Visual representations of dynamic actions from static ... - CiteSeerX

Perception, 2005, volume 34, pages 835 ^ 846

DOI:10.1068/p5039

Visual representations of dynamic actions from static pictures

Claude Bonnet, Carlos Paulos, Christelle Nithart

Institut de Physique Biologique, Universite¨ Louis Pasteur, UMR 7004 du CNRS, 12 rue Goethe, 67000 Strasbourg, France; e-mail: [email protected] Received 15 January 2003, in revised form 18 November 2004

Abstract. We investigated how subjects used their knowledge of biomechanical constraints when judging whether different items were in balance or in the process of falling, as a function of their angle of slant. In the first experiment, the stimuli were pictures of postures of a human body, of a wooden mannequin, and of a skeleton. The results show that for these 3 items, fall responses appeared for a smaller slant angle for a backward slant than for a forward one. This difference may reflect the influence of biomechanical constraints. To verify whether the asymmetry of the responses to the mannequin and the skeleton was genuine or due to some semantic context effect, a second experiment was run with only pictures of a wooden mannequin. The same asymmetry was observed. In a third experiment, falling judgments were obtained for pictures of a human body and of a structurally comparable artifactual object. The asymmetry of the fall responses appeared only for the human body.

1 Introduction Interpreting body positions of other people is an important activity in everyday life. Humans, and likely many other species, seem to be quite skilled in this task. Recognising actions from pictures, which is the aim of the present experiments, raises several related questions. First of all, one should not forget that recognising objects from pictures is largely a cultural task (eg Deregowski 1999), ie it necessitates the use of mental representations. Second is the question of knowing whether or not pictures of the human body are processed, at least in part, differently from those of other (inanimate) objects. Different arguments suggest that this is the case. As for faces, infants aged about 1 year show a preference for normal human body shape as opposed to scrambled body shapes (Slaughter et al 2002). The inversion effect frequently considered as face-specific also appears for body images (Reed et al 2003). The relations among parts of human bodies are different from those in other objects (Reed et al 2004). Neuroimaging studies demonstrate that the brain structures activated by (inanimate) object images are different from those activated by human body images (Kourtzi and Kanwisher 2000; Senior et al 2000, 2002; Paulos et al 2003). However, the distinctions between body perception processing and object perception processing may be questioned from different points of view. The distinctions may relate more generally to a difference between processing animate versus inanimate òbjects' (eg Farah et al 1996), or between natural and manufactured òbjects' (eg Joseph 2001). They may also reflect an effect of expertise. The latter point is illustrated in the debate about face processing. Some authors state that a region of the fusiform gyrus (the fusiform face area or FFA) is specifically involved in face processing (Kanwisher et al 1997), while others (Gauthier and Tarr 2002) try to demonstrate that this area is more generally involved in processing òbjects' which are in the expertise domain of the subject. As humans, we are undoubtedly expert in visual recognition of human bodies. The third and more central question for the present paper is the recognition of human actions (movements) from pictures. This ability to recognise human actions is well documented for stimuli containing explicit cues to body motion. Since the pioneering work of Johansson (1973), many researchers have investigated the capacity of the

