perceiving human locomotion: priming effects in direction

Brain and Cognition 44, 192–213 (2000)

doi:10.1006/brcg.2000.1228, available online at http://www.idealibrary.com on

Perceiving Human Locomotion: Priming Effectsin Direction Discrimination

Karl Verfaillie

University of Leuven, Leuven, Belgium

Published online August 15, 2000

During the perception of biological motion, the available stimulus informationis confined to a small number of lights attached to the major joints of a movingactor. Despite this drastic impoverishment of the stimulus, the human visual appara-tus organizes the swarm of moving dots in a vivid percept of a human figure. Inaddition, observers effortlessly identify the action the figure is involved in. Aftera historical introduction and a short walk through the literature, data from a primingexperiment are presented. In a serial two-choice reaction-time task, participants werepresented with a point-light walker, facing either to the right or to the left andwalking either forward or backward on a treadmill. Subjects had to identify thedirection of articulatory movements. Reliable priming effects were established inconsecutive trials, but these effects were tempered by the relation between primingand primed walker. The reaction time to a walker was shorter when the walker inthe preceding trial moved in the same direction and was facing in the same direction.The findings are discussed in relation to recent data from neuropsychological casestudies, neuroimaging, and single-cell recording. 2000 Academic Press

Key Words: visual action identification; perception of biological motion; directiondiscrimination; priming.

The movements of biological creatures have captivated a lot of scientificinterest at least as early as the ancient Greeks, as evidenced by Aristotle’sDe Motu Animalium. The discovery of photographic techniques during theprevious century has given a strong impetus to the investigation of the loco-motion of living organisms. The well-known sequences of high-speed photo-

The writing of this article was supported by Concerted Research Effort Convention No.GOA 98/01, the Belgium Programme on Interuniversity Poles of Attraction Contract No. P4/19, and the Fund for Scientific Research of Flanders. I am grateful to Kim Waeytens for testingparticipants, to Anja Daems and Mathias Dekeyser for stimulating discussions on visual actionidentification, and to Raffaella Rumiati and two anonymous reviewers for useful commentson an earlier version of the text.

Address correspondence and reprint requests to Karl Verfaillie, Department of Psychology,University of Leuven, Tiensestraat 102, B-3000 Leuven, Belgium. E-mail: [email protected].

1920278-2626/00 $35.00Copyright 2000 by Academic PressAll rights of reproduction in any form reserved.

PERCEPTION OF BIOLOGICAL MOTION 193

FIG. 1. A photograph taken by Etienne-Jules Marey between 1886 and 1890 showingsuccessive snapshots of a pole-vaulting man (Archives du College de France).

graphs of moving humans and animals which Eadweard Muybridge madein the second half of the 19th century are exemplary of this development(published in 1887 in a monumental, 11-volume work entitled Animal Loco-motion; cf. Muybridge, 1955, for a selection of plates of human figures inmotion; also see Mather & West, 1993, on Muybridge’s pictures of animallocomotion).

In Muybridge’s work, each individual posture of the moving figure wasimprinted on a different plate by a different camera, which made it difficultto relate the figure’s successive positions to time in a precise manner. TheFrench doctor Etienne-Jules Marey (1894) used only one camera equippedwith one photographic plate. The shutter was opened periodically afterequally spaced temporal intervals, resulting in a discontinuous trajectory rep-resenting the successive spatial positions that the figure occupied at the mo-ments the shutter was open. Fig. 1 shows an example.

Marey’s technique had a serious drawback. When the velocity of the mov-ing object was relatively low and the surface which the object occupied onthe photographic plate was relatively large (as in the case of human locomo-tion), the successive snapshots of the object superimposed when the fre-quency of the shutter openings increased, which made the resulting photo-graph ‘‘confusing.’’ However, the number of snapshots per unit of time couldbe increased by artificially decreasing the spatial area that the object occupied

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FIG. 2. Georges Demeny, one of Marey’s collaborators, dressed in a black costume withwhite markers and lines attached to the head and the left limbs; the picture was taken in 1883(musee de Beaune).

on the plate. For the case of human locomotion, Marey adopted the followingtechnique: The actor was dressed in tight-fitting black clothing, small incan-descent bulbs (or other radiant marks) were attached to his/her major joints,and he/she was photographed in a darkened environment.1 Figure 2 depictsMarey’s point-light walker and Fig. 3 shows an example of the resultant

1 Marey credits Soret for being the first to have used this technique: ‘‘L. Soret a le premierrecouru a cette disposition. La nuit, dans une salle de theatre uniquement eclairee par quelqueslanternes rouges, il etudiait les mouvements de la Choregraphie en appliquant sur la tete etsur les pieds des danseuses de petites lampes a incandescence. Soret obtint de cette manieredes trajectoires fort curieuses ou les courbes decrites s’entrelacaient avec une elegante regu-larite’’ [L. Soret was the first to use this set-up. At night, in a theater hall illuminated withonly a few red lanterns, he studied choreographic movements by attaching small incandelescentlights to the head and the feet of the dancers. Soret obtained quite curious trajectories in whichthe curves were interwoven in a strange manner but with an elegant regularity] (Marey, 1894,p. 77).


