for the children with the developmental disorder, which are then compared to their actual

scores. We adopted this approach in our group comparisons

163

Participants

Thirty-eight 8- to 16-year old children with an ASD (32 males) were recruited

through an autism register, speech pathologists and participation in previous research

projects at the University of Western Australia. Thirty children met the DSM-IV-TR

(American Psychiatric Association, 2000) criteria for Autistic Disorder, two met the criteria

for Asperger�s Disorder and six were diagnosed with Pervasive Developmental Disorder �

Not Otherwise Specified 2 . These diagnoses were independently confirmed using the

Autism Diagnostic Interview � Revised (ADI-R, Lord, Rutter, & LeCouteur, 1994).

Children either met full criteria for autism (N = 34) or scored above the cut-off in two of

the three symptom domains (N = 4). Descriptive statistics for the three domain scores of

the ADI-R are as follows: (1) social interaction: M = 20.16, SD = 6.36; (2) communication:

M = 15.94, SD = 5.04; (3) repetitive behaviours: M = 6.51, SD = 2.65. Children with a

diagnosis of any major medical condition (e.g. epilepsy), other developmental disorder (e.g.

ADHD) or visual difficulties, or who were taking medication likely to affect visual or

cognitive performance were excluded from the final sample.

One hundred and thirty-two 8- to 16-year old typically developing children were

recruited from a metropolitan school. Parents completed a brief screening questionnaire and

children with a history or diagnosis of developmental disorders, language difficulties,

medical or genetic conditions, or difficulties hearing or seeing were not included in the 132

children who comprised the final data set. Written, informed consent was obtained from the

parents of all children participating in the study in accordance with the policies of the

University of Western Australia Ethics Committee. Children had normal or corrected-to-

normal vision, and wore optical correction when required.

The groups were well matched for chronological age, t(168) = .96, p = .34, and non-

verbal ability, t(168) = .31, p = .76, as measured by the Matrix Reasoning subscale of the

Wechsler Intelligence Scales for Children (WISC-IV, Wechsler, 2003, see Table 1). There

was a greater proportion of females in the typically developing group compared to the ASD

group, ÷2(1) = 14.12, p < .01. Children�s verbal ability was also measured, using the

Vocabulary subscale of the WISC-IV, to ensure their receptive language was at a level

where they would be capable of comprehending task instructions. The ASD group had

significantly lower verbal ability than the TD group, t(168) = 5.46, p < .01 (see Table 1), 2 There were no significant differences on any of the dependent variables between the three groups with different ASD diagnoses, hence all of the analyses reported are for the three groups combined.

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consistent with the language difficulties required for diagnosis of ASD. Nevertheless, all

children were considered high-functioning in that they were attending mainstream schools.

Table 1. Participant Characteristics

Measures Children with ASD

(N = 38)

Typically Developing

Children (N = 132)

Age (years)

Mean (SD) 11.88 (2.54) 12.15 (1.94)

Range 8.17 - 16.92 8.83 - 15.83

WISC Vocabulary (scaled score)

Mean (SD) 8.95 (2.75) 11.47 (2.40)

Range 4 - 15 6 - 19

WISC Matrix Reasoning (scaled score)

Mean (SD) 10.34 (2.91) 10.06 (2.55)

Range 3 - 16 2 - 18

Apparatus

All psychophysical stimuli were presented on an LG L1730SF 271mm x 340mm

resistive touch screen driven by a Sony Vaio VGNSZ34GP laptop computer. The screen

resolution was 1024(w) x 768(h) pixels and had a refresh rate of 75Hz. Stimuli were drawn

using MATLAB 6.1 (Mathworks, Natick, MA) and displayed using the WinVis

(Neurometrics Institute, Oakland, CA) toolbox. Responses were recorded via touch screen

input to the Matlab protocol. Extensive pilot testing was conducted with experienced adult

observers to ensure that thresholds obtained with this equipment were comparable to those

obtained with CRT-based displays.

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General Procedure

The WISC Vocabulary subscale, WISC Matrix Reasoning subscale and Children�s

Embedded Figures Test (CEFT, Witkin et al., 1971) were given in that fixed order first,

followed by the psychophysical tasks in a randomised order. In order to simplify the

mapping of target to response, the current study used a two-spatial-forced-choice (2SFC)

paradigm whereby the target appeared in one of two possible spatial locations during the

same presentation interval. Pilot testing indicated that thresholds were not affected in

typical adult observers by this presentation method when compared to a two-interval-

forced-choice procedure.

For the psychophysical tasks, testing was conducted in a darkened room with the

child seated 0.75m from the screen. Viewing was binocular and feedback concerning the

accuracy of the response was given after each trial. Children were informed at the outset

that it is impossible to respond correctly to all psychophysical stimulus presentations. The

method of constant stimuli was used to control stimulus presentation and 75% correct

thresholds were obtained from the psychometric function by fitting the equation:

Y = Ǥͷ Ǥହଵାୣ୶୮ቀೞషೣ ቁ (6.1)

where threshold yields the 75% correct level, exp is the exponential function, ó is a scalar

determining the slope of the psychometric function, y is the number correct out of 15, and x

is the signal level. There were seven stimulus levels, with 15 trials for each. The goodness

of fit of the psychometric function for each observer�s threshold was estimated by

calculating R2. If the proportion of variance accounted for by the goodness of fit of the

psychometric function was less than 60% then the threshold for that observer was not

included in the analyses. Children were given two opportunities to meet this criterion if

required.

Stimuli & Procedure

CEFT

The first stimulus set uses a cardboard triangle as the target and 17 laminated cards

depicting coloured meaningful figures, 4 for demonstrations, 2 for practice trials and 11 for

test trials. The second set uses a cardboard house as the target and 19 laminated cards, 4 for

demonstration, 1 for a practice trial and 14 for test trials. Consistent with standard

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procedure for the test, children were first asked to name the complex picture, and then to

locate the hidden figure (triangle or house) embedded in the picture as quickly as possible.

The time taken by each child to find the target in each case was recorded (in seconds),

although if a child was unable to locate the target within 90 seconds they were credited with

a time of 90 seconds. The number of incorrect identifications was also recorded.

Glass Pattern Task

A Glass pattern is composed of a number of dot pairs (dipoles), the orientations of

which are specified relative to imaginary lines projecting from the centre of the pattern to

the centre of each dot pair (Dickinson & Badcock, 2007). Using fMRI, radial and

concentric geometric patterns have been shown to strongly activate area V4 (Tse et al.,

2002; Wilkinson et al., 2000), and specialised detectors in V4 for concentric shapes have

been suggested (Kurki & Saarinen, 2004). Therefore, the current study employed

concentric Glass patterns as a measure of ventral pathway global processing. Concentric

Glass patterns are created when the dot pairs are oriented at 90o to vectors emanating from

the image centre (see Figure 1).

Figure 1. An example of a Glass pattern with no coherent signal pairs (left) and a Glass

pattern with 100% coherent concentric signal pairs (right)

Two spatially separated Glass pattern stimulus displays were presented

simultaneously, each of which consisted of dipoles presented at randomly selected locations

within the display apertures. The target display contained a variable proportion of

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concentrically oriented dipoles while the remaining dipoles were randomly oriented. All

dipoles were randomly oriented in the other (noise) display. The displays were presented

within two 6.48° diameter circular apertures positioned side-by-side and had an edge-to-

edge separation of 2.28o. Each of the two stimulus displays consisted of 50 white dipoles of

luminance 60 cd/m2, presented on a grey background of luminance 30 cd/m2. Each circular

dot measured 0.16° in diameter, resulting in a dot density of 0.66 dots/deg2. The separation

between the dots in a dipole was 0.27o and the presentation duration was 426.66 ms.

Following the presentation of each stimulus pair, the child decided which of the two

contained dots that were �starting to form the rings of a lollipop� by touching the left or

right side of the screen. There was a 1s period between a response being registered on the

touch screen and the presentation of the next stimulus during which a blank screen was

shown at the background luminance. An individual�s coherence threshold was the lowest

proportion of dot dipoles required to be oriented concentrically in order for the observer to

correctly identify that pattern with 75% accuracy.

Orientation Discrimination Task

The orientation discrimination task was designed to be a local version of the Glass

pattern task, for which the orientation of a dipole is the local signal that is globally

accumulated. Accordingly, for the orientation discrimination task, two dot pairs were

presented simultaneously, one at 0o (or vertical), the other at a positive or negative angle

from vertical (see Figure 2). As in the Glass patterns, each dot measured 0.16° in diameter

and the centres of the dots in each pair were separated by 0.27o. The dot dipoles were

positioned side-by-side and separated by 2.28o. The vertical position of the dipoles was

independently jittered up to 0.27o on each successive trial to prevent horizontal alignment

being used as a cue. The luminance and presentation duration exactly matched that of the

Glass pattern task.

