Further Considerations of Visual Cognitive Neuroscience

in Aided AAC: The Potential Role of Motion Perception

Systems in Maximizing Design Display

VINOTH JAGAROOa,c and KRISTA WILKINSONa,b*

aCommunication Sciences & Disorders, Emerson College, 120 Boylston St., Boston, MA 02116, USA,bUMASS Medical School/Shriver Center, Waltham, MA and cBehavioral Neuroscience Program &

Department of Psychiatry, Boston University School of Medicine, Boston, MA

Current augmentative and alternative communication technologies allow animation withinvisual symbol displays. Clinicians therefore have the option of incorporating motion-basedeffects into AAC displays. Yet there is no research in the field of AAC to guide this clinicaldecision-making, in terms of the number or types of animated symbols that would best suitspecific communication needs. A great deal is known within the discipline of cognitiveneuroscience about how humans perceive motion, however. In this paper we propose thatthe field of AAC might exploit these known principles of motion perception, and weidentify some potential uses of different types of motion. The discussion is presented withinthe context of neuro-cognitive theory concerning the neurological bases for motionperception.

Keywords: Cognitive Science; Visual Processing; Motion Perception

INTRODUCTION

Many aided augmentative and alternative com-munication (AAC) systems incorporate visualsymbols placed on a physically delimited space;that is, the symbol display. For many users ofAAC the visual symbols constitute not just theprimary expressive mode of communication, butalso the avenue for acquiring receptive language(cf. Romski & Sevcik, 1993). Because each andevery message preparation requires a commu-nicator to find and identify specific symbolswithin the visual display, the study of visualprocessing is likely as critical to understandingfunctional outcomes within aided communica-tion as the study of auditory processing is to thestudy of aural/oral communication modes. Inthis paper, we examine one aspect of visualprocessing, that of motion perception, and itsrelevance for AAC.

OVERVIEW AND RATIONALE

An important technological advance in aidedAAC has been the introduction of dynamicdisplay systems. In these high-technology dis-plays, the pages displaying both the visualsymbols and any auditory voice output are storedwithin the memory of the device itself. Each pagedisplay is linked to the others electronically, andchanging from one page to another is a matter ofpressing a symbol (or several) on the display.One of the most compelling features of dynamic

displays is their ability to present symbols thatincorporate motion. Simple animated symbolshave become available with the development ofhigh-technology dynamic displays; examples in-clude systems including Dynavox DynasymsTM,which have been available for some time now, theLibrary of International Picture Symbols (LIPS),and the BoardmakerTM PCS symbols. The

*Corresponding author. Krista M. Wilkinson, Communication Sciences & Disorders, Emerson College, 120 Boylston St., Boston,MA 02116, USA. Tel: þ1 617 824 8288. E-mail: krista_wilkinson@emerson.edu

Augmentative and Alternative Communication, March 2008 VOL. 24 (1), pp. 29 – 42

ISSN 0743-4618 print/ISSN 1477-3848 online � 2008 International Society for Augmentative and Alternative CommunicationDOI: 10.1080/07434610701390673

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visual and spatial dynamism offered by moderncomputer graphics technology make for anentirely new scale of innovation for displays.Despite their increasingly widespread availabil-

ity, there is a virtual absence of associatedresearch that might help guide clinicians in theirdecision-making when incorporating motion-based symbols. How many motion-based symbolsshould be presented on a single display? Is theresome maximum limit, beyond which the visualdisplay simply becomes overwhelming? Shouldthe motion-based symbols on a display move allat once, or sequentially? What kinds of motionare best used for specific purposes?Given the emphasis in the discipline of AAC on

evidence-based practice (Schlosser, Wendt,Angermeier, & Shetty, 2005), the lack of researchin support of these daily clinical decisions is acritical and potentially serious concern. Our propo-sal is fairly simple: we should examine the richpotential source of evidence and ideas from the basicscience of visual processing. Indeed, a number ofcognitive principles and neural systems underlyingmotion perception have been identified, proposed,or studied within visual cognitive neuroscience. Ourimmediate goal in this paper is to introduce andexploit basic science concerning visual motionperception as it might relate to the construction ofaided AAC displays. Our longer-term aim is toextend the notion of a dynamic display beyond thecurrent conceptualization of a system that allowsmovement from one page to another. Carefullyincorporating motion dynamics, as a characteristicof symbols or scenes themselves, is an importantnext step in the development of technology servingpeople with communication support needs.We begin by detailing a range of specific

possible applications of motion effects to AACsymbols. We then describe different types ofmotion, highlighting in particular those withimportant implications for aided communication.We wrap up with a description of the relevantneuro-cognitive theory concerning the neurologi-cal and theoretical bases upon which our sugges-tions for clinical practice are based. Thisdiscussion provides the context for translationalresearch that would examine empirically the waysin which basic research in motion detection couldpositively influence clinical construction of aidedAAC displays. We view this proposal as a startingpoint, insofar as we hope to generate ideas anddiscussion not just around the points we raise, butconcerning other aspects of motion that mightalso be valuable. We also hope this proposalmight generate research that would determineempirically whether the possibilities we putforward do, in fact, have direct clinical implica-tions for people who use aided AAC.

