J. Child Psychol. Psychiat. Vol. 40, No. 5, pp. 743–755, 1999

Cambridge University Press

' 1999 Association for Child Psychology and Psychiatry

Printed in Great Britain. All rights reserved

0021–9630}99 $15.00­0.00

Local Bias in Autistic Subjects as Evidenced by Graphic Tasks:Perceptual Hierarchization or Working Memory Deficit?

Laurent Mottron, Sylvie Belleville, and Edith Me!nard

Universite! de Montre! al and Ho# pital Rivie' re-des-Prairies, Montre! al, Canada

In the present study, copying tasks were used to assess hierarchical aspects of visualperception in a group of 10 nonsavant autistic individuals with normal intelligence. InExperiment 1, the hierarchical order of graphic construction and the constancy of this orderwere measured for the copying of objects and nonobjects. In comparison to controlparticipants, autistic individuals produced more local features at the start of the copying.However, they did not differ from controls with respect to graphic constancy. Experiment 2measured the effect of geometrical impossibility on the copying of figures. Results revealedthat autistic individuals were less affected by figure impossibility than were controls.Therefore, these experiments seem to support the notion of a local bias for visual informationprocessing in individuals with autism. Two interpretations are proposed to account for thiseffect. According to the hierarchical deficit hypothesis, individuals with autism do notmanifest the normal global bias in perceiving scenes and objects. Alternatively, the executivefunction hypothesis suggests that autism brings about limitations in the complexity ofinformation that can be manipulated in short-term visual memory during graphic planning.

Keywords: Autistic disorder, drawing, executive function, hierarchical processing, neuro-psychology, visual perception, working memory.

Abbreviations: ADI-R: Autism Diagnosis Interview-Revised.

Introduction

Individuals with autism are known to engage inatypical behaviors in the visual domain, including hand-flapping in front of their eyes, lateral vision, gazeavoidance, and preferences for certain visual patterns,such as flickering stimuli and spinning objects. In ad-dition, the regions and types of features used as cues forface and emotion identification appear to be abnormal inthese individuals (Baron-Cohen, Wheelwright, & Jolliffe,1997; Langdell, 1978). Finally, individuals with autismexhibit special abilities in the visuospatial domain (Frith& Happe! , 1994).

Although these behaviors are not entirely specific toautism, they are by far more common in this conditionthan in any other developmental syndrome. Despite thefact that these clinical signs suggest perceptual anomalies(Young & Ellis, 1992), it has been proposed that low-levelperceptual processes are intact in autism because autisticindividuals are usually able to reproduce auditory pat-terns and label visual patterns (Frith, Baron-Cohen, &Paul, 1987).

The conclusion that perception is intact in individualswith autism might depend on the degree of elaboration inthe perceptual models in the literature. Indeed, the last 20

Requests for reprints to: Dr Laurent Mottron, Cliniquespe! cialise! e des troubles envahissants du de! veloppement, Ho# pi-tal Rivie' re-des-Prairies, 7070 Boul. Perras, Montre! al (Qc),CANADA, H1E 1A4 (E-mail : mottron1!istar.ca).

years have seen a marked increase in the knowledge ofvisual perception (Ellis & Young, 1988; Humphreys &Bruce, 1989), which has resulted in the fragmentation ofvisual perception into numerous distinct operations, eachof which is susceptible to impairment by neurologicaldiseases.

One area in which the refinement of knowledge was themost crucial was in the study of high-level perceptualprocesses (i.e. the recognition of the perceptual charac-teristics of objects). This is now known to involve severalsubcomponents, such as the construction of perceptual 2-D and 3-D representations, selective visual attention,feature grouping, and the hierarchization of local andglobal parts. Only a fraction of these mechanisms hasbeen empirically investigated in the autistic population.In addition, it is now recognized that a number ofperceptual deficits may leave the naming of objectsunimpaired while severely altering other abilities such asface perception (Farah, 1990).

As visual perception has become better defined, thehypothesis of a deficit in visual perceptual processes inautism has received increased empirical support (for areview, see Mottron & Belleville, 1998). Indeed, indivi-duals with autism present several peculiarities in theprocessing of complex visual stimuli. For example, theydo not benefit from the segmentation of visual stimuli(Shah & Frith, 1993), nor do they perceive visual illusionsto the same extent as controls (Happe! , 1996). Moreover,they detect hidden figures more easily than normalparticipants (Jolliffe & Baron-Cohen, 1997), although

743

744 L. MOTTRON et al.

this finding is not significant when the diagnostic criteriaare less stringent (Brian & Bryson, 1996).

Additional support for the presence of perceptualanomalies in autism comes from the localization ofcortical lesions in patients with pervasive developmentaldisorders. Although some authors do not report acharacteristic pattern in the distribution of these lesions(Bailey, Philips, & Rutter, 1996; Minshew, Sweeney, &Bauman, 1997), our group has noted that brain lesions inautism are localized in cortical areas implicated in higher-order visual processing (Jambaque! , Mottron, Ponsot, &Chiron, 1998; Mottron et al., 1997). Moreover, it hasbeen observed repeatedly that autism is associated withnumerous diseases in which a common feature seems tobe a central or peripheral visual deficit. For example,several symptoms of autism like echolalia, pronounreversals, and motor stereotypies have been reported incongenital blindness (Brown, Hobson, Lee, & Stevenson,1997) and in delayed visual maturation (Cass, Sonksen, &McConachie, 1994). These studies have typically beenused as demonstrations that these symptoms are notspecific to autism, but they could also be taken as anindication that a number of symptoms in autism may becaused by visuoperceptual impairments. Recent refine-ments in models of visual perception and empiricalinvestigations for perceptual atypicalities in autism areboth reorienting toward a systematic examination ofhigher-order visual processes.

