Perception & Psychophysics 2000, 62 (5), 889-899

Orientation specificity in biological

MARINA PAVLOVA and ALEXANDER SOKOLOV University ofTiibingen, Tiibingen, Germany and Institute ofPsychology, Russian Academy ofSciences, Moscow, Russia

We addressed the issue of how display orientation affects the perception of biological motion. In Ex­ periment 1, spontaneous recognition of a point-light walker improved abruptly with image-plane dis­ play rotation from inverted to upright orientation. Withina range of orientations from 180°to 90°,it was dramatically impeded. Using ROC analysis, we showed (Experiments 2 and 3) that despite prior fa­ miliarization with a point-light figure at all orientations, its detectability within a mask decreased with a change in orientation from upright to a range of 90°-180°. In Experiment 4, a priming effect in bio­ logical motion was observed only if a prime corresponded to a range of deviations from upright orien­ tation within which the display was spontaneously recognizable. The findings indicate that display ori­ entation nonmonotonically affects the perception of biological motion. Moreover,top-down influence on the perception of biological motion is limited by display orientation.

The ability to correctly identify an event is ofgreat eco­ mans and animals depicted in animated cartoons (Mitkin logical importance for an organism, because it supports & Pavlova, 1990), despite the highly reduced and unusual functional behavior in a continuously changing environ­ structural information available. By 3-5 months of age, ment. Since the classic work ofGestalt , there infants can discriminate a computer-simulated point­ has been phenomenological evidence for veridical per­ light walker from a similar display in which local rigid­ ception of simple events depicted only by rigid motions ity between dots is perturbed (Bertenthal, Proffitt, & of several points. As will be shown later, the visual sys­ Kramer, 1987) or from displays ofidentical absolute mo­ tem is highly sensitive to information about the invariant tions with scrambled spatial relations between the moving structure in complex everyday events, such as biological points (Bertenthal, Proffitt, & Cutting, 1984). Even cats motion patterns that consist ofa set ofmoving dots on the (Blake, 1993) and bottlenose dolphins (Herman, Morel­ main joints ofan invisible walker. Samuels, & Pack, 1990) are able to perceive point-light Despite the potential perceptual ambiguity, humans displays. readily extract the invariant structure from biological Humans recognize biological motion despite the distor­ motion. In his initial study, Gunnar Johansson (1973, tions caused by reverse transformation (showing the film 1976) showed that adults need only 0.1-0.2 sec to iden­ backwards) or by changing the presentation rate to about tify displays with filmed biological motion. Mather and 0.5 times faster or slower than normal (Pavlova, 1992, West (1993) extended these findings to perception ofan­ 1995). Ahlstrom, Blake, and Ahlstrom (1997) reported imated point-light figures of quadrupeds. Preschoolers that observers can discriminate between canonical and 3-4 years of age were able to recognize point-light hu- phase-scrambled point-light figures, and that they tolerate variations in dot contrast and . Much research has established that with upright orien­ Parts ofthe study were presented at the 12th-14th Annual Meetings tation ofa target, observers can detect a point-light figure of the International Society for Psychophysics and at the I9th-2 Ist embedded in a simultaneous moving-dot mask (Berten­ European Conferences on . M.P. was supported by the thal & Pinto, 1994; Cutting, Moore, & Morrison, 1988; Deutsche Forschungsgemeinschaft (436RUS 17/14/97)and by the Max­ Planck-Gesellschaft. Parts of the paper were done at the Institut fur Mather, Radford, & West, 1992; Neri, Morrone, & Burr, Arbeitsphysiologie, Dortmund, the Institut fiir Psychologie, Christian­ 1998; Thornton, Pinto, & Shiffrar, 1998). Yet,when a dis­ Albrechts-Universitat zu Kiel, and the MPI for Biological Cybernetics, play was presented for less than 0.8 sec, the ability to de­ Tiibingen. We thank Niels Birbaumer, Isabelle and Heinrich Biilthoff, termine the apparent direction (facing) ofa camouflaged C. Richard Cavonius, Walter H. Ehrenstein, Dieter Heyer, Laurence walker fell to chance level (Cutting et aL, 1988). Maloney, Rainer Mausfeld, and Sverker Runeson for stimulating dis­ cussions and Bennett Bertenthal for providing us with the walker-gen­ On the other hand, it has recently been shown in a num­ erating program. We especially thank John C. Baird for valuable advice ber ofpsychophysical and perceptual studies that display on an earlier version ofthe manuscript and Arseny Sokolov for moral inversion dramatically impedes the perception ofbiolog­ support. We also thank two anonymous reviewers for helpful comments. ical motion. Upside-down presentation prevents infants Correspondence concerning this article should be addressed to M, Pav­ lova, Institute of Medical Psychology and Behavioral Neurobiologyl from discriminating a point-light walker from similar MEG-Center, University of Tiibingen, Gartenstr, 29, D-72074 Tiibin­ displays (Bertenthal et aL, 1984; Bertenthal et aL, 1987; gen, Germany (e-mail: [email protected]). Bertenthal, Proffitt, Spetner, & Thomas, 1985). Adults

