A NEW TEST OF OBJECT DECISION: 2D SILHOUETTES FEATURING A MINIMAL VIEW

Elizabeth K. Warrington and Merle James

(The National Hospital for Neurology and Neurosurgery, London)

INTRODUCTION

It is widely accepted that there is a major discontinuity in the procedures that achieve object recognition between perceptual processing, in which a fully spe­cified structural description is achieved and semantic processing in which such a structural description acquires meaning (e.g. Marr, 1982). Neuropsychologi­cal evidence provides strong support for this discontinuity. Perceptual process­ing deficits are observed in patients with Right Hemispere lesions and semantic processing deficits in patients with Left Hemisphere lesions (e.g. De Renzi, Scotti and Spinner, 1969; Warrington and Taylor, 1978). However, using tasks that require a verbal response or object identification to establish the integrity of perceptual processing can be problematical. In the former case, the dys­phasic, in the latter case the visual associative agnosic would be disadvantaged. Indeed attempts to establish the status of perceptual competence in patients with a visual associative agnosia have been somewhat unsatisfactory. The patient's ability to copy complex drawings can in some cases provide indirect evidence of perceptual integry. But the standard of drawing skills in many (perhaps the ma­jority) is far too weak to be a source of useful data. The Ghent overlapping fi­gures test has possibly fewest drawbacks, since the task does not require an ov­ert verbal response; however, performance may be confounded by covert nam­ing and response biases (e.g. De Renzi, Scotti and Spinnler, 1969).

It has been suggested that object decision tasks offer an appropriate means of assessing the integrity of a structured percept (e.g. Riddoch and Humphreys, 1987a; Sartori and Job, 1988). Such tasks require the subject to detect which is the real object (picked from distractor nonsense objects) not what is the object. If such object decisions can be made on the basis of reference to stored struc­tural descriptions and if such structural descriptions are lateralised to the right hemisphere, it follows that patients with right hemisphere lesions will be im­paired on object decision tasks.

In the present investigation a consecutive series of patients with unilateral cerebral lesions was tested using an object decision procedure. Our aim was to clarify the issue as to whether failure on object decision tasks occurs at the level of achieving a structural description or in processing for meaning.

Cortex, (1991) 27, 377-383

378 E.K. Warrington and M. James

MATERIAL AND METHOD

Experimental Groups

(1) A consecutive series of 107 patients with unilateral cerebral lesions was tested. All patients physically fit enough to be tested in the Psychology department and able to coop­erate with the task demands were included (severe dysphasia was not a basis for exclusion). All patients who had not been educated in the normal English school system (at present a minimum of 10 years) were excluded, as were patients with a previous medical history that might compromise the cerebrum. Further details of the Right and Left Hemisphere lesion groups are given in Table 1. There was no age difference between the 2 groups and the incid­ence of field defects is very similar.

In all cases a CT scan became available and the right and left hemisphere groups were allocated to an anterior or posterior group as follows:

Anterior: patients with predominantly frontal, fronto-parietal or fronto-temporal le­sions (20 patients had Left Hemisphere lesions and 15 Right Hemisphere lesions).

Posterior: patients with predominantly parietal, temporal, temp oro-parietal , occipito­parietal and occipito-temporallesions (32 patients had Left Hemipshere lesions and 40 had Right Hemisphere lesions).

(2) 60 patients in whom extra-cerebral neurological conditions had been diagnosed were tested as. controls. The same exclusion criteria as for the lesion groups were applied. The mean age of the control group was somewhat younger than either of the experimental groups, thus any lesion group differences cannot be attributed to unfavourable age effects.

Control group N=60

Left hemisphere N=52

Right hemisphere N=55

Object Decision Test

TABLE!

Clinical Details of Experimental Groups

Mean age Incidence of aphasia

44.6 (13.9) 0

48.3 (13.4) 20

48.9 (14.4) 0

Procedures

Field deficits

0 7 hemianopic

3 quadrantanopic 12 hemianopic

4 quadrantanopic

The object decision test is an adaptation of our 3-D shadow image projector test (War­rington and James, 1986) in which scale models of common objects were rotated from a po­sition of maximum foreshortening to an angle where just sufficient features were present for it to be identified.