INTRODUCTION TO VISUAL COGNITION,

MOTION STUDIES, AND THEIR

RELEVANCE FOR AAC

As Wilkinson and Jagaroo (2004) noted, cogni-tive neuroscience encompasses a broad range ofareas from basic sensory and motor processing, tocomplex, high-level cognition. It attempts to drawlinks between functional domains such as lan-guage, memory, visuospatial processing andemotion and their underlying neural systems.The interdisciplinary approach of cognitive neu-roscience brings together diverse areas in theneurosciences, psychology, cognitive and compu-ter sciences, neuropsychiatry and neurology. As aresult, behavioral outcomes can be analyzed andunderstood not only from the standpoint of overtbehavior, but also as they relate to the neuralcircuitry constraining or producing the behavior.Similarly, understanding of neural functioning isinformed by theories of cognitive functioning.The visual system is a significant component of

human perceptual and cognitive functioning, andcognitive neuroscience views it as the pre-eminentsensory modality. More cortical tissue is given tovisual and spatial processing than to any of theother sensory modalities (see Felleman & VanEssen, 1991; Ungerleider & Haxby, 1994). Re-search in visual cognition has made significantadvances in understanding visual processing and itseffects on behavioral outcomes like stimulusperception, identification, classification, and laterrecall. Each of these outcomes is critical for users ofAAC, who must find symbols within an array,identify them as specific symbols, determine themeaning of each symbol, and then recall thesymbol the next time it is required. As Wilkinsonand Jagaroo (2004) argued, the principles identifiedin laboratory studies likely have relevance becausein many cases accessing aided AAC involves anexplicitly visual, input –output channel.Recent advances in the field of cognitive

neuroscience have tied various forms of motionto discrete cortical modules. From a neuralstandpoint, motion-perception systems are ro-bust; they are present very early in development(Braddick, Birtles, Wattam-Bell, & Atkinson,2005) and are far less susceptible to disruptionthan higher cognitive domains like language,memory and attention. Even with the range ofclinical populations that rely on aided AAC, it isexceptionally rare that the cortical mechanismsinvolved with motion perception are disrupted(see Zihl, Van Cramon, & Mai, 1983; Zihl, VanCramon, Mai, & Schmid, 1991).The robustness of motion-detecting neural

systems supports our proposition that motion-based symbols could be beneficial for users of

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AAC. Yet, equally important is the possibility thatdifferent populations might benefit differently frommotion-based symbols. Motion-based symbolsmight either be ineffective or possibly counter-productive for some individuals. For instance, itappears that individuals with autism may have aselective deficit in motion processing (e.g., Milne,Swettenham, & Campbell, 2005). Other thanindicating that motion processing seems to beselectively impaired in autism, however, the litera-ture offers no guidance about how this motion-processing deficit would play out from a clinicalstandpoint. Would such a selective deficit in motionprocessing interfere with the effectiveness of motioncues on a display? If so, motion cues might beunnecessary or counterindicated. Understandingthe relationship between specific etiological profilesand motion processing would be critically impor-tant information for clinicians who are consideringmotion symbols. Clearly, professionals within thefield of AAC are in need of an evidence-basedapproach to the inclusion of motion for any client,whether that client’s neural motion-perceptionsystems are intact or impaired.

VISUAL MOTION AND COGNITIVE

PROCESSES RELEVANT TO AAC

Numerous aspects of cognitive functioning can betriggered or enhanced by motion effects. These

include simple motion effects that draw attentionto a stimulus as well as more complex or subtleeffects of interpretation of a stimulus. Someexamples of both simple and complex displayeffects discussed in this section are presented inFigures 1 and 2. Of necessity, the stimuli in thesefigures are static; readers will have to imagine themotion depicted within the figures (this constraintillustrates quite nicely the importance of animatedstimuli, as they are difficult to depict using still-frames). In this section, we begin by identifyingthe simple and complex effects that can beinduced by motion, in the context of basicscientific research. We then turn to a discussionof how these simple and complex motion effectsmight be considered, applied, or exploited forAAC display design. As before, little to noresearch within AAC has been conducted; thus,our intent is to raise some possible avenues forresearch to fill this critical gap.

Simple Effects: Drawing Attention to a Stimulus

One important role that motion can play is indrawing attention to a particular stimulus.Various types of motion can be used for capturingattention; three of these are illustrated in Figure 1.A stimulus may grow larger and then smaller,either once or even through small pulses; thus, inFigure 1, the mouse enlarges then shrinks back toits original size. Another way of attracting

Figure 1. Examples of simple effects that capture attention through motion: traversing a path, rotation, and size changes.

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attention through motion is through rotation.For instance, an airplane may rotate in degree ofpresentation or a ball may spin, or a ballerina

twirl. Where appropriate, we may have a stimulustraverse a path, such as a car that appears tomove forward.

Figure 2. Complex effects that can be elicited through motion: functional and behavioral characteristics of events like shoveling orflying (A); Causation and physical dynamics such as the movement of a ball after being kicked (B); three-dimensional or depthcharacteristics such as opening of a door or shift in view while ascending a staircase (C); Mental rotation and representations of eventssuch as the movement of a ball rolling (D); Higher-order semantic categories elicited by multiple movements in a scene (E).