889 Copyright 2000 Psychonomic Society, Inc. 890 PAVLOVAAND SOKOLOV and 3- to 6-year-olds failed in the recognition ofinverted (Proffitt et aI., 1984). Even infants can discriminate be­ animated cartoons (Mitkin & Pavlova, 1990; Pavlova, tween a computer-simulated canonical walker and a figure 1989). When presented through multiple apertures at without occlusion (Bertenthal et aI., 1985). On the other orientations of 0°, 90°, and 180°, only an upright-ori­ hand, the lack ofocclusion in filmed biological motion ented walker was reliably identified (Shiffrar, Lichtey, & is ineffective for its perception (Runeson, 1994), appar­ Heptulla Chatterjee, 1997). Under observation through a ently because ofredundant stimulus information in nat­ Dove prism, even performance in such a relatively easy ural displays. task as discrimination between texture-defined canonical Dynamic constraints that reflect sensitivity to a match and phase-scrambled walkers was degraded (Ahlstrom between event kinematics (configuration oftrajectories et aI., 1997). Sumi (1984) reported that inversion ofthe and velocities) and dynamics (mass- and force-related original Johansson films led to an impression of unnat­ information) may also limit nonveridical interpretations ural movements even when observers were aware of the of biological motion. Sensitivity to dynamic properties manipulation of a display or when responses were re­ revealed from kinematics was demonstrated when esti­ stricted to categories ofhuman/nonhuman motion. Previ­ mating the relative weight of moving balls in collisions ous experience with upside-down displays affected their (Flynn, 1994) or the weight ofan object lifted by a point­ recognition very little. Furthermore, inversion ofpoint­ light person (Runeson & Frykholm, 1983). Earlier work light displays disrupted the ability to judge basic emotions indirectly favors the role ofdynamic constraints in event represented in dance (Dittrich, Troscianko, Lea, & Mor­ perception. Observers were able to distinguish humans gan, 1996), types ofhuman actions (Dittrich, 1993), and in action from animated point-light puppets (Johansson, the gender of a walker (Barclay, Cutting, & Kozlowski, 1976). Gender discrimination in filmed displays signifi­ 1978). cantly decreased with a change in presentation rate (three Taken together, these findings raise the issue of the times slower than normal), mainly because ofdistortions nature oforientation specificity in the perception of'bio­ in the perception ofthe gravitational force (Barclay et aI., logical motion. The pattern of intrastimulus kinematics 1978; see also Mather et aI., 1992). Bingham, Schmidt, remains the same regardless ofdisplay orientation. Thus, and Rosenblum (1995) reported that among three view­ contrary to the assumptions ofthe computational structure­ ing conditions (upright display and observer, upright dis­ from-motion models (Hoffman & Flinchbaugh, 1982; play and inverted observer, and inverted display and up­ Sugie & Kato, 1987; Webb & Aggarwal, 1982), extracting right observer), the last one produced the most errors in structure from biological motion does not depend only on the identification of simple point-light events (such as the relative motions ofits elements defined by local rigid­ falling leaves or a pendulum). They suggested that event ity or by an elongated axis of symmetry. Obviously, the recognition depends on absolute display orientation in the perceptual system implements additional constraints for gravitational field, rather than on the relative orientation the unambiguous interpretation ofbiological motion. Two of the display and the observer. Much less is known, types of such constraints can be distinguished: (1) eco­ however, about how the perception of biological motion logical constraints that represent sensitivity ofthe visual depends on such constraints. More specifically, when the system to regularities in the environment (e.g., occlusion or natural orientation ofan event is changed, does the dis­ gravity) and (2) knowledge-based or internal constraints, crepancy between perceived kinematics and dynamics in­ which can be defined as a prior knowledge about the fluence the perception ofbiological motion? outer world. Ecological constraints appear to affect primar­ The present set offour experiments, in which we used ily bottom-up processing ofa point-light display, whereas an image-plane rotation of the display, was designed to knowledge-based constraints reflect a top-down influence systematically study how display orientation affects the on biological motion. Although much research has im­ perception ofbiological motion. Experiment I was aimed plicitly assumed that the processing constraints in the at examining how spontaneous recognition of a point­ perception ofcomplex events are closely interconnected light walker varies with a gradual change in orientation. and hierarchically nested, only a few data are available on In Experiments 2 and 3, a masking paradigm was used to their interrelation. For example, a priori information about investigate how display orientation affects recovery ofa display orientation was found to be insufficient for reliable known point-light figure. The essential difference ofthe recognition ofan inverted walker (Pavlova, 1993). task from spontaneous recognition is that it deals primar­ We suppose that ecological constraints playa promi­ ily with top-down processing of biological motion. Ex­ nent role in event perception. Actually, displays with periment 4 was conducted to establish whether and, ifso, filmed biological motion are much less ambiguous than how a priming effect in the perception ofbiological mo­ computer simulations. Computer-generated point-light tion depends on prime orientation. displays usually lose the higher harmonic motions ofthe minorjoints ofthe foot or natural gait asymmetry (Prof­ EXPERIMENT 1 fitt, Bertenthal, & Roberts, 1984). Among other param­ Spontaneous Recognition ofa Rotated Walker eters, occlusion between moving dots during a walking cycle significantly improves recognition ofa synthesized The primary goals ofthis experiment were to examine point-light walker and reduces its perceptual instability (I) how spontaneous recognition varies with a change in ORIENTATION AND BIOLOGICAL MOTION 891 orientation and (2) whether a range ofdeviations from up­ cated a highly significant effect ofdisplay orientation on right exists within which a display is spontaneously rec­ perceptual instability [F(6,9) = 7.l54,p < .01]. Display ognizable. orientation also affects