Following the presentation of each stimulus pair, the child decided which of the two

contained dots that were �not straight up and down� by touching the left or right side of the

screen. An individual�s threshold was the minimum angle required to be able to correctly

discriminate vertical from tilted with 75% accuracy.

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Figure 2. An example of an oriented dot pair (left) and vertical dot pair (right).

Results

The data for each group were screened for normality and for outliers. Any scores

more than three standard deviations from the mean were excluded. The means and standard

deviations for each task are presented in Table 2.

Regression Analyses

Using the method advocated by Brock et al. (2007), a simple linear regression

analysis was conducted for each of the dependent variables using gender, age, and verbal

and non-verbal ability within the data set of the typically developing children. The

regression equation for each dependent variable was then used to predict a score for each

ASD individual on the basis of the gender, age and ability variables. Next, residuals were

calculated by subtracting the expected score from the observed score for each child with an

ASD, and then standardising it by dividing by the standard error of the regression estimate.

If ability on each of the experimental tasks has developed in line with that predicted by the

psychometric variables, then the mean standardised residual scores for the ASD children

should be zero.

The mean standardised residual time for children with an ASD on the CEFT (CEFT

RT) was -.33 (95% CI = -.62 to -.04), which is significantly below zero on a one sample t-

test, t(35) = 2.30, p = .03, d = .30, indicating that they were faster than typically developing

children to detect the embedded figures (see Figure 3 for the means and 95% CIs of the raw

RTs). The mean standardised residual for number of incorrect CEFT identifications for the

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ASD group did not differ significantly from zero, t(36) = .99, p = .33, d = .13, suggesting

that there was no difference between the typically developing children and the ASD

children on this measure.

Table 2.

Unregressed means (95% CIs) for the CEFT, Glass pattern and orientation discrimination

tasks

Task Children with an ASD Typically Developing

CEFT RT (s)

N 36 126

Mean (95% CI) 9.36s (7.87-10.85) 11.15s (10.16-12.13)

Range 1.92 � 19.89s 3.04 � 27.71s

CEFT errors

N 37 123

Mean (95% CI) 2.81 (1.89-3.73) 3.30 (2.76-3.84)

Range 0 � 10 0 � 15

Glass Patterns

N 32 117

Mean (95% CI) 39.92 (35.96-43.88) 41.1 (39.28-42.94)

Range 18.76 � 68.90 21.56 � 69.48

Orientation Discrimination (deg)

N 33 116

Mean (95% CI) 3.34o (2.69-3.99) 2.71o (2.58-2.99)

Range .83-8.30o .56 � 5.85o

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Figure 3. Mean unregressed reaction times on the CEFT for the TD and ASD groups (lines

show 95% confidence intervals).

The ASD children did not differ from zero in their mean standardised residual Glass

pattern thresholds, t(31) = .67. p = .51, d = .09, indicating that they did not vary

significantly from the typically developing group on this task (see Figure 4 for the means

and 95% CIs of the raw thresholds). The mean standardised residual orientation

discrimination threshold for children with an ASD was .64 (95% CI = .02 to 1.26), which is

significantly above zero, t(32) = 2.11, p = .04, d = .28. The ASD group required a greater

angle in order to discriminate between vertical and tilted than the typically developing

children (see Figure 5 for the means and 95% CIs of the raw thresholds)3.

3 The group differences reported using the regression analyses produce the same pattern of results as when groups matched for age and non-verbal ability were compared using ANCOVA with gender and verbal ability as covariates. The ANCOVA analyses confirmed that the ASD children were faster on the EFT, F(1, 158) = 4.02, p < .05, Np

2 = .025, had poorer thresholds on the orientation discrimination task F(1, 145) = 5.85, p < .05, Np

2 = .039, but were not significantly different from typically developing children on the Glass pattern task, F(1, 145) = .52, p = .47, Np

2 = .004, or in EFT errors, F(1, 156) = 3.07, p = .081, Np2 = .019.

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Figure 4. Mean unregressed thresholds on the Glass pattern task for the TD and ASD

groups (lines show 95% confidence intervals).

Figure 5. Mean unregressed thresholds on the orientation discrimination task for the TD

and ASD groups (lines show 95% confidence intervals).

Correlations

Preliminary correlational analyses were performed to determine whether any of the

demographic variables were associated with the dependent variables. Because CEFT RT

was negatively associated with age, WISC Vocabulary and WISC Matrix Reasoning raw

scores, Glass pattern thresholds were negatively associated with WISC Vocabulary raw

scores, and orientation discrimination thresholds were negatively associated with WISC

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Matrix Reasoning raw scores, these three demographic variables were included as control

variables in partial correlations. No other correlations between the psychometric variables

and thresholds on the visual tasks or CEFT were significant. Jarrold, Gilchrist and Bender

(2005) reported a unique pattern of correlations in their ASD group compared to their

typically developing sample with respect to visual search abilities and EFT performance.

Therefore, partial correlations were examined in each sample separately and are reported

with the raw correlations in Table 3.

There was one significant raw correlation between Glass pattern thresholds and

CEFT RT in the direction of superior performance on the Glass pattern task being

associated with faster CEFT responses. While this is not in the direction that would be

expected if poor global processing contributes to better CEFT performance, the correlation

probably reflects variance shared with some of the demographic variables, since when these

variables are controlled for, the correlation is rendered nonsignificant. CEFT RT did not

correlate with orientation discrimination task thresholds in either the TD or ASD group

when age and verbal and nonverbal ability were controlled for. Additionally, Glass pattern

and orientation discrimination thresholds were not significantly correlated in either group,

suggesting that perhaps such fine grained local orientation sensitivity is not critical to

successful performance on this version of the Glass pattern task.

Table 3. Raw correlations and partial correlations controlling for age, and verbal and

nonverbal ability for the ASD and TD groups

Glass Pattern Orientation Discrimination

TD group

CEFT RT .19* (.14) .15 (.08)

Glass Pattern -.02 (-.05)

ASD group

CEFT RT .14 (.003) .05 (.07)

Glass Pattern .06 (.007)

*p < .05

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Discussion

The aim of the current research was to assess ventral stream functioning using tasks that

clearly target local and global processing but minimise differences between stimuli. The

relationship between performance on local and global ventral stream tasks and EFT

performance was also examined in order to determine whether particular patterns of

strengths and weaknesses in visual processing, as outlined by the WCC and EPF theories,

can explain superior EFT performance in ASDs. The first important finding was that the

advantage on the EFT in a group of children with high functioning ASD was replicated

using an analytic approach that is arguably more sensitive than those that have been

employed previously. The children in the clinical group performed better on the EFT than

was predicted by their age, gender, and verbal and nonverbal ability. The finding that the

group with ASDs displays a strength on the EFT relative to controls reinforces the

importance of considering the assets associated with autism when attempting to delineate

neurocognitive accounts of the disorder.

The second finding of this study was that the ASD group had more difficulty on the

orientation discrimination task relative to the TD group. If EPF best accounts for superior

performance of individuals with an ASD on the EFT, then it would be expected, contrary to

what was found, that children with an ASD would exhibit superior thresholds and thus

better local processing ability on the orientation discrimination task. The observed pattern

of results is also inconsistent with the WCC theory, which suggests that local processing in

ASDs may remain intact, or in some situations even be enhanced, despite impaired abilities

in global perception. This task assessed the orientation discrimination capabilities likely to

be supported by cells early in the cortical visual system (most likely V1), and one might

expect that these results point towards a general impairment of orientation coding in the

ventral visual stream. If this was the case, then it might also be expected that the early

difficulties in orientation discrimination would flow on to affect processing involving

orientation discrimination at later stages, such as Glass pattern perception. It is important to

note, however, that while orientation discrimination thresholds are of the order of a few

degrees, the the orientation tuned units in V1 that interact to support this form of

discrimination (Regan, 2000) have a bandwidth of approximately 15-30o (DeValois, Yund,

& Hepler, 1982). Because of this, the global accumulation process involved in the

perception of Glass patterns is tolerant to variation in local orientation cues. Therefore, if

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±3.34o (the mean local orientation discrimination threshold for the ASD group) of jitter was

added to each dipole within the Glass pattern, the average coherence of a concentric pattern

probably would not change. Thus, relatively small anomalies for the ASD group in the

processing of orientation for individual dipoles would not be expected to affect

performance on the Glass pattern task. This is consistent with the results showing that the

ASD group did not differ significantly from the control group in their performance on the

Glass pattern task in addition to the lack of correlation between thresholds for the two tasks.

This pattern of findings suggests that any difficulties seen on the orientation discrimination

task are not impacting performance at later stages of visual processing.

The finding that a proportion of the ASD group in the current study demonstrated less

sensitivity to orientation relative to the TD group may appear to be inconsistent with the

findings of other studies assessing V1 capabilities in individuals with ASDs. For example,

Bertone et al. (2005) reported enhanced capabilities in the ventral form pathway at the V1

level using first-order luminance-defined stimuli. Conversely, if the line segment stimuli

employed by Spencer et al. (2000) and Milne et al. (2006) and the Gabor stimuli employed

by Del Viva et al. (2006) do assess components of both V1 contour integration as well as

V4 global grouping (see Introduction, above), then the lack of group differences reported

by these authors may reflect intact V1 contour integration mechanisms in the ASD groups.

