ORIGINAL ARTICLE

The effect of discordant sensory information in graphic production:two distinct subject groups

Received: 23 November 2000 / Accepted: 13 November 2002 / Published online: 19 February 2003� Springer-Verlag 2003

Abstract The present study investigated the underlyingprocesses used to cope with discordant sensory infor-mation induced in a mirror-drawing task. Two experi-ments were carried out in which adults copied simplegeometrical figures made up of either horizontal andvertical segments or oblique segments meeting at a rightangle in both a normal and a mirror condition. Exper-iment 1 identified individual differences in relation topreferred graphic movement directions; some subjectspreserved the visual directions that occurred in normaldrawing by reversing the direction of drawing move-ments (perceived-direction group), while others pre-served normal drawing directions that producedreversed visual directions (performed-direction group).Experiment 2 was performed to elucidate whether thesetwo distinct behaviors resulted from different strategiesused to cope with visuo-proprioceptive discordances.The main results showed that preference for the per-ceived directions led to longer pauses, slower movementvelocity, greater movement dysfluency, and greaterspatial orientation accuracy. By contrast, longer reac-tion time and greater angular accuracy characterizedperformance in the performed-direction group. Theseresults were interpreted as indicating that two distinctinformation-processing strategies can be used when re-solving sensory discordance in graphic production.

Keywords Mirror drawing Æ Feedback control ÆFeedforward control Æ Visuo-proprioceptive conflicts

Introduction

Are visual information and proprioceptive informationboth used in graphic production? Van Doorn and Keuss(Van Doorn, 1992; Van Doorn & Keuss, 1992, 1993)suggested that subjects are able to compensate for theabsence of visual information in handwriting by an in-creased contribution of proprioceptive processing, asrevealed by the increase in axial pen force, movementtime, and writing size. Similarly, other experiments haveshown that subjects compensate for the absence ofproprioceptive cues through an increased contributionof visual processing. Teasdale, Forget, Bard, Paillard,Fleury, and Lamarre (1993) revealed that when visionwas available in a graphic task, a deafferented patientand control subjects performed similarly, while withoutvision, the spatial control of movements was morestrongly impaired in the patient than in the controls.Thus, the movements of the hand within the limits of thegraphic space depend on afferent information providedby the visual as well as by proprioceptive systems. Thisraises the question of the respective role of each systemin drawing movement production.

Some authors have reported that visual and pro-prioceptive information may control different aspects ofmovement execution. For example, Meulenbroek andThomassen (1991) suggested that two independent spa-tial reference systems were involved in straight-lineproduction. One system, designated as the anatomicalreference system, corresponded to the anatomicalstructure of the effectors (the hand or the fingers), andresulted in the preferential production of oblique lines.The second system, called the geometrical referencesystem, resulted in a bias towards horizontal and verticalorientations. Meulenbroek and Thomassen (1991) havesuggested that oblique orientations are strong candi-dates for predominantly motoric control with a reducedsensitivity to the availability of visual information, whilehorizontals and verticals are more likely to be underperceptual control, with a strong dependency on visual

Psychological Research (2003) 67: 291–302DOI 10.1007/s00426-002-0129-y

Lucette Gullaud-Toussaint Æ Annie Vinter

L. Gullaud-Toussaint (&)Laboratoire d’Analyse de la Performance Motrice Humaine,MSHS, 99 Avenue du Recteur Pineau, 86000 Poitiers, FranceE-mail: Lucette.Toussaint@mshs.univ-poitiers.fr

A. VinterLaboratoire d’Etude des Apprentissages et du Developpement,CNRS/UMR 5022, Dijon, France

information. The authors demonstrated that the preva-lence of visual control depended on the type of segmentsto be drawn in a situation where visual and proprio-ceptive modalities were available.

Other studies revealed that the role played by visualfeedback on graphic accuracy concerns the topokineticcomponent of movements (i.e., the spatial orientation),while the morphokinetic component (i.e., the shapeproduction) seems to be preserved whatever the visualexposure conditions (Broderick & Laszlo, 1987; Paillard,1991; Van Doorn & Keuss, 1993). In a previous exper-iment devoted to the study of sensory information insimple drawing production (geometrical figures com-posed of two segments meeting at a right angle), Gul-laud (1998) confirmed that the withdrawal of visualinformation did not modify the angular accuracy ofdrawings (morphokinetic component), although visionwas necessary for calibration of the displacement of thepen (line orientations or topokinetic component) withinthe limits of the graphic space in the case of modelscomposed of horizontals and verticals. With regard tooblique models, the author noted that, in the absence ofvision, compensatory strategies were sufficient to ensurethe spatial orientation accuracy of the drawings. Thesestrategies were probably based on an increase in pro-prioceptive control, as suggested by the increasedmovement time and writing size (see also Van Doorn,1992).