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Cognitive Underpinnings: The Stimulus MovementEffect

Many of the simple motion effects illustrated inFigure 1 are justified by a basic principle calledthe Stimulus Movement Effect (Nealis, Harlow, &Suomi, 1977). That is, the perceptual system has abuilt in bias to direct attention to motion changesand sudden stimulus changes that occur in theenvironment (Franconeri, 2004). Consequently, ifone object in a group of stationary objects is set inmotion, it generally commands greater attentionby virtue of its dynamic action.Evidence supporting the positive effects of

motion effects is found consistently in bothhuman and non-human primate studies (forexample, Kardos, da Pos, Dellantonio, & Saviolo,1978a, b; Washburn, 1991, 1993; Washburn,Hopkins, & Rumbaugh, 1989). These studies allillustrate that general task learning, task perfor-mance, discrimination between stimulus items,and memory for the stimulus items improve whenthe relevant task stimuli are given movement.Thus, performance is enhanced when a relevantstimulus item is conferred some form of motion,as opposed to the condition when it remainsstationary.The effect has been explained in a few ways.

Nealis et al. (1977) suggested that stimuli thatexhibit novelty, sudden changes in properties orposition, and so forth will automatically elicitattention. A variation of this attention hypothesiscame from theories that emphasized habituationand dishabituation in attending to stimuli (seeCowan, 1988). In this view the observer habi-tuates to stationary or static stimuli, therebyminimizing their selection for greater cognitiveprocessing. With movement comes dishabituationand cognitive selection. Washburn et al. (1989)suggested the stimulus movement effect could beexplained by the increase in exposure durationbrought about by movement (a non-attentionalexplanation). In a comprehensive series of experi-ments Washburn (1993) tested the various ex-planations of the stimulus movement effect; theresults supported an attentional allocation ex-planation that added to his earlier hypothesis.Kardos et al. (1978a,b) proposed an explanationdifferent from the general attentional view: Thatmemory for spatial location is affected bypositional changes brought about by movement,forcing the observer to rely more heavily on otherstimulus cues such color and form. In thisexplanation, the enhancing effect of movementis indirect.As with many complex cognitive effects, it is

likely that the motion effects under discussionhave multiple explanatory factors. Attentional

allocation, duration of exposure, stimulus proper-ties indirectly enhanced through movement, etc.,may all play differential roles depending onvarious task and stimulus conditions. The morerelevant point is that the stimulus movementeffect may be a fundamental perceptual propertyand an adaptive response in coding the visualenvironment and learning. (Numerous othermovement-related learning enhancement effectscan be found in cognitive literature but the fullrange of the topic is not within the scope of thispaper.)

Complex Effects: Eliciting Cognitive Processes

A motion effect added to a stimulus item canserve to elicit the stimulus’ associated functionalor behavioral characteristics (a shovel is used todig or pick up soil; left of panel A in Figure 2),behavioral characteristics (a bird flies off byflapping its wings and then lands and wings stopflapping; right of panel A in Figure 2), andprinciples of causation or physical dynamics(kicking a ball causes it to lift off into the air,then bounce and roll; Figure 2 panel B). Otherphysical properties such as gravity can also beillustrated, as through the falling of the ball.Panels C –E of Figure 2 present a few other

complex display effects and cognitive processesthat can be elicited through motion. Motion canserve to enhance 3D and depth effects and may beexperienced as real world visual perception. Forinstance, Panel 2C illustrates two types of depthperspectives: in an animation of a door opening(left of Panel 2C), the top edge of the doorbecomes slightly lower in height and its outerlength appears shorter as it swings toward theviewer; as a person climbs up a flight of stairs(right of panel 2C), the image is reoriented to givethe viewer that same changes in perspective andview that the climber faces with each step. Aturning object or object part can simulate internalor cognitive representations, mental rotations andtransformations of the object. In panel 2D, theperception of a rolling ball requires mentalrepresentation of various logical changes in theball’s position and the overall rotational trans-formation of the object. The same can be appliedto view of a car taking a turn.Simultaneous movement of two or more objects

or of multiple features of an object or scene canstimulate the perception of larger, more complexsemantic categories (e.g., a street scene couldinclude many cars and pedestrians moving andsome trees blowing in the wind). A subset ofobjects moving simultaneously can also stimulatedynamic perceptual organization in accordancewith classic Gestalt principles. Objects moving

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together can be grouped together to form ahigher-order perceptual category (e.g., in ananimated playground scene in panel 2E, simulta-neous movement of a few objects, the swings, thesee-saw and the slide, can contribute to thesemantic category of a playground).Note that as we move into complex motion

effects, we also move into slightly more abstractrepresentations, in terms of the icon. How do werepresent a property such as ‘‘flight’’, when theproperty of action is being represented (the flying)rather than the specific individual performing thataction (the bird, the plane, Superman)? Thestimulus characteristics that motion effects canelicit need not be tied only to physical propertiesof an object. In animated examples such as astone being thrown, a bird taking off, or super-man in his flight posture, the property of flightcan be conveyed by certain key motion cues.These could be the displacement of an object fromthe ground following a naturalistic movementtrajectory in air; they could be backward linestreaks symbolizing forward propulsion (against ablank background, symbolizing the sky or space),or the pointed forward/upward configuration ofan object with its background flowing in theopposite direction. While the type of flight differsfor each of the three examples, they all adhere tosome basic patterns of displacement and trajec-tories within expected temporal frames. Likemany other motion effects, they also force theperceptual system into representing momentum(see Hubbard, 1995). In these examples, thespatio-temporal patterns and the trajectories areintegrally tied to generating a sense of momen-tum, and they all come together to signify thegeneral principal of flight.