The contradictory results concerning low-level ventral stream processing in ASDs highlight

the fact that different basic capabilities were assessed in each study, and perhaps suggest

that there may not be a uniform pattern of augmented, typical or impaired performance

across the full set of capabilities identified with V1 in ASDs. It would be useful for future

research to delineate the specific nature of tasks that results in superior, equivalent and

impaired performance in ASD groups. With respect to the current findings, perhaps these

results indicate a more specific difficulty in a subset of individuals with an ASD (36% of

the ASD group had thresholds above the upper end of the 95% confidence interval of the

TD group) in the perception of vertical orientation. Visual misperception of verticality has

been linked to difficulties in posture and balance following stroke (Bonan, Guettard,

Leman, Colle, & Yelnik, 2006; Bonan et al., 2007). While not required for a diagnosis, one

of the most commonly reported characteristics of children with an ASD is abnormalities in

motor control (see Nayate, Bradshaw, & Rinehart, 2005, for a review), particularly in

posture (Minshew, Sung, Jones, & Furman, 2004) and balance (Molloy, Dietrich, &

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Bhattacharya, 2003). Perhaps these characteristics are related to an element of lower-level

visual perception concerning the perception of vertical. It would be interesting for future

research to explore the relationship between motor control and difficulties in visual

perception of the vertical in ASDs.

Another finding of this study was that children with an ASD did not differ significantly

from typically developing individuals in their ability to perceive the coherent structure in

concentric Glass patterns. If a deficit in global processing in the ventral visual stream best

accounts for performance on the EFT, as proposed by WCC theory, then it would be

expected that thresholds on the Glass pattern task would have been higher in the ASD

group relative to the comparison group. The non-significant difference in comparing the

mixed ASD group with controls on Glass pattern thresholds is consistent with similar

comparisons reported by Tsermentseli et al. (2008) and Spencer and O�Brien (2006) using

Glass patterns comprised of dot triplets. However, when these authors considered

individuals with autism (N = 9 for Tsermentseli et al. and N = 15 for Spencer & O�Brien)

separately to those with a diagnosis of Asperger�s disorder, they found that the autism

subgroups exhibited elevated global processing thresholds on this task. We did not find a

similar pattern of subgroup differences when those with autism were considered separate to

individuals with Asperger�s disorder or Pervasive Developmental Disorder - Not Otherwise

Specified in the current study. Importantly, lack of power is not a likely explanation for the

non-significant results in the current study, as Tsermentseli et al. tested 21 children with an

ASD, and Spencer and O�Brien tested 15 children with autism and 10 with Asperger�s

disorder, whereas 32 children with an ASD were tested on the Glass pattern task in the

present study. There was sufficient power in the current study (.92; calculation based on

effect sizes, see Cohen, 1988, for a description) to detect a difference between the two

groups had there been one present (power estimation based on effects reported by

McKendrick, Badcock, & Gurgone, 2006, using similar Glass pattern stimuli in

migraineurs versus controls).

Perhaps the discrepancies in performance of the sub-groups between the current study

and those reported by Tsermentseli et al. (2008) and Spencer and O�Brien (2006) can be

attributed to methodological differences. An important methodological consideration

concerns the Glass pattern stimuli. One difference between the present study and the other

two is that we presented our stimuli within separate apertures, whereas Tsermentseli et al.

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and Spencer and O�Brien embedded the target stimulus within a patch of noise. However,

Dickinson, Broderick and Badcock (2009) demonstrated that placing a Glass structure in a

field of noise does not affect the coherence thresholds of typical observers when compared

to Glass patterns presented in apertures. Therefore it is doubtful that the difference in

stimulus presentation can account for the discrepancy in findings between the current study

and Tsermentseli et al. and Spencer and O�Brien. A second difference in methodology was

that our Glass patterns were presented for 426.7 ms, approximately 176 ms longer than was

the case for the other two studies. It is not clear what impact this difference would have

since Aspell, Wattam-Bell and Braddick (2006) demonstrated for concentric Glass patterns

that longer presentation durations are required to accurately determine thresholds for

smaller stimuli, whereas shorter durations can be used for larger stimuli. Because the Glass

patterns used in the current study were almost half the size (diameter = 6.48o) of the Glass

patterns used by Tsermentseli et al. and Spencer and O�Brien (diameter = 12.12o), it was

appropriate to use longer presentation durations consistent with the optimal durations

identified by Aspell et al. (2006, see Figure 3) for concentric stimuli. Thus, given the

difference in stimulus size across studies, it is not clear whether the difference in

presentation durations is critical in explaining the different patterns of results. Furthermore,

individuals with autism have been found to perform visual search tasks faster than their

respective matched control groups (Jarrold et al., 2005; O�Riordan, 2004). Consequently, it

could be expected that shorter presentation durations would benefit the ASD groups, rather

than contribute to the impaired thresholds obtained by Tsermentseli et al. and Spencer and

O�Brien. Therefore, it also seems unlikely that this methodological difference can account

for the discrepancy in results across the two studies, although it may be important for future

studies to investigate systematically the impact of presentation duration and stimulus size

on the perception of Glass pattern stimuli in individuals with an ASD.

A final methodological consideration concerns the nature of the participant groups in

the three studies. In the present study, we predicted task performance for each ASD child

based on gender, age, verbal ability and nonverbal ability, according to the contribution of

each variable to task performance in the TD group. The difference between the predicted

and observed scores for individuals in the ASD group was then calculated to determine

whether their performance varied from what would typically be expected. Conversely, the

Tsermentseli et al. sample was matched to their control group using the Vocabulary and

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Block Design subscales of the Wechsler Abbreviated Scale of Intelligence (Wechsler,

1999) for a much larger age range (17 � 40 years) than the present study (8 � 17).

Individuals with autism have been shown to have a peak in ability on the Block Design

subscale of the WAIS and similar tasks, with reference to other abilities (Morgan et al.,

2003; Pellicano, Maybery, Durkin, & Maley, 2006; Shah & Frith, 1993). Therefore, it is

possible that the IQ estimate that Tsermenseli et al. used over-estimated general ability in

the ASD participants. This may have resulted in their comparison group being of higher

general ability relative to the autism group, and it could be for this reason that the control

group outperformed the autism group on the Glass pattern task. Additionally, Spencer and

O�Brien matched their groups for verbal mental age using the British Picture Vocabulary

Scale (BPVS). However, Mottron (2004) found that when compared to the Wechsler scales,

picture vocabulary scales considerably overestimate the intelligence level of children with

an ASD. Accordingly, the groups in Spencer and O�Brien�s study may not have been

matched as accurately as they could have been, thus biasing the results towards a group

difference.

However, in the present study a concern was raised when the distribution of thresholds

within the typically developing group on the Glass pattern task was examined in more

detail. Lewis et al. (2004) reported that by 9 years of age, typically developing children�s

sensitivity to concentric Glass patterns reached adult-like performance, for which

thresholds for concentric stimuli on average range from 12% to 20% (Badcock et al., 2005;

Wilson & Wilkinson, 1998; Wilson, Wilkinson, & Asaad, 1997). The mean for the

typically developing children in the current study was double (40%) what would be

expected based on Lewis et al�s research. The reason for the poor performance of the

typically developing group on this task is unclear. We are also uncertain whether the

thresholds for both the ASD and TD groups are high for the same reason and therefore our

finding of a non-significant difference between the two groups is accurate, or whether the

ASD group exhibited high thresholds for a different reason to the TD group. As a result of

the uncharacteristically high thresholds in the TD group, we wanted to assess global form

processing using an alternative methodology to confirm the outcome of equivalent global

pattern processing thresholds in individuals with an ASD and typically developing

individuals. Radial frequency patterns (Wilkinson, Wilson & Habak, 1998) have been

shown to provide an independent measure of global processing at the same, intermediate

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level in the visual system as Glass patterns (Wilkinson et al., 2000), and form the basis of a

follow-up study reported in Chapter 7.

Finally, WCC theory can be used to predict a negative correlation between EFT

performance and Glass pattern thresholds, whereas EPF can be used to predict a positive

correlation between EFT performance and thresholds on the orientation discrimination task.

Pellicano et al. (2005) reported that higher thresholds on a global dot motion task were

associated with superior ability to detect embedded figures. Similar results would have

been expected for the present study if a general limitation in global processing contributes

to superior performance on the EFT, as can be argued from WCC. In contrast to Pellicano

et al.�s findings, we did not find a relationship between the Glass pattern thresholds and

CEFT RT in the current study when age and verbal and nonverbal ability were controlled

for. Given that we did not find group differences on the measure assessing global ventral

stream capabilities, it is not surprising that a significant relationship between Glass pattern

thresholds and EFT RT was not found for the ASD group. Similarly, no relationship was

observed between orientation discrimination thresholds and CEFT RT in the current study.