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visual system to recognise dynamic actions from impoverished stimuli in which only kinetic cues were available, while the form of the body was not shown. A dozen lights were fixed at the major joints of a human actor who was filmed while performing different activities. When such dynamic stimuli were shown to observers, they could identify many different types of activities, discriminate the gender of the actor, and recognise friends as well as themselves (eg Cutting and Proffitt 1981; Dittrich 1993; Thornton et al 2002). The special status of body motion was also revealed in apparentmotion studies (Shiffrar and Freyd 1990, 1993; Chatterjee et al 1996) in which the biomechanical constraints of the human body appeared to influence the perceived path of the apparent human motion. What does it mean to recognise a motion or an action in a static picture? While artists have devoted much effort to this aim, few systematic investigations have explored this issue (eg Cutting 2002). The dynamic balance or broken asymmetry of the shown body images appeared to be an important factor. Few researchers have investigated human competence in recognising actions from pictures (eg Rosenblum et al 1993). However, neuropsychological studies stressed differences between recognising objects and recognising (mostly naming) actions from pictures. For instance, Campbell and Manning (1996) reported the case of a patient with optic aphasia who had a strong deficit in naming object pictures but a preserved capacity for naming actions. On the other hand, Manning and Warrington (1996) reported another case in which the patient was impaired for naming actions but the naming of objects was preserved. For neuropsychological diagnosis a set of one-hundred action pictures has been standardised (Druks and Masterson 2000). The issue of recognition of actions from pictures has gained increased interest with fMRI studies. Kourtzi and Kanwisher (2000) and Senior et al (2000, 2002) found that when subjects viewed still photographs of people resting or performing an action, brain activation was observed in the MT/MST complex when the picture contained an implied movement of a human body. Importantly, this brain region is known to be involved in the processing of real visual motion (eg Derrington et al 2004). Such results are expected if one hypothesises that mental representations are subserved by brain structures `normally' activated by corresponding stimuli. From a behavioural point of view, Shiffrar and her colleagues (Chatterjee et al 1996; Kourtzi and Shiffrar 1999) demonstrated in two experiments results illustrating one consequence of such an approach. As a general rule, the perceived path of an apparent movement is the shortest one, ie a linear track. However, when the apparently moving object is a member of a human body, for long interstimulus intervals the perceived path of the member may be curved so as to respect biological constraints, ie not to go through a body part. The authors mentioned above found that similar perceived paths of apparent motion were obtained with high-quality photographs of humans and of a wooden mannequin. These results suggested that human observers interpreted the movement of the mannequin as if it were a real human movement. In other words, a wooden mannequin can evoke similar representations of body biomechanical constraints as do realistic views of the human body itself. Here, we investigated whether or not the recognition of a falling process uses our implicit knowledge about biomechanical constraints if the item in the picture is a human body slanted at different angles. The biomechanical constraint used here lies in the fact that body balance can be maintained for larger slant angles in the forward direction than in the backward one. Second, we investigated to what extent a wooden mannequin or a skeleton shown in the same posture as a human body is judged as an inanimate object, or as having anthropomorphic dynamic properties. Finally, we compared the falling judgments between pictures of a human body and those of an inanimate (artifactual) object.


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2 Experiment 1 In the first experiment, we compared judgments of fall (versus balance) in pictures of a human body, of a wooden mannequin, and of a skeleton seen laterally. Biomechanically, it is commonly known that a human body, from the upright posture (vertical), can maintain its balance over a larger range of slants in the forward direction than in the backward one. Human adults are expected to possess, at least implicitly, such knowledge. Consequently, one would expect that the slant limit between balance and fall (later called `point of subjective imbalance'öPSI) will correspond to a larger angle of the body when slanted forward than when slanted backward. The falling postures are restricted to a rigid rotation around the hip. In line with the similarity of the effect found between human-body and wooden-mannequin apparent motion (Chatterjee et al 1996; Kourtzi and Shiffrar 1999), we also addressed the issue of how to recognise whether a wooden mannequin or a skeleton will show the same fall asymmetry as a human body. While mannequins and skeletons are clearly identified as inanimate objects, they may activate biomechanical representations similar to those of a human body, in such a way that their anthropomorphic significance would prevail over their physical properties. 2.1 Stimuli and method Images of a naked man, of a wooden mannequin, and of a skeleton, all shown in full colours, seen laterally, were drawn with the MetaCreations Poser 3 software, so that the overall height and the relative proportions of the body parts remained the same for the 3 items. The height of the stimuli was 4.5 deg when vertical. They were facing either rightward or leftward and could be shown at 4 different angles of slant of the trunk and head from the hip: 08, 208, 408, and 608. The null orientation (08) corresponds to the vertical; other angles are given with respect to the upright posture. One leg was maintained in a vertical position and the rotation around the hip was rigid, with the arm kept aligned with the trunk. The other leg was bent in a direction opposite to the slant angle so as to act as a partial counterweight. A sample of these stimuli is shown in figure 1. The experiment was run on a high-resolution (10246768 pixels) computer display (refresh rate 60 Hz) controlled with Inquisit V1.32 by Millisecond Software. Picture stimuli were presented for 166 ms each in the centre of the monitor. The luminance background of the monitor was 10 cd mÿ2. This was the only source of light in the room. Subjects placed their head on a head-and-chin rest 120 cm from the monitor. On each trial, subjects reported, following a two-alternative-choice procedure, whether the figure was in balance or falling. Ten subjects (psychology students) took part in the experiment. They were volunteers and naive to the hypothesis under investigation. They had normal or corrected-to-normal vision. Two dependent variables were measured: choice (balance or fall) and the corresponding reaction times, with two vertically placed keys of the computer keyboard. Each of the 48 stimulus conditions (3 items64 slant angles62 sides of facing62 directions of fall) was presented in random order and the sequence repeated ten times. A 1300 ms delay separated the response from the presentation of the subsequent stimulus. 2.2 Results A first ANOVA was done on the complete frequency data. It showed that the facing factor was not significant (F1, 9 2:95, p 5 0:12) and did not show any significant interaction with other factors. Consequently, the results were pooled on this condition and submitted to a three-factor analysis of variance (items6direction of slant6level of slant). The results are shown in figure 2. The 3 items (body, mannequin, skeleton) exhibit a marginally significant difference (F2, 18 2:93, p 5 0:08). There are more fall responses when the slant angle fall is backward than when it is forward (F1, 9 12:81, p 5 0:006).