Cognitive Underpinnings: Complex CognitiveProcesses

Clearly, the value of motion effects extends wellbeyond simple perceptual and attentional en-hancements. Higher cognitive and spatial opera-tions can be triggered and even exercised bydynamic motion in a stimulus object or scene.Movement can help convey functional propertiesof objects, relationships between the objects in ascene, and causal patterns in kinetic action. It isnot simply the case that capture of attentionderives from the moving object as the objectcreates a moving 2D image. This changing 2Dretinal image is interpreted by the observer in thecontext of a 3D environment (Palmer, 1999).Many higher-level cognitive processes must thenbe forced into action to aid the perceptualprocess. For instance, Palmer (1999) noted that‘‘Once moving objects have been perceived and

attended, they are usually classified . . .motionmay play a role in object classification, althoughprecisely how this might occur is not well under-stood’’ (pp. 468 – 469).

Extension of Basic Science Research for Aided

AAC

Perhaps the very first statement concerningmotion within AAC that warrants explicitacknowledgement is that incorporating motionin symbols must be done carefully. The basicscience clearly suggests that too many movingsymbols presented simultaneously will be visuallyoverwhelming. Furthermore, motion that is pre-sented for too long risks a viewer habituating to itand thus ignoring the intended cue. A criticallyimportant step in developing an evidence-basedapproach to incorporating motion in AAC is,therefore, research that examines the number ofitems that might conceivably be animated on anaided AAC display before visual overload and/orhabituation occurs. Alternately, we might explorewhether sequential presentation of animatedcuing might be a better approach, despite thefact that sequential animation would necessarilyintroduce a delay as the motion moves, symbol bysymbol. Beyond outlining the basic parameters ofnumber of animated symbols and their relation-ship to one another (simultaneous versus sequen-tial presentation), some other interestingapplications of the simple and complex motioneffects also warrant direct study. Some of theways in which motion could be used to helpchildren use dynamic displays more effectively arehighlighted here, in hopes of sparking furtherresearch or ideas that might arise from them.

Fading of Motion

Perhaps importantly to any growing display, akey point to remember is that the uses of motionto attract attention may serve an immediate butnot necessarily permanent function. For instance,attracting a viewer’s attention to a symbol isimportant only until that symbol’s location and/or meaning becomes well-learned; at that point,an explicit attentional cue is unnecessary. Remov-ing motion that no longer serves its originalfunction may therefore also be a criticallyimportant component of display construction.One way in which actual motion cues might be

faded is through the use of arrows to indicatemotion. Static arrows as a way of indicatingdirection or motion are used universally ineveryday applications from electronic devices toroad signs. For instance, the shutting of one’seyes (for BLINK or SLEEP) might be indicated at

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first by the actual motion but then schematicallythrough static arrows. In the context of AAC wemay ask whether motion perception might beelicited by using static arrows to indicate theopening or closing of a person’s eyes?The answer depends, again, on the complexity

of the stimulus, the movement in question, amyriad of other stimuli, and perceptual variables.It is certainly argued in cognitive neurosciencethat the mental/neural representation of move-ment trajectories can be triggered by symbolicdirectional cues; that is, arrows may primethe representation of a movement pattern (seeJagaroo, 2004 for a brief discussion). On the onehand, therefore, arrows offer a rudimentary andeffective way of indicating motion or direction.On the other hand, optimizing this effect requiresan understanding of interaction of many othermany other contextual variables, yet to be workedout; again, this is an area ripe for the developmentof an evidence base.

Motion in Scanning

Motion cues as a means for facilitating scanninghave actually already been studied within AACin a recent study of scanning in young nondis-abled children by McCarthy et al. (2006), whocompared children’s selection performances un-der traditional and enhanced linear scanning. Inthe traditional scanning condition, each targetstimulus was highlighted one-at-a-time by a redsquare that enclosed the stimulus’ perimeterwhile a voiceover spoke the stimulus’ name ina neutral intonation. In the enhanced scanning,each target stimulus was highlighted one-at-a-time by having the stimulus grow larger, byvirtue of it appearing to ‘‘move in sequence fromthe selection array (the background) to theforeground’’ (p. 275), while a voiceover spokethe stimulus name in a question intonation.Performance was found to be significantly betterunder the enhanced rather than the traditionalscanning condition. This pattern is consistentwith basic research in visual cognition andprovides empirical support for the argumentspresented here.

Motion as Cues to Symbol Meaning/Identity

Motion could also be used to introduce newsymbols, either on a traditional grid display or inthe natural scene approach recently introduced byDrager, Light, Speltz, Fallon, and Jeffries (2003).In the naturalistic scene displays, symbols are notpresented in individual locations within a grid butrather embedded as ‘‘hot spots’’ within a photo-graph or schematic drawing of a real-life scene.