FIG. 3. One of the first biological-motion photographs, taken by Etienne-Jules Marey in1883 (Archives du College de France).

photograph. Subsequent studies with this point-light technique for a longtime have mainly focused on the production of movement rather than on itsperception.

Gunnar Johansson (1973, 1975) was the first to use the point-light methodto reveal aspects of the perception of biological motions. Analogous to theearlier techniques, Johansson attached small light-emitting bulbs or light-reflecting patches to the bodies of moving human actors and filmed themunder such conditions that only the lights were perceptible for observingsubjects. Johansson discovered that static displays of the stimulus figure donot produce impressions of a human figure. Observers mostly perceive arandom collection of disconnected dots. When the lights are in motion, how-ever, the visual system organizes the swarm of moving dots in a vivid perceptof a human figure involved in recognizable activities.

Subsequent research has revealed the richness of the perceptual interpreta-tion. First, it has been shown that observers can recover several characteris-tics of the human figure engaged in motion. Observers to some degree canrecognize themselves and their friends on the basis of the point-light motions(Beardsworth & Buckner, 1981; Cutting & Kozlowski, 1977). Even whenthe moving stimulus figure is unfamiliar to the subject, biological motioncarries information about the figure’s gender (Barclay, Cutting, & Kozlow-ski, 1978; Cutting, Proffitt, & Kozlowski, 1978; Kozlowski & Cutting, 1977,1978; Mather & Murdoch, 1994; Runeson & Frykholm, 1983), personalitytraits such as his/her gender-schematic type (Frable, 1987), age (Montpare &Zebrowitz-McArthur, 1988), and emotion (Dittrich, Troscianko, Lea, &Morgan, 1996; Montpare, Goldstein, & Clausen, 1987; Walk & Homan,1984).

Second, apart from the figure in motion, biological motion also specifies

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characteristics of the action. First, Johansson (1973) demonstrated that ob-servers can easily identify different categories of everyday actions (also seeDittrich, 1993), such as walking, dancing, and painting. Second, Hoenkamp(1978) and Todd (1983) investigated observers’ ability to discriminate be-tween subtle differences within classes of related activities, such as the differ-ence between walking and running. Third, it has been shown that point-lightmotions specify detailed characteristics of a particular action such as theamount of force exerted on an object during the action of lifting, carrying,or throwing: On the basis of the point-light motions of the actor, viewersaccurately estimate the weight of the lifted object or the distance traveledby the (invisible) thrown object (Bingham, 1987; Runeson & Frykholm,1981, 1983).

The robustness of the ability to recover a human figure in action has beeninvestigated further by examining the effect of several manipulations: differ-ent types of simultaneous masks consisting of moving point lights (Berten-thal & Pinto, 1994; Cutting, Moore, & Morrison, 1988); the temporal delaybetween display frames (Thorton, Pinto, & Shiffrar, 1998); variations in thepolarity, spatial frequency, and disparity of the point lights (Ahlstrom,Blake, & Ahlstrom, 1997); the number of displayed points with each pointhaving a ‘‘limited lifetime’’ across consecutive frames (Neri, Morrone, &Burr, 1998); the location of the point lights on the body (Cutting, 1981); thenumber and type of displayed limbs (e.g., only showing two ipsilateral vscontralateral limbs; Pinto & Shiffrar, 1999); variations in the depth positionof the point-lights along the observer’s line of sight (Bulthoff, Bulthoff, &Sinha, 1998); or presenting a stick-figure version behind apertures (Shiffrar,Lichtey, & Heptulla-Chatterjee, 1997). Tasks include, among others, to de-termine the left–right orientation of the figure and to discriminate betweena canonical walker, on the one hand, and a spatially scrambled, a phase-shifted, an upside-down, or a ‘‘nonhuman’’ (in which the upper body of thefigure is facing in a direction opposite to the direction of the lower part ofthe body) version, on the other hand. Bertenthal (1993) provides a detailedoverview of relevant infant research.