the number ofdisplay interpreta­ tions [F(6,9) = 7.759,p < .01]. However, these measures Method ofperformance varied independently with a change in ori­ SUbjects. Twenty paid subjects (18-25 years of age, with an entation [F(6,9) = 4.308,p < .05; Figure IB]. This indi­ equal number of males and females) with normal or corrected-to­ cates that the effect oforientation on instability was caused normal vision participated. None had previous experience with by perceptual switching from one interpretation to the point-light displays. They were run individually. other, rather than simply by holding a number ofdifferent Stimuli and Apparatus. A computer-generated biological mo­ tion display consisted ofan array of II dots on the head and main interpretations. joints of an invisible walker. The simulation program was created Inspection of Figure 2 indicates that spontaneous using an algorithm described by Cutting (1978). A walking figure walker recognition improved abruptly with changing ori­ was seen moving as if on a treadmill, facing right. A gait cycle was entation. Each data point ofthis figure corresponds to the accomplished in 40 frames, with a frame duration of36 msec. This percentage of subjects who, at least once, interpreted a resulted in a walking speed of about 42 complete cycles per minute, display at a particular orientation as a walking figure. De­ which corresponded to a normal walking speed that ranged from 30 to 70 cycles per minute (Inman, Ralston, & Todd, 1981). With the spite the relatively long viewing period, at orientations of upright orientation, a walker subtended a visual angle of 4.0 0 in 150°, 120°, and 90°, the pattern was very rarely perceived height and 2.80 in width at the most extended point ofa gait cycle. as a walker. Instead, the display was described in a variety To avoid a framing effect, the point-light figure was displayed on a of ways, ranging from "swinging ofdots back and forth" circular-screen monitor 18 em in diameter. The monitor was me­ to "rotation of a stick or a hand." In a couple of cases, chanically rotated in the image plane in order to stabilize the spa­ when presented with a 90° orientation, it was interpreted tial resolution of the stimuli across changes in orientation. An ob­ server sat in a dimly illuminated room at a distance of90 ern from as a very strange swimmer or drummer. the screen. His or her head was fixed in a head-and-chin rest. When the display was presented at 60° and 30° orien­ Procedure. Two independent groups of subjects were presented tations, most observers reported seeing the walking fig­ with the point-light display, which was rotated in 30 0 steps in the ure. However, with the 60° orientation, the mean response image plane. In both groups, orientation was varied from 1800 to time (RT) to a first impression of a walker was about OO-for one of them, clockwise, and for the other, counterclockwise. 0 19 sec, which is much longer than that usually reported Rotation from upright to a 180 orientation was not used, because for the perception ofupright filmed or synthesized point­ a pilot study showed a carry-over effect: Once the display had been recognized as a walker at a particular orientation, this interpreta­ light displays (Johansson, 1976; Mitkin & Pavlova, 1990; tion became dominant for the subsequent orientations. For each ori­ Perrett, Harries, Benson, Chitty, & Mistlin, 1990). As was entation, the viewing period was 60 sec. The observers were asked expected from the pilot study, the results showed a car­ to press a key whenever one interpretation changed to another (e.g., ryover effect regarding walker recognition: Once the dis­ when a bouncing or a swinging ofthe moving dots was seen in tum). play was recognized as a walker at a particular orientation, The notion of instability was clarified by demonstrating two well­ this interpretation became dominant or was preserved for known examples of ambiguous static pictures: Rubin's face-vase picture and the Necker cube, which are usually bistable. The ob­ the ensuing orientations. servers were told that the displays might have one, two, or even more than two interpretations. Number of keypresses was used as a Discussion measure for perceptual instability. Proffitt et al. (1984) used a sim­ The results ofExperiment I show that with changing ilar task in their study ofhow the occlusion of dots in a point-light orientation from inverted to upright, spontaneous recog­ display affects its perception. Decisive reasons for using such a pro­ nition ofa point-light walker improves abruptly. Orien­ cedure in the present study were twofold. First, it avoids making the tations from inverted to 90° dramatically impeded spon­ task one of explicit identification. Second, because ofthe potential ambiguity of a point-light display, perceptual instability by itself taneous recognition despite the much longer viewing appears to be an informative characteristic. After each presentation, period than that usually needed for veridical perception of the subjects were asked to briefly describe what they had seen and to upright displays (Johansson, 1976; Pavlova, 1992; Shiffrar indicate how many different interpretations ofthe pattern they had. eta!., 1997). However, starting from 60° and 30° orientations, most Results observers reported seeing a point-light walker. These Figure 1 shows that the mean number of keypresses findings suggest that the power ofdynamic constraints is increased as orientation varied from inverted to 90° and graded, or in other words, their effectiveness in event then decreased almost entirely to upright orientation. The recognition is limited. When a display is rotated in the difference between the two groups of subjects was not image plane, in contrast with filmed down- or uphill significant [F(1,9) = 0.467, P > .05]. In both groups, movement ofa real walker, there is some mismatch be­ perceptual instability as a function ofdisplay orientation tween perceived kinematics and dynamics. However, can be approximated by a parabolic curve: By using the within a limited range ofdeviations from upright orienta­ method of orthogonal polynomials, significant linear tion, such a mismatch does not appear to substantially im­ (p < .05) and quadratic (p < .05) trends were found. As pair bottom-up processing ofbiological motion. can be seen in Figure 1, the peak ofinstability occurred at These results are consistent with the findings reported 90° orientation. An analysis of variance (ANOVA) indi- by Pavlova (1992). Reverse transformation (showing the 892 PAVLOVA AND SOKOLOV