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(b) Figure 1. Example of the three items (human body, mannequin, skeleton) used in experiment 1 and shown with the same slant angle (408); (a) forward slant; (b) backward slant. In the experiment, the items were in colour.

The frequency of the fall responses increases with the slant of the stimuli (F3, 27 65:19, p 5 0:0001). There is no significant interaction between the items and the direction of fall (F2, 18 1:29, ns). However, there is a significant interaction between the items and the slant factor (F6, 54 3:55, p 5 0:005) as well as between the direction of slant and the level of slant (F3, 27 7:42, p 5 0:0009). In order to summarise the results, sigmoid psychometric functions were fitted to the frequency-of-fall responses in order to estimate the PSI defined as the slant angle leading to 50% of fall responses. The PSI did not vary with the items for backward slant (PSI 19:18). For forward slants, we found PSI 27:98 for the mannequin, PSI 32:18 for the human body, and PSI 38:28 for the skeleton (averaged PSI 32:68). No statistical tests have been carried out on the PSI. The mean RT was 647 ms. The RTs (fall and balance responses included) did not vary significantly with the type of item (F2, 18 5 1). They were slightly faster for a backward fall than for a forward fall (629 ms versus 666 ms; F1, 9 3:90, p 5 0:08). RTs tended to vary curvilinearly with the angle of slant (F3, 27 5:77, p 5 0:0035).


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1.0 body (forward) body (backward) mannequin (forward) mannequin (backward) skeleton (forward) skeleton (backward)

Frequency of fall responses

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Figure 2. Mean frequency of the fall responses as a function of body slant. Error bars are omitted for the sake of clarity. Averaged sigmoid psychometric functions for each direction of fall were fit in order to estimate the PSI at 50% fall responses. Dotted lines show the averaged PSI for backward and forward responses.

They were maximum in the middle range of slants. The slant factor interacts significantly with the two other factors: with the type of item (F6, 54 3:32, p 5 0:007) and with the direction of fall (F3, 27 4:36, p 5 0:01). 2.3 Discussion The differences between the 3 item stimuli were small. However, for all 3 items a large asymmetry between a forward and a backward fall has been observed. The real falling slant angle of a human body in such postures was difficult to estimate a priori. We then introduced a control with a living person requested to adopt the same posture as our human-body pictures. While the slanted postures are difficult to produce (the subject can hardly avoid producing a compensating small bend of the back), the psychophysical results seem to be rather closer to the real falling angle of a living human body. The slight, but significant, differences between the 3 items can only be explained speculatively. They appear mainly for the forward falls. As seen in figure 2 (and from the PSI), the distribution of the fall responses for the skeleton and for the mannequin is symmetrical with respect to that for the human body. They may result from an impression of weight (or a size ^ weight factor) attributed to the item: a skeleton being judged lighter than a human body and the latter judged lighter than a wooden mannequin. But this does not explain why such a factor is not present (to a significant degree) in backward slants. However, the differences between forward and backward fall are much closer for the 3 items. In conclusion, as expected, subjects estimated the PSI from static pictures of a human body approximately correctly. Moreover, their estimates were very similar for pictures of a human body, of a wooden mannequin, and of a skeleton. The last two stimuli are clearly considered to have anthropomorphic characteristics and activate similar biomechanical representations to those of a human body. However, the assimilation of the properties of a mannequin and of a skeleton to those of a human body could be due to some semantic context effect. Since images of the mannequin and of the skeleton were mixed with human body images within the same experimental block of trials, the latter may have encouraged subjects to give more consideration to the anthropomorphic properties of the mannequin and the skeleton than to their physical (inanimate) properties.