Neuro(psycho)logical research is beginning to reveal the neural mecha-nisms underlying the perception of biological motion. First, brain-damagedpeople have been described whose performance on lower level motion tasksis very poor, but who are able to recover the action from a biological-motiondisplay. Zihl and colleagues (Rizzo, Nawrot, & Zihl, 1995; Zihl, von Cra-mon, & Mai, 1983; Zihl, von Cramon, Mai, & Schmid, 1991) described acerebral akinetopsic (‘‘motion-blind’’) patient with extensive bilateral extra-striate lesions (primarily in the dorsal pathway) including the suspected hu-man homolog of V5/MT (Marcar, Zihl, & Cowey, 1997). LM shows a pro-found deficit in several motion tasks. For instance, she cannot discriminatebetween shapes defined by kinetic boundaries in a moving random-dot dis-play. Surprisingly, however, LM is able to identify the action in a biological-


motion display (McLeod, Dittrich, Perrett, & Zihl, 1996). A similar patientwas first reported by Vaina, Lemay, Bienfang, Choi, and Nakayama (1990).AF has extensive damage to the dorsal occipito-parietal cortex sparing thetemporal lobe and is impaired in ‘‘low-level’’ motion tasks, while he showsnormal performance in the identification of actions shown under point-lightconditions.

Second, by means of positron emission tomography, Bonda, Petrides, Os-try, and Evans (1996) measured cerebral metabolic activity in human sub-jects observing biological motion. In comparison to a condition in whichparticipants saw goal-directed hand actions, the perception of whole-bodymotions under point-light conditions (a person dancing) resulted in increasedactivity in limbic structures such as the amygdaloid region (possibly relatedto the observer’s emotional reaction to the ‘‘social’’ stimulus of a dancingperson). In addition, cerebral blood flow increased in the rostrocaudal portionof the superior temporal sulcus and the adjacent temporal cortex, largelyconsistent with the regions that are spared in patients LM and AF.

Third, single-cell recording studies also reserve an important role for thesuperior temporal sulcus in the perception of biological motion. Indeed, fol-lowing observations first reported by Bruce, Desimone, and Gross (1981),Perrett and colleagues recorded neurons in the anterior upper bank of theSTS (STPa) of macaque monkeys, which responded selectively to visuallypresented body movements (Perrett et al., 1985; see Kendrick & Baldwin,1989, for similar findings in sheep). Oram and Perrett (1994) reported thatone-third of the body-selective cells were still responsive to bodies shownunder point-light conditions (also see Perrett, Harries, Benson, Chitty, &Mistlin, 1990). In the same area, cells have also been found that respondto the sight of faces (e.g., Oram & Perrett, 1992) or static human bodies(Wachsmuth, Oram, & Perrett, 1994).

The face and body neurons in STPa display an interesting pattern of gener-alization over versus selectivity for particular viewing conditions (Perrett etal., 1989, provide an overview). First, the cells show considerable generaliza-tion across changes in relatively low-level variables such as contrast andcolor. Second, the activity of the neurons is fairly independent of the stimulusobject’s position, both in the image plane and in depth, as indicated by theirlarge receptive fields and the observation that changes in size and viewingdistances have only small effects. This high tolerance for several stimulusvariations contrasts with a third characteristic of a majority of the STPa neu-rons: Most of the cells are highly specific for a particular depth orientationof the face or body. For instance, neurons have been discovered that aremaximally responsive to a walker facing to the right with respect to theobserving monkey, but whose activity to a walker facing to the left do notexceed spontaneous activity, whereas other cells show the opposite responsepattern. A minority of cells have been found that respond to all views of aparticular object, apparently exhibiting orientation-independent coding.

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The finding that the majority of the STPa neurons display an orientation-specific response pattern suggests that basic-level recognition of a face or abody is accomplished by accessing an orientation-dependent, but high-level(e.g., position-independent), object representation. Cells responsive to multi-ple views of a face or a body could then be viewed as providing viewpoint-independent descriptions of the stimulus object. Perrett et al. (1989) sug-gested that the flow of information from orientation-dependent to orientation-independent representations proceeds in a hierarchical fashion. In otherwords, the activity of the orientation-independent neurons could be the resultof combining the output of several orientation-sensitive neurons, each tunedto a particular view of the object. The finding that orientation-selective cellshave slightly shorter response latencies than viewpoint-independent cells(Perrett et al., 1991) supports this hypothesis.

Verfaillie (1993) collected converging behavioral evidence for the conjec-ture that recognition of a point-light walker is accomplished by accessingan orientation-dependent object representation. Participants were involvedin a serial two-choice reaction-time task. On each trial, a biological-motionconfiguration was presented and subjects had to decide as rapidly as possiblewhether the stimulus depicted a regular human walker or a ‘‘nonhuman’’version (in which the upper body of the figure was facing in a directionopposite to the direction of the lower part of the body; the point-lights corre-sponding to the head and the upper limbs specified a ‘‘normal’’ human upperbody facing to the right, and the point-lights of the lower limbs designateda ‘‘normal’’ human lower body facing to the left or vice versa; also seeMather, Radford, & West, 1992). All figures were upright and were presentedas moving on a treadmill. Both the human and the nonhuman walkers couldappear in one of two possible depth orientations: Half of them were facingto the right with respect to the viewer and half were facing to the left. Inaddition, half of the figures were moving forward and half of them weremoving backward. Even though the figures were not translating across thescreen and were moving on a rolling treadmill instead, the direction of move-ment could be specified by manipulating the so-called articulatory move-ments of the body parts. In the case of forward articulation, the body partsmoved in such a way that the treadmill seemed to move forward (i.e., thefigure would be ‘‘following his nose’’ if the figure was translating throughthe environment); in the case of backward articulation, the treadmill seemedto move in the opposite direction (i.e., the figure would be translating back-ward if there was no treadmill).