These overall results suggest that visual feedbackcontrols specific aspects of graphic movement execution.Vision seems to play a dominant role in calibrating thetopokinetic component of drawing, especially when thefigures are composed of horizontal and vertical segments(geometrical reference system, Meulenbroek & Tho-massen, 1991; Gullaud, 1998). The role of propriocep-tion in graphic movement execution is less known owingto methodological difficulties in isolating proprioceptiveinformation. In order to elucidate the role of proprio-ception, the use of a discordant sensory informationparadigm appears appropriate, because the resolution ofvisuo-proprioceptive conflicts implies the dominance ofeither the visual or the proprioceptive modality (Uhlarik& Canon, 1971; Redding & Wallace, 1988a, 1988b).

Lajoie, Paillard, Teasdale, Bard, Fleury, Forget andLamarre (1992) provided an illustration of visual infor-mation dominance when drawing under a mirror con-dition. Using a six-pointed star, these authors observedthat a deafferented patient had no problem in copyingthe pattern, while controls encountered certain difficul-ties in producing fast and accurate movements. Theauthors suggested that the greater difficulty encounteredby controls in the production of obliques as opposed tovertical segments revealed the dominance of vision, be-cause resolving the conflict implied a multiple directionalchoice for obliques (combination of up, down, left, andright directions), whereas it implied a binary choice forverticals (reversal of the top-down and down-top direc-tions). Before being able to reach a level of perfor-mance similar to the deafferented patient, the control

participants had to learn how to use reversed visualfeedback by recalibrating their proprioceptive map.

Gullaud and Vinter (1996, 1998) examined the mir-ror-drawing production of simple open geometrical fig-ures, which were visually less constraining than theclosed figures used in the Lajoie et al. experiment (1992).They were interested by the syntactical level, which de-scribes graphic behavior in terms of a system of rules(Van Sommers, 1984), i.e., starting rules (start at thetopmost and/or leftmost point), progression rules (drawfrom top to bottom and/or from left to right), and athreading rule (draw without pen-lifts). These rulesgovern drawings and result from an interaction betweenbiomechanical and visual constraints (Thomassen, Tib-osch & Maarse, 1989; Vinter & Meulenbroek, 1993). Forinstance, going from left to right induces less biome-chanical constraints (flexion movements) and optimalvisual control. In some situations, subjects have tochoose between biomechanical facilities and visuallyguided movement, as was the case in Gullaud and Vin-ter’s experiments (1996, 1998), where the mirror-drawingcondition induced left to right and right to left reversals.The directional analysis of graphic movement showedthat two subject groups could be distinguished. Somesubjects seemed to favor vision (the perceived-directiongroup): they visually preserved the perceived movementdirections as they occurred in the normal condition,which thus induced a reversal of the directions drawn onthe sheet of paper. In contrast, other subjects seemed tofavor biomechanical constraints (the performed-directiongroup): they tended to preserve the directions drawn onthe sheet of paper as they did in the normal condition,this strategy provoking a reversal of the directions per-ceived in the mirror.

Thus, the current literature suggests that visualdominance may vary as a function of the geometricalfigure to be drawn (horizontal-vertical versus obliquefigures) as well as of the type of subjects (perceived-versus performed-direction group). Using a directionalanalysis, Experiment 1 aimed to show that two groups ofsubjects effectively emerge in a mirror-drawing task. TheGullaud and Vinter experiment (1996) included too fewsubjects in each group to provide substantial informa-tion. A more systematic study performed with a largersample of participants was needed. As previously ob-served, we expected that most of the subjects would vi-sually preserve the movement directions and movementsequences of mirror-drawing production as they do un-der the normal condition (perceived-direction group),while directions and movement sequences should bepreserved and not be strongly dependent on the presenceof the mirror for the other subjects (performed-directiongroup). The analysis of errors in mirror productionseemed appropriate in the present experiment. The greatdifficulties encountered by the perceived-direction groupwhen drawing oblique segments in the mirror conditioncould reveal the dominance of vision (Lajoie, Paillard,Teasdale, Bard, Fleury, Forget & Lamarre , 1992), whilethe performed-direction group should encounter greater

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difficulties in the production of horizontal and verticalsegments, visual control being dominant for these typesof segment (Meulenbroek & Thomassen, 1991; Gullaud& Vinter, 1998).

Experiment 1

Method

Subjects

Thirty-two right-handed adults (mean age 23.9 years, range 18–28years) who were students at the University of Bourgogne partici-pated in the experiment, for which they received course credits.They were unaware of the aims of the study. All of them sponta-neously used their right hand to write their name on a sheet ofpaper, were familiar with a language using only left-to-righthandwriting and did not have visual or motor problems.

Stimuli

Eight models composed of two segments meeting at a right anglewere used in this experiment. Half of the models, called ‘‘hvstructure,’’ were composed of the four possible combinations ofhorizontal and vertical line segments (e.g., the four corners of asquare shape), whereas the other half, called ‘‘ob structure,’’ werecomposed of the four possible combinations of oblique line seg-ments (e.g., the four corners of a diamond shape). The line segmentlength was 1 cm for all models. Each model was printed in blackink on a 6·6 cm white card, and models were presented one at atime. Subjects were instructed to copy each model one at a time andas accurately as possible. The starting points, movement directions,and movement sequences were free and pen-lifts were allowed be-tween two segments.