A right-oriented displays presented either as usual or in re­ .. verse transformation. Recognition ofa display was mainly . .', determined by the apparent direction ofcommon motion ofa point-light quadruped. For example, the display that was recognized as a cat moving to the right with tail lifted EJ0 was interpreted as a giraffe moving to the left when the 180 film was shown backward. These findings suggest that 150 0 1500 in interpreting biological motion, the perceptual system may neglect a mismatch of kinematics and dynamics, presumably in favor ofmore ecologically or functionally 0 120 meaningful factors. Other measures ofperformance, how­ ever, indicated that observers are sensitive to this mis­ match. For example, RT to a point-light display presented backward was systematically longer (Pavlova, 1992). Likewise, in the present experiment, with orientations of 30° and 60°, the display was perceived as unstable. Given the strong carry-over effect, which was observed across a change in orientation, the perceptual instability with these orientations appears to be fairly remarkable. One might ar­ gue that this effect was simply induced by a self-instruction to perceive a display as unstable or by experience with preceding orientations with which instability was high. Yet, our data contradict these arguments. When the dis­ play was finally presented upright, the perceptual insta­ bility was close to zero (see Proffitt et aI., 1984). . '':. .. EXPERIMENT 2 [J Detection ofa Point-Light Walker Within a Mask B ~ Key-presses -o-Interpretations In Experiment 2, a masking paradigm was used to ex­ amine how orientation affects recovery ofa camouflaged a: 6 point-light walker. Because a simultaneous mask is sup­ w posed to impair bottom-up processing ofbiological mo­ aI 5 tion (see, e.g., Bertenthal & Pinto, 1994; Thornton et aI., :e 4 1998), in order for a camouflaged figure to be detected, ;::) observers must be familiar with it. The essential differ­ 3 z ence ofthe task from spontaneous recognition is that it z 2 deals primarily with top-down processing of biological II( motion, although bottom-up and top-down processes w 1 :e usually are closely interconnected and probably cannot o be completely isolated. 180 150 120 90 60 30 0 Cutting et al. (1988) reported that one of the most ef­ fective masks to camouflage a walker's facing is com­ o R lEN TAT ION (deg) posed ofspatially scrambled dots on the joints ofa walker. Figure 1. The results of Experiment 1. (A) Perceptual instabil­ Their pilot study showed that even such masks as these ity (the mean number of keypresses during the viewing period) as were almost ineffective in camouflaging an upright fig­ a function of display orientation. The solid line represents the ure. Bertenthal and Pinto (1994) found, however, that ob­ data for one group of subjects, the dashed line for the other. Each servers failed to detect a camouflaged walking figure and experimental point is based on data from 10 subjects. (B) Per­ to judge its facing when the display was presented upside ceptual instability (closed circles) versus the number of different interpretations (open circles). Vertical bars show ±SE. Each ex­ down. In this experiment, we examined sensitivity to a perimental point is based on data from 20 subjects. camouflaged walker, which was rotated in the image plane over a range between upright and inverted displays.