One manner in which qualitative differences in themechanisms that subtend visual perception can be as-sessed is through copying tasks of visually presentedobjects or drawings. Copying is a complex process (for areview, see Gue! rin, Ska, & Belleville, in press) involvingthe perception of the model, the construction of aninternal representation and its maintenance in visualworking memory, as well as graphic decisions andplanning. The internal representation is enhanced bylong-term memory inputs with respect to the perceptualand functional properties of objects (Kosslyn & Koenig,1992; Van Sommers, 1989). Copying tasks thereforeprovide a rich source of information for the assessment ofseveral cognitive processes.

Each of these processes can be isolated with the use ofproper experimental conditions (Van Sommers, 1989).For this reason, copying tasks have been widely used inthe exploration of cognitive functions, especially visualperception, in brain-damaged children and adults. Re-searchers have shown that different types of perceptualdeficits yield distinctive graphic anomalies. Visual ag-nosia, which is engendered by the destruction of thetemporal-occipital area of the brain, is an impairment inthe recognition of objects. Different types of visualagnosia have been identified, and each produces specificabnormalities in copying tasks. For example, aperceptiveagnosia results from altered perceptual construction of avisual representation and profoundly impairs graphiccopying. Integrative agnosia, on the other hand, resultsfrom difficulties in the integration of different parts of apercept. In comparison to normal participants, who drawstructural global components of a figure first, participantswith integrative agnosia begin by copying random parts.The copy can be quite similar to the original, however thecopying sequence is altered, which seems to reflect

modifications of hierarchical aspects of perceptual repre-sentations (Humphreys & Riddoch, 1987).

Graphic peculiarities similar to those observed inintegrative agnosia have been reported in EC, a savantautistic draughtsman with exceptional graphic abilities(Mottron & Belleville, 1993, 1995). The accuracy of EC’sdrawings surpassed that of overtrained control partici-pants, both in detail and proportion. He always started tocopy a drawing with random details and progressed bycontiguity. When the sequence of graphic constructionwas compared through several trials, EC showed graphicinconsistency. Indeed, his copying sequence varied acrosstrials, as opposed to normal or overtrained controls whoexhibited a fixed copying sequence. EC also presentedatypicalities in short-scale reaction time tasks that as-sessed visuoperceptual processing. First, he did notexhibit the hierarchical global-to-local interference effectfound in control participants. Specifically, in a conditionin which the local and global level of a pattern wereincongruent, EC’s global target detection was affectedadversely, whereas his detection of local level targets wasrelatively intact. This is in stark contrast to performanceby typically developing participants, in whom the reversepattern is obtained. Second, EC was unable to detectgeometric impossibility, a property that emerges from thejuxtaposition of several local features of briefly exposed‘‘ impossible figures ’’. EC’s various peculiarities in theprocessing of visual stimuli were accounted for by amodel emphasizing the absence of hierarchization invisual perception.

Hierarchization refers to the fact that different aspectsof a figure are not processed to the same degree. Figuresand visual scenes are generally constructed with local andglobal features, and research has shown that normalparticipants process global aspects faster than theyprocess local aspects of a figure. These processing levelshave been shown to be independent and neurologicallydistinct (for a review see Robertson & Lamb, 1991).

According to the hierarchization deficit hypothesis(Mottron & Belleville, 1993), the subcomponents (e.g.local parts) of an information unit (e.g. global pattern)are not organized hierarchically in individuals withautism. In the visual domain, global and local parts ofvisual stimuli are presumed to be processed independentlyand equivalently. This hypothesis is similar to someaspects of the central coherence deficit hypothesis (Frith,1989), which predicts that information at a particularlevel is processed in isolation from higher levels ofinformation.

Hierarchization deficit hypothesis predicts an absenceof bias or asymmetrical interference because both globaland local levels are equivalent. Under some experimentalconditions, particularly when using stimuli that containmore local than global features, it may predict a localbias ; however, interference would be absent. Globalcoherence deficit hypothesis predicts a local bias, due toan inversion of the normal tendency to relate informationof a given level to higher levels of organization.

EC’s case study allowed us to show graphic andperceptual particularities, as well as possible impairmentsin hierarchical processing, in one individual with autism.Although these findings are believed to generalize toother autistic savant draughtsman (Pring, Hermelin,

745GRAPHIC TASKS IN PERSONS WITH AUTISM

Buhler, & Walker, 1997; Selfe, 1978), it is unclear whetherthey can be extended to the general autistic population.Therefore, the present study used copying tasks toevaluate hierarchical visual processing in a group ofnonsavant individuals with autism.

Experiment 1

Normal participants typically begin to copy figures bytracing their global traits, particularly the outline (Rey,1959). This strategy tends to remain stable across suc-cessive copies of the same object, and is dependent uponthe normal processing and hierarchization of globalfeatures. This experiment assessed the hierarchical orderof graphic construction in a copying task, as well as theconstancy of this sequence. We predicted that the absenceof a global bias in visual perception would result in amore local graphic construction strategy. Thus, autisticparticipants should produce more local features early intheir graphic production than should normal controls.Additionally, the absence of a perceptual hierarchyshould be reflected in a random and unstable copyingsequence across successive reproductions of the sameobject.

The perceptual familiarity of the figures was alsomanipulated in order to investigate the presumed role oflong-term perceptual representations on hierarchizationand constancy. The copying of familiar objects wastherefore compared to that of abstract figures (non-objects) that were constructed with the features compos-ing the objects.