After the subject had completed the task, a data-sort program identifieddifferent types of transitions between trials and I examined to what degreethe RT to classify stimulus n as a human or nonhuman walker was influencedby the correspondence of stimulus n to the immediately preceding stimulusn 2 1. I confine myself here to the RT to human configurations that werepreceded by a trial that required the same ‘‘human’’ response. Relative to


FIG. 4. Mean facilitation in relation to the neutral baseline as a function of orientationand articulation correspondence between priming and primed walkers in the object-decisiontask used in Experiment 1 of Verfaillie (1993); bars indicate 95% confidence intervals. 5Ori,same orientation; ≠Ori, different orientation; 5Art, same articulation; ≠Art, different articula-tion.

transitions where the stimulus figure on trial n 2 1 (the priming figure orprime) and the stimulus figure on trial n (the primed figure or target) haddifferent depth orientations, there was a significant benefit when the primedconfiguration was preceded by a human walker in the same orientation. Infact, the RT to the target in the first type of transition did not differ fromthe RT to a human target preceded by a trial in which an ‘‘abstract’’ objectwas shown and participants were required to give the same ‘‘human’’ re-sponse to the object (cf. infra). Treating this last transition as the neutralbaseline, there appeared to be facilitatory priming with same-orientationpairs and neither facilitation nor inhibition when the priming and primedfigures had different orientations. The data for one of the experiments isreproduced in Fig. 4 (compare the two bars with ‘‘same orientation’’ withthe two bars with ‘‘different orientation’’).

In contrast, the manipulation of the direction of articulation (articulatingforward vs backward) had no effect. Participants were not faster to identifythe stimulus on trial n as a human walker when the figure was articulating

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in the same direction as the walker in the preceding trial n 2 1 than whenthe figures were articulating in opposite directions. Moreover, the benefit ofidentically oriented priming and primed figures was as large when they werewalking in the same direction as when they were walking in different direc-tions (the difference between the first two bars in Fig. 4 was not significant).Other experiments showed that the orientation-dependent priming effect wasalso not affected when priming and primed figures had different point-lightlocalizations on the body or when the portion of the step cycle shown in theprimed stimulus did not exactly match the portion of the step cycle shownin the priming stimulus.

On the one hand, the data converge with the neurophysiological findingsthat suggest that object recognition is accomplished by accessing high-level(e.g., independent of articulation direction), orientation-dependent represen-tations. On the other hand, the observation that the correspondence betweenthe articulation direction of the priming walker and the articulation directionof the primed walker had no effect at all may be somewhat surprising. Afterall, during the perception of biological motion, observers not only recovera human figure, but also identify the action the figure is involved in (and thedirection of articulation is a salient characteristic of the action of walking).

This paradox can probably be related to the fact that object and actionrecognition serve different computational goals. Because of the nonrigid na-ture of human bodies, there exists an infinity of possible three-dimensionalmanifestations of each body. The purpose of basic-level object recognition,on the one hand, is to categorize the stimulus object as a human body and,therefore, to make abstraction of the specific 3D arrangement of body parts.Irrespective of the 3D organization of body parts, the object is classified as ahuman body. Recognizing a body action, on the other hand, does not requireabstracting away from the spatial relations between the body parts. To thecontrary, action recognition boils down to categorizing a particular 3D mani-festation of a human body as a specific pose or a specific phase of a certainaction (e.g., Marr & Vaina, 1982).

Participants in Verfaillie’s (1993) study had to discriminate between ahuman and a nonhuman walker, a task that is more akin to object recognitionthan to action identification. The difference between a human and a nonhu-man walker has to do with the spatial structure of the objects, not with theparticular action they are involved in (participants were instructed to discrim-inate between two objects, irrespective of the way they were moving).2 From

2 Obviously, object and action recognition are not independent. On the one hand, the kindsof actions that are possible are limited by structural properties of the body itself. Shiffrar andFreyd (1990, 1993; also see Heptulla-Chatterjee, Freyd, & Shiffrar, 1996; Kourtzi & Shiffrar,1999) have shown that even the perception of actions is constrained by biomechanical proper-ties of the human body. In an apparent motion paradigm, participants saw an alternating se-quence of two body postures. For instance, in the first frame, a picture was shown of a personwith her right arm rotated about the elbow over a particular angle of rotation and the picture


this perspective, it is not surprising that the correspondence between primingand primed walker in terms of direction of articulation had no effect on thehuman–nonhuman decision.