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3 Experiment 2 A second experiment was then designed to verify whether the falling judgments on the mannequin were due to a semantic context effect as mentioned above or whether they were more genuine. The issue was then to know whether a similar asymmetry between forward and backward falls would be observed for an inanimate object when no picture of a human body was shown, in parallel, which could suggest an anthropomorphic interpretation of the mannequin. In this second experiment, stimuli were photographs of a real wooden mannequin, alone and in a large variety of postures. The use of a real mannequin allowed us to know the real limit between a physical balance posture and a falling posture for such an inanimate object; this was a kind of `point of physical imbalance', defined by the fact that at larger slants the mannequin would fall unless suspended with invisible wires. 3.1 Stimuli and method The stimuli were digital photographs of a wooden mannequin viewed laterally. The mannequin had a height of 31 cm and was photographed, with a digital camera, from a distance of 100 cm. Four sets of actions were used. In each, 4 levels of progressive mannequin slant were employed. At the 2 smaller slants, the mannequin was physically in balance, while at the other 2, it would have fallen if not maintained by invisible wires. The slants of the body ^ trunk line were approximately 08, 208, 408, and 608 from gravitationally defined vertical. It should be stressed that the manipulation of the articulated wooden mannequin, which had to be in balance at 2 slant postures, was done at the expense of perfect symmetry between forward and backward postures. For 2 sets of stimuli, the direction of fall was forward (Fa and Fb), while for the other 2, it was backward (Ba and Bb). In each view, the mannequin clearly had at least one foot on the ground. In fact, for 2 sets of stimuli, when in balance at the 08 slant, the mannequin has two feet on the ground (Fa and Ba) while for the 2 others, it has only one foot on the ground (Fb and Bb). The number of feet on the ground is better considered as a variety factor, which allowed us to obtain 2 samples of posture for each direction of slant (the number of feet on the groundöa 2 feet versus b 1 footöapplies only for the balance position at 08). An example of the 4 types of stimuli is shown in figure 3 for the same slant level (408). As seen in the figure, the postures are more complex than in the previous experiment, since we had to change the positions of the arms and legs in order to maintain some physical imbalance, at least for a 208 slant. Finally, since the mannequin faced either left or right, we obtained 32 different stimuli. The experiment was run on a high-resolution (10246768 pixels) computer display (refresh rate 60 Hz) controlled with Inquisit V1.32 by Millisecond Software. Picture stimuli were presented for 166 ms each in the centre of the monitor. Their visual size was 3.8 deg. Twenty new subjects (psychology students) took part in the experiment. They were naive to the hypothesis under investigation. On each trial, following a twoalternative-choice procedure, subjects reported whether the mannequin figure was in Fa

Fb

Ba

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Figure 3. Example of the 4 types of stimuli for the same body slant (408) in experiment 2. Fa and Fb are 2 exemplars of forward falls; Ba and Bb are 2 exemplars of backward falls.


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balance or falling. Two dependent variables were measured: choice (balance or fall) was registered by pressing one of two horizontally placed keys on the computer keyboard, which also allowed the corresponding reaction times to be measured. Half of the subjects were instructed to use the left key for the fall response and the right key for the balance response. This was reversed for the other half of the subjects. The 32 stimuli were presented in random order and the sequence repeated ten times. A 1300 ms delay separated the response from the presentation of the subsequent stimulus. 3.2 Results There were no statistical differences between the two groups of subjects with respect to the side of the response key (left or right) either for the frequency of responses (F1, 9 1:91, ns) or for the reaction times (F1, 19 1:33, ns). The data were collapsed across response-key assignment and the facing factor, and pooled over these two factors and submitted to a three-factor ANOVA (variety6direction of slant6level of slant). The mean results are shown in figure 4. 1.0 Fa Fb Ba


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Figure 4. Mean frequency of fall responses as a function of the body slant for each of the 4 stimulus categories (see figure 3). The error bars are standard errors. Averaged sigmoid psychometric functions were fit in order to estimate the PSI at 50% fall responses for backward (dotted line) and forward (continuous line) falls.