Procedure

The subjects participated in two successive conditions within asingle experimental session. In the normal condition, they were ableto look normally at their drawing performance. In the mirrorcondition, they performed the experimental task looking at themodel, their hand, and the trace through a mirror. The mirror waslocated at the left of the sheet of paper and was placed perpen-dicular to the graphic workplane. As illustrated in Fig. 1, a maskprevented the subjects from having a direct view of the model andof their drawing movements.

The subjects began the task by copying four random series ofthe eight models in the normal condition. In a second phase and inorder to familiarize them with the difficulties involved in the mirrortask, they copied 12 one-segment models (four horizontals, fourverticals, and four obliques) in the mirror condition. Afterfamiliarization, they drew the eight models presented in a randomorder four times under the mirror-drawing condition. Because oneof the 32 subjects was unable to draw in the mirror condition de-spite numerous trials, 1,984 drawings (31 subjects · 2 conditions · 4trials · 8 models) were retained for the analysis.

Data analysis

Each drawing was recorded by means of a videotape. For eachmodel, we coded the direction and the sequence of the drawingmovements. The mean frequencies of occurrence of verticals [tothe top (90�) and to the bottom (270�)], horizontals [to the right(0�) and to the left (180�)] and obliques [from bottom left to topright (45�), from top left to bottom right (315�), from top right

to bottom left (225�) and from bottom right to top left (135�)]were thus computed. In the coding of movement directions,minor angular deviations were not considered. We were mainlyinterested in coding whether a vertical was drawn, for instance,in the 90� or in the 270� direction. For convenience, we will termperformed movements the movements realized on the sheet ofpaper, and perceived movements those visible in the mirror. Asillustrated in Fig. 2, the performed and perceived drawingmovements are conflicting in the mirror-drawing condition. Ex-ceptions are for the vertical directions (90�, 270�). Because thesame information is given by an analysis of either the performedor the perceived movement directions, we restricted the analysisto the frequencies of occurrence of performed movement direc-tions only.

The tape recording allowed us to pick up and compute thenumber of errors in movement direction. Errors were character-ized by an entire movement or by a ‘‘micro–movement’’ (mini-mum length of 1 mm) performed in the wrong direction ororientation when compared with the model. In the latter case, adeviation of 22.5� on both sides of each standard segment (i.e., 0�,45�, 90�, 135�, 180�, 225�, 270�, and 315�) was defined, and anerror was coded when the segment orientation exceeded this de-viation.

Fig. 1 Illustration of the experimental set-up used in the mirror-drawing task. An opaque box was positioned over the paper sheet,which prevented the subject from having a direct view of the model,his writing hand, and the pen. An opening at the left of the opaquebox allowed the subject to visually control his/her movement froma mirror located at the left (8 cm) of the sheet. The small rectangleat the far end of the mirror is the place where the models appeared.A videotape (experiment 1) and an xy digitizer connected to acomputer (experiment 2) were used to record the drawingproductions

Fig. 2 Illustration of discordances between ‘‘performed’’ and‘‘perceived’’ movement directions

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Results

Movement directions

The main goal of the first experiment was to checkwhether two groups of subjects could be identified in themirror condition on the basis of the reversal or nonre-versal of performed movement directions, as observed ina previous study (Gullaud & Vinter, 1996). Conse-quently, for each subject, the frequency of occurrencesof each direction (0�, 45�, 90�, 135�, 180�, 225�, 270�,and 315�) produced on the sheet of paper (performeddirections) when drawing in the mirror condition wascomputed. The data were collected in a 31 (number of

subjects) by 8 (number of directions) frequency table.This table was composed of 248 cells, including thefrequency of occurrences of each direction for eachsubject. It was analyzed using a correspondence analysis(a type of principal component analysis using frequencytables as data; for details on the procedure and analysis,see Greenacre, 1984; Benzecri, 1973). This procedureillustrates the presence of different groups of subjects asa function of the movement directions they mainly usedin the mirror condition. The results are presented inFig. 3.

The first factorial axis explained 47.99% of the vari-ance. As expected, this axis divided subjects into twogroups. The first group included 16 subjects who pre-dominantly produced left-oriented segment directions(135�, 180�, and 225�). In contrast, the second groupincluded the 15 remaining subjects who predominantlyproduced right-oriented segment directions (0�, 45�, and315�). The first and second groups were termed theperceived-direction and performed-direction groupsrespectively. Subjects in the perceived-direction groupmight have favored left-oriented performed movementdirections because the resulting perceived directionsagreed with those realized in the normal condition. Bycontrast, subjects in the performed-direction groupmight have favored left-oriented perceived movementdirections because the resulting performed directionsagreed with those realized in the normal condition. Thisinterpretation was tested in the following part by anexamination of the frequency of performed directions.

Figure 4 illustrates the mean frequencies of occur-rence for performed directions produced by each group.ANOVAs were carried out with Group (perceived di-rection versus performed direction) as the between-subjects factor, and with Condition (normal versusmirror) and Block of trials (four blocks) as the within-subjects factors. Significant interactions were investi-gated by means of Newman-Keuls post hoc tests.