If superior local abilities contribute to EFT performance, then the lack of a relationship

between EFT performance and orientation discrimination thresholds may indicate that

another property of the visual system other than local orientation discrimination is

responsible for the enhanced performance of individuals with an ASD on embedded figures

tasks.

One challenge for theories that attempt to account for both the deficits and assets

seen in ASDs is to provide a neurological basis for what are traditionally cognitive

accounts. In the present study, we have used psychophysical tasks as an indication of

underlying brain function in order to discern the nature of the visual processes

characteristic of ASDs, and their relationship to performance on the EFT. While the focus

was on local and global visual processing so as to address the WCC and EPF theories, an

alternative neurocognitive account has been proposed by Minshew et al. (1992, 1997;

Williams, Goldstein, & Minshew, 2006). These authors propose that ASDs are

characterised by impairments in complex information processing and that difficulties arise

when integration of information is required, while simple information processing is spared.

Support for this theory, and indeed the other theories arguing that individuals with autism

have difficulty in integrative processing (Bertone et al., 2005; Frith, 1989; Happé & Booth,

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2008) can be found in brain imaging studies. Within these studies, local connectivity within

neural assemblies is differentiated from long-range connectivity between functional brain

regions (Belmonte et al., 2004). Connectivity (or the degree of synchronisation of the time

series of the neural activation) between various cortical regions participating in tasks

involving language, working memory and problem solving has been found to be lower for

individuals with autism than control participants (Cherkassky, Keller, Kana, & Minshew,

2007; Koshino et al., 2005; Just, Cherkassky, Keller, & Minshew, 2004; Just, Murias,

Webb, Greenson, & Dawson, 2007). Thus, the results from fMRI suggest relative

underconnectivity between brain regions in individuals with ASDs when compared to

typically developing controls (see Belmonte et al., 2004; Hughes, 2007; Minshew, and

Williams, 2007, for reviews). Perhaps combining psychophysical tasks with imaging

techniques would assist in clarifying some of the inconsistencies surrounding visual task

performance in the ASD literature.

To conclude, the present research replicated the finding of superior performance on the

EFT in a group of children with an ASD. The ASD and TD groups did not differ

significantly on a measure assessing global processing in the ventral visual stream, and

children with an ASD had unexpectedly high thresholds on a measure of local ventral

visual stream processing. Thus, we were unable to establish a profile of visual ability

associated with the form processing pathway in ASDs that was consistent with either WCC

or EPF theory, nor to account for the nature of group differences in EFT performance.

Recently, we reported that individuals with high levels of self-rated autistic-like traits

showed an advantage in detecting embedded figures (Grinter et al., in press; Grinter, Van

Beek, Maybery, & Badcock, 2009). This advantage occurred in conjunction with decreased

sensitivity to coherent Glass patterns and equivalent performance on a measure of lower-

level ventral stream processing when compared to individuals who reported low levels of

autistic-like traits (Grinter et al., in press). In this adult population, faster EFT performance

was related to poorer thresholds on the Glass pattern task, suggesting that difficulties in

global processing may account for some of the variance in EFT performance associated

with autistic-like traits. The Glass pattern stimuli in the Grinter et al. (in press) study were

identical to those employed in the current study. Perhaps the story is different for children,

but it may also be the case that testing adults scoring at two ends of the spectrum of

autistic-like traits better overcame some of the issues associated with comparing clinical

180

and control populations (see Walter, Dassonville, & Bochsler, 2009, for a discussion) than

could be achieved with the statistical techniques employed in the current study.

181

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Chapter 7.

Perception of shapes targeting local and global processes in autism spectrum disorders

Emma J. Grinter, Murray T. Maybery, Elizabeth Pellicano,

Johanna C. Badcock and David R. Badcock

Chapter 7 could not be included in the digital version of this thesis for copyright reasons. Please refer to the physical copy of the thesis, held in the University Library.

212

Chapter 8.

General Discussion

213

The central objective of this thesis was to determine whether the capabilities of the

ventral visual stream associated with autism are best conceptualised by a profile of abilities

consistent with weak central coherence (WCC; Frith, 1989; Frith & Happé, 1994) or

alternatively, with enhanced perceptual functioning (EPF; Mottron, Dawson, Souliéres,

Hubert, & Burack, 2006). More specifically, the aims of this research were to: (1) consider

whether developmental disorders share a deficit in dorsal stream functioning or rather

whether a profile exists that is specific to different conditions, and in doing so, identify

methodological improvements that can be made with respect to examining ventral stream

processing; (2) ascertain whether the visual-cognitive abilities (such as superior Embedded

Figures Test performance) seen in autism spectrum disorders (ASDs) are shared by

individuals in the general population who score highly on a self-rated measure of autistic-

like behavioural traits, and determine whether WCC or EPF is associated with visual-

perceptual abilities (such as performance on visual psychophysical tasks) in this population;

and (3) examine whether WCC or EPF in ventral stream visual-perceptual function is the

likely mechanism underlying visual-cognitive abilities by assessing the integrity of the

ventral pathway in children with an ASD, using tasks that engage processing at lower and

higher cortical areas within the visual form pathway. These aims were investigated through

a series of independent, but related studies. The implications of the overall findings from

this body of work will be considered in this chapter following a review of the key results

from each study.

Summary of Findings

The review of the literature in Chapter 2 considered whether the developmental

disorders share a common impairment in dorsal visual stream functioning, as suggested by

Braddick, Atkinson and Wattam-Bell (2003). Atkinson et al., (1997) argued that threshold

measurement on global form and motion tasks might provide a diagnostic instrument for

developmental disorders, but later suggested that the use of these tasks as diagnostic tools

is limited by a similar pattern of poor motion perception thresholds across a number of

disorders (O'Brien, Spencer, Atkinson, Braddick, & Wattam-Bell, 2002). However, the

review of the literature in highlighted that not all levels of the visual pathways have been

adequately assessed across the five disorders for which early to mid-level visual abilities

have been assessed: developmental dyslexia, ASDs, developmental dyspraxia, Williams

214

syndrome and fragile X syndrome. In particular, recent research suggests that there does

not appear to be a general dorsal stream weakness in individuals with an ASD; instead, a

pattern of weak global grouping and intact local processing in the dorsal stream suggests

that global visual processing might be impaired in ASDs (Bertone, Mottron, Jelenic, &

Faubert, 2003; Pellicano, Gibson, Maybery, Durkin, & Badcock, 2005). The literature is

less consistent with respect to the ventral visual stream, and the studies reviewed led to the

suggestion that perception of the line segment stimuli commonly used to assess global form

processing may actually be facilitated by significant input from the neurons in V1 sensitive

to local contours and which are linked by horizontal connections to enhance extended

contours (Li & Gilbert, 2002; Loffler, 2008). With respect to the other developmental

disorders considered, impairment in the dorsal stream was most clearly associated with

dyslexia and FXS, whereas further research is needed to decisively state that dyspraxia and

Williams syndrome have visual impairments arising from difficulties in this pathway.

Therefore, while it is possible that the dorsal stream profile in ASDs might be

distinguishable from the profile for other developmental conditions, the review chapter

confirmed the need for a more comprehensive analysis of ventral visual stream processing

in ASDs and other conditions, using methodological approaches that specifically assess

global grouping in the ventral cortical pathway.

One of the concerns raised in Chapter 2 was the susceptibility of the staircase

method, often used to estimate visual task thresholds, to mistakes or inattentiveness early

on in the task. In order to avoid this impacting on our studies with children, we wanted to

use the method of constant stimuli (MOCS) to assess thresholds on the psychophysical

tasks. The purpose of the pilot study reported in Chapter 3 was to compare thresholds

derived under the staircase and MOCS procedures in adult observers. The MOCS method

produced lower thresholds than the staircase method on the Glass (1969) pattern tasks, but

there was no difference in thresholds for the two procedures for the global dot motion task

and radial frequency (RF; Wilkinson, Wilson, & Habak, 1998) patterns. In all instances the

estimated thresholds fell within the ranges reported in the literature for similar tasks. These

results suggested that the MOCS procedure is reasonable to use with children for the tasks

we administered, as it is able to provide accurate threshold estimates while being robust to

early errors or attention lapses.