2-D silhouettes were constructed from the original 3-D shadow images by tracing the projected outline of the object at an angle of rotation at which approximately 750/0 of the normal control group could identify it. The resulting 2-D silhouettes are close to an object recognition threshold and can be regarded as representing a minimal view.

The test stimuli consist of arrays of 4 silhouettes, one of which was a 2-D drawing of a real object and 3 distractor items which were constructed to be similar object-like shapes but are in fact entirely imaginary (Warrington and James, 1991). There are 20 such arrays and subjects were required to 'point' to the real object and to guess if they were uncertain (see Figure 1). They were not required to name or identify the object. The number of correct choices was recorded as the Object Decision test score.

Object decision test and laterality 379

• Fig. 1 - Examples of object decision test.

Unconventional View Test

The unconventional view test which consists of 20 familiar objects photographed from an 'awkward' angle was administered to obtain a measure of perceptual skills on a task that had previously been shown to be impaired in patients with right hemisphere lesions (War­rington and Taylor, 1973, 1978; Warrington and James, 1988). Subjects were required to identify the object either by naming, gesture or describing its use.

Arithmetic

The Arithmetic subtest of the W AIS was included to obtain a cognitive measure on a task that had previously been shown to be impaired in patients with left hemisphere lesions (e.g. Warrington, James and Maciejewski, 1986).

RESULTS

Object Decision

The distribution of the control group scores on this test was skewed towards high scores, the median value being 19120 correct. The mean correct score and standard deviation for each group and subgroup are given in Table II. The Mann-Whitney U Test was computed to assess the significance of the group dif­ferences. The values of 'z' and the significance of each group and subgroup comparison are given in Table III. The right hemisphere group was significant­ly worse than the left hemisphere group and the right posterior subgroup was significantly worse than the left posterior subgroup. One negative finding to be

380 E.K. Warrington and M. James

TABLE II

Mean Correct Scores and Standard Deviations for Each Group and Subgroup

Object Unconventional WAIS decision views arithmetic

Control group N=60 18.1 (1.8) 18.8 (1.6) NT

Left hemisphere N=52 17.7 (2.1) 18.0 (2.5) 8.6 (4.2)

Right hemisphere N=55 16.2 (3.0) 15.9 (4.5) 10.2 (2.9)

Left anterior N=20 18.5 (1.4) 18.6 (2.0) 9.9 (3.6)

Right anterior N= 15 17.6 (2.5) 18.0 (3.6) 10.7(3.1)

Left posterior N=32 17.2 (2.3) 17.6 (2.7) 7.8 (4.4)

Right posterior N=40 15.6 (3.1) 15.1 (4.5) 10.0 (2.8)

TABLE III

Values 0/ 'z' and Significance Levels/or Each Group and Subgroup Comparison

Control vs. right hemisphere Control vs. left hemisphere Left hemisphere vs. right hemisphere Left anterior vs. right anterior Left posterior vs. right posterior

* p < .05, 2 tailed test. ** p < .01. .** p < .001.

Object decision

3.8*** 1.15

2.8** 0.56 2.4**

Unconventional WAIS views arithmetic

3.8*** NT 1.1 NT

2.6** 2.1* 0.23 0.62

2.6** 2.39*

noted is that there was not a significant decrement in the left hemisphere group as compared with the control group.

A significantly greater number of patients with Right Hemisphere lesions (14) than Left Hemisphere lesions (3), had a significant deficit in that they scored at or below the 5th percentile score of the control group (chF= 8.72 p<O.OOI).

Unconventional Views

The distribution of the control group scores on the unconventional view test was skewed, the median value being 19120 correct. An identical pattern of re­sults has been obtained on the unconventional view test as on the object deci­sion test. The central finding again is a lateralised and localised deficit in the right posterior subgroup (see Table II and III). The incidence of patients with a

Object decision test and laterality 381

significant deficit was also very similar to that observed on the object decision test (15 right hemisphere cases and 6 left hemisphere cases).

Arithmetic

The control group scores on this test were normally distributed and the mean scores and standard deviations for the control group and the right hemisphere group were close to that of the standardisation sample (mean 10, SD 3). The opposite pattern of results was obtained on this test as compared with the two tests of visual object perception. The left hemisphere group scored significantly below the right hemisphere group and the left posterior subgroup scored sig­nificantly below the right posterior subgroup (see Table II and III).