Method film backward) was expected to cause a discrepancy be­ Subjects. Seven subjects (2 women and 5 men; mean age, tween event kinematics and natural dynamics, which af­ 31.2 years), including both authors, with normal or corrected-to­ fects perception ofbiological motion. Instead, adults and normal vision were recruited as volunteers to participate in the ex­ 6-year-olds readily extracted an invariant structure in up- periment. They were run individually. ORIENTATION AND BIOLOGICAL MOTION 893

I- baum, 1986). For further data processing, the jackknife c 100 w estimation of the area under the ROC curve, Az , was taken as a sensitivity index (Figure 5). a: 80 a: A one-way ANOVAperformed on individual values of A revealed a highly reliable effect ofdisplay orientation o 60 z o on detectability [F(4,6) = 16.476,p < .01]. Sensitivity I­ 40 was already reduced at 45° orientation. The difference be­ Z tween Azs for the upright and the 45° orientations was W o 20 highly significant [t(6) = 4.549,p < .01]. Figure 4 shows a: that the ROC curves for orientations of 90°, 135°, and w o "":;~--l_-L----I.._"--...J 180°are situated close to one another. Wedid not find any Q. significant difference in sensitivity between these orien­ 180150120 90 60 30 0 tations. o R lENTAT ION (deg) Despite the decrease with image-plane rotation, sen­ sitivity with all the orientations exceeded chance level. Figure 2. Percentage of walker recognition as a function of dis­ play orientation (Experiment 1). The dashed line represents the This was assessed using the x2-statistic, which involved threshold probability for significance at the 5% level according both the sensitivity index and the slope ofthe binormal to a Z criterion. ROC curve (Metz & Kronrnan, 1980). The lowestX2 value was for the 135° orientation and was equal to 23.447 (p < .01). Apparatus and Stimuli. Two types of stimuli were used (see Figure 3). One of them (the target) represented an II-dot point-light Discussion walker, the characteristics (size, speed, etc.) ofwhich were the same The main finding ofthis experiment was that despite as those in Experiment J. This figure was simultaneously camou­ prior familiarization with a point-light figure, at each par­ flaged by a mask of66 moving dots that corresponded to spatially ticular orientation its detection declined with image-plane scrambled points on the joints of a walker. Thus, the mask shared rotation. Moreover, this decrease was nonmonotonic: the same particular parameters of motion as those ofthe canonical point-light figure. The masking level, defined as the number of With 90°-180° orientations, sensitivity to a camouflaged moving dots, was chosen so as to enable a comparison of the data figure was essentially the same. with the findings ofBertenthal and Pinto (1994). The other type of High sensitivity to a camouflaged point-light walker stimuli (noise) was a 77-dot mask alone. For the generation ofstim­ with upright orientation corresponds to earlier findings uli, a computer simulation program was created, using the algo­ (Bertenthal & Pinto, 1994; Cutting et a!., 1988; Mather rithms described by Cutting (1978; Cutting et aI., 1988). In both et aI., 1992; Neri et aI., 1998; Thornton et aI., 1998). A stimulus displays, moving dots were distributed within a region about 5.0° in height by 6.8° in width. The viewing period was about significant difference in detectability between upright I sec, which was comparable with the overall display duration used and inverted displays also agrees with Bertenthal and Pin­ by Bertenthal and Pinto. The experimental setup was essentially the to's results. Unlike Bertenthal and Pinto (Experiment 1), same as that in Experiment J. however, we found that even with inversion, observers Procedure. At five display orientations between upright and in­ were sensitive to the presence of a camouflaged figure. verted (0°, 45°, 90°, 150°, and 180°), the observers saw a sequence The discrepancy between our results and those obtained of displays. Halfofthe displays consisted ofa camouflaged point­ light walker, whereas the other half were only masks. The observers by Bertenthal and Pinto can be attributed to some method­ participated in seven experimental sessions (for about 40 min each) ological differences (e.g., two vs. five display orienta­ with five randomly presented blocks-30 trials for each orienta­ tions or 2AFC vs. confidence-rating procedures). One tion. With each orientation, the observers completed a total of 210 difference ofpossible importance is the number oftrials: trials. Each block was preceded by a 3-sec exposure to the same ori­ 100 versus 210 in our study. To control for this possibil­ ented point-light figure without a mask. In a confidence-rating pro­ ity, we performed the data analysis with only the first 90 cedure, the observers had to judge whether a walker was present. A trials. With all the orientations, sensitivity was signifi­ 5-point equal-spaced unipolar scale was used (5,from 100% to 80% confident in the presence ofa walker; 4,from 80% to 60%; 3,from cantly lower, but it exceeded chance. In the work by 60% to 40%; 2,from 40% to 20%; and l,from 20% to 0%). No Bertenthal and Pinto, the procedure of familiarization feedback was given regarding the subject's performance. with an unmasked walker was not explicitly described. Keeping in mind that in order for a camouflaged figure to Results be detected, subjects must know which target they have to To compare detectability for a point-light walker as a look for, we suggest that the familiarization phase might function ofdisplay orientation, we pooled individual data substantially influence detectability. from the 7 observers by averaging the frequencies with In performing the task, a subject was free to use what­ which each observer gave each rating. Figure 4 shows the ever strategy he/she preferred. Despite or probably even receiver-operating characteristic (ROC) curves obtained owing to the mask complexity, the observers could ex­ for each orientation. A jackknife procedure was employed tract the most salient event parameter to facilitate per­ to calculate statistically unbiased parameters of ROC formance. Presumably, in some cases, they did not need curves from pooled rating-method data (Dorfrnan& Ber- to recover the whole fine structure. One possibility is that 894 PAVLOVA AND SOKOLOV

. ., .. ..·.· \ ...... *.· ."· · ..·...... · +.· . . ·· . . . · · ...· . ··

Figure 3. The left panel depicts a static frame from a walking cycle of a canonical point­ light figure. The points represent the locations ofthe major joints. A 66-dot moving mask si­ multaneously camouflaged the ll-dot walker. The right panel depicts a 77-dot mask alone. The displays are represented in reverse contrast: Observers were shown patterns consisting of bright moving dots against a dark background.