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Before beginning the empirical studies examining ventral stream processing in the

autism spectrum, the opening study of this thesis, reported in Chapter 4, investigated the

notion that the traits associated with ASDs exist on a continuum spanning from autism to

typical development (Baron-Cohen, Wheelwright, Skinner, Martin, & Clubley, 2001; Frith,

1989; Wing, 1988). While there is growing evidence that autistic behavioural traits can be

measured quantitatively in the general population and that these traits form a normal

distribution (Baron-Cohen, Hoekstra, Knickmeyer, & Wheelwright, 2006; Baron-Cohen et

al., 2001; Constantino, Przybeck, Friesen, & Todd, 2000; Constantino & Todd, 2003, 2005;

Hoekstra, Bartels, Verweij, & Boomsma, 2007; Posserud, Lundervold, & Gillberg, 2006;

Williams et al., 2005; Woodbury-Smith et al., 2005), it had not previously been established

whether these behavioural traits have the same underlying cognitive substrate in the general

population as for the diagnosed condition. Using the Embedded Figures Test (EFT; Witkin,

Oltman, Raskin, & Karp, 1971) and the Block Design subscale of the WAIS (Wechsler,

1997), the first experiment reported in Chapter 4 demonstrated that individuals from the

general population scoring high in autistic-like traits do indeed locate hidden figures and

reconstruct block designs faster than those scoring low in autistic-like traits, consistent with

evidence of exceptional performance on these tasks in ASD samples. The second

experiment reported in Chapter 4 replicated the advantage on the EFT in the group scoring

high on the Autism-spectrum Quotient (AQ; Baron-Cohen et al., 2001) compared to those

scoring low, and showed that this difference occurred irrespective of verbal and non-verbal

ability. This extension of the visual-cognitive characteristics seen in ASDs to individuals in

the general population who self-report mild autistic-like traits provides additional support

for the notion of continuity between ASDs and typicality, and highlights the viability of

conducting studies of high AQ scorers, in addition to ASD populations, to further our

understanding of visual functioning in ASDs.

In the second study of this thesis, reported in Chapter 5, the focus was on assessing

visual-perceptual processing in the ventral stream specifically for high and low AQ groups.

Global dot motion stimuli and the EFT were administered to determine whether a similar

profile of performance on these tasks exists in individuals scoring high on the AQ to that

seen in ASDs. Glass patterns were used to assess the integrative capabilities of the ventral

cortical stream, in addition to a pulsed-pedestal contrast-sensitivity task assessing lower-

level inputs to the ventral visual stream. The results replicated the finding that people who

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score high on the AQ are faster to identify embedded figures, and in addition high AQ

scorers had poorer global motion and global form thresholds compared to low AQ scorers.

However, the two groups did not differ on the pulsed-pedestal contrast-sensitivity task that

does not require global processing. These findings are consistent with the emerging pattern

in the literature showing that diminished global dorsal stream sensitivity in combination

with superior EFT performance appears to be unique to ASDs, providing additional validity

concerning the use of high versus low AQ groups to examine the relationship between

ventral visual stream functioning and autistic traits. Moreover, the results indicated that

individuals with high levels of autistic-like traits have difficulties with global integration in

both the ventral and dorsal visual pathways. Difficulties in global processing in the ventral

stream may partly explain superior performance on the EFT in that we found a small but

significant negative correlation between Glass pattern thresholds and time taken to locate

the hidden figure. Given the lack of evidence for superior low-level ventral stream ability in

the high AQ group, and the difficulties experienced by this group in global processing,

these findings are consistent with WCC rather than ELP.

However, in the third study (reported in Chapter 6), which examined local and

global ventral stream processing in typically developing (TD) and ASD children using

Glass patterns and a local orientation discrimination task, the results were not so clear-cut.

While the ASD group exhibited the characteristic enhanced ability to detect embedded

figures, they had equivalent thresholds on the Glass pattern task, and experienced more

difficulty on the orientation discrimination task relative to the TD children. The Glass

pattern task requires global accumulation of local orientation cues, and therefore the

processes tapped by the simple orientation discrimination task could provide inputs to the

processes involved in the Glass pattern task. Nevertheless, as mentioned in Chapter 6, this

global accumulation process is tolerant to variation in local orientation cues, so the

differences that were present on the low-level task may have averaged out on the Glass

pattern task. These findings were not consistent with either the WCC or EPF theories, in

that WCC can be used to predict impairment on the Glass pattern task in conjunction with

at least equivalent thresholds on the orientation discrimination task, and EPF can be used to

predict superior performance on the orientation discrimination task. Finally, there was no

consistent relationship between either of the psychophysical measures and performance on

the EFT. Importantly, the mean threshold for the TD group was unexpectedly high on the

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Glass pattern task. As a result, it was unclear whether the thresholds for both groups were

high for similar reasons and the finding of an equivalent performance on the Glass pattern

task was accurate, or whether the ASD group exhibited high thresholds for a different

reason to the TD group. In order to confirm the finding of intact global processing in the

ventral stream in individuals with ASDs, global form processing was assessed in the study

reported in Chapter 7 using stimuli that provide an independent measure of function at the

same, intermediate level of the visual system as Glass patterns.

Therefore, the final study of the thesis (reported in Chapter 7) also examined local

and global processing in the ventral visual stream in ASDs, and was the first to use radial

frequency (RF; Wilkinson et al., 1998) patterns in this population. The application of these

stimuli had the benefit of assessing another aspect of local processing in the ventral stream

distinct from the visual perception of vertical, which a proportion of children with an ASD

appeared to have difficulties with in Study 3. The results of Study 4 indicated that children

with an ASD required greater shape deformation to identify RF3 patterns compared to TD

children, consistent with difficulty in global processing in the ventral stream. No group

difference was observed for RF24 patterns, suggesting that local ventral stream processing

of this nature is intact in the ASD group. It is notable that the thresholds for the TD group

were similar to those reported in the literature for adult observers for both the RF3 and

RF24 tasks (Bell, Badcock, Wilson, & Wilkinson, 2007), suggesting that the issues

associated with the perception of Glass patterns in Study 3 were not present in Study 4. The

outcomes of Study 4 are consistent with WCC and add substantially to the position that a

deficit in global pattern processing within the ventral cortical pathway is present in autism.

Implications for WCC and EPF theories

Recently, Happé and Frith (2006) reviewed the original conceptualisation of WCC

as an inability to extract global form or meaning. Instead, they proposed that WCC

represents a bias towards detailed-focused processing. This revision represents a position

similar to the EPF account, in that superiority in local processing is posited to occur

without a concomitant deficit in global processing. Conversely, the evidence from the

present collection of studies indicates that poor global processing in the cortical ventral

visual stream is more characteristic of both individuals scoring high in autistic-like traits

and ASD children than is superior abilities in local visual perception. In particular, the high

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AQ group had difficulty integrating local dipole information in order to perceive the overall

structure in Glass patterns as well as deficits in global motion perception associated with

the dorsal stream, and children with an ASD had difficulty perceiving the deformation in

RF3 shapes that required global pooling of local information. In neither Study 2 nor 4 was

superior local processing associated with autistic traits, and in Study 3 the children with an

ASD displayed difficulties on the local orientation discrimination task. While this latter

result does not appear to reflect a pervasive impairment in the ventral visual stream in this

population (the ASD children displayed equivalent abilities compared to the TD group on

the RF24 task), it is incompatible with the predictions of EPF2. These patterns of results

indicate that perhaps theories that allow for difficulties in complex, global processing are

more applicable to visual abilities in high AQ and ASD populations than those that posit

enhanced local processing alone.

Indeed, Happé and Booth (2008) revisited Frith�s original concept of WCC and

posited that the idea of reduced integrative processing has �been prematurely abandoned in

the recent focus on superior local processing in autism� (p. 50). In reviewing the literature,

these authors question whether the necessary data exist from tests that independently tap

global and local processing. They identify that many of the tasks used to assess central

coherence thus far place local and global processing in competition, such that task

performance characteristic of ASDs may reflect either reduced integration or a greater

ability to focus on local details. The results from the present studies concur with the

assertion that WCC has been prematurely abandoned, and provide a useful paradigm from

which to revisit it. One advantage of assessing functioning of the visual system is that tasks

tapping local and global processing are able to distinguish any potential global impairment

or local enhancement associated with ASDs. This is because the visual system is organised

in a predominantly hierarchical way, and for this reason local processing occurring early in

the system can be assessed somewhat independently of the processes involved in

integrating these local elements to form a global, coherent whole further along in the

hierarchy. Because global processing relies on the input from lower levels, it is reasonable

to expect that anomalies in local processing should carry through to impact global abilities.

2 The results from the Glass pattern task in Study 3 indicated that the ASD group had intact capabilities in global form processing, which appears to be inconsistent with WCC. However, as noted in Chapter 6, the TD group had unusually high thresholds on this task making it is difficult to conclusively state that thresholds were equivalent across the two groups.

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Thus, if local processing is superior in ASDs, processing should be intact or even superior

on tasks assessing global visual abilities, a pattern of performance not incongruent with the

EPF account.

However, the results from Studies 2 and 4 involving the ventral stream and studies

assessing contrast sensitivity and global motion perception in the dorsal stream (e.g.