The purpose of the experiment reported in the present article was to exam-ine priming in an action-decision task rather than in an object-decision task.To maximize the comparability with the experiments in Verfaillie (1993), Idecided to use the same human configurations as target stimuli in a serialtwo-choice reaction-time task. Under the assumption that human locomotionis an action with high biological significance for observers and that the direc-tion of walking is an important characteristic of human locomotion, the pres-ent experiment focuses on the discrimination between forward and backwardarticulation. On each trial, participants were presented with a human walkerfacing either to the right or to the left and articulating either forward orbackward. Subjects had to indicate as rapidly as possible whether the point-light figure was walking forward or backward. The stimuli were identical tothe human walkers in Verfaillie (1993); there were no nonhuman walkersin the present experiment. Moreover, in Verfaillie (1993), an object-decisiontask was used (the human walkers had to be classified as ‘‘human’’), whereasthe present study employed an action-decision task (the human walkers hadto be classified as articulating forward or backward).

If the absence of an effect of articulation correspondence between primingand primed walkers in Verfaillie (1993) indeed can be traced back to theuse of an object-decision task (in which abstraction is made of the relativemotion of the body parts), then an effect of articulation correspondence canbe predicted in the action-decision task (which hinges on an interpretation ofthe relative movements of the limbs). Priming effects have been documentedbefore in experiments in which participants were asked to identify the action,both when actions were shown under point-light conditions (Nilsson, Olofs-son, & Nyberg, 1992; Olofsson, Nyberg, & Nilsson, 1997) and with staticpictures of human actions (Daems & Verfaillie, 1999), but these experi-ments employed a long-term priming paradigm instead of the short-termpriming paradigm used in Verfaillie (1993). In addition, an effect of orienta-

in the second frame showed the same person with her right hand in a different orientationvis-a-vis the elbow. Care was taken that perception of apparent motion along the shortestpossible path between the two frames would violate an anatomical constraint (limited by theelbow joint). Under short-range apparent-motion conditions, participants reported perceivingthe shortest, but anatomically impossible, path. Under long-range apparent-motion conditions,subjects reported seeing the physically possible path, even though this perception violated theshortest-path constraint. This indicates that anatomical constraints influence the perception ofthe movements. On the other hand, in most cases it is possible to make a relatively clear-cutdistinction between object and action classification. The same kind of creature can be involvedin different actions and the same kind of action can be performed by different species. Forinstance, several circus acts consist of infrahuman animals imitating human actions (e.g., cy-cling dogs, bipedally walking elephants, and clapping chimps).

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tion correspondence would suggest that the visual system transcends the 2Dprojection of a particular 3D arrangement of body parts by accessing one ofseveral, linked orientation-dependent representations rather than one view-point-invariant model of the action (Daems & Verfaillie, 1999; Verfaillie,1992).

In sum, whereas only an effect of orientation correspondence in the ab-sence of an effect of articulcation correspondence was observed in the object-decision task of Verfaillie (1993), both orientation and articulation corre-spondence between priming and primed walkers are predicted in the action-decision task used in the present experiment.

EXPERIMENT

On each trial, subjects were presented with one of six possible movingstimulus configurations. Two-thirds of the trials depicted a human point-walker in a sagittal view: a right-facing, forward-articulating walker; aright-facing, backward-articulating walker; a left-facing, forward-articulat-ing walker; or a left-facing, backward-articulating walker. The figures werenot translating; instead, they appeared to move on a treadmill. Subjects hadto decide as rapidly as possible whether the figure was walking forward orbackward on the treadmill, by pressing one of two buttons. On one-third ofthe trials, one of two abstract-geometrical stimulus configurations, movingin a nonrigid way, was presented. Participants had to press the right buttonwhen they saw one of the abstract configurations and the left button whenthey identified the other abstract configuration.

After participants had completed the task, a data-sort program identifieddifferent types of transitions of two consecutive trials. The reaction time(RT) to the second (primed) stimulus of such a pair of trials was analyzedas a function of the figure’s correspondence to the preceding (priming) con-figuration. There were four possible transitions of two walkers: Both walkershad the same depth orientation and articulation direction, had the same depthorientation but articulated in opposite directions, moved in the same directionbut were facing in opposite directions, or had both different orientations anddifferent articulation directions. In addition, the two abstract-geometricalstimulus configurations functioned as neutral primes: The transitions inwhich a walker was preceded by such a neutral prime provided a baselinefor the other transitions.