the observers detected a walking figure only as a global entations [t(6) = 5.112, 3.812, and 4.034, respectively, cloud ofmoving dots. To eliminate this possibility, in Ex­ p < .05]. We did not find any difference in sensitivity be­ periment 3, we included a partly distorted walker embed­ tween 90°, 135°, and 180°. ded in a noise display. We supposed that observers have to A two-way ANOVA indicated that with all the display perceive hierarchical connections between moving dots in orientations, sensitivity was systematically lower than in a canonical figure in order to discriminate between dis­ Experiment 2 [F(4,6) = 6.41,p < .05]. The interaction of plays with canonical and distorted walkers. experiment X orientation was not significant [F(4,6) = 0.707,p> .05]: The curves representing sensitivity to a EXPERIMENT 3 camouflaged walker in Experiments 2 and 3 are parallel Canonical Versus Distorted Walker to one another (Figure 5). Yet, with all the orientations, performance exceeded Method chance level. The lowest value ofX2 was for the 135°ori­ Subjects. Seven subjects (I woman and 6 men, 23-44 years of entation [X~ = 26.55, P < .05]. The effect oforientation age) with normal or corrected-to-normal vision were recruited as on detectability was most pronounced within the first 90 volunteers. They were run individually. trials, although sensitivity was always above chance. The Stimuli and Procedure. The experimental setup, stimuli, and 2 [X~ procedure were essentially the same as those in Experiment 2. The lowest value of X was for 135° = 7.588, P < .05]. only difference was that a partly distorted walking figure was em­ Learning to detect a walker proceeded more rapidly for 0°, bedded in a noise display. Pairwise relations between moving dots 45°, and 90° than for 135° and 180°: Differences in sen­ on the arms of this figure were perturbed (Figure 6) by setting the sitivity between the first 90 and the final 120 trials were parameter D (shoulder excursion as a multiple ofhip) of a walker­ significant for these orientations [t(6) =4.561,3.474, and generating program to 15 instead of 1.5 for the canonical figure. A 2.834, respectively,p < .05]. 66-dot mask camouflaged both the canonical and the distorted fig­ ures. As in the previous experiment, prior to each experimental block, the observers were exposed (for 3 sec) to a similarly oriented non­ Discussion camouflaged canonical walker, but not to a distorted one. On each Experiment 3 produced basically the same pattern of trial,the subjectshad tojudge whethera canonical walker waspresent. results as Experiment 2. Despite prior familiarization with point-light figures, detectability of a camouflaged Results walker nonmonotonically leveled off with changing ori­ Figure 4 shows the ROC curves obtained for each dis­ entation. However, it remained above chance even at 135°. play orientation. The jackknife estimations of the area Given the close similarity ofthe target and the noise dis­ under the ROC curve (Figure 5) indicate that sensitivity plays, the high sensitivity to the canonical figure appears decreased nonmonotonically with varying orientation amazing. Yet, it should be stressed that in both Experi­ from 0° to 135°and then increased slightly at 180°. A one­ ments 2 and 3, the observers had to detect known pat­ way ANOVA revealed a significant effect ofdisplay ori­ terns. In this case, recovery ofthe coherent structure is entation on detectability [F(4,6) = 8.04,p < .01]. connected primarily with top-down processing of bio­ As in Experiment 2, detectability was already reduced logical motion. A comparison with the findings of Ex­ at the 45° orientation. The difference betweena,s for up­ periment 1, which showed that with orientations between right and 45° orientations was significant [t(6) = 2.721, 180° and 90°, the observers failed in spontaneous recog­ p < .05]. In turn, at the 45° rotation, sensitivity was sig­ nition of a point-light walker, suggests that orientation nificantly higher than that at the 90°, 135°, and 180° ori- influences bottom-up processing of biological motion ORIENTATION AND BIOLOGICAL MOTION 895

A both ofthem. However, this observation requires special empirical clarification. .8 Q) The first three experiments of this series were con­ cerned primarily with either bottom-up or top-down pro­ as .6 a: cessing ofbiological motion. Experiment 4 was designed to examine in more detail the interrelation between top­ .4 down influence on biological motion and the constraints :I: connected with display orientation. .2 EXPERIMENT 4 0 Priming in Detection ofa Camouflaged Walker 0 .2 .4 .6 .8 False-Alarm Ra te In Experiment 4, in addition to image-plane rotation of a camouflaged walker, we implemented a long-term prim­ ing paradigm. This paradigm implies that prior viewing of B a stimulus facilitates performance later on. We hypothe­ sized that ifecological constraints playa prominent role .8 Q) in biological motion processing, a priming effect would occur only with an upright-oriented prime. Alternatively, as .6 a: ifknowledge-based constraints were decisive, a priming effect should be observed with all the prime orientations. ... .4 :J: Thus, Experiment 4 was conducted to establish whether a priming effect in the perception ofbiological motion de­ .2 pends on prime orientation. 0 Method ..6 0 .2 .4 .8 1 Subjects. The subjects were 25 paid volunteers (18-32 years of False-Alarm Rate age) with normal or corrected-to-normal vision. They were re­ cruited from the Max Planck Institute subjects pool. None had pre­ Figure 4. Receiver-operating characteristic curves obtained in vious experience with these types ofdisplays. The observers were (A) Experiment 2 and (B) Experiment3. Data are represented by tested individually. circles for the upright display orientation, diamonds for 45", tri­ Stimuli and Procedure. The experimental setup, stimuli, and angles for 90", asterisks for 135°, and squares for 180". procedure were essentially the same as those in Experiment 2. On a trial, a subject had to press as rapidly as possible a "yes/no" key to indicate whether the walker was present. Following this, they more strongly than it influences top-down processing. At gave a confidence rating on a 6-point equal-spaced unipolar scale (6, absolutely confident; 1, absolutely unconfident). The experi­ the same time, detection ofa known structure is affected ment consisted of three sessions of five randomly presented by orientation in similar ways, as are some characteristics blocks-32 trials for each orientation. The key difference between of spontaneous perception (e.g., perceptual stability). this experiment and Experiment 2 was that prior to a block oftrials, Both detectability and perceptual stability decreased the subjects were primed (for 10 sec) with a sample ofa noncam­ with a change oforientation from upright to 90°/135° and ouflaged walker that was upright, oriented 45° or 90°, or inverted. then slightly increased up to display inversion (see Fig­ ures 1 and 5). The results suggest that processing constraints in the 0 1 perception of biological motion, which are connected 0 IX with orientation (e.g., an elongated axis ofsymmetry or .9 dynamic constraints), are hierarchically nested (see also IX w .8 Pavlova, Sokolov, & Biilthoff, 1998). Although with c image-plane rotation, dynamic constraints lose their z .7 strength, other processing constraints become more pow­ ~ erful. For instance, the lower sensitivity at a 135° than 0( .6 at a 180° rotation may be accounted for by an axis-of­ w IX symmetry constraint that is implemented by the visual 0( .5 system at 180°. Likewise, it seems that owing to the in­ o 45 90 135 180 efficiency of this constraint, perceptual instability of a point-light walker was higher at 90°-150° than at 180° ORIENTATION (deg) (Experiment 1). Itis noteworthy that in both Experiments Figure 5. Jackknife estimates of the area under the ROC 2 and 3, sensitivity was lower at the 135°orientation than curves obtained in Experiments 2 (open circles) and 3 (closed cir­ at inversion, despite the lack ofeveryday experience with cles). Vertical bars show ±SE. 896 PAVLOVA AND SOKOLOV

. . . > . o· .. .. o· ., .. . · ~ . . - \ . ,~· .~ . ..* · . ..* > · ,. .· . ,. . · . , ...... > > . · > · > · , . > ·., . '. .-.· > • . . . . . · 0 . · · . · , - . . 0 - . _-0 . . - . - .