Method

Participants. Two groups participated in the study. Theclinical group was composed of 10 high-functioning autisticteenagers and adults (9 males, 1 female ; mean age, 19±4 years)with normal intelligence (performance IQ" 90). All parti-cipants were 12 years of age or older, to ensure that all haddeveloped the capacities to copy the third dimension. Thediagnosis of autism was obtained with the Autism DiagnosisInterview-Revised (ADI-R; Lord, Rutter, & Le Couteur, 1994),a diagnostic instrument operationalizing the criteria for autism.The ADI-R is standardized at three levels : question formu-lation, coding system, and diagnostic algorithm. Participantsabove the cutoff in the three relevant areas meet the ICD-10(World Health Organization, 1994) diagnostic criteria forautism. The ADI-R was administered by one of the authors(LM) who has obtained a reliability score of ±9 with the creatorsof the instrument. The presence of symptoms relevant to thediagnosis was clinically verified by an experienced clinician.This resulted in the exclusion of 1 individual from the clinical

Table 1Characteristics of Participants with and without Autism

Age Nonverbal IQ Laterality

Group Mean SD Mean SD Mean SD

With autism 19±41 7±72 112±20 12±47 71±60 52±94Without autism 18±48 6±67 111±91 5±28 53±64 75±00

group, which was originally comprised of 11 autistic part-icipants. None of the participants satisfied the DSM-IV(American Psychiatric Association, 1994) criteria for the Asper-ger syndrome. The language items of the ADI were used ascriteria. Only participants with a non-zero score for items 12(delay for word) and 13 (delay for two-word sentences) of theADI-R were included. The control group was composed of 11individuals without autism matched for gender, age, laterality(Oldfield, 1971), and Performance IQ with the autistic in-dividuals (Table 1). No statistically significant differenceswere found between the two groups on the stated variables.All control participants and their first-degree relatives werescreened for current or past neurological, developmental,or psychiatric disorder. All participants were given $10 fortheir participation.

Material. Four black-and-white line drawings of objectswere selected from the Snodgrass and Vanderwart (1980) list(Fig. 1a). The objects (plug, toaster, drum, and kettle) werechosen based on their visual complexity and familiarity. Thecomplexity of the selected objects was average (mean com-plexity score¯ 2±6, the highest complexity score in theSnodgrass and Vanderwart scale being 5). A pilot study showedthat both the controls and the clinical group were able torecognize the objects and draw the pictures. To create thenonobject counterparts, each object was decomposed into itsbasic features. A feature was defined as a straight or curvedline without an inflection point, delimited by junction pointswith other features. Each nonobject was then created byregrouping the features into a new configuration. Each non-object was created with a general outline broadly comparableto its corresponding object (Fig 1b). Thus, the object and thenonobject were comparable with respect to the number andnature of their component features and with respect totheir global outline. On average, each stimulus was composedof 31±5 features. Each model was reproduced in black onthe upper half of a white 21¬27 cm sheet of paper. Thestimuli subtended approximately 10¬10 cm. A line separatedthe upper and lower half of the sheet, and the participantswere asked to copy the drawing just below the model, on thelower half.

Procedure. All participants were tested in a quiet room inwhich they were seated comfortably at a table. The drawingsheets were given to participants one at a time, and they receivedinstructions to copy the drawings with a lead pencil, as quicklyand as accurately as possible. They were also informed that theycould not erase their drawings or start over. The sheet of paperwas fastened to a clipboard to ensure that the participantswould not move it around. A time limit of 3 minutes wasinitially scheduled but none of the participants reached thislimit, as all copies were performed in 15 to 128 seconds. Theparticipants were allowed two practice trials on different stimulibefore the experiment, after which each experimental figure hadto be copied three times in immediate succession to allow anevaluation of copy constancy. Based on a preliminary pilotstudy, the copying task was adjusted to make the instructionsclear and unambiguous, and to counterbalance the presentationorder to avoid interference effects. The eight stimuli (fourobjects, four nonobjects) were then presented to the participants

746 L. MOTTRON et al.

(a) (b)

(c) (d)

Figure 1. Examples of the stimuli and of the scoring procedure used in Experiments1 and 2: (a) object ; (b) nonobject ; (c) possible figure; (d) impossible figure. Each

feature is identified by a number and indexed as global (Gn) or local (L

n).

in a semirandomized fashion, so that corresponding objects andnonobjects were never issued successively, and more than twoobjects or two nonobjects were never issued in succession. Allparticipants were videotaped as they completed the task, andthe films were used for the final notation.

Results

Scoring procedure. A scoring procedure was devisedin order to evaluate precisely the variables of interest. Foreach stimulus, each feature (as defined previously) wasgiven an identification number and was categorized aseither global or local (Fig. 1b). The features forming thetwo-dimensional contour of the stimuli were consideredglobal, whereas the features forming the internal detailswere considered local. The sequence of feature pro-duction was thus coded for each copy. The amount oftime needed to complete each drawing was also computedon-line and verified on videotape, to the nearest second,using an electronic stopwatch.

From this scoring procedure, a control measure of thequality of the production was computed first. Thismeasure was obtained by counting the mean percentageof omitted features in each level (global vs. local) and

each condition (objects vs. nonobjects). This ensured thatthe two groups were equivalent in their copying abilityand that quantitative differences in production could notaccount for the qualitative differences observed betweenthe two groups.

Measures of graphic construction involved copyingtime, graphic hierarchization, and constancy. Copyingtime was defined as commencing when the participantsdrew their first line and terminating when they verballyannounced that their copy was completed. Graphichierarchization was assessed by computing the proportionof global and local features copied in the first, second,and third section of the copying. Because the number ofglobal and local features was different for each stimulus,this measure was computed individually for each stimu-lus. The sections were obtained by dividing in three equalparts the number of features copied in one trial. Forexample, the first section of a given trial corresponded tothe first third of the total number of features copied. Ameasure of constancy was also computed to evaluate thestability of graphic planning across successive copies ofthe same stimulus. A constancy index was obtained by theratio of fixed transitions (FTs) performed over thenumber of possible FTs. This ratio is expressed in theformula [(number of times that feature ‘‘x ’’ is followed by

747GRAPHIC TASKS IN PERSONS WITH AUTISM

Table 2Mean Percentage of Omitted Features in the Copying of Objects and Nonobjects, inParticipants with and without Autism

Objects Nonobjects

Group Global features Local features Global features Local features

With autismMean 3±20 4±60 0±40 2±20SD 6±99 5±21 1±26 1±75

Without autismMean 1±40 3±50 4±40 1±10SD 2±55 4±06 5±66 2±02

feature ‘‘y ’’ across the three trials)}(number of trials®1)¬(total number of features produced®1)]. Since thenumber of possible FTs varied for each stimuli, theconstancy index was computed individually for each ofthe eight stimuli using the smallest number of featurescopied.