Bertone et al., 2003; Pellicano et al., 2005) are painting a reasonably consistent picture that

does not support the central tenets of ELP theory. When significant differences have been

reported for global processing, the differences have invariably been in the direction of

impaired performance for the ASD sample, whereas for local processing the differences

have reliably been in the direction of superior performance for the ASD sample, relative to

neurotypical comparison groups. EPF does not allow for impairment in global processing,

whereas WCC is not incongruent with enhanced local processing abilities, and thus the

findings from studies assessing visual abilities in ASDs are compatible with WCC. The one

exception to this pattern is the results from the local orientation discrimination task in

Study 3. The difficulty the ASD group had in the perception of vertical orientation contrasts

with results previously reported of superior lower level ventral stream processing (e.g.

Bertone, Mottron, Jelenic & Faubert, 2005). However, the neurones early in the visual

system have a number of response properties (such as orientation and direction detection,

colour sensitivity, contrast sensitivity and speed of processing) and individuals with an

ASD may not vary on all of these capabilities when compared to control groups. While it is

possible that individuals with an ASD exhibit a specific impairment in the perception of

vertical, the disparity in results for this study indicates that the many different functions

performed early on in the visual system in the ventral stream warrant further investigation

in ASDs.

The emerging literature concerning the neuroanatomy of ASDs also favours WCC

over EPF. These studies typically compare the brain activation of high-functioning ASD

groups with matched control groups using fMRI. The measurement of interest is the

functional connectivity (or the degree of synchronisation or correlation of the times series

of the activiation) between various cortical areas involved in task completion in each group.

Differences between individuals with an ASD and control groups have been found in the

patterns of activation and in the synchronisation of the activation across the cortical

networks recruited to perform tasks involving language (Just, Cherkassky, Keller, &

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Minshew, 2004; Kana, Keller, Cherkassky, Minshew, & Just, 2006), working memory

(Koshino et al., 2005; Koshino et al., 2008), problem solving (Cherkassky, Keller, Kana, &

Minshew, 2007), action planning (Villalobos, Mizuno, Dahl, Kemmotsu, & Müller, 2005),

and social cognition (Castelli, Frith, Happé, & Frith, 2002). These studies provide evidence

of a general problem with functional underconnectivity between neocortical systems in

ASDs (Minshew & Williams, 2007). The results from fMRI studies have been interpreted

as reflecting reduced connectivity in higher order circuitry and intact or enhanced lower

order circuitry. Thus, the underconnectivity theory predicts that �any facet of psychological

or neurological function that is dependent on the coordination or integration of brain

regions is susceptible to disruption, particularly when the computation demand of the

coordination is large� (Just et al., 2004, p. 1817), whereas functions that do not require such

coordination and reliance on frontal, integrating centres, can be performed adequately or in

some instances even extraordinarily well (Just et al., 2004). Common to these fMRI studies

is an anomaly in the integration of information in ASDs. However, given that the

underconnectivity theory pertains to multiple cortical regions, the definition of integration

is more complex than the global processing construct used throughout the studies reported

in this thesis.

Clearly, however, there are instances in which enhanced local processing in the

visual domain results in superior performance in individuals with an ASD, particularly on

tasks such as featural visual search (Jarrold, Gilchrist & Bender, 2005; O�Riordan, Plaisted,

Driver & Baron-Cohen, 2001). While the results from the current series of experiments

suggest that weak global processing characterises visual abilities in ASDs, it is possible that

superior local processing abilities develop as one way to overcome impairments in global

perception. Thus, while WCC may be the predominant factor contributing to the atypical

nature of visual capabilities in ASDs, further research needs to be conducted to elucidate

the relative contributions of weak global processing and enhanced local processing to the

profile of abilities for this family of disorders. Milne, Scope, Pascalis, Buckley and Makeig

(2009) recently compared the visual evoked potentials elicited by Gabor patches of varying

spatial frequency in individuals with an ASD and IQ-matched typically developing

children. Interestingly, Milne et al. showed that latency to the peak stimulus-induced

response was reduced in individuals with an ASD. This finding of faster rise times for

electrical activity in V1 indicates that it would be worth investigating further the role of

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speeded processing when considering the contributions of local processing to visual task

performance in ASDs.

WCC as a unified explanation of ASD symptomatology

As outlined in Chapter 1, cognitive theories (namely Theory of Mind, Executive

Dysfunction and WCC; see also the Extreme Male Brain hypothesis, Baron-Cohen, 1999)

have aimed to account for some or all of the core features of ASDs. While the different

theories appear to each explain one of the core features well, the ability of these theories to

provide a unitary explanation encompassing the entire triad of impairments is effectively

limited (see Happé & Ronald, 2008, for a detailed discussion). Thus, a pertinent question

concerning the symptoms that characterise ASDs is whether they constitute a single

dimension, or multiple dimensions. Happé and colleagues (Happé & Ronald, 2008; Happé,

Ronald, & Plomin, 2006) recently posited that the different aspects of ASD may have

distinct causes at the genetic, cognitive and neural levels that map on to the autistic triad of

impairments. Evidence for the fractionation of the autistic triad of impairments arises from

(1) modest correlations between the three areas of autistic-like traits in both the general

population (Ronald et al., 2006a; Ronald, Happé, Price, Baron-Cohen, & Plomin, 2006b)

and ASD samples (Wing & Gould, 1979), (2) evidence for multiple factors underlying

autistic behaviours (Austin, 2005; Hurst, Mitchell, Kimbrel, Kwapil, & Nelson-Gray, 2007;

Mandy & Skuse, 2008, but see Constantino et al., 2004), (3) twin studies demonstrating

that each aspect of the triad is heritable (Ronald et al., 2006a; Ronald, Happé, & Plomin,

2005; Ronald et al., 2006b), (4) family studies suggesting that the genes contributing to

ASDs segregate among relatives and have distinct influences on the different parts of the

phenotype (Bolton et al., 1994; Pickles et al., 2000; Szatmari et al., 2000), (5) genetic

studies suggesting that different genetic loci may be associated with the core behaviours

defining the ASDs triad (see Szatmari, 1999, for a review) and (6) neuroimaging work

suggesting discrete neural substrates for the cognitive functions associated with the

different cognitive accounts of ASDs (Castelli et al., 2002; Just, Cherkassky, Keller, Kana,

& Minshew, 2007; Lee et al., 2007; Manjaly et al., 2007).

One implication of ASDs being considered multidimensional is that each of the

proposed underpinnings need not be specific to ASDs, but rather it is the combination of

deficits that is unique to the condition (Happé & Ronald, 2008). That several elements of

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the autism phenotype can exist in lesser form in siblings and first- and second-degree

relatives, as well as in the general population, suggests there is much variation in the

strength that these traits can manifest themselves. While the most extreme of each is

evident in the diagnosis of autism, a multidimensional approach allows for heterogeneity

along the spectrum. If the traits can vary in their nature and intensity then this introduces

the possibility that not all individuals will show strong traits in each domain. It follows then

that just as there may be some independence of the behavioural features, with each varying

in severity, then so too may there be some independence of the different forms of atypical

visual and cognitive processing associated with ASDs, with the extent of atypicality

varying across individuals also. This is consistent with the findings of the present studies.

Specifically, not all of the participants in our high AQ and ASD groups demonstrated

impaired global processing or enhanced local processing on the tasks designed to assess

WCC. In the research reported in Chapter 4, 87-89.5% of the high AQ group had scores

falling outside the 95% confidence interval of the low AQ group on the Embedded Figures

Test (EFT). In the experiment reported in Chapter 5, 77% of the high AQ group reached

this criterion on the EFT, 69% on the global dot motion task, and 65% on the Glass pattern

task. In study described in Chapter 6, 39% of the ASD group had Children�s EFT scores

outside the 95% confidence interval of the typically developing group, and for the

experiment reported in Chapter 7, 62.5% of the ASD group reached this criterion on the

RF3 task. Thus, while WCC may be better able than EPF to account for the pattern of

strengths and weaknesses seen in ASDs on certain visual-cognitive and visual-perception

tasks, impaired global processing need not be a defining feature of all individuals with an

ASD. The increased variability introduced by considering a multidimensional aetiology of

ASDs allows for some individuals to show extreme visual traits, whereas some may exhibit

such traits only under certain task demands, while others may not exhibit these visual traits

at all despite being severe in other behavioural traits associated with the condition.

Thus, it would be important for future research to elaborate the contribution that

each purported deficit (i.e. WCC, Theory of Mind, and Executive Dysfunction) makes to

the severity of behavioural symptoms seen in ASDs. This issue was not addressed in the

studies assessing autistic-like traits in the general population as the participants in Studies 1

and 2 were selected to be high or low in total AQ score, which amalgamates autistic-like

traits across multiple dimensions (see below). This meant that the high and low AQ groups

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differed significantly and substantially on each of the dimensions, and therefore it was not

appropriate to examine relationships between visual-cognitive and visual-perceptual task

performance and scores on individual dimensions of the AQ. Future research may focus on

the notion that, as mentioned above, the evidence for multiple factors underlying

behavioural autistic traits is also apparent in the general population, including when the AQ

has been employed. While Baron-Cohen et al. (2001) suggest five subscales for the AQ,

factor analytic studies have found two- (Hoekstra, Bartels, Cath & Boomsma, 2008), three-

(Austin, 2005; Hurst et al., 2007), and four-factor (Stewart & Austin, 2009) models.