For instance, the symbol for milk might actuallybe embedded as a glass of milk placed on the tablein the kitchen, while the symbol for refrigeratorwould be the refrigerator sitting to the right of thetable. The claim is that the scene itself providescontextual support to young aided languagelearners that is similar to the support availableto infants and toddlers learning spoken commu-nication modes.Although visual processing can differ between

grids and scenes (see, e.g., Wilkinson & Jagaroo,2004) motion could conceivably be incorporatedin both. For instance, motion cues might be usedto capture attention when new symbols are beingadded to a display, or when a communicator failsto use a symbol reliably. If a user is unaware thata symbol is present on a display, she or he isunlikely to use it. Using a motion cue, the new orunderused symbol can be highlighted, thusexploiting a perceptually driven cue to potentiallyspark the communicator’s attention to the sym-bol’s presence.Beyond simply drawing attention to the pre-

sence of a symbol, motion can also be used toenhance symbol meaning by enhancing thetransparency of the symbol. Movement of thehands on a wall clock, for instance, may help cuethe viewer to the meaning of what mightotherwise appear to be a simple square stimulus.Similarly, a simple round stimulus may be morereadily identified as a BALL when the movementcue of bouncing is present. Another possibilitywould be to cue word-class category; symbolsintended to convey verb/action meanings mayinvolve motion (McCarthy, personal communica-tion). For instance, the action of sitting might becued by the movement of the figure as it sits downon the chair, in comparison to a static symbolthat might refer to the chair itself.

Summary

We have described how various neurocognitiveprocesses are involved with and exercised throughmotion perception. The mechanisms by whichmotion confers these advantages are described inthe next two sections, which outline the varioustypes of motion and the neuroanatomic correlatesand theoretical models of motion detection,respectively.

TYPES OF MOTION AND THEIR

IMPLICATIONS FOR AAC

There are various forms of motion perception,various neural mechanisms involved with motionperception, and many levels of interaction

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between motion perception systems. With somany facets to motion perception and so muchyet to be understood, there exists no singleclassification system that fully encompasses thetopic. Various types of motion have beendescribed and many models of motion detectionhave been proposed. The summary of typesof motion, derived from Goldstein (2002);Lu and Sperling (1995); Nagano, Hirahara, andUrushihara (2004); Nakayama (1985); andPalmer (1999), is presented here. This overviewis structured so as to inform the question of thoseaspects of motion perception that readily apply toAAC design and those that do not apply.

Real Movement and ‘‘Non-Real’’ Movement

A basic differentiation of types of motion is thatof (a) real movement; and (b) apparent move-ment, induced movement, or movement after-effects. Real movement is the perception of actualmovement as an object physically moves acrossan observer’s visual field or within a region of thefield. Observing a rolling ball or a blinking eye areexamples of observing real movement. Realmovement often produces continuous motion.Continuous motion may follow a straight orcurved path or may be harmonic in nature,typically up-and-down or back-and-forthmovement.In contrast to real movement, apparent motion,

motion after-effects, and induced motion; areillusory, perceptual, movement phenomena. Ap-parent movement, also called stroboscopic mo-tion, refers to the perception of motion createdwhen a series of snapshots are presented insuccession, provided that certain limits areobeyed regarding rate of alternation and distanceof separation of images (in cases where images areseparated by distance to create the effect ofmovement across a path). The use of cycledanimations within AAC, in which the animationis created by repetition of a short sequence offrames, exploits the human ability to perceivemotion and could be the basis for some of themotion effects already described in Figure 2, inparticular, frames A, B, or D. Even relativelycrude effects can, therefore, be used to inducemotion perception.Induced movement is the phenomenon of

perceived motion of an object that is actuallystationary in proximity or in front of an actualmoving object. For example, fast moving cloudscan induce the perception of movement of thestationary moon as they overlap the view of themoon. Motion aftereffects result from prolongedfixation on particular types of stimulus items,such as the actual flow of waterfall or a

rotating spiral. The fixation induces neuraladaptation or triggers certain patterns of neu-ral responses that remain active for a fewmoments after fixation on the original stimulusis withdrawn.

Giving Primacy to Real Movement

At this point, AAC may most effectively concen-trate on real movement. Illusory movement suchas dramatic apparent motion, induced movement,and movement aftereffects may be far toodistracting for practical clinical application.Perhaps some role for apparent movement mightbe explored once the use of motion in AACsymbols becomes better understood. Real move-ment can be applied in numerous ways to enhancesymbols: An item could be set in motion so that itslowly traverses a path on the screen; for example,a car moving across a screen or a kite movingfrom the bottom left to the top middle and then tothe bottom right of the screen. An item couldbe set to expand and contract and so as tobe emphasized, for example, in a display of fiveanimals, a selected animal could expand over theother stimulus items and then contract back to itsoriginal size (as in McCarthy et al., 2006). Simpleharmonic motion effects can be applied, forexample, to the wings of a bird may be set toflap gently or a simple shaking motion could beapplied to a ringing bell.

Local versus Global Motion

Some movement stimulus arrays can also bedescribed in terms of local and/or global motion.Local motion refers to individual movements ofthe pieces of a larger stimulus configuration, andglobal motion is the overall effect of movementof the larger stimulus configuration (i.e., thecollective effect of local movements). A com-monly used stimulus pattern that includes botheffects is a random arrangement of white dotssuperimposed on a large black circle. In such anarray, when the dots move randomly, in some-what evenly distributed directions, and within alimited range, a global flow is perceived inaddition to the local movement (see Watanabeet al., 2002). In visual motion studies, theconstruct of local – global motion applies speci-fically to the kind of scenario described pre-viously, where local effects contribute to globaleffect. However, the terms local movement andglobal movement are used more flexibly to refer,respectively, either to movement of a part orwithin a region of a stimulus item, or tomovement of the entire stimulus or entire visualfield.