All subjects performed left-oriented movements morefrequently in the mirror than in the normal condition[horizontal, F(1, 29)=86.86, p<.001; oblique 45�–225�,F(1, 29)=33.62, p<.001; oblique 135�–315�, F(1,29)=198.42, p<.001]. A significant interaction between

Fig. 3 Simultaneous representation of subjects and graphic move-ment directions as a function of the first and second factorial axis.Numbers 1–31 indicate the individual subjects. D0, D45, D90,D135, D180, D225, D270, and D315 correspond to movementdirections performed to 0�, 45�, 90�, 135�, 180�, 225�, 270�, and315�, respectively

Fig. 4 Polar representation ofthe frequency of performedmovement directions as afunction of Condition (normalvs mirror) and Group(perceived-direction vsperformed-direction)

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Group and Condition for horizontals [F(1, 29)=10.08,p<.005], top left to bottom right obliques [F(1,29)=41.84, p<.001] and top right to bottom left obliques[F(1, 29)=10.87, p<.005] showed that the left-orientedmovement directions were more frequent in the per-ceived-direction group than in the performed-directiongroup when drawing in the mirror. Whatever the segmentorientation, no other significant effect was observed, andin particular there was no effect of Block of trials (p>.09).

Movement sequencing

For each model and condition, the dominant movementsequences observed in the perceived-direction and

performed-direction groups were established. The resultsare presented in Fig. 5. The standard sequence corre-sponds to the usual movement sequence produced in thenormal condition. Because no differences were observedbetween the groups for each model (p>.22), the fre-quencies of occurrence of the dominant sequences in thenormal condition were averaged for all subjects (secondcolumn), thereby simplifying graphic presentation. Theperceived and performed sequences correspond tomovements as they were seen in the mirror or realized onthe sheet of paper respectively. Note that whatever themodels, the dominant movement sequences were pro-duced without pen-lifts. Figure 5 shows that the modelscomposed of horizontals and verticals only (models a, b,

Fig. 5 Illustration of thedominant movement sequencesrealized by perceived-directionvs. performed-direction groupsfor each model in normal(standard sequence) and mirrorconditions (performed andperceived sequences). For agiven model, dotted drawingshowed that performed orperceived sequences neveroccurred in the normalcondition, while boldcontinuous drawing showedthat the standard sequenceswere preserved in mirrorproduction

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c, d) were reversed in the mirror, because of the mirrortransformations induced on the left-right and right-lefthorizontal directions.

Figure 5 reveals that, for both groups, the standardmovement sequences were preserved in the mirrorcondition for the models c, d, g, and h [F(1, 29)=1.77,p>.19]. By contrast, performance varied as a functionof Condition and Group for models a, b, e, and f [F(1,29)>16.02, p<.001]. For each of these four models,subjects in the perceived-direction group visually pre-served the sequences used in the normal condition forthe same model (indicated by bold continuous drawingon Fig. 5). But because of the mirror transformation,the resulting performed sequences were new in com-parison to the normal condition (indicated by dotteddrawing on Fig. 5). Subjects in the performed-directiongroup preserved the performed sequences as they did inthe normal condition for the same model (models a, b,e, and f). Their movement sequence choices did notseem to depend on what they saw in the mirror, theperceived sequences never being preserved (as illus-trated by dotted drawings).

Number of errors

No error was recorded in the normal condition. Themean number of errors (i.e., entire movement and mi-cro-movement performed in the wrong direction ororientation) made in the mirror-drawing task was com-puted. An ANOVA was carried out with Group as thebetween-subjects factor, and Structure and Block oftrials as the within-subjects factors. Whatever theGroup, the mean number of errors was greater for ob-lique models (1.44) than for horizontal-vertical ones(0.87) [F(1, 29)=24.45, p<.001], and decreased withpractice (block 1=1.8, block 2=1.1, block 3=1.0, block4=0.7) [F(3, 87)=16.69, p<.001]. Contrary to ourexpectations, no effect of Group was observed (p>.12).However, differences between groups appeared withregard to on-line movement corrections: the micro-movements were more frequently observed in theperceived-direction group (57%) than in the performed-direction group [39.7%; v2(2)=9.23, p<.005].

Discussion

Experiment 1 showed that two distinct subject groupswere observed when participants had to cope withvisuoproprioceptive discordance in a mirror-drawingtask. Some subjects preserved the visual directions andsequences that occurred in normal drawing by reversingthe direction of drawing movements (perceived-direction group), while others preserved normal draw-ing directions and sequences, which produced reversedvisual directions (performed-direction group). Theanalysis of movement directions and sequences suggeststhat vision could be the principal determinant for the

perceived-direction group when resolving the visuo-proprioceptive conflicts, to the detriment of biome-chanical constraints. An additional argument supportingthe idea of the visual guidance of graphic movements inthe mirror-drawing condition for this group was pro-vided by the occurrence of micro-movements for seg-ments produced in the wrong direction when comparedwith the model. By contrast, subjects in the performed-direction group produced significantly less on-linemovement corrections, the drawing of the requestedmodel being reversed in consequence. These lattersubjects noted that their drawing did not match themodel only when it was entirely completed.