Figure 6. Representation of the stimuli used in Experiment 3. The left panel depicts a static frame from a walking cycle of a canonical point-light figure; the right panel depicts a partly distorted walking figure. Both figures were simultaneously camouflaged with a 66-dot moving mask.

Thus, each offour groups ofsubjects saw a point-light walker with­ plays with increasing deviation ofthe prime from the up­ out a mask in one of these particular orientations. The observers right orientation [F(3,5) = 5.444, p < .05]. We did not were explicitly informed about the relative orientations ofthe prim­ find any significant difference in sensitivity to 45°-, to ing and the primed displays. Except for the group that was primed 90°_,and to 180°-oriented displays as a function ofprime with a 90°-oriented walker (7 subjects), all the groups consisted of 6 observers. orientation [F(3,5)= 2.046, 0.077, and 1.045, respectively, p> .05]. For 135°-oriented displays, sensitivity was also Results independent ofprimes [F(3,5) = 1.52,p > .05], although Figure 7 shows the ROC curves obtained with differ­ we did not use a specifically oriented one. ent-oriented primes. As can be seen there, a pronounced We also performed an analysis ofRTs for correct re­ priming effect occurred only with the upright-oriented sponses (hits only). For each subject and experimental prime: The ROC curve for the same-oriented display is condition, we processed only those RT values that did not situated higher than those for other orientations. The jack­ exceed the cutoffvalue ofthree standard deviations from the mean RT (Figure 9). A two-way ANOVA did not re­ knife estimates ofthe area under the ROC curve, Az ' are shown in Figure 8. veal any differences in RT as a function ofprime [F(3,5)= In the group primed with an upright-oriented walker, 0.542, p > .05] or display orientation [F(4,5) = 2.572, the highest sensitivity was found for similarly oriented p> .05]. displays. A one-way ANOVA revealed a reliable effect of display orientation on sensitivity: It decreased with in­ Discussion creasing deviation ofthe display from the upright orienta­ The findings of Experiment 4 clearly indicate that tion [F(4,5) = 7.827,p < .05]. A within-group analysis only an upright-oriented prime strongly affects detection showed highly significant differences between Azs for ofa camouflaged point-light walker: It significantly im­ primed and nonprime orientations [t(5) = 8.889, 9.676, proves performance for the same-oriented displays. It is 9.676, and l7.88l,p < .05, between 0° and 45°, 90°,135°, fairly remarkable that 90°- and 180°-oriented primes did and 180°, respectively]. not facilitate performance for the similarly oriented dis­ In the group primed with the 45°-oriented walker, we plays. This evidence provides strong support in favor of also found a significant effect of display orientation on the primacy of ecological constraints in biological mo­ detectability [F(4,5) = 7.412, p < .05]. Sensitivity was tion processing. statistically the same for 45°-and upright-oriented displays At first glance, contrary to our initial hypothesis, but higher than for all the other orientations. In the groups a long-term priming effect was observed with a 45°• primed with 90°- and 180°-oriented walkers, an ANOVA oriented prime. Yet, this finding conforms to the data of did not indicate any significant difference in sensitivity Experiment 1, which showed that despite a mismatch be­ [F(4,5) = 0.132 and 0.264, p > .05, respectively]. Fig­ tween event kinematics and natural dynamics at 30° and ure 7 shows that the ROC curves for all orientations are 60° rotation, the display was spontaneously recognizable. situated very close to each other. However, sensitivity to upright-oriented displays in this A between-group analysis showed that sensitivity to up­ experiment was significantly lower with a 45° prime than right displays was higher with the same-oriented prime with the upright one. This suggests that stimulus infor­ than with the other primes (Figure 8). A one-way ANOVA mation, which is recovered at the 45° orientation, does not revealed a decrease ofsensitivity to upright-oriented dis- induce as high performance as with an upright display. ORIENTATION AND BIOLOGICAL MOTION 897

1 1

.8 .8 Q) Q)

I'lS I'lS .6 cr:- .6 cr:- .4 .4 ~- ~- .2 .2 00

o .._.&.---I_---L_--&..._..1 0 o .2 .4 .6 .8 0 .2 .4 .6 .8 1 False-Alarm Rate False-Alarm Rate

.8 .8 Q) Q) -I'lS .6 I'lS .6 cr: cr:- .4 - .4 -~ ::I: .2 .2

0 0 0 .2 .4 .6 .8 0 .2 .4 .6 .8 1 False-Alarm Rate False-Alarm Rate Figure 7. The results of Experiment 4: the receiver-operating characteristic curves obtained in four independent groups of subjects that were primed with 0°_,45°_,90°_,and 180°-oriented walkers. Data are represented by circles for the upright display orientation, diamonds for 45°, triangles for 90°, asterisks for 135°, and squares for 180°.