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Selective Application of Local and GlobalMovement Effects

Perceptually simple localized motion that hasclear bearing to the action of the larger stimulusitem may have direct implications for AAC. Forexample, in either grid or schematic scenepresentation, a simple drawing of a car couldmove across the screen (global movement) andthe wheels could slowly rotate (local movements).In a schematic scene involving shopping mallscene, there could be a few forms of simple localmovements, such as a moving escalator, shopperswalking around and some doors opening, allfunctionally contributing to the contextual mean-ing even if they do not, themselves, serve as ‘‘hot-spot’’ symbols with specific meanings of theirown. (What specific meanings are highlighted, ofcourse, will depend on the goals of the display,and thus customized for each individual client’sdisplay.) Simple global movements, such as theexpansion of an object and the global movementsinvolved in real movement, may also be suitable.The number of local movements within an objector scene that contribute to the functional wholewill differ depending on the complexity of thewhole object of scene and the skills of theindividual. The ways in which such motion woulddiffer for symbols presented on a grid orembedded in a scene, of course, require systematicanalysis. Due to the perceptual intricacies, globalmotion flow that derives from local motion effectsmay be less well-suited to AAC.

Optic Flow, Self Motion and Motion Flowfields

Optic flow in the classic Gibsonian sense (Gibson,1966, 1979) occurs when, for example, anobserver walks straight ahead and focusesstraight ahead, and all the elements of the visualfield (called the optic array) appear to flow pastthe observer. Gibson emphasized the importanceof information gained from perceived surfaces inguiding locomotion and influencing the per-ception of movement. The focus of expansion isthe centrally located point of focus in front ofthe observer from which all motions vectors in theoptic flow field appear to emanate. It helps theobserver identify his/her upcoming path. WhileGibson’s ecological approach to visual perceptionmay not be a driving force in the modern study ofvisual motion perception (Blake, 1994), it hasnonetheless provided an original and enduringperspective on visual motion and visual percep-tion in general (Nakayama, 1994). As outlined inthe upcoming section, it bears great relevance toAAC because of its emphasis on visual dynamicscreated by the immediate environment.

Simplified Optic Flow Effects

The dramatic optic flow effect of a car racingdown a road and the optic array rapidly flying pastthe driver is clearly not an effect fitting for AACdisplays. Consider a simplified version of opticflow: in an animated scene, a very slow optic flowpattern can be applied to create the effect ofperson walking down a street. Distinct objects onthe side of the street, such as a car, a fire hydrant,some stores, and some people, can slowly movepast as the scene progresses. A slow optic flowarrangement can give sequential appearance to anumber of objects, slowly appearing and disap-pearing. Not only can this make for an interesting,entertaining visual display, it can also simulate areal-world perception of the moving observer. Inthis form, exposure time to each object may beshorter than that of the same object in a staticdisplay. However, an optic flow arrangementoffers an unparalleled breadth of exposure andnavigational realism. It also does this in aregulated, streaming, non-overwhelming fashion.The importance of optic flow has most appar-

ent implications within the naturalistic or sche-matic scene displays described earlier, in whichsymbols are embedded within an actual photo-graph or drawn scene. Although the naturalisticscenes are scanned photographs or drawings,there is clear potential for integrating optic flow,particularly as it relates to movement betweenpages. Currently in dynamic displays, this isaccomplished by hitting a menu key; upon itstouch, the page on display disappears and isreplaced by the next one. Imagine a dynamicdisplay in which the menu key is actuallyembedded in the doorway of a naturalistickitchen scene. When the communicator touchesit, the door appears to open and, through an opticflow effect, it appears as if the viewer is movingdown the hallway into the living room. The opticflow could allow the movement between pages tohave the same contextual support as the natur-alistic scene, on the same principal. This is not tosuggest that optic flow would be used to eliminatethe pages themselves, or to create a virtual realityversion of the communication situation (whichwould introduce a host of new issues that are toobroad in scope to address here). Rather, theproposal is to apply the same principles of acontextually supportive scene as a means to movefrom page to page as for the construction of thepages themselves.

Biological Motion

Biological motion refers to the perceptual sys-tem’s uniquely tuned sensitivity to the complex

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movement combinations and geometric relationsembedded in the bodily and limb movements ofhumans and other animals. Johansson’s (1973)landmark study demonstrated that people canaccurately distinguish human movement frominanimate movement when perceiving only a setof point lights attached to the joints of a humanfigure, as the figure moved in a dark room.Biological movement has since been extensivelystudied. Perceptual attunement to biologicalmotion is so fine that information about genderand mood of the subjects, or the type of animal inthe case of animal movement, can be extractedfrom point-light displays (see Pavlova, Krageloh-Mann, Sokolov, & Nirls, 2001, for a review).Infants as young as 4 – 6 months of age showsensitivity to biological motion (Fox&MacDaniel,1982), and by the age of 5 years, a child’srecognition level for biological motion is thoughttobe fully developed (Pavlova et al., 2001).Fromallaccounts, the capacity toperceive biologicalmotionappears to be innate and distinct.

Flexibility in the Application of Biological Motion

For simple animated sequences, it seems unne-cessary to try to achieve great realism in themotion. If a symbol of a dog moves across thescreen, even crude motion that approximates awalking movement may be more than sufficient toenhance the effect of the symbol, especially foryounger users of AAC. For older children oradults, a closer approximation of animal move-ment may enhance the realism. (It may be odd toenhance realism in one type of AAC symbol andnot in another, and this is an issue that needs tobe resolved with further research.) If children aredrawn to simple animated movements in cartoon-depicted animals, crude in comparison to realisticbiological motion, it may be counterproductive tonaturalize all animal movements just because theviewers possess neural modules for biologicalmotion perception. This example illustrates thedilemma clinicians will face when applying mo-tion effects to symbols: To what degree and towhat level should biological movement approx-imate nature? Much research is needed to answerthis question.