Method

Subjects. Eight students of the Psychology Department of Leuven University participatedin the experiment. All had normal or corrected-to-normal vision. They were unaware of thefact that priming effects would be investigated.

Apparatus and stimuli. Stimuli were generated on an ETAP-ATRIS screen with a refreshrate of 75 Hz, noninterlaced, and a rapidly decaying P4 white phosphor. A dual-response panelwas connected to the PC and registered RT and type of answer.


FIG. 5. The stimulus configurations of the experiment; four different images taken fromfour different moments in the ‘‘step cycle’’ are shown for a human walker (A) and the ‘‘for-ward’’ (B) and ‘‘backward’’ (C) neutral configurations.

The biological-motion stimulus was a point-light version of a person walking in the fronto-parallel plane (i.e., walking in a direction orthogonal to the observer’s line of sight). The figurewas presented as walking on a treadmill in the center of the screen. The walker was generatedusing an adaptation of an algorithm described by Cutting (1978). The stimulus was composedof 11 dots corresponding to the head; the closest shoulder and hip; and the right and leftelbow, wrist, knee, and ankle. The most distant shoulder and hip were not shown becausethey were always occluded. The points on the visible hip and the remote limbs were occludedat the appropriate moments during the step cycle. The walking velocity was 533 ms per stepor 1.88 steps each second. The walker subtended a height of 4 cm (2° of visual angle) anda width of 1.5 cm (0.75°) during the double support phase (in which both legs are on theground). The four versions of a walker were assembled by factorially combining the figure’sorientation in relation to the viewer (right or left facing) and the direction of articulatorymovements (forward or backward). In backward-walking versions, the static displays constitut-ing forward walking were presented in reverse order. At the beginning of each trial, the walkerstarted moving in a randomly determined posture. Figure 5A shows a sample of static imagestaken out of a step cycle of a right-facing walker.

Figures 5B and C show some static views of the two abstract-geometrical stimulus configu-rations, which were used as neutral primes. Both stimuli were made up of five point-lights.

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The central point remained stationary. One point was rotating back and forth between 150°and 210° (with 0° at 12 o’clock) around the central point. The angular velocity was heldconstant at 113°/s (3° each two frames). Another point was rotating at the opposite side ofthe central point between 30° and 330°. Two additional dots were rotating back and fortharound the previous two moving points in a direction opposite to the rotation of the latterpoints. The difference between the two abstract configurations was based on the relation be-tween the two dots rotating around the central point. In one stimulus (Fig. 5B), both dotsrotated in the same direction at a particular moment (clockwise or counterclockwise). In theother stimulus (Fig. 5C), the dots rotated in opposite directions. The global figures had aboutthe same projected size as the biological-motion walkers. The time to rotate back and forthonce was equal to the time of one step cycle of the walker.

Procedure and design. Participants were tested individually, sitting at a distance of 114 cmfrom the display. Their head was fixed in a chin-and-forehead rest to keep the viewing distanceconstant. Stimuli were viewed binocularly. The room was completely darkened and stimuliwere presented on a black background.

Stimulus configurations appeared one at a time in a random order. Participants decidedwhether the figure was articulating forward or backward by pressing one of two buttons. Thefigure continued walking until the subject responded. The RT was the time between stimulusonset and the moment a response button was pressed down. At this point, the response–stimulus interval (RSI) of 400 ms was started and the screen was cleared. The subject receivedimmediate feedback by means of a 50-ms tone (a high-frequency tone if correct, a low-frequency tone if incorrect). Half of the subjects pressed the right button for forward walkingand the left button for backward walking. This mapping was reversed for the other half ofthe subjects. Moreover, all participants had to press the button corresponding to forward walk-ing when the first abstract stimulus (Fig. 5B) was displayed and the other button when thesecond abstract pattern (Fig. 5C) was shown. The transitions of a human walker preceded byan abstract pattern were treated as the neutral baselines (cf. infra). Note that the task onlystressed the decision to the current stimulus; no retention of previous trials was necessary.During instructions, both accuracy and speed were stressed.

The experiment involved six sessions, each planned on a separate day. A session consistedof six blocks of 140 trials. Therefore, each subject was administered a total of 5040 trials.The first two sessions, the first two blocks of each subsequent session, and the first five trialsof each block served as training and were not included in the analysis.

Results

For each subject, a cutoff value set at 3 standard deviations above themean RT of the correct responses was computed. Subsequent analyses onlyincluded correct RTs not exceeding the cutoff value (88% of the trials). Allresponses in a transition had to satisfy those conditions. For instance, onlytransitions where the response to both the priming and the primed figurewere correct and did not exceed the cutoff value were included in the analysisof priming effects.