Contrary to our expectations, a higher number oferrors (in the mirror condition) for oblique modelswhen compared with horizontal-vertical models wasobserved in both groups. Recall that, in accordancewith Lajoie and co-workers’ hypothesis (1992), thegreater difficulties observed for obliques could suggestthe dominance of visual information when resolvingdiscordances. From a visual point of view, obliquesare effectively the most complex segments to drawbecause the conflict entails a multiple choice, whereasfor horizontals resolving the conflict entails a binarychoice. The aforementioned result of the present studysuggests the dominance of visual information whenresolving the conflict, whatever the movement direc-tion favored by the subjects. Moreover, the distinctionbetween the geometrical and the anatomical systems ofreference reported by Meulenbroek and Thomassen(1991) in straight line production was not confirmedwhen simple geometrical figures were drawn through amirror. It may be that the complexity induced by theconflicting task leads subjects to favor vision, the‘‘Queen of the senses,’’ whatever the line-segmentorientation (horizontal-vertical or oblique).

Taken together, these results suggest that althoughvisual information was important regardless of themovement directions favored by the subjects in themirror condition, it may have played a different role foreach group. As suggested by movement directions andon-line corrections, performance of subjects in the per-ceived-direction group could have been under visualguidance in the mirror drawing task. By contrast, theimportant number of directional errors of entire move-ments produced in the mirror condition for the per-formed-direction group shows that these subjects usedvisual information before and later during movementproduction, probably to check whether their graphicproduction and the models were congruent. It is possiblethat biomechanical constraints are stronger in the lattergroup, which could explain the preservation of per-formed movement directions and the necessity for thesubjects to plan the resolution of visuoproprioceptivediscordance before drawing production. This hypothesiswas investigated in the second experiment, by analyzingspatial accuracy and temporal features of drawingmovements.

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Experiment 2

The second experiment investigated the underlyingprocesses used by the perceived-direction and per-formed-direction groups when solving visuoproprio-ceptive conflicts. As already mentioned, the availabilityof visual information during movement execution isdeterminant with regard to the topokinetic componentof graphic behavior, whereas the morphokinetic com-ponent seems to be preserved whatever the visualexposure conditions (Paillard, 1991; Gullaud, 1998). Wethus expected that the importance of on-line visualcontrol in the perceived-direction group would bemanifested through a greater accuracy of spatial orien-tation than in the performed-direction group.

In the present experiment, the analysis of temporalfeatures of drawing production, including movementprogramming and movement execution, was consideredof prime importance in understanding how both groupsproceeded to solve visuoproprioceptive conflict. Did thesubjects solve the sensory discordance before or duringgraphic production, illustrating a programming-basedstrategy or a feedback-based strategy respectively? Notjust the latency phase (reaction time) but also later partsof the response (movement time and duration of pausesbetween two segments) needed to be analyzed to answerthis question. We expected that the greater importanceof on-line visual corrections for the perceived-directiongroup would be reflected in longer movement time, nu-merous velocity peaks, and longer duration of pauses,the last-mentioned being necessary in order to recali-brate the movement in another direction. By contrast, ifthe preservation of performed movement directions hassome basic determinants grounded in the biomechanicalsystem (performed-direction group), this should implythe elaboration of a feedforward plan taking into ac-count the mirror transformations before movement ini-tiation, in order to respect the syntactical rules.

In a previous experiment, Plamondon and Clement(1991) showed that movement directions affect move-ment time without disturbing performance accuracy.They reported that more time was needed to produceright to left than left to right graphic progressions.Consequently, the location of the starting point and themovement direction were imposed on participants inExperiment 2, in order to explore how changes inmovement directions affected graphic performance in themirror-drawing task for both groups. The reader isreminded that each group behaves differently with regardto the directions most often produced on the paper sheetwhen mirror drawing (Gullaud & Vinter, 1996), withright to left directions in the perceived-direction groupand left to right directions in the performed-directiongroup. For each model, participants were thus forced touse their preferred movement sequence on the papersheet (i.e., from the left in the perceived-direction group,and from the right in the performed-direction group) andtheir nonpreferred sequence (i.e., from the right in the

perceived-direction group, and from the left in the per-formed-direction group).

Method

Subjects

Eighteen of the 31 subjects who participated in the first experimentwere selected on the basis of the frequency of movement directionsfavored in the mirror-drawing condition. This frequency wascomputed using models a, b, e, and f (see Experiment 1, Fig. 5), forwhich subjects behaved differently and preserved either a per-formed or a perceived movement sequence. Ten subjects in theperceived-direction group of Experiment 1 were kept in Experiment2 because in more than 62.5% of cases (mean 84.8%) they adoptedthe same perceived movement sequence as in the normal condition.The eight remaining subjects were from the performed-directiongroup of Experiment 1. They preserved a performed movementsequence in more than 62.5% of the cases (mean 75%).