Despite these differences, all studies agree on �social skills� and �details/patterns� factors

and the �communication� factor is largely agreed on (but see Hoekstra et al., 2008, who

report two higher-order factors). It would therefore be possible to select participants who

score, for example, high on social skills and low on details/patterns and compare them to

participants scoring high on details/patterns and low on social skills and to a further group

of participants scoring low on both dimensions. To extend the current research in this

manner, if WCC is most related to the attention to detail traits seen in high AQ scorers, then

it could be expected that higher scores on the details/pattern factor of the AQ would be

associated with superior EFT performance and poorer global visual processing thresholds.

Examining the relationship between task performance and symptomatology would facilitate

our understanding of the underlying perceptual and cognitive mechanisms associated with

the behavioural traits seen on the ASD spectrum.

Despite the need for further research examining the relationship between

symptomatology and task performance, the results from Studies 3 and 4 revealed no

significant relationships between the EFT, or local and global visual processing abilities

and domain scores on the ADI-R. Given that the results from the visual-perceptual tasks in

Study 3 were not consistent with either the WCC or EPF accounts, the lack of correlation

with the ADI-R subscales in this study is perhaps unsurprising. However, the absence of a

clear relationship between RF thresholds and symptom severity in Study 4 provides limited

evidence for the relationship between WCC and behavioural symptomatology in ASD. The

ADI-R is currently the �gold standard� (de Bildt et al., 2004) for diagnosis of ASDs in both

clinical and research settings and is therefore a particular strength of the research reported

in this thesis. Nevertheless, the interview is based entirely on parental report and may

therefore be susceptible to potential biases relating to the parent�s perception of his/her

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child�s disability and the parent�s ability to recall information about the child�s early

development. Another possible problem is that the ADI-R focuses on behaviours at age 4-5,

whereas we assessed visual abilities many years later. Different children might show

different rates of change with development, which may interfere with the correlations.

While it may have been therefore more appropriate to measure symptom severity using

more objective techniques such as direct observation, it would have only been possible to

observe the children in a limited range of situations, thus limiting our capacity to detect

infrequent but important events that may be indicative of the child�s acute symptoms. This

highlights the importance of obtaining information regarding symptom severity from

multiple sources to ensure that an accurate representation of the child�s symptomatology,

both past and present, is obtained. Two other studies have recently attempted to address this

need in ASD groups. Pellicano, Maybery, Durkin and Maley (2006) reported that indices of

autistic symptomatology obtained using the ADI-R were unrelated to performance in any of

the three cognitive domains they assessed: theory of mind, executive dysfunction and

WCC. When participants were assessed three years later, scores on cognitive measures

were not correlated with scores on behavioural measures, including one measure of direct

observation (Pellicano, 2009). If we view ASDs from a multidimensional causality

perspective, then for WCC theory to be successful it should be related to at least one

dimension of the range of behavioural symptoms that characterise the condition. It is this

link between WCC and the behavioural features of ASD that is yet to be firmly established.

Implications for the design of ASD studies

Many issues arise when clinical populations are compared to various types of

control populations (Brock, Jarrold, Farran, Laws, & Riby, 2007). The first potential

challenge lies in the heterogeneity of the clinical group, particularly an ASD group that

contains individuals with different diagnoses of autism, Asperger�s syndrome or Pervasive

Developmental Disorder Not Otherwise Specified. Even individuals with the same

diagnosis can exhibit considerable variance in cognitive abilities and severity of

impairments (Happé et al., 2006; Ring, Woodbury-Smith, Watson, Wheelwright, & Baron-

Cohen, 2008). Given the relatively small numbers of participants typically tested in studies

of perceptual ability in ASDs, this within-group heterogeneity could potentially impact the

research outcomes if it is not accounted for. One way to correct this issue of heterogeneity

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might be to identify the sub-groups and study each more intensively, a practice that is

occurring more frequently in the literature. Secondly, there is the difficulty in matching

groups for IQ. One common approach to deal with this issue is to include a control group

matched to the mental age of the ASD group; however, this can lead to differences in

chronological age. These differences can be problematic in the context of measuring, for

instance, visual abilities that develop with age (see Bertone, Hanck, Cornish & Faubert,

2008, and Lewis et al., 2004, for examples of visual abilities that develop with age). An

alternative to using mental-aged matched control groups would be to instead compare an

ASD group to a control group of typically developing children matched for chronological

age, but this could in turn introduce a confound if thresholds are instead correlated with

mental age. As a result, statistical analyses are often used as a convenient alternative to

control for the effects of age and abilities. Brock et al. (2007) highlight the numerous issues

that arise when traditional statistical approaches such as analysis of covariance, are used to

attempt to control for the aforementioned imbalances in age and ability. With this in mind,

the experiments reported in this thesis employed two different solutions in an attempt to

overcome these concerns.

Firstly, in the studies assessing visual abilities in the ASDs population, we used

regression analyses to compare performance for the clinical and typical samples as per the

method outlined by Brock et al. (2007). In doing so, we predicted the performance for each

ASD child based on his/her gender, age, verbal ability and nonverbal ability, according to

the contribution of each variable to visual task thresholds in the TD group. We then

calculated the difference between the predicted and observed scores for individuals in the

ASD group to determine whether their performance on the visual tasks varied from what

would typically be expected. Thus, despite the two groups not being matched in gender or

verbal ability, we were able make meaningful group comparisons. We found group

differences on the EFT, the orientation discrimination task and the RF3 task using this

method. This standardisation approach side-steps concerns with group differences in

matching variables, and is therefore a very useful method for studying developmental

disorders in a cross-sectional design (see also Thomas et al., 2009).

The second approach we took to study the relationship between autistic traits and

visual processing was to examine relationships between autistic characteristics and visual

abilities within a typically developing population. The strength of this approach is that it

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avoids the complications of age and IQ differences (Walter, Dassonville, & Bochsler,

2009). We established in Study 1 that individuals scoring high in autistic-like traits show

differences in visuospatial ability relative to individuals scoring low in autistic-like traits

that are similar to the differences found when ASD samples are compared to control

groups. Not only did these results provide another line of support for the concept of an

autism spectrum that spans from autism through lesser variants to the typical population,

but they also validated using this population to examine the relationship between autistic-

like traits and visual ventral stream processing in Study 2. It is important to note that in

using a design based on groups separated in their AQ scores, we are not claiming that

individuals with high AQ scores are equivalent to a clinically diagnosed ASD group.

Nevertheless it is plausible that the behavioural, cognitive and perceptual differences

distinguishing high and low AQ groups may take the same form, while perhaps less

extreme, as differences observed between ASD and control groups. It will be informative if

future research can provide additional characterisation of the high AQ group by

administering the ADI-R or similar measures to parents of participants to determine any

difficulties experienced during childhood, or by assessing current life functioning to see if

the self-reported mild autistic traits impact in similar ways to traits seen in ASDs.

It was a relative strength of the research reported in this thesis that two designs were

used to assess the performance of groups on the �autism spectrum� on visual tasks using

similar methodologies. However, our research did suffer from the limitation that children

with an ASD were recruited to participate only if their verbal and non-verbal abilities fell

within the normal range. High functioning children with an ASD were preferred as their

language and intellectual capabilities ensured they would be able to understand task

instructions and could successfully engage with the visual tasks. While restricting the

sample to high-functioning ASD is common throughout the literature, it remains unclear

whether the findings from the present study would generalise to those children who also

suffer from additional learning difficulties.

Implications for the Broader Autism Phenotype (BAP)

The results from the first two studies of this thesis point towards high AQ

individuals showing a complex pattern of strengths and weaknesses in visual-cognitive and

visual-perceptual capabilities when compared to low AQ scoring individuals, similar to that

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seen in individuals with an ASD diagnosis when compared to control groups. Recently, two

other studies have added to the idea that high AQ scorers in the general population share

similar characteristics to ASDs. Walter et al. (2009) assessed the relationship between

autistic-like traits and susceptibility to visual illusions. There are mixed findings

concerning whether individuals with an ASD are less susceptible to illusions than

neurotypical individuals (see the �visual-cognitive abilities� section in the Introduction).