First, the RT to each version of the human walker was examined indepen-dent of potential priming. The mean RT for the four human walkers wascomputed for each subject and was subjected to an analysis of variance(ANOVA) with in-depth orientation (right vs left facing) and articulationdirection (forward vs backward walking) as independent variables. Like inmost experiments in Verfaillie (1993), latencies to forward-walking figures(M 5 577 ms) were shorter than to backward-walking figures (M 5 596


ms), but the difference was not significant, F(1, 7) 5 2.54, MSe 5 1069.25,p . .15. For the main effect of orientation and for the interaction betweenorientation and articulation, F , 1.

Second, and more importantly, the influence of a priming trial on the RTin the immediately following primed trial was examined. There were sixdifferent transition types, defined by the relation between priming and primedconfiguration. The first four transition types always consisted of a successionof two walkers and were the result of combining two factors: Same or differ-ent orientation and same or different articulation direction in priming andprimed human figure. In addition, there were two neutral-priming transitiontypes. The first neutral-priming transition consisted of a response to a walkerpreceded by an abstract pattern that required the same response. This transi-tion functioned as baseline for the transitions in which participants made thesame response to priming and primed human figures (i.e., when both walkersarticulated in the same direction). The second neutral-priming transition con-sisted of a response to a walker preceded by an abstract pattern that requiredthe other response than the response to the primed walker. This transitionfunctioned as baseline for the transitions in which participants made a differ-ent response to the priming walker than to the primed human figure (i.e.,when priming and primed walkers articulated in opposite directions). Figure6 shows the difference between the RT to the primed walker in the appro-priate neutral baseline and the RT to the primed walker in the four experi-mental transitions, averaged across subjects. This difference is an estimateof the facilitatory or inhibitory priming; 95% confidence intervals are alsogiven. Inspection of the figure indicates the following. First, in comparisonto the neutral same-response baseline, there was a benefit of 62 ms whenthe figure had the same orientation and direction of articulation in primingand primed trial. Second, the facilitation in transitions in which the orienta-tion changed but the articulation direction of priming and primed walkermatched was smaller but still amounted to 18 ms. Third, even when primingand primed figures articulated in different directions but had the same depthorientation, there was still a facilitatory priming effect of 15 ms. Fourth, incomparison to the neutral baseline, there was even a very weak benefit ofrepeating a human figure with a different direction of articulation and depthorientation (M 5 8 ms).

An ANOVA on the difference between each subject’s neutral baselineRTs and the individual RTs as unit of analysis (because of an unequal numberof observations per cell, the general linear model approach to analysis ofvariance was adopted; Kirk, 1982) yielded a reliable main effect of articula-tion correspondence, F(1, 7) 5 9.41, MSe 5 314624.81, p , .02: The articu-lation direction of a figure that moved in the same direction as the figure inthe preceding trial was identified faster than when the figure in the precedingtrial articulated in the opposite direction (M 5 40 ms vs 12 ms). The maineffect of orientation correspondence was also significant, F(1, 7) 5 12.26,

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FIG. 6. Mean facilitation in relation to the appropriate neutral baseline as a function oforientation and articulation correspondence between priming and primed walkers in the actiondecision task; bars indicate 95% confidence intervals. 5Ori, same orientation; ≠Ori, differentorientation; 5Art, same articulation; ≠Art, different articulation.

MSe 5 191635.14, p , .01: There was more facilitatory priming when prim-ing and primed walkers had the same orientation than when they were facingin opposite directions (M 5 38 ms vs 13 ms). Most importantly, the signifi-cant interaction, F(1, 7) 5 29.32, MSe 5 45778.12, p , .001, indicates thatthe effect of articulation correspondence and the effect of orientation corre-spondence were not simply additive. In fact, substantial priming was ob-tained only when priming and primed walker matched both on articulationdirection and on depth orientation.

Discussion

First, in comparison to the neutral baseline, the identification of the direc-tion of articulation of a biological-motion walker was facilitated when thefigure in the preceding trial articulated in the same direction. Even when


priming and primed walker were facing in opposite directions, there was asmall benefit when they had the same direction of articulation. This effectof articulation correspondence diverges from the findings in Verfaillie (1993)and is probably related to the difference between recognizing the object andrecognizing the action in a biological-motion display. On the one hand, rec-ognizing a biological-motion walker as a human figure does not necessitatedetailed interpretation of the specific 3D arrangement of body parts, so thatarticulation correspondence between priming and primed walker does nothave an effect on priming in the object-decision task as used in Verfaillie(1993). On the other hand, deriving the direction of articulation, as an inher-ent characteristic of the action of walking, is crucially dependent on the cor-rect interpretation of the movements of the body parts relative to one another,resulting in a substantial priming effect of articulation correspondence in theaction-decision task. Note that the latter effect is not merely due to responsepriming because the facilitatory priming effect was defined as the benefit ofarticulation correspondence in comparison to a neutral-priming condition inwhich a biological-motion walker was preceded by an abstract-geometricalobject that required the same response.