Stimuli and apparatus

As mentioned previously, four models of Experiment 1 (models a,b, e, and f) were retained in the second experiment. Two modelswere composed of horizontal and vertical line segments (‘‘hv’’structure), and the two remaining models were made up of obliqueline segments (‘‘ob’’ structure).

The task was performed on an xy digitizer (Calcomp Draw-ingboard) connected to an IBM-PC computer. The xy coordinateswere sampled at a frequency of 100 Hz, with a spatial accuracy of0.2 mm (for technical details, see Teulings & Maarse, 1984).

Procedure

The experimental conditions were similar to those used in the firstexperiment: a normal and a mirror condition. Each drawing trialstarted with a high-frequency acoustic signal, which indicated tothe subjects that they had to begin their drawing production. Whenthey had finished, an interactive recording procedure allowed theexperimenter to stop the process of data acquisition.

As illustrated in Fig. 6, two opposite movement sequences wereused in this experiment and the starting point aswell as themovementdirections were imposed for each model by arrows. The term pre-ferred sequence indicates that subjects copied the model using asequence that they spontaneously produced in the mirror task(Experiment 1), the reverse being true for the nonpreferred sequence.

Because two distinct behaviors (reversal vs nonreversal of per-formed sequences) were observed in the mirror condition as afunction of the Group, the preferred sequence was the visuallyperceived one in the perceived-direction group, while it corre-sponded to the performed sequence in the performed-directiongroup. It should be remembered that, as shown in Experiment 1, allthe subjects behaved similarly in the normal condition.

In this experiment, 64 models were drawn by subjects (4 models· 2 sequences · 4 trials · 2 conditions). The total number of re-corded movement sequences was 1,152. All models were presentedin a random order in each condition.

Data analysis

Each drawing was identified and smoothed with a second-orderfilter with a transition band between 8 and 20 Hz. The filtereddata were displayed alongside tangential velocity profiles. Aninteractive analysis program1 was used to extract different tem-poral variables for each segment. The temporal variables were

1The recording apparatus (interface and pen), acquisition and dataanalysis programs were made by the Nijmegen Institute for Cog-nition and Information (NICI).

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the reaction time (ms) and the duration of pauses (ms) betweenthe two segments of each model, the tangential velocity (cm/s),and the number of velocity peaks for each segment (each peakbeing due to successive acceleration and deceleration phases indrawing movements). Two measures of drawing accuracy werecomputed: the spatial orientation of the drawing in the work-space and the size of the angles (in degrees). For both of thesedependent variables, the root mean square error (RMSE) wasanalyzed, because it is considered the best overall measure ofperformance accuracy (Henry 1975; Schmidt 1988). The othervariables were the mean number of errors, as computed in thefirst experiment (the total number of entire movements and‘‘micro-movements’’ performed in the wrong direction or orien-tation when compared with the model) and the frequency ofmicro-movements only (on-line movement corrections).

The dependent variables were analyzed by means of sepa-rate ANOVAs with Group (perceived direction vs performeddirection) as the between-subjects factor, and Condition (normalvs mirror), Structure of models (hv vs ob), and Sequence (pre-ferred vs nonpreferred) as within-subjects factors. Newman-Keuls post hoc tests were used to investigate the significantinteractions.

Results

Spatial accuracy of drawing productions

As presented in Fig. 7, most of the errors were madewhen subjects used a nonpreferred sequence and whenthey drew oblique models [F(1, 16)=6.83 and 19.72,p<.001, respectively]. However, the occurrence of errorstended to be greater for oblique models than for hor-izontals and verticals when subjects in the performed-direction group drew in a nonpreferred direction [Group· Sequence · Structure, F(1, 16)=3.81, p=.066].

A qualitative analysis of errors confirmed theimportance of visual guidance of mirror-drawingmovements for the perceived-direction group. Micro-movements were more frequent in the perceived-direc-tion group (42.37%) than in the performed-directiongroup (7.5%) [v2(2)=17.12, p<.001]. These resultsreproduced those of Experiment 1.

The subsequent analyses included the drawings pro-duced without errors, that is 1,053 drawings out of the1,152 collected. The RMSE relative to the spatial orien-tation of models in the workspace is displayed in Fig. 8A.On average, the RMSE was higher in the mirror

Fig. 6 Illustration of the models used in experiment 2 and of thepreferred versus nonpreferred sequence for each model. These twosequences were determined as a function of subjects’ performancespontaneously realized in the first experiment. Arrows indicated thestarting point and the movement directions for each model

Fig. 7 Mean number of errors produced in the mirror-drawingcondition as a function of Group (perceived-direction vs per-formed-direction), Structure (hv vs ob) and Sequence (preferred vsnonpreferred)

Fig. 8A, B RMSE of the spatial orientation of models (A) and ofangular accuracy (B) as a function of Group (perceived-direction vsperformed-direction), Condition (normal vs mirror), and Structure(hv vs ob)

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condition [F(1, 16)=28.45, p<.001], for oblique models[F(1, 16)=31.68, p<.001], and in the performed-direc-tion group [F(1, 16)=5.48, p<.05]. The differencebetween groups mainly resulted from mirror-drawingproductions [Group · Condition, F(1, 16)=4.17,p=.058]. Both groups performed differently as a functionof Structure [Group · Structure, F(1, 16)=6.54, p<.025].The difference between hv and ob structure in theperformed-direction group was only tendential(Newman-Keuls, p=.066) and it disappeared in themirror condition, while oblique models were significantlyless accurate than horizontal-vertical models in bothnormal and mirror conditions in the perceived-directiongroup (p<.001). No other significant effect was observed,and in particular there was no effect of Sequence (F<1).