These authors attempted to clarify whether a link between autism and illusion susceptibility

exists by assessing the relationship between perception of visual illusions and autistic traits

within the general population. The traits they assessed were systemising (the drive to

analyse variables and the rules governing a system, Baron-Cohen, Richler, Bisarya,

Gurunathan, & Wheelwright, 2003), empathising (the ability to infer what other individuals

are thinking or feeling, Baron-Cohen & Wheelwright, 2004), and autistic-like traits

assessed with the AQ. These trait scores were then correlated with performace on the visual

illusion tasks. The visual tasks administered were ones in which the illusion is induced by

contextual elements, such as the Ponzo, Poggendorf, Zöllner, Müller-Lyer, and Ebbinghaus

illusions. Walter et al. reported that higher levels of the systemizing trait were associated

with less susceptibility to a subset of visual illusions, and thus they suggested that

individuals scoring high in systemising are not influenced by the different contexts of the

illusions to the same extent as individuals scoring low on the systemising trait. These

results support the notion of an imbalance in the use of local and global cues in visuospatial

perception associated with autistic-like traits. However, the findings of Walter et al. also

suggest that atypical patterns of visuospatial abilities may relate specifically to systemising

traits. Therefore, one valuable direction for future research would be to include measures of

systemising traits in addition to the AQ when assessing visuo-spatial perception in the

general population. Finally, Stewart, Watson, Allcock and Yagoob (2009) reported that

high AQ scorers performed better than low AQ scorers on the traditional version of the

Block Design task. However, the high AQ group did not benefit from segmentation of the

image into its constituent parts on a modified version of the task, whereas the low AQ

group was significantly faster on this adaptation. These results replicated the effect seen in

children with autism who also do not obtain benefit from pre-segmenting the block design,

whereas typically developing children do (Shah & Frith, 1993). The findings from Stewart

228

et al.�s research are consistent with those reported in Chapter 4, and provide additional

evidence for the cognitive phenotype in ASDs extending to the general population.

Accordingly, in line with the research demonstrating that the behavioural traits of

ASDs exist on a continuum, with no evidence of a bimodal distribution separating clinical

from nonclinical presentation (Skuse, Mandy, & Scourfield, 2005), it is becoming apparent

that the cognitive, in particular visual-cognitive abilities that are typical of individuals with

an ASD also extend to the typical population (see also Best, Moffat, Power, Owens, &

Johnstone, 2008). Moreover, recent research has demonstrated that, for high AQ scorers,

self-reported concerns with social abilities translate into difficulties in real-life experiences.

Jobe and White (2007) found that high AQ scorers report more feelings of loneliness

relative to low AQ scorers. For the high AQ group, high scores on the social skills subscale

of the AQ (indicating greater difficulty with social skills) made unique contributions to the

variance in loneliness scores. Harborow, Locke and Maybery (2009) administered a

simulated online ball throwing game in which participants could be either included or

excluded by the other characters. They found that individuals scoring extreme in autistic-

like traits exhibited higher levels of self-reported physiological arousal than those scoring

low in autistic-like traits, regardless of whether they were included or excluded from the

game by other participants. These authors posit that it is difficulties related to the social

skills domain of the AQ that may mediate the experience of anxiety in social situations for

these individuals. The findings from these studies substantially add to the argument that the

social impairment seen in ASDs may best be viewed along a continuum that extends into

the general population.

The increasing evidence for a continuity of behavioural, cognitive and perceptual

traits that have genuine, observable implications for high AQ scorers suggests that it is now

becoming difficult to avoid the implication that these individuals are indistinct from

individuals with the BAP identified for some first-degree relatives of children with an ASD

(e.g. Bishop et al, 2004; Bolton et al., 1994). Despite not sharing the common element of a

family member with an ASD, high AQ scorers (and indeed high scorers on the other

measures of autistic-like traits such as the Social Responsiveness Scale, Constantino et al.,

2003) self-report traits that are qualitatively and quantitatively similar to those reported for

parents, siblings and other family members of autistic probands (e.g. Bishop et al., 2004;

Constantino et al., 2006). While there is currently no identified trait that enables researchers

229

to distinguish individuals with the BAP from high AQ scorers, there is no evidence to

suggest that high AQ scorers are distinct from individuals with the BAP. Most of the

research to date recruits university students scoring high and low in autistic-like traits

without asking them whether they have family members with a diagnosis of an ASD.

Collecting this additional information, as well as conducting longitudinal studies examining

whether there is a raised incidence of ASD diagnoses among the children of high AQ

individuals would be helpful in clarifying whether high AQ scorers are equivalent to the

BAP. Additionally, imaging and anatomical studies may assist in determining any

underlying structural similarities or differences between high AQ and BAP populations.

Baron-Cohen et al. (2006) demonstrated that parents of children with Asperger�s Syndrome

showed atypical patterns of brain function while performing the EFT and while performing

an emotion recognition task, relative to parents of typically developing children. Similar

results have been found in unaffected siblings of individuals with autism, who showed

decreased gaze fixations along with diminished fusiform activation, akin to an ASD group,

when compared to control participants on a face processing task (Dalton, Nacewicz,

Alexander, & Davidson, 2007). Analogous fMRI studies in individuals from the general

population scoring high versus low in autistic-like traits will help clarify whether these

individuals should also be referred to as the �Broader Autism Phenotype�.

If a relationship connecting high AQ individuals to BAP family members, and more

importantly to individuals with an ASD was firmly established, then high AQ scorers could

represent an alternative phenotype to be used in comparison with typically developing

individuals in behavioural, genetic and brain imaging studies (Best et al., 2008). Given the

difficulty of conducting neuropsychological assessments of lower functioning children with

an ASD, imaging studies are typically limited to individuals diagnosed with high

functioning autism or Asperger�s disorder, or to relatives of children with an ASD. Often

the power of such studies can be constrained by small sample sizes (e.g. Baron-Cohen et

al., 2006), a problem that can be further compounded by requiring the removal of children

from analyses who are unable to remain motionless during the process (e.g. Gaffrey et al.,

2007). High AQ scorers provide an alternative, easily accessible population for studying the

brain mechanisms behind the milder behavioural and cognitive traits characteristic of the

autism spectrum. Additionally, given that not all of the cognitive traits associated with each

of the social, communication and restricted and repetitive interests and behaviours domains

230

occurs in every single individual with ASD (see Mandy & Skuse, 2008, for a discussion),

the BAP (Scheeren & Stauder, 2008) or high AQ traits (Grinter et al., in press; Grinter, Van

Beek, Maybery, & Badcock, 2009), comparing individuals in the general population who

score high versus low in autistic-like may help establish the factors that contribute to this

variation in cognitive abilities.

Summary and Conclusions

In summary, the finding that ventral visual stream processing appears to be

anomalous in individuals with an ASD, consistent with the pattern of performance seen in

the dorsal visual pathway, has important implications regarding the conceptualization of

visual processes in ASDs. The results suggest that there is some evidence of impaired

global processing, as well as enhanced local processing on the autism spectrum. The studies

reported in this thesis focused on the visual system because the mechanisms underlying

visual perception are better understood than many other functions. The pattern of findings

using this approach is more consistent with the WCC account than with the EPF account,

and thus may provide important insights regarding the processing of information in ASDs.

Therefore, these results provide additional support for Happé and Booth�s (2008)

suggestion that the notion of WCC in ASDs deserves to be revisited.

These results also have potential application to the early identification of autism

spectrum conditions. In particular, a unique profile of impaired global processing in both

the dorsal and ventral visual streams may provide the basis for developing tests of visual

functioning that will assist in discriminating ASDs from other developmental disorders for

children for whom there are early developmental concerns. Indeed, McCleery, Allman,

Carver and Dobkins (2007) demonstrated that infants at risk for autism as a result of having

a sibling with a diagnosis can be assessed using the preferential looking paradigm. They

found that 6-month-old infants at risk for ASDs because they had an older sibling

diagnosed with the condition demonstrated abnormal sensitivities to luminance contrast

associated with the magnocellular visual pathway. While these differences do not appear to

be present in older children with ASDs (see Pellicano et al., 2005), early anomalies in the

organisation of the brain may be an indicator for later developmental difficulties. Following

the developmental trajectories of such children in order to determine the relationship

between performance on early assessment measures and later diagnoses will be very

231

important in developing early diagnostic measures. Additionally, it has been demonstrated

that the earlier an ASD is diagnosed and treated, the better the likely outcomes for the child

(Dawson & Osterling, 1997). The assessment of visual abilities may provide an additional

tool for early identification of ASDs. However the results from the present research,

particularly Study 3, demonstrate that the tests used must be sensitive and specific enough

to detect differences between clinical and control groups, and it is the profile of

performance that is crucial for these tasks, rather than functioning on isolated visual tasks.

Finally, the fact that mild, autistic-like characteristics appear to be distributed

throughout the population in the behavioural, and now as demonstrated in this thesis, the

cognitive and perceptual domains, opens a whole new realm of experimental possibilities.

If research can use individuals in the general population scoring high in autistic-like traits

to enhance our understanding of the mechanisms underlying such traits, we may be able to

move one step closer to determining the factors that increase vulnerability to the

complicated and puzzling conditions that are ASDs. Establishing what it is about these high

AQ individuals that enables them to share so many of the traits of ASDs but not exhibit the

clinical syndrome is likely to demand a research program integrating information from

genetics, neuropsychology, neuroanatomy and behaviour in order to enable a full

understanding of functional differences along the autism spectrum.

232

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