The frequency of the fall responses is greater for backward fall than for forward fall (F1, 19 55:16, p 5 0:0001). The number of fall responses increases with the slant angle (F3, 57 63:82, p 5 0:0001). The variety factor (a, b) is not significant (F1, 19 1:11, ns). It interacts with the direction of fall (F1, 19 10:27, p 5 0:005). In fact, this interaction is due to the difference between the two varieties of forward fall (see figure 4). Finally, there is a significant interaction between the direction of fall and the angle of slant (F3, 57 8:76, p 5 0:0001). The latter interaction is well explained when sigmoid psychometric functions are fitted to the frequency of fall responses. In this way, it is possible to estimate the PSI, defined as the slant angle comprising 50% of fall responses. Globally, PSI 33:88 for the forward falls, and PSI 14:88 for backward falls. The mean RT was 679.5 ms. The RTs (fall and balance responses included) did not vary significantly with the direction of fall (F1, 19 0:06, ns). They varied nonlinearly with the angle of slant (F3, 57 6:91, p 5 0:0005), with longer RTs for 208 and 408 slants. The variety factor was significant (F1, 19 12:66, p 5 0:002). As regards the frequency of fall responses, this difference is due to the difference of the two varieties of the forward falls. Finally, RTs increased with the angle of forward falls, but decreased with the angle of backward falls (F3, 57 24:14, p 5 0:0001).

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3.3 Discussion Subjects systematically reported balance and fall responses in relation to the slant of the mannequin. However, they more frequently reported a fall when the mannequin was tilted backward than when it was tilted forward. While the point of physical imbalance was about midway between the slant angles of 208 and 408, it did not matter whether the physical fall was forward or backward: mean PSI 14:88 for a backward fall and 33.88 for a forward one. This backward-fall bias held even when the mannequin was physically in balance at 208. The difference between forward and backward conditions appeared in spite of the fact that the different conditions were randomised over trials. The present results confirm the results of the previous experiment, even at the metric level of the PSI. The RTs are also consistent with those of the previous experiment. Essentially they reflect the certainty of the responses. At variance with the previous experiment, the imbalance between forward and backward fall responses is present at the 08 slant. Such a difference is due to the fact that the different postures of the mannequin were empirically obtained in order to approximate the levels of slant and the balance or fall constraint. The magnitude of this difference is about the same at 08 and at larger slant angles. However, the results suggest that observers visually analysed the mannequin as if it was a living human body. The responses do not reflect their taking into account of the physical balance, since the point of physical imbalance is approximately equal for the forward and backward slants ( 308). The difference in PSI, which is close to that reported in experiment 1, suggests that subjects merely take into account the biological constraints of a human posture. The first factor of imbalance is the shortest distance between the ankle and the heel, compared to its distance to the tip of the toes. But a second, more dynamic factor, can be invoked. In a forward slant of the body, a fall can be avoided more easily by the movement of a leg, whereas fall is nearly inescapable for a backward slant. It should be stressed that the instructions given to the subjects were neutral with respect to the interpretation of the `figure' (wooden mannequin or human body). Hence, for the present task, the mannequin stimulus is not treated as a manufactured (inanimate) object, but rather as a living one. Even in this completely static situation, just as in the dynamic ones mentioned in section 1, the `figure' appears to have activated representations of a human body, including its biomechanical constraints. 4 Experiment 3 In order to confirm the previous interpretation about treatment of the mannequin as a living body, a third experiment was run. Here, images of a falling human body were compared to those of a structurally similar artifactual object. The responses to the latter should reflect only physical constraints of gravity, while the responses to the former should take into account biomechanical constraints. In other words, the PSI is expected to correspond to a similar slant for backward and forward slants of the object, whereas asymmetry is expected for the human body. While in the previous experiment the postures of the mannequin were rather complex, forcing observers to take into account leg and arm positions, in this control experiment the slant of the human body was restricted to a rigid rotation around the hip (see figure 5). 4.1 Stimuli and method Images of a naked man seen laterally were drawn with the MetaCreations Poser 3 software. The height of the human body was 4.5 deg. It was facing either rightward or leftward and shown with 4 different angles of slant around the hip: 08, 208, 408, and 608 from gravitationally defined vertical. Both his legs were vertical. An artifactual object of similar shape was drawn having a sphere as a `head', a cylinder as a `trunk',