Structure-from-Motion (SFM) and Kinetic Depth

(KD)

When visual motion influences or organizes one’sperception of form and depth, the perceptualphenomena are called the SFM and KD effect,respectively. A shadow of a cube projected onto atranslucent screen may appear flat but when thecube is rotated, the 2D shadow takes on a 3D

characteristic (SFM); sets of dots arranged alonga particular gradient within a circle, movinglaterally at particular speeds, create the effect ofa rotating globe or ball (KD). Both SFM and KDwere originally highlighted by Gibson (1954) andhave since been met with computational theoriesof visual motion (Ullman, 1979). Clearly, KDmay also be described by the local-global motionconstruct above.

Application

Structure-from-motion and kinetic depth effectsare complex, motion-induced effects, and perhapsthe least readily applied within AAC. Given thatthe other forms of motion can be applied in waysthat range from very simple to very complex, theyseem better suited as immediate research avenues.We do not dismiss the application of complexmotion or multiple, simultaneous motion effectsin AAC. We merely suggest that much more basicresearch is needed to explore the possible use ofthese motion effects in AAC.

Summary

Some aspects of motion perception theory haveimmediate implications for the enhancement ofsymbols in AAC displays while others are some-what remote. Some visual stimuli that showcomplex forms of motion, rapid motion changes,multiple motion areas, and so forth might becounterintuitive to the mission of AAC and ill-suited to the general population for which AACdisplays are designed. Excessive motion, move-ments that are too fast or possibly even too slow,or movements that are too complex, may amountto confounding factors in the AAC visual scene.The remaining issues we have raised provideample opportunities for further research to clarifythe application of basic science to application.

NEUROANATOMIC CORRELATES OF

MOTION PERCEPTION

The motion perception phenomena that we havediscussed so far are based on specific patterns ofneuroanatomical functioning. In this section wedescribe the anatomical and physiological sub-strates thought to underlie some of these phe-nomena. This section may have little immediateclinical application, but we feel it is critical toinclude for the benefit of readers who might beinterested in pursuing the issues from a neurolo-gical perspective. That is, we seek to promotediscussion and research within both applied AACand neurological perspectives, and thus offer this

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section for those readers who are interested in theneurological underpinnings of the phenomena wehave discussed so far.

Motion Processing in the Eye: Luminance Patterns

and Retinal Detectors

Many models of motion detection relate to retinalreceptor dynamics. The foundation for many suchmodels lies in Reichardt’s (1957, 1961) pioneeringdescription of a simple circuit for motion detec-tion. Such models highlight the unique pattern offiring of retinal cells for different spatiotemporalconfigurations (see Dowling, 1979; van Santern &Sperling, 1985). The arrangement of the retinallayers and their connections are such that theretina is finely tuned to detecting changes inimage position as an object crosses any part of thevisual field. When a red ball moves across a whitewall, the difference in luminance between the balland its background is highlighted as the image ofthe ball traverses the retina. The detection ofthese luminance differences within a spatiotem-poral frame leads to the perception of movementas cells are activated in a particular sequence. Thistype of motion, detected via spatiotemporalretinal cues, is called first-order motion or Fouriermotion (Lu & Sperling, 1995).Some types of motion perception cannot be

accounted for by first-order motion detection.Consider a scenario where one complex object ofvarious luminance patterns moves in front of acomplex background also made up of variousluminance patterns. Here, the average luminanceof the object and the background can be the same,and simple spatiotemporal changes in luminancecannot account for motion detection. This type ofmotion is referred to as second-order motion ornon-Fourier motion (see Cavanagh & Mather,1989; Chubb & Sperling, 1989; Smith, 1994).Second-order motion systems likely entail com-plex integration of texture, contrast and relativemotion across multiple regions of the visual field.Whether first- and second-order motion aredetected by the same or overlapping neuralmechanisms or different neural mechanisms, isnot clear at this point.Additionally, Lu and Sperling (1995) have

suggested a possible mechanism for third-ordermotion, one that tracks features in motion; andCavanagh (1992) has suggested an attention-based motion perception system. However, de-marcation of first- or second-order motionremains the major organizing principle in modelsof visual motion.The distinction between first- and second-order

motion is just one construct that evolved out ofnecessity to describe motion detection in relation

to figure-ground luminance values and relatedcomplexities. There are many types of motionthat cannot simply be described as either first-order or second-order motion. Models of motionmay also be described on the basis of neuralspecialization for different types of motion (seeClifford & Ibbotson, 2003, for a review). This hasbeen a rapidly emerging approach to motionperception in cognitive neuroscience, focusing onmotion processing in the brain.