Correlations

Spearman's rank correlation between the object decision test and the uncon­ventional view test was computed for the control group, the left hemisphere group and the right hemisphere group. These values were .46, p< .0001; .42 p< .001; and .72; p< .0001, respectively.

DISCUSSION

A new test of object decision has been administered to patients with unila­teral cerebral lesions and their performance compared with a normal control group. The right hemisphere group was significantly impaired on this task as compared with the left hemisphere group whose performance was not signifi­cantly different from the normal control group. Despite the fact that many of the left hemisphere patients had multiple deficits in part reflected by their ov­erall poor performance on the Arithmetic subtest of the W AIS, there was a very low failure rate on the Object Decision task in this group. Conversely in the right heinisphere lesion group there was a high failure rate: indeed their failure rate was almost as high as on the more demanding task of identifying uncon­ventional views. The correlation between these 2 perceptual tests was signifi­cant in all 3 groups and highest in the impaired right hemisphere group. We would therefore assume that these two tasks maximise the same component processes of visual object recognition. However, the Object Decision test has wider applications in so far as it requires no verbal response and uses a forced­choice technique.

The present investigation extends the generality of our previous findings us­ing the shadow image projector. In both cases performance hinges on the iden­tication of a 'minimal' view object. In the first procedure, a 3-D shadow image of an object is rotated (either through its lateral or vertical axis) until it could be identified. It was argued that identification occurred when a feature or a cluster of features was visible in the 3-D image to ensure differentiation from other similar objects (Warrington and James, 1986). The minimal view objects used in the present experiment were 2-D silhouettes constructed to be close to a

382 E.K. Warrington and M. James

normal subject's 'threshold', in so far as any further foreshortening would ob­scure the critical features the normal subject requires for identification. This manipulation in the context of an object decision task appears to be as effective as the 3-D shadow images in eliciting a right hemisphere deficit.

The object decision task devised for this investigation differs in principle from those used in previous investigations. The 'real' object was a minimal view and the distractor items nonsense shapes having no identifiable features of ac­tual objects. Hitherto the non-objects in object decision tasks were composite figures derived either by interchanging different parts of real objects (e.g. a cow's body on a horse's head) or a spurious additional feature (e.g. a kangaroo plus a human leg) (e.g. Riddoch and Humphreys, 1987a, 1987b; Sartori and Job, 1988).

These object decision tasks using incongruous figures purport to probe structural rather than semantic knowledge. However, we would question whether this type of object decision necessitates reference to stored structural knowledge. If each part alone of a composite figure can be identified as the particular part of the object (say a cat's head or a dog's body), then the object decision task could be 'solved' verbally or by access to visual semantic know­ledge without recourse to structural knowledge of the whole. Consequently failure on such tasks might be more appropriately attributed to impaired se­mantic knowledge than to a damaged structural description of the object.

Laterality studies have established one major anatomical discontinuity in the sub-component processes of visual object recognition. There is general con­currence that pre-semantic perceptual processing in which a structural descrip­tion is fully specified is sub served by right hemisphere structures and the as­signment of meaning by left hemisphere structures (e.g. De Renzi, Scotti and Spinnler, 1968; Warrington and Taylor, 1978). The present group study has es­tablished that performance on the minimal view object decision task is im­paired in patients with right hemisphere lesions. We would therefore argue that the integrity of structural descriptions can be assessed by performance on this task.

A group study comparing patients with right and left hemisphere lesions could clarify the issue as to whether failure on object decision tasks that jux­tapose incongruent parts of objects occurs at the level of achieving a structural description or in assigning meaning to it.

ABSTRACT

A new test of object decision in which a minimal view 2D silhouette of an object is pre­sented together with 3 nonsense shapes has been administered to a consecutive series of pa­tients with right and left hemisphere lesions. There was a significant deficit in the right hem­isphere lesion group as compared with the left hemisphere group. It is concluded that this object decision test provides a measure of the integrity of pre-semantic perceptual processing systems.

Acknowledgements. We wish to thank the physicians and surgeons of the National Hos­pital for Neurology and Neurosurgery for permission to study and report our findings on pa­tients under their care. This research was supported by a grant from the Brain Research Trust of the Institute of Neurology.

Object decision test and laterality 383

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Elizabeth K. Warrington, Merle James, The National Hospital for Neurology and Neurosurgery, Queen Square, Lon­don WCIN 3BG, U.K.