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Figure 5. Example of the 2 items used in experiment 3, slanted at 408 from the `hip' and shown here grey-scaled. In the experiment, they were in colour.

another sphere as `hips', an inverse cone as a `leg', and a horizontal small cylinder as a `foot', so that a forward versus backward fall could be distinguished at least a priori (see figure 5). Both sets of items were shown, in full colours, as falling either forward or backward. They landed on a horizontal orange pedestal given as a reference line. Hence, the experiment contained 32 different conditions resulting from the combination of four factors: the items (human body or artifactual object), the direction of the fall (forward ^ backward), direction of facing (left ^ right) and the 4 degrees of slant. These 32 conditions were presented in a random order and repeated ten times according to the method of constant stimuli. We used an experimental procedure similar to that in experiment 1. Each stimulus was shown for 160 ms. Two dependent variables were measured, choice (balance or fall) and the corresponding reaction times, with two vertically placed keys of the computer keyboard. A further ten subjects (psychology students) took part in the experiment. None of them had participated in the previous experiments. Stimulus size, luminance, and other specifications were the same as those of experiment 1. Instructions specified to subjects that the object as well as the human body were not fixed to the ground; and that the object was rigid, wooden, and had an equivalent weight to that of the human body. 4.2 Results Since, as in experiment 1, the facing factor was not significant and did not interact with other factors, the results have been pooled in this condition and submitted to a new analysis of variance (items6direction of slant6level of slant). These results for the frequency of fall responses are shown in figure 6. The 2 items (human body versus object) did not exhibit a significant difference (F1, 9 0:25, ns). The direction of slant had a significant effect (F1, 9 13:48, p 5 0:005) and the frequency of fall responses increased with the angle of slant (F3, 27 60:19, p 5 0:00001). The interaction between the items and the direction of fall was significant (F1, 9 12:44, p 5 0:006), as well as the interaction between the direction of slant and the levels of slant (F3, 27 7:81, p 5 0:0006). The interaction between the items and the levels of slant was not significant (F3, 27 1:32, ns). In short, while the PSIs were nearly equal for the object (32.28 and 34.88), for the human body in forward fall PSI 52:78, and in backward fall PSI 20:98.

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Figure 6. Mean frequency of fall responses as a function of body slant for the two types of items. Sigmoid psychometric functions were fit in order to estimate the PSI at 50% fall responses.

The mean RT was 591.9 ms. The RT was nearly the same for the 2 items (F 5 1). It was shorter for backward falls (576 ms) than for forward ones (607 ms). It was longer for the middle-range slant levels than for the extreme (F3, 27 15:86, p 5 0:00001). There was no interaction between items and direction of slant (F 5 1). The interaction between the items and the levels of slant was significant (F3, 27 3:99, p 5 0:02). 4.3 Discussion Experiment 3 showed that falling judgments for an inanimate (artifactual) object were identical for forward and backward falls, whereas a strong asymmetry between these two directions has been observed for human-body pictures. Although the object was not familiar to the subjects, they consistently evaluated its fall. There was no difference in the frequency of fall responses for backward versus forward fall. For the object, RTs were longer for forward falls (612 ms) than for backward ones (580 ms) (F1, 9 4:89, p 5 0:06). The difference was most pronounced for the two highest slant levels, ie when the majority of the responses were ``fall''. Although the only cue for the asymmetry of the object was its `foot', this result suggests that subjects did not ignore the asymmetry. However, it did not influence the frequency of the fall responses for the object. On the other hand, for the human body, although the difference in RTs for forward and backward falls was not significant, the interaction of the direction factor with the level of slant was marginally significant (F3, 27 2:63, p 5 0:07). The latter difference results from the fact that longer RTs were observed at 408 for forward slant and at 208 for backward slant, consistent with the difference in the PSI. Hence, according to the asymmetry in the falling responses for the human body, these results confirm that assumptions made take into account knowledge of biomechanical constraints. 5 General discussion and conclusions Subjects judged consistently whether or not the items shown in pictures were in balance or falling. The results suggest that when the item is the image of a human body, biomechanical constraints are taken into account, so that a backward fall is judged to occur at a smaller slant angle than a forward fall. Moreover, anthropomorphic inanimate objects, like a wooden mannequin or a skeleton, gave rise to similar fall judgments