(1) Quality. An analysis was performed first on thepercentage of omitted features. As can be seen inTable 2, the overall graphic production quality wasexcellent, with the mean omission rate always lessthan 7% of the features. Nevertheless, neithergroup reached a ceiling, which allowed for stat-istical comparisons. Analyses of the nonparametrictype were used because the participants made veryfew errors and the data were not normally dis-tributed. Group comparisons were performed us-ing local and global features as dependent vari-ables. This was done separately for objects andnonobjects (Table 2). Results indicated no groupdifferences for objects (Mann-Whitney U¯ 51±5and 49±0 for local and global features respectively)and nonobjects (Mann-Whitney U¯ 30±0 and 34±0for local and global features respectively).

(2) Copying time. The time needed to complete thecopy in each trial was compared across groups. Athree-way ANOVA was performed with Group(autistic individuals vs. matched controls) asbetween-subject factor and Material (objects vs.nonobjects) and Trial (1, 2, 3) as repeated factor.The analysis showed a main effect for Trial, F(2,38)¯ 35±24, p!±0001, explained by the increasinglyfast copying over trials, but no main effect forGroup, F! 1 and Material, F(1,19)¯ 2±035, p¯±17. There was also a significant Group by Trialinteraction,F(2,38)¯13±75, p!±0001. The results,shown in Fig. 2, suggest that across trials, theperformance of autistic individuals did not improveas much as it did for the matched controls.Statistical analysis of simple effects revealed asignificant Trial effect in matched controls, F(2,38)¯ 48±84, p!±001, but no Trial effect in autisticparticipants, F(2,38)¯ 2±37. However, the Groupdifference was not significant in all trials (F! 1 inall cases). Finally, the Material by Trial interactionreached significance, F(2,38)¯ 3±49, p!±05. Thisinteraction appears to be due to the more pro-nounced decrease in copying time, across trials, fornonobjects, F(2,38)¯ 54±94, p!±001, than for

Figure 2. Copying time across trials for objects and non-objects.

objects, F(2,38)¯ 11±27, p!±001. Copying timefor objects and nonobjects did not differ in Trial 1and 2 [F! 1 and F(1,19)¯ 2±79, respectively].However, the difference approached significance in

748 L. MOTTRON et al.

Figure 3. Proportion of global features copied, across timesections, for objects and nonobjects.

Trial 3 [F(1,19)¯ 4±35, p¯±051], in which allparticipants were slightly faster at copying non-objectsthanobjects.NeithertheGroupbyMaterial,nor the three-way interactions were significant,F! 1 in both cases.

(3) Graphic hierarchization. This measure evaluatedthe tendency to begin copying by tracing the outlinevs. the internal features. The number of global andlocal features was not equivalent (local featureswere more numerous), thus, the statistical analyseswere performed separately for the global and locallevels.

The percentage of global features produced inthe three sections (as described earlier) of thecopying task was computed. Since the proportionof local and global features was the same in allthree trials with a given stimulus, the data wereaveraged across trials. A three-way ANOVA was

Figure 4. Proportion of local features copied, across timesections, for objects and nonobjects.

performed with Group (autistic individuals vs.matched controls) as a between-subject factor, andMaterial (objects vs. nonobjects) and Section (firstvs. second vs. third section of the production) aswithin-subject factors. In the following analyses,Greenhouse-Geisser correctionswere appliedwhennecessary in order to correct for the heterogeneityof variance, and this did not modify the results. Amain effect for Section was observed, F(2,38)¯101±26, p!±0001, with normal participants pro-ducing more global features in the first section ofthe task (see Fig. 3). However, no main effect ofGroup [F(1,19)¯ 1±629, p¯±217] or of Material[F(1,19)¯ 2±182, p¯±155) was obtained. As pre-dicted in our hypotheses, the Group by Orderinteraction was significant, F(2,38)¯ 4±58, p!±05.Analysis of simple effects and inspection of Fig. 3reveal that autistic participants produced fewerglobal features than matched controls during thefirst section [F(1,19)¯ 4±69, p!±05], but more

749GRAPHIC TASKS IN PERSONS WITH AUTISM

than controls during the last section of theirproduction [F(1,19)¯ 5±81, p!±05]. However,neither the Group by Material, nor the higher-order interaction, reached significance (F! 1 inboth cases).

An identical statistical analysis was performedto compare the proportion of local features pro-duced in the first, second, and last section ofgraphic production. A three-way ANOVA wasperformed, using Group (autistic individuals vs.matched controls) as a between-subject factor, andMaterial (objects vs. nonobjects) and Section (firstvs. second vs. third section of the production) aswithin-subject factors. As indicated in Fig. 4,results were comparable to those observed in theanalysis of the global features. The ANOVArevealed a main effect of Section, F(2,38)¯ 65±69,p!±0001. However, as predicted in our hypo-theses, the Group by Section interaction was sig-nificant, F(2,38)¯ 4±67, p!±05. The analysis ofsimple effects and inspection of Fig. 4 reveal thatautistic participants produced more local featuresthan matched controls during the first section oftheir production [F(1,19)¯ 6±31, p!±05], and thatthere was also a tendency for autistic individuals toproduce less local features than controls during thethird section [F(1,19)¯ 3±92, p¯±06]. A Materialby Order interaction was also observed, F(2,38)¯9±04, p!±001. All participants tended to producefewer local features during the first section of theirdrawing of nonobjects than of objects [F(1,19)¯16±76, p!±01] whereas the reverse was true for thethird section of the production [F(1,19)¯ 7±47,p!±05]. However, the Group by Material andhigher-order interactions failed to reach signifi-cance (F! 1 in both cases).