Motion Processing in the Brain

There are numerous areas of the brain involvedwith motion perception and motion guidance.Some areas are involved directly with detectingmotion while other areas subserve cognitiveprocesses that may operate in conjunction withmotion processing (Culham, He, Dukelow, &Verstraten, 2001).Although there is much left to learn, the general

architecture of motion processing in the cortex issteadily being discovered. While our focus islimited to some of the highlights related to motionapplication in AAC symbol structure, a moredetailed appraisal of these issues can be found inthe following original sources: Felleman and VanEssen (1987), Movshon and Newsome (1996),Culham et al. (2001), Clifford and Ibbotson(2003), Kourtzi and Kanwisher (2000), Tanakaand Saito (1989), and Duffy and Wurtz (1991).Much of the data summarized here derives fromstudies on non-human primates, but the data areconsidered very relevant to the understanding ofmotion perception in humans.From the retina, projections go largely to the

lateral geniculate nucleus (LGN) of the thalamus.The magnocellular layer of the LGN is onlypartially tuned to detecting directional effects butplays a critical role in the summation of directioninformation. The midbrain nuclei in the pretec-tum that receive a small ocular projection, arehighly selective to direction of movement acrossthe visual field and play a major role in ocular-motor movements. LGN signals then project tothe primary visual cortex (area V1, also calledarea 17), where neurons show a high degree ofdirectional tuning (Hubel & Wiesel, 1962, 1965).This has since been demonstrated across manyspecies (see Hubel & Wiesel, 2004, for a review).As neurons from V1 project to V3, the directionalselectivity of cells becomes progressively greater.Neurons from area V1 project directly to a

middle temporal area called area MT (V5 or thehuman motion complex – hMTþ, in humans),which then projects directly to an adjacent area,area MST (medial superior temporal). Areas MTand MST, also known as the MT-MST motion

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complex or MTþ, are the most prominent andwell-studied cortical areas for high-level motionprocessing. Both area MT and area MST havelarge proportions of directionally selective neu-rons. MT neurons are particularly driven by first-order motion. However, unlike area V1, area MTis not simply directionally selective and does notsimply extract motion information from localizedregions of the visual field. With wide input fromarea V1, area MT also appears to computeaggregate directions based on information acrossthe visual field. It responds to specific combina-tions of direction and form; hence it is responsiveto both local and global motion. MT neurons alsorespond to movement flow patterns that occurwith optic flow motion and visual expansion as anobject approaches the viewer, or the rotation ofthe entire visual field.The major advance made by area MST is an

even greater degree of global motion processing.Like area MT, MST is also highly specialized forprocessing optic flow and self-motion input,expansion, contraction, and rotation or largesectors of the visual field. However, area MSTresponses are tuned to even broader sectors of thevisual field and to more complex motion patternconfigurations.Motion perception neural systems are shared in

other aspects of visual perception, but also appearto be very robust. It is extremely rare that anindividual suffers from a disorder or motionperception following brain damage. One remark-able clinical case (often referenced in literature) isthe case patient L.M., whose bilateral posteriorparieto-temporal and occipital damage, followinga stroke, left her unable to perceive motion in thevisual modality (Zihl et al., 1983). To L.M.,moving stimuli would either appear frozen (still)or as a sequence of still shots.

Summary

Visual motion processing, like almost every otherform of perception, follows a neural sequence thatbecomes progressively more complex as thestimulus moves from primary receptors to asso-ciation areas of the cortex. As with all of vision,all forms of visual motion perception begin withretinal dynamics – the starting point for motiondetection. Even simple forms of motion percep-tion involve numerous cortical areas, startingwith area V1. The most complex cortical areas formotion have large receptive fields and respond toa variety of complex forms and directions, andcreate aggregate extractions based on motiondata from the entire visual field. Parts of thehigher centers for motion processing are specia-lized for distinct types of motion, such as visual

expansion and optic flow. While the interactionbetween motion processing and other aspects ofcognition is not fully understood, it is apparentthat motion processing subserves a range ofcognitive functions and recruits many perceptualprocesses.

CONCLUDING COMMENTS

Given the limited evidence base regarding bestpractices in incorporating motion in AAC, wecannot offer answers to important and relevantquestions that our discussion might raise. Forinstance, we do not yet know how many symbolsshould be animated, how long animation main-tains its effectiveness as a cue, or how to fade outmotion cues on a display. There is no guidance asto the best size for animations, how precisely theyshould mimic actual movement, or whetheranimation should be incorporated differently inscenes versus grids. The visual processing profilesof many of the clinical populations of interest inAAC have yet to be delineated carefully, thus theextent to which processing of motion cues bynondisabled viewers extends to viewers withcommunication disabilities of various origins,remains to be examined.Despite the sparse evidence base, technology is

increasingly offering clinicians the ability toincorporate animated symbols. Clinicians aretherefore required to make decisions aboutincorporating motion in the absence of anyresearch guidance. It is for this reason that it isso essential to begin a translation effort in whichthe field of AAC explores and exploits basicscience, even though the basic science has notbeen conducted with the goals and needs ofcommunication intervention in mind. From thatliterature, we know that when motion occurswithin in a visual scene, it calls into play primaryperceptual processes and also higher-level cogni-tive processes. The benefits of motion are tied tosuch factors as greater attention, novelty, con-trast, relative emphasis, and functional realismthat movement in symbols or scenes can confer.They are also tied to higher cognitive processesrecruited during motion processing.Enhancing the effectiveness of AAC learning is

likely best served when motion is applied in waysconsistent with neurocognitive and perceptualprinciples in processing motion. The wide avail-ability of user-friendly software both within andoutside the field of AAC makes this linkage evermore critical to establish and exploit. Our hope isthat an integrative effort like the one proposedhere could help fill out the much-neededevidence base within AAC, and provide cognitive

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neuroscience with a directly applied means oftranslating their basic science for functionaloutcomes.

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