Figure 8B depicts the RMSE of angular accuracy. Nomain effect of Group was revealed by ANOVAs (F<1).For all subjects, the angular error was higher in themirror condition [F(1, 16)=16.59, p<.001] and for ob-lique models [F(1, 16)=16.94, p<.001], but it was notmodified by Sequence (F<1). The RMSE varied as afunction of Condition and Structure [F(1, 16)=4.38,p=.053], and between Group, Condition, and Structure[F(1, 16)=4.29, p<.05]. The effect of Structure appearedto be similar in the normal and mirror conditions in theperceived-direction group (p>.26). By contrast, subjects

in the performed-direction group had difficulty indrawing horizontal-vertical models in the mirror condi-tion when compared to performances in the normalcondition [F(1, 16)=15.26, p<.001]. No other sig-nificant effect appeared.

Analysis of temporal variables

Mean reaction times are displayed in Fig. 9A. ANOVArevealed that they were longer in the mirror than in thenormal condition [F(1, 16)=37.83, p<.001], and for anonpreferred sequence than for a preferred one[F(1, 16)=9.81, p<.005]. However, the marginally sig-nificant interaction between Condition and Sequence[F(1, 16)=3.98, p=.056] showed that the increase inreaction times when using a nonpreferred sequencemainly occurred in the mirror-drawing condition. Dif-ferences appeared between the perceived-direction andthe performed-direction group in the mirror task only[F(1, 16)=4.48, p<.05], the increase in reaction timesbeing significantly more important in the performed-direction group. No effect of Structure [F(1, 16)=1.06,p=.32] and no other interaction were revealed by theANOVAs ( p>.33).

The mean duration of pauses, illustrated in Fig. 9B,also increased in the mirror condition for all subjects[F(1, 16)=16.83, p<.001], but it increased significantlymore in the perceived-direction group than in the per-formed-direction group [F(1, 16)=4.58, p<.05]. When anonpreferred sequence was imposed, an increase in theduration of pauses was observed [F(1, 16)=4.46,

Fig. 9A–D Reaction times (A), duration of pauses (B), tangentialvelocity (C), and mean number of velocity peaks (D) as a functionof Group (perceived-direction vs performed-direction), Condition(normal vs mirror), and Sequence (preferred vs nonpreferred)

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p<.05], especially in the mirror condition and for theperceived-direction group [Group · Condition · Se-quence, F(1, 16)=3.98, p=.06]. No other significanteffect appeared.

As shown in Fig. 9C, slower movements were ob-served in the mirror-drawing condition [F(1, 16)=15.60,p<.001], for a nonpreferred sequence [F(1, 16)=4.60,p<.05], and in the perceived-direction group [F(1,16)=5.90, p<.05]. In the mirror task, velocities de-creased significantly more in the perceived-directiongroup than in the performed-direction group [F(1,16)=4.22, p<.05], especially for a preferred sequence[Group · Condition · Sequence, F(1, 16)=8.33, p<.01].For all subjects, the mean tangential velocity was higherfor oblique models (3.9 cm/s) than for horizontal-verti-cal models (3.6 cm/s) [F(1, 16)=14.88, p<.001].

Finally, the analysis of the mean number of velocitypeaks (Fig. 9D) revealed that movements were lessfluent in the mirror condition [F(1, 16)=27.86, p<.001]and for oblique models [hv structure =2.4, ob struc-ture =3.0; F(1, 16)=11.85, p<.005]. Movements ten-ded to be more dysfluent in the mirror condition forthe perceived-direction group than for the performed-direction one [F(1, 16)=3.93, p=.064]. Moreover, thesignificant interaction between Group, Condition, andSequence [F(1, 16)=6.27, p<.025] showed that theincrease in movement dysfluency in the mirror condi-tion for the perceived-direction group was greater forthe preferred sequence, while this was true for thenonpreferred sequence in the performed-directiongroup. No other significant effect was revealed by theANOVAs (p>.23).

Discussion

Experiment 2 confirmed that subjects’ drawing behaviordiffered as a function of their group (perceived-directionversus performed-direction) with regard to temporalparameters and performance accuracy. While no differ-ence between groups appeared in the normal condition,the subjects from each group reacted differently whenresolving the visuoproprioceptive discordance. Theoccurrence of micro-movements, the longer duration ofpauses, the slower movement velocity, and the greatermovement dysfluency show that the perceived-directiongroup encountered more difficulties during movementexecution in the mirror-drawing task. By contrast, themajor changes in reaction times observed in the per-formed-direction group show that much of the conflictwas resolved during movement planning.