The present findings indicate that a long-term priming affected by a change in display orientation. Specifically, effect in biological motion occurs only if a prime corre­ with 90°-180° orientations, (1) spontaneous recognition sponds to a limited range of deviations from the upright was dramatically impeded (Experiment 1), (2) sensitiv­ orientation within which a display is spontaneously rec­ ity to a camouflaged figure was essentially the same (Ex­ ognizable. The only earlier work on priming in biological periments 2 and 3), and (3) the primes corresponding to motion concerns short-term effects for a point-light walk­ these orientations did not improve detectability ofa point­ ing figure rotated in depth (Verfaillie, 1993). For the first light walker embedded within a mask (Experiment 4). time, we established a differential priming effect in bio­ With upright orientation, however, all the observers re­ logical motion, which depends on image-plane display ported seeing a walking figure, which was perceived as orientation. Contrary to common beliefbased on image­ stable and unambiguous. Despite prior familiarization plane rotation ofstatic objects with explicit structure (e.g., with a rotated figure at each particular orientation, the Jolicoeur, 1988), we found that the priming effect in bio­ highest sensitivity to camouflaged walker was observed logical motion is partly independent ofthe relative orien­ with an upright display. Moreover, only an upright-oriented tation of priming and primed displays. Moreover, an RT prime yielded a pronounced priming effect. analysis indicated that the priming effect is not connected The data suggest that dynamic constraints in event with a process ofmental rotation or normalization. 1f such recognition are graded in their influence. When a display dependence did exist, one would expect an increase in RT is rotated in the image plane, in contrast with the filmed with increasing relative orientation between priming and down- or uphill movement ofa real walker, there is some primed displays. This was not the case in our study. mismatch between perceived kinematics and dynamics. Within a limited range ofdeviations from the upright ori­ GENERAL DISCUSSION entation (30° and 60°), such a mismatch does not appear to substantially impair recognition ofbiological motion. In a set offour experiments, we demonstrated that the Accordingly, a long-term priming effect (Experiment 4) perception of biological motion was nonmonotonically was observed not only with an upright-oriented prime, 898 PAVLOVA AND SOKOLOV

(.J .9 ...... o-prlme ered as a within-subjects priming design, Experiments 2 0 ~45 and 3 clearly show that despite prior familiarization with a:: -'-90 a noncamouflaged walker, detectability is reduced from .8 a:: -0-180 upright to 90°-180°. Verfaillie (1993) documented short­ w term priming, which occurred only when upright prim­ 0 z .7 ing and primed walking figures shared the same in-depth ::::I orientation. The effect was not suppressed when priming and primed walkers differed in their position in the vi­ c( .6 w sual field and the starting position in the step cycle. His data suggest that viewpoint-specific constraints are more a: .5 c( powerful than position cues or, in other words, that a hi­ o 45 90 135 180 erarchy of orientation- and position-dependent cues oc­ OR lEN TAT ION (deg) curs in biological motion. Likewise, the constraints con­ nected with image-plane orientation were observed to be Figure 8. Jackknife estimates of the area under the receiver­ much more powerful than other constraints, such as po­ operating characteristic (ROC) curves in Experiment 4. Closed circles represent values obtained in the group with 0°_,open dia­ sition cues or occlusion (see, e.g., Bertenthal et aI., 1985; monds with 45°_, closed triangles with 90°_,and closed squares Heptulla Chatterjee, Freyd, & Shiffrar, 1996; Mather with 180°-oriented primes. Vertical bars show ~SE. et aI., 1992). The present data converge with observations that a priming effect does not occur for photographs de­ picting unnatural human poses that are impossible to but also with a prime rotated 45°. However, sensitivity to perform (Daems & Verfaillie, 1999) or for distorted dy­ upright-oriented displays was significantly lower with namic actions (Nilsson, Olofsson, & Nyberg, 1992; Olofs­ the 45° prime than with the upright one. This suggests that son, Nyberg, & Nilsson, 1997). Kourtzi and Shiffrar stimulus information, which is recovered at 45° rotation, (1999) also have reported that although apparent motion does not provide as high performance as with an upright facilitates linkage of multiple views of a human body, orientation. Likewise, perceptual instability ofa display priming for these views is restricted by biomechanical rotated 30° and 60°, which was observed in Experiment I, constraints. We did not observe priming effects with the indicated that observers are sensitive to the mismatch be­ displays representing such unnatural events as walking tween event kinematics and dynamics that occurs with upside down or walking on a cliffin the same manner as these orientations. The latter findings provide additional with an upright orientation in the gravitational field. Taken support in favor of constraints connected with display together, our findings show that top-down influence on orientation in the perception ofbiological motion. the perception ofbiological motion is limited by display For the first time, we established a differential long­ orientation. term priming effect in biological motion, which depends on image-plane display orientation. In Experiment 4, it REFERENCES occurred only when a prime corresponded to a limited AHLSTROM, v., BLAKE, R., & AHLSTROM, U. (1997). Perception ofbio­ range of deviations from the upright orientation, within logical motion. Perception, 26, 1539-1548. which the display is spontaneously recognizable. Consid- BARCLAY, C. D., CUTTING,J. E., & KOZLOWSKI, L. T. (1978). Temporal and spatial factors in gait perception that influence gender recogni­ tion. Perception & Psychophysics, 23,145-152. ..-. BERTENTHAL, B. I., & PINTO,J. (1994). Global processing ofbiological ~ 3.3 motions. 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