Second, there was an effect of orientation correspondence. The identifica-tion of the articulation direction of a walker was facilitated when the figure inthe preceding trial had the same depth orientation. Even without articulationcorrespondence, orientation correspondence still produced a small facilita-tory effect.

Third and most importantly, the effect of articulation correspondence wasseriously tempered by the orientation correspondence between priming andprimed walker: Without orientation correspondence, articulation correspon-dence was much less beneficial. Under the working hypothesis that primingis based on a reactivation of the representations mediating target identifica-tion, the interaction between orientation correspondence and articulation cor-respondence suggests that the stored representations that afford the identifi-cation of the object as a human figure and the representations that mediate therecognition of the action performed by the human figure both are orientation-dependent. This conclusion is in agreement with the long-term priming ef-fects reported by Daems and Verfaillie (1999).

The finding that orientation-dependent representations play a prominentrole in the perception of biological motion converges with the observationsmade in a transsaccadic-integration paradigm (Verfaillie, 1997; Verfaillie &De Graef, in press; Verfaillie, De Troy, & Van Rensbergen, 1994). On eachtrial, we monitored participants’ eye movements while they were looking ata point-light walker. At a specific moment in the trial, subjects made a sac-cade to a new location within the same walker. During the saccade, a displaychange occurred in a visual attribute of the human figure on some of thetrials and subjects had to indicate whether they noticed that there had beena change. Because the input of information is suppressed during saccades,

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the transient that normally accompanies such a display change was not per-ceptible and detection that something was changed could only be based onthe integration of presaccadic and postsaccadic information. The underlyingrationale was that, if a particular object attribute is inherent to the objectrepresentation that is carried across a saccade, then saccade-contingent trans-formations of the attribute should be as easy to detect as when the changeoccurs during steady fixation. We observed that saccade-contingent displace-ments of the walker were hard to detect, indicating that an object’s positionis only maintained coarsely across saccades. In contrast, an intrasaccadicdepth rotation of the walker around her or his top–bottom axis (so that inthe postsaccadic fixation the figure was facing in a slightly different directionthan in the presaccadic fixation) were readily noticed, suggesting that trans-saccadic memory codes a biological-motion walker in a particular in-depthorientation. Finally, Verfaillie et al. (1994) observed that transsaccadic mem-ory for the posture was fairly accurate in that participants were relativelygood at predicting the postsaccadic posture of the walker (the posture thatthe figure naturally reaches when she or he continues walking during theobserver’s saccade). The latter finding indicates that a relatively rich repre-sentation of the action of walking survives a saccade.

As noted in the Introduction, Perrett et al. (1989) proposed a hierachicalscheme in which orientation-dependent stored representations of objects areaccessed (although abstraction is made of other stimulus variations, like im-age-plane position and size) prior to the activation of representations that aremore independent of the observer’s vantage point. Perrett et al. put forward asimilar scheme for action recognition, e.g., for identifying a particular actionas ‘‘walking forward.’’ Recognizing the latter action would be cruciallydependent upon the construction of a viewer-dependent description of‘‘walking forward’’ (e.g., a leftward facing body walking leftward). Underthis scheme, it is not surprising that the majority of the neurons selectivefor whole-body movements such as walking (identified by Oram & Perrett,1992) were sensitive both to the direction of movement and the depth ori-entation of the moving body. This converges with the priming effects ofboth articulation and orientation correspondence revealed in the present ex-periment. (Maybe the very small benefit observed in transitions withoutorientation and articulation correspondence reflects the activation of a view-point-independent representation of the human body activated later in thehierarchical scheme.)

In summary, our behavioral data are largely consistent with the findingsfrom single-cell recording (Oram & Perrett, 1992). Together with the conclu-sions reached on the basis of neuroimaging experiments (Bonda et al., 1996)and neuropsychological case studies (McLeod et al., 1996; Vaina et al.,1990), this indicates that the superior temporal sulcus plays a prominent rolein the computation and representation of viewpoint-dependent models for


purposes of visual recognition.3 Given the fact that this area receives inputboth from the dorsal ‘‘where’’ system and from the ventral ‘‘what’’ system,the perception of biological motion presents itself as an interesting test casefor studying the integration of ‘‘where’’ and ‘‘what’’ information. In addi-tion, future research should not only focus on interactions between differentperceptual processing streams, but could also further unravel interactionsbetween the perception of actions and the production of actions (not onlythe actual execution of motor actions, but also the imagination of movements,cf. Decety, 1996; also see Parsons et al., 1995). Indeed, it has been suggestedthat the processes underlying the perception of human actions are biologi-cally complementary to the production of actions (Bertenthal & Pinto, 1993;also see Reed & Farah, 1995).

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