(4) Constancy. In order to assess copy constancy(Table 3), a 2 (Group: autistic individuals vs.matched controls) by 2 (Material : objects vs.nonobjects) ANOVA was performed with thepercentage of fixed transition as the dependentvariable. Results revealed that neither the Group[F(1,19)¯ 1±36] nor the Material main effects(F! 1) were reliable. Table 3 indicates that autisticparticipants were slightly less consistent thanmatched controls when copying nonobjects but theGroup by Material interaction was nonsignificant,F(1,19)¯ 3±59, p¯±073.

Discussion

Autistic individuals could copy objects and nonobjectsat the same speed and with the same accuracy as matchedcontrols. Since clinical participants were neither worse(motor clumsiness) nor better (higher graphic training)than control participants on these aspects, the qualitativedifferences they exhibited in copying must be interpretedin terms of cognitive differences instead of generaldifferences in graphic abilities.

Before discussing the main findings, secondary resultsproduced in particular by material and practice effectswill be reviewed. Variations in material yielded someinteresting results. The copying time for objects and

Table 3Mean Percentage of Fixed Transitions across Trials, inParticipants with and without Autism

Material

Group Object Nonobject

With autismMean 67±34 64±92SD 10±18 7±46

Without autismMean 66±96 72±05SD 7±87 6±30

nonobjects did not differ in the first two trials, whichseems to confirm that difficulty level was equivalent.However, nonobjects were copied faster than objects onthe third trial, which indicates a more substantial im-provement in copying speed across trials for nonobjects.This result appears paradoxical, yet one possible ex-planation is that nonobjects can be copied with a two-dimensional graphic planning strategy. The possibilitythat objects may not correspond entirely to participants’internal representations may also play a role in this effect,as object copies would therefore have to satisfy on-linecomparisons with an internal representation of thestimulus. However, this does not explain why nonobjectswere copied faster only on the last trial. This may beexplained by the fact that nonobjects are less influencedby semantic properties, which might make the productionof nonobjects more dependent upon procedural memory.Procedural memory refers to the slow and graduallearning of actions and habits. In that sense, it can beargued that repeatedly copying an unknown stimulussolicits the contribution of procedural memory. Interest-ingly, the training effect shown by matched controls wasnot seen in individuals with autism, neither with objectnor with nonobject stimuli. This might reveal an ab-normal procedural memory. To our knowledge, thismemory system has never been explored in individualswith autism.

With respect to the initial hypotheses, there were twomain findings in Experiment 1. First, graphic constancywas identical in both groups. The absence of graphicconstancy previously found in an autistic savantdraughtsman (Mottron & Belleville, 1993) could there-fore not be generalized to the autistic population. Thissuggests that EC’s low constancy level might be related toan interaction between his autistic anomalies and hisgraphic expertise. Second, the clinical group presented agraphic strategy that differed from that of the controlgroup. Indeed, for both types of stimuli, autistic indivi-duals produced more local features than controls in thefirst section of their copying and less than controls in thelast section. In other words, they did not favor globalfeatures as much as controls did at the beginning of theirproduction. This modification in the sequence of graphicproduction might result from a deficit in perceptual or inexecutive processes. Each of these interpretations willnow be discussed.

At the perceptual level, these results could be accountedfor by impairments in hierarchical processing that affectthe perception of the global aspects of figures. A

750 L. MOTTRON et al.

hierarchical processing deficit was also proposed toaccount for the piecemeal production of brain-injuredpatients with integrative agnosia (Humphreys & Rid-doch, 1987). In neural models of visual perception, twoprocessing streams have been identified based on theiranatomical localization and role in vision (Ungerleider &Mishkin, 1982). The ventral stream is an occipital-temporal pathway presumed to encode the physicalproperties of objects (the ‘‘what ’’ system), whereas thedorsal stream is an occipital-parietal pathway responsiblefor the encoding of spatial properties of objects. Brainlesions of the ventral system are known to modifyperceptual hierarchization in perception and graphicconstruction (Robertson & Delis, 1986). More specifi-cally, lesions of the right inferior temporal regions areknown to produce selective deficits in perceiving anddrawing global aspects of figures, whereas lesions of theleft inferior temporal regions impair local production andperception. Thus, the local bias observed here in autisticindividuals might result from impairments of the rightventral processing stream.

According to Van Sommers (1989), the manner inwhich the construction sequence is elaborated alsodepends on depiction decisions. This term comprisesvarious planning operations specific to the graphiccopying of an object. One of these operations is theselection and ranking of segmented parts of the figure inorder to reproduce them in the appropriate order. Thisoperation appears to be of an executive nature. Anotherpossible ‘‘executive ’’ interpretation assimilates this localstrategy to a mild form of constructional apraxia that iscomparable to that resulting from frontal lesions. Thisinterpretation appears less plausible, since it has beenproposed to explain perseverative errors in spatial rela-tions (Gainotti & Tiacci, 1970), and perseverative errorswere not observed in the current study. Nonetheless, themere possibility that copy order could be interpreted inan ‘‘executive ’’ framework prompted the design ofanother task that was aimed at assessing the effect of themanipulation of global properties upon graphic per-formance.

Experiment 2

In normal participants, parts of figures are related toone another at an early level of visual perception. Forexample, global aspects of a pattern interfere with itslocal aspects before figure identification (Lamb, Robert-

Table 4Mean Percentage of Omitted Features in the Copying of Possible and ImpossibleFigures, in Participants with and without Autism

Possible figures Impossible figures

Group Global features Local features Global features Local features

With autismMean 0±00 0±00 1±00 1±80SD 0±00 0±00 2±31 2±53

Without autismMean 0±20 0±90 0±30 0±60SD 0±63 2±23 0±95 1±90

son, & Knight, 1989; Robertson & Lamb, 1991). Parts ofvisual displays are also positioned in a common referenceframe (Kosslyn & Koenig, 1992). In order to evaluate therelations among parts of a visual stimulus in individualswith autism, Experiment 2 investigated the effect of figureimpossibility on copying time. Impossible figures aregeometric, 3-D line drawings in which parts conform to 3-D rules, but in which the relations between parts violatethese rules. For example, in the impossible stimulus(derived from Penrose triangle) depicted in Fig. 1d, eachof the four angles conforms to 3-D rules, but the figuredoes not. Thus, impossibility is an emergent property ofthe whole figure, which necessitates the perceptual in-tegration of parts. Consequently, normal participantsshould find impossible figures more difficult to copy thanpossible ones.