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to a human body: same asymmetry between forward and backward falls and nearly identical PSI. As seen from the comparison of experiments 1 and 2, this result cannot be attributed to contextual semantic factors, since a wooden mannequin presented alone (experiment 2) produced similar results to those observed when presented in the context of a human body (and a skeleton). An artifactual object does not show an asymmetry between forward-fall and backward-fall judgments. Of course, the choice of the inanimate object for the control experiment is critical. The artifactual object we chose did not really have a front or back, as, for instance, a grandfather clock would have. However, the object had a shape asymmetry as a result of the presence of its foot. We have indicated above that such a shape asymmetry was likely to have been taken into account by the subjects while not affecting their falling judgments. `Foot' asymmetry is likely to be more critical than `face' (front ^ back) asymmetry. In a control experiment (unpublished), the slant of the body and of the artifactual object was created by a rigid rotation around the ankle. Then, both the body and the object exhibited the forward ^ backward asymmetry to about the same extent. But the averaged PSI was smaller for the object (4.48) than for the body (5:78). While not analysed in great detail in the present paper (see Bonnet and Paulos 2004), the variations in RTs reflect the subject's certainty about his/her response (fall versus balance). RTs are larger around the middle range of slants or, more precisely, at the PSI. They are shorter at both extremes of body slants. From a more general point of view, the present results are consistent with the hypothesis of a specific processing of human body images as compared with (inanimate) objects, provided that the latter do not evoke anthropomorphic representations. The present experiments add a further factor to those already mentioned in favour of this specificity: preference for normal human-body shape as opposed to scrambled ones (Slaughter et al 2002), the body-inversion effect (Reed et al 2003), the different processing of the parts (Reed et al 2004), and the (specific) selective impairment in some neurological patients (ie Campbell and Manning 1996; Manning and Warrington 1996). The present results also stress the fact that the dynamical properties of the human body are generalised to inanimate objects having an anthropomorphic shape whether they are real, as in the apparent movement experiments of Chatterjee et al (1996) and of Kourtzi and Shiffrar (1999), or implicit, as in the present experiments. In fact, the posture of the body in a picture activates mental representations of a movement, as shown in neuroimaging studies (Kourtzi and Kanwisher 2000; Senior et al 2000, 2002; Paulos et al, 2003). Static postures do not activate such implicit movement representations (Kourtzi and Kanwisher 2000). Of course, this is a common everyday experience used empirically, for instance, by artists or in comics. On the basis of many examples from painting and sculpture, Cutting (2002) stressed the criterion of dynamic balance for efficacy in evoking a movement in human or animal figures. Such a factor works because we have previous knowledge (implicit or explicit) of the dynamic properties of a human body which is used in our understanding of pictures. These representations also help humans to prepare their bodies for actions in their environment. Acknowledgments. We thank Maggie Shiffrar and two anonymous referees for their very helpful comments on previous versions of the paper, and to Snezana Mijailovic for running experiment 2. References Bonnet C, Paulos C, 2004 ``Reaction time as a measure of uncertainty'', in Fechner Day 2004, Proceedings of the Twentieth Annual Meeting of the International Society for Psychophysics Eds A M Oliveira, M P Teixeira, G F Borges, M J Ferrero (Coimbra, Portugal: International Society for Psychophysics) pp 320 ^ 324 Campbell R, Manning L, 1996 `Òptic aphasia: A case with spared action naming and associated disorders'' Brain and Language 53 183 ^ 221

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