In addition to these temporal differences, the groupsdiffered with regard to spatial accuracy. Subjects inthe performed-direction group were less accurate inreproducing model orientation than those in the per-ceived-direction group, and they did not exhibit anymodifications of the morphokinetic component (angularaccuracy), especially when drawing the oblique models.In accordance with the literature showing that on-line

visual feedback plays a major role in spatial orientationaccuracy (Broderick & Laszlo, 1987; Gullaud, 1998;Paillard, 1991; Van Doorn & Keuss, 1993), the presentexperiments confirm that the perceived-direction groupmakes greater use of visual information than the per-formed-direction group in graphicmovement production.

Differences between the production of oblique mod-els and horizontal-vertical models were expected fromthe literature. In straight line production (Meulenbroek& Thomassen, 1991) and in simple geometrical drawingtasks performed in nonconflicting conditions (Gullaud,1998; Gullaud & Vinter, 1998), it was shown that thevisual system was dominant for horizontal and verticalline segments, while motoric control was prevalent forobliques. Consequently, we expected that horizontal-vertical drawing production would be easier for theperceived-direction group, and the drawing of obliqueseasier for the performed-direction group. However, inthe present experiment no significant Group · Structureof models interaction was revealed by the ANOVAs (onerrors and temporal variables). For both groups, errorswere more frequent for obliques than for horizontalsand verticals, and movements were less fluent for theformer than for the latter. These results suggest thatvisual control predominates over motoric control for allsubjects, whatever the directions they spontaneously usewhen drawing through a mirror.

Movement sequences (preferred vs nonpreferred)were imposed on participants in order to explore howchanges in movement direction affected the graphicperformance of perceived-direction and performed-direction groups. As expected (Plamondon & Clement,1991), graphic accuracy (i.e., spatial orientation and sizeof angle) did not change as a function of left to right orright to left movement progressions in either group,while changes in movement execution parameters (i.e.,tangential velocity and number of velocity peaks) weremainly observed in the perceived-direction group forpreferred and nonpreferred sequences (Newman-Keuls,p>.05). Thus, difficulties encountered in terms ofmovement execution in the mirror task by the perceived-direction group cannot be explained by the productionof movements from right to left. Increases in reactiontimes in both groups and in the duration of pauses in theperceived-direction group were observed when a non-preferred sequence was used in the mirror condition.These results suggest that when subjects were forced touse a sequence that they did not spontaneously producein the mirror, they encountered more difficulties in theplanning of movement direction.

General discussion

Both experiments confirm that two different informa-tion-processing strategies appear when adults arerequired to draw in a conflicting task. With regard tomovement sequence, some of them favor the visuallyperceived sequence, while others favor the performed

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sequence. The importance of visual guidance in theperceived-direction group is revealed by the analysis ofthe spatial accuracy of drawing production, especiallywith regard to spatial orientation of movements in thegraphic workspace. Moreover, the temporal data showthat the perceived-direction group encounters moredifficulties during movement execution in the mirror-drawing task. These findings suggest that subjects inthe perceived-direction group rely mainly on a visualfeedback control of movement when they have toresolve visuoproprioceptive discordances. By contrast,visual guidance does not seem to play a major role inthe performed-direction group. The changes in reactiontimes observed in this latter group show that much ofthe conflict is resolved during motor programming.Subjects in this group would rely mainly on a feed-forward control of movements when drawing in themirror.

Such results seem in accordance with prism adap-tation studies in which some authors have reported thattwo motor control strategies can occur (Redding &Wallace, 1996). In this case, when both starting andtarget locations were simultaneously visible, the relativedifference between locations were used to initiate afeedforward movement plan (i.e., feedforward controlstrategy). When starting locations were not visible, thedifferences between target location and effector loca-tion, when available, were directly used for feedbackcorrections (i.e., feedback control strategy). However,whereas the nature of strategic control appeared to betask-specific in prism exposure, the present experimentson mirror drawing suggest a more subject-dependentapproach to account for the type of strategies used tosolve visuoproprioceptive conflicts.

These results call for further investigations, andespecially for developmental experiments. Research inmotor development has revealed age-related differencesin the use of feedforward and feedback processes (Bard,Hay & Fleury, 1985; Hay, 1979, 1984; Olivier, Audiffren& Ripoll, 1998), with a critical period in development ofmovement control at around 7–8 years. The changes inspatial accuracy and timing in movements were inter-preted as a changing dominance of pro- (5-year-olds)and retroactive control (7-year-olds), followed by amixed control in older children (11-year-olds). Investi-gation of mirror-drawing production in children seemsinteresting to observe how changes in feedforward andfeedback processes can affect performance in the case ofvisuoproprioceptive conflicts. Before the age of 6 years,most children should belong to the performed-directiongroup because of the dominant role of feedforwardprocesses, while at 7–8 years most children should be-have like the perceived-direction group because of thedominant role of feedback processes. Moreover, wemight expect to observe a differential dichotomy in olderchildren, as was the case in adults. Wondering whyadults react differently when resolving visuoproprio-ceptive discordance also opens up a fruitful avenue ofresearch.

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