Material and Procedure

The stimuli used in this experiment were four possiblegeometric figures and their impossible counterparts, matched innumber of features and general outline (see Fig. 1c and 1d). Apreliminary study was performed to ensure that all figures wereunambiguous with respect to their geometric possibility orimpossibility. The procedure was the same as in Experiment 1,with the exception that each drawing was copied only once.

Results

(1) Quality. First, nonparametric analyses were per-formed on the percentage of omitted features foreach type of stimuli. Group comparisons wereperformed, for local and global features, in thecopying of possible and impossible stimuli (Table4). All participants omitted very few features for alltypes of stimuli. Results indicated no group differ-ences for the number of features omitted in thecopying of possible figures (Mann-Whitney U¯50±0 and 45±0 for local and global features re-spectively) or impossible figures (Mann-Whitney U¯ 48±5 and 39±5 for local and global featuresrespectively).

(2) Copying time. The time needed to complete thecopies was compared across groups in a 2 (Group:autistic vs. matched controls) by 2 (Material :possible vs. impossible) ANOVA with Material as

751GRAPHIC TASKS IN PERSONS WITH AUTISM

Figure 5. Copying time for possible and impossible figures.

a repeated factor. The results are depicted in Fig. 5.The ANOVA showed a main effect for Material,with possible figures generally reproduced fasterthan impossible figures, F(1,19)¯ 65±44, p!±0001. There was also a significant Group byMaterial interaction, F(1,19)¯ 4±86, p!±05. Ana-lysis of the simple effects and inspection of Fig. 5reveal that matched controls [F(1,19)¯ 55±64, p!±001] were more affected than autistic participants[F(1,19)¯ 16±53, p!±01] by the geometric im-possibility of the impossible figures. The othermain effects and interactions did not reach signi-ficance.

Discussion

The ‘‘ impossibility effect ’’ was expected to be reducedin autistic individuals because they have difficulty relatingthe elements of a figure. Hence, the clinical group shouldexperience less difficulty in copying impossible figuresthan should matched controls. This task allowed us toassess the processing of holistic properties independentlyof executive functions, since abnormal planning strategiesshould not affect the copying of impossible figures morethan possible ones. It should be noted that a copy task offigures that are impossible at the geometrical level doesnot tap into the same processes as those in imaginationtasks, which consist of drawing something that does notexist, as in Scott and Baron-Cohen (1996) and Leeversand Harris (1998).

Results indicated that normal participants neededmore time to copy impossible figures than possible ones.This has to be interpreted in terms of an ‘‘ impossibilityeffect ’’ because possible and impossible figures wereidentical with respect to their other pictorial properties(nature and number of features). The stimuli weretherefore adequate to assess a possible modification ofthis effect in autistic individuals. The findings revealedthat individuals with autism were less sensitive to geo-metric impossibility than were control participants. Thiscan be accounted for parsimoniously from a perceptualperspective, in which individuals with autism do notintegrate local parts into a perceptual representation ofthe complete figure to the same extent that controls do.

Indeed, they copy each part separately without integrat-ing it with other local parts to form a global pattern. Asstated in Experiment 1, this can be related to a deficit ofthe ventral visual stream, which is involved in thehierarchization of features.

However, impairment in the dorsal or ‘‘where’’ visualstream could also account for the findings of this secondexperiment. The integration of parts in a single referenceframe has been attributed to a component of the dorsalsystem, namely spatiotopic mapping (Kosslyn & Koenig,1992). An example of dorsal stream impairment issimultagnosia, a deficit in which two parts of a visualdisplay cannot be perceived simultaneously. Patients withsimultagnosia who are asked to perform copying tasksomit large parts of the figure because they cannot perceivemore than one aspect of the pattern at a time. A similarhypothesis, ‘‘ tunnel vision’’, has been proposed byRincover and Ducharme (1987) to account for the factthat autistic individuals consider only one cue whenpresented with pairs of visual cues. This interpretation,however, is hardly compatible with the copying accuracyexhibited by the autistic participants in the present study.

Another possibility, involving long-term memory, isthat the geometric rules underlying the impossibilityeffect are defective or inactivated. Difficulties in thecopying of impossible figures would result from a conflictbetween what is perceived (impossible figure) and what isstored (representation of possibility). This representationof possibility might not be activated to the same extent inautistic individuals, thus reducing their sensitivity togeometrical impossibility. Data from EC partially sup-ports this interpretation (Mottron & Belleville, 1993).Indeed, EC could not detect the geometrical impossibilityof briefly exposed (100–800 msec) impossible figures.However, increasing the time of exposure to 15 secallowed for a correct judgement of impossibility. Thissuggests that even though graphic possibility is repre-sented in long-term memory, it might not be matchedproperly or quickly enough with perceptual stimuli.

General Discussion

The current experiments assessed the effect of per-ceptual characteristics on the graphic performance ofhigh-functioning nonsavant individuals with autism. In acopying task with objects and nonobjects as stimuli,autistic individuals produced more local features thannonautistic controls in the first third of their copying.However, their constancy in graphic construction wassimilar to that of controls. In a copying task with possibleand impossible figures as stimuli, individuals with autismperformed as accurately and as quickly as the controls forpossible figures and copied impossible figures faster thancontrols.

In order to interpret these results, the various sub-systems normally implicated in copying tasks must beconsidered. Van Sommers (1989) proposed a sequentialmodel of copying that was inspired from current modelsof face and object recognition (Humphreys & Bruce,1989; Marr, 1982). According to this model, the to-be-reproduced figure is first processed through a visualanalysis of its 2-D, 2"

#-D and 3-D properties. After its 3-D

construction is understood, the representation of a figure

752 L. MOTTRON et al.

can be matched with a visual recognition unit stored inlong-term memory if the representation corresponds to afamiliar item. This stored visual representation in turnactivates the network of semantic knowledge connectedwith this recognition unit. At this stage, the picturedfigure is represented, recognized, and interpreted, andgraphic processes as such may operate. Decisions aremade about the various possible ways to draw the object(e.g. line drawing vs. texture) and depiction processes areinitiated through the use of stored motor programs.These two final steps are presumed to require the integrityof executive functions. Our findings will now be discussedin the light of these perceptual and nonperceptualprocessing steps.

Perceptual Interpretation

The findings obtained in the present study may beinterpreted as resulting from a deficit at the perceptuallevel, just as abnormal perception of the holistic proper-ties of objects has been suggested to account for theabnormal part-whole relationship in the graphic con-struction of individuals with integrative agnosia (Grailet,Seron, Bruyer, Coyette, & Frederix, 1990). An atypicalconstruction of the visual figure, preliminary to its copy,should result in atypical graphic performances. In thatsense, one interpretation involves the ‘‘dorsal ’’ subsystemdealing with the spatial properties of objects. A deficit inthe processing of spatial relations between parts of anobject might indeed favor a local features approach. Asecond perceptual interpretation involves components ofthe ‘‘ventral ’’ system responsible for the processing of thephysical properties of objects and the hierarchical pro-cessing of visual stimuli (i.e. the differentiation betweenlocal and global properties). Results of Experiments 1and 2 seem in agreement with an imbalance between thetwo property levels in autism, to the detriment of theglobal or holistic properties. Indeed, in Experiment 1,autistic individuals gave less prevalence to global prop-erties than did controls at the start of their graphicconstruction. In Experiment 2, autistic individualsshowed a reduction of the impossibility effect, which canbe explained by a diminished perception of the holisticproperties of objects.

As in Frith et al. (1987), it can be argued thata pure perceptual deficit would result in abnormalobject recognition. Indeed, the part-whole relationshipdeficit reported in brain-damaged individuals was foundin patients who failed to recognize objects (Riddoch &Humphreys, 1987), which was not the case of the clinicalgroup studied herein. However, Lamb et al. (1989) havereported brain-damaged patients suffering from hier-archical deficits who were not all typical agnosics. Itcould also be argued that the final graphic result ofautistic participants would be impaired if they sufferedfrom abnormal perceptual processes. However, thisargument does not stand, as the assessment of brain-damaged persons has shown that patients with abnormalpart-whole processes could copy resembling figures.However, like the autistic individuals tested herein, theydid so by using an abnormal strategy (Humphreys &Riddoch, 1987).

Therefore, it appears justifiable to interpret the presentresults from copying tasks in terms of perceptual deficits.An abnormally low encoding level of the global propertiesof objects is compatible both with the hierarchizationdeficit hypothesis (Mottron & Belleville, 1993; Mottron,Burack, Stauder, & Robaey, 1999) and the centralcoherence theory (Frith, 1989; Frith & Happe! , 1994).

The former account predicts an equivalent amount ofprocessing for both global and local levels. Because localfeatures are more numerous than global ones, an absenceof bias toward the global level should favor the locallevel. Nevertheless, the hierarchization deficit hypothesisalso predicts an absence of interference from global tolocal level. Central coherence theory predicts a local biasper se. Unfortunately, other studies conducted in ourlaboratory do not support this interpretation. In theseexperiments, autistic participants were tested in tasksinvolving the perception of hierarchical stimuli, that is,stimuli in which a target was located either at the local orglobal level. In these studies, reaction time to detecttargets located at the global level was not slower inautistic individuals than in controls. We also found anormal global to local interference effect (Mottron et al.,1999), and a normal global bias was reported byOzonoff and collaborators (Ozonoff, Strayer, McMahon,& Filloux, 1994). This apparent inconsistency betweenreaction time studies and copying tasks might relate tothe time-scale of the paradigms used. Indeed, a number ofother studies with an autistic clinical group have foundperceptual deficits compatible with a global impairment.This was particularly found in block design tasks (Shah &Frith, 1993) as well as with hidden figures (Jolliffe &Baron Cohen, 1997; Shah & Frith, 1983) and visualillusions (Happe! , 1996). All of these paradigms rely upondependent measures with large time-scales.

The apparent insensitivity of autistic individuals togeometric impossibility that was observed in Experiment2 might depend upon differences in perceptual long-termknowledge.Adefective or delayed activation of geometricrules could account for the fact that individuals withautism are less affected than controls by geometricimpossibility. These rules might be stored in semanticmemory as analogic properties shared by the ‘‘objectsrecognition units ’’ (Humphreys & Bruce, 1989) or asabstract rules. However, this interpretation cannot ac-count for the results obtained in Experiment 1, wheregeometrical rules were not involved.

Executive Interpretation

Perceptual interpretations are concerned with theperceptual construction of representations. In contrast,executive interpretations are concerned with the planningprocesses that are necessary to act upon constructedperceptual representations. A recent and influential lineof research suggests that difficulties in high-level planningand the control of behavior could account for the autisticsyndrome (Hughes, Russell, & Robins, 1994; Ozonoff,Pennington, & Rogers, 1991; Prior & Hoffman, 1990).According to this theory, autistic individuals suffer fromimpairments in a specific set of functions referred to as‘‘executive ’’ functions. These functions are believed to

753GRAPHIC TASKS IN PERSONS WITH AUTISM

rely upon the maintenance of several representations inworking memory.

The effect of planning deficit in graphic tasks isillustrated in research by Scott and Baron-Cohen (1996)and Leevers and Harris (1998) on the ability of autisticindividuals to draw imaginary entities. In the formerstudy, Scott and Baron-Cohen asked participants todraw an entire figure from start to finish, and the findingsrevealed that the clinical group performed at a lower levelthan the control group. Leever and Harris’ task consistedof completing a figure in such a way as to render itimpossible. In these conditions, where planning require-ments were minimized, the clinical and control groupsperformed at a similar level. Leevers and Harris attributethis divergence to differences in planning demand be-tween the tasks used in the two studies.

An interpretation of the present results could thereforebe proposed along these lines. The lack of primacy ofglobal features in autistic graphic construction might berelated to deficits in graphic planning or to idiosyncraticplanning strategies. It can be argued that to begin to copya drawing with global features requires a strategicdecision on the part of the subject, especially if oneconsiders that local features are more numerous thanglobal ones. For the same reason, a planning impairmentwould result in the subject beginning its copy by any partand thus would tend to favor the more numerous localfeatures, therefore producing a ‘‘piecemeal ’’ copy with alocal bias. This interpretation, however, cannot accountfor the results of Experiment 2. Indeed, why wouldplanning deficits decrease the difficulty of reproducingimpossible figures?

Level of Complexity Interpretation

A slightly different version of the executive interpret-ation attributes the idiosyncratic graphic strategies ofindividuals with autism to a limitation in the level ofcomplexity of the operations performed upon spatialrepresentations (see Zelazo, Burack, Benedetto, & Frye,1996, for a related model). Copying tasks require theconcurrent activation of several elements in workingmemory in order to plan the motor components of thetask successfully. For example, in order to represent acomplete figure within the space limitations of a sheet ofpaper, the proportions of a drawing must be consideredin relation to the dimensions of the paper. In Experiment1, limitations in the capacity to maintain local and globalfeatures of the model in working memory, in order toplan graphic construction, might not allow for theelaboration of the ‘‘global features first ’’ strategy. Thiswould result in a copying sequence favoring local fea-tures. In Experiment 2, the attenuation of the effect ofimpossibility might be caused by a deficit in the ability tomaintain several local parts in working memory duringgraphic planning. This would nullify the geometricconflict between the local part of impossible figures. Thisexplanation is compatible with the atypical weaknesses orstrengths of autistic individuals in visuospatial tasks thatare dependent upon larger time-scales, as opposed toreaction time experiments. Time-scale of visuospatialworking memory processes (for example, block design) isin seconds whereas that of perceptual processes (for

example, the detection in hierarchical stimuli) is inhundreds of milliseconds.

This interpretation could also account for the highperformance of individuals with autism in the blockdesign subtest, which involves the reproduction of ageometric model with different faces of blocks. In thistask, disembedding parts of the model from the wholefacilitates its reproduction, and segmenting the modelappears to suppress the advantage exhibited in this taskby autistic individuals (Shah & Frith, 1993). This couldbe due to limitations in the number of parts that can bemaintained simultaneously in working memory.A similarinterpretation could be used to explain the good per-formances of autistic participants with hidden figures.Furthermore, it could account for the difficulties thatautistic individuals present in joint attention and in thedetermination of intention from gaze direction. Indeed,these abilities require the simultaneous manipulation ofseveral regions of space and of several local regions of avisual pattern.

New theoretical developments of the notion of com-plexity allow for an extension of the previous interpret-ation to include additional limitations exhibited byautistic individuals in nonspatial tasks. Halford (in press)proposed to account for limitations of working memorycapacity by referring to the relational complexity of itscontent. Relational complexity is determined by thenumber of arguments contained in a relation (i.e. thenumber of dimensions that can vary independently). Forexample, a unary relation is a proposition with oneargument, such as the representation of a state (example:‘‘a is large’’). A secondary relation contains two argu-ments (example: ‘‘a is larger than b’’) such that for eacha, there is one b satisfying the relation. According to thisauthor, mastering of relational complexity in humanworking memory increases with development, up to andincluding the capacity for quarternary relations. Whenthe complexity of an operation exceeds that available tothe subject, complexity is reduced by segmentation andchunking operations. This classification of mental opera-tions may explain why certain items of the Tower ofHanoi, or high-level cognitive tasks like counterfactualreasoning and false belief tasks, are acquired particularlylate in development.

It is possible that autistic individuals are limited in thelevel of complexity of the operations they are able toperform in short-term memory (for a related interpret-ation, see Mottron, 1988). According to this hypothesis,they would perform the copy task by a sequence of binaryrelations between (a) one feature of the to-be-copiedfigure and (b) one feature they are currently copying,without relating this feature with other parts of the figure.By contrast, normal participants would copy a figure bya sequence of ternary relations, relating (a) one part of theto-be copied figure, (b) other parts of this figure, and (c)one feature of the copy, resulting in an ‘‘ impossibilityeffect ’’. The same interpretation could explain whyautistic individuals perform at a high level in block designand embedded figure tasks, but not in false-belief tasks,counterfactual reasoning, and the Tower of Hanoi.Indeed, these latter tasks involve ternary or quaternaryrelations. In support of this interpretation, a negativecorrelation exists between time necessary to find em-

754 L. MOTTRON et al.

bedded figures and the ability to solve mentalizingproblems (Jarrold, Jimenez, & Butler, 1998).

Conclusion

The present study provides empirical evidence thatindividuals with autism use visual information differentlyto controls in that they show a general tendency toward alocal bias on several types of tasks. This may be relatedeither to a deficient processing of the global properties ofobjects or to an impairment in the maintenance of severalrepresentations in spatial working memory. It appearsthat the second interpretation can more easily account forthe results generally reported in the literature. Furtherexperimentswill be necessary to disentangle the respectiverole of visual perception and working memory in theidiosyncratic processing of visual information shown byautistic individuals.

Acknowledgements—This work was supported by grantsfrom the Fonds de la Recherche en Sante! du Que!bec—CentreQue!becois de la Recherche Sociale, by two Chercheur-Boursiergrants from the Fonds de la Recherche en Sante! du Que!bec,awarded to LM and SB, and by a studentship from the FCARand Groupe de Recherche en Neuropsychologie Expe! rimen-tale awarded to EM. Special thanks to B. Ska for kindlyproviding the ‘‘ impossible ’’ figures, and to B. A. Bacon and J.Boseovski for editing the English version of the manuscript.

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Manuscript accepted 23 October 1998