Manual People Watching: Social, Perceptual, and Neurophysiological Studies of Body Perception

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Ban, H. Journal of Information and Television Engineers , 69 6 , Training transfers the limits on perception from parietal to ventral cortex. Current Biology , 24, Mechanisms for extracting a signal from noise as revealed though the specificity and generality of task training.

Journal of Neuroscience , 33 27 , Troje, N. Shape-independent processing of biological motion. Johnson, M.

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Shiffrar Eds. Oxford University Press. Hirai, M. Body configuration modulates the usage of local cues to direction in biological motion perception. Psychological Science , 22 ,— Frames of reference for biological motion and face perception. Journal of Vision , 10 6 , 1— Characterizing global and local mechanisms in biological motion perception. Journal of Vision , 9 5 :8, Acceleration carries the local inversion effect in biological motion perception. Journal of Vision , 9 1 , Perception of animacy and direction from local biological motion signals.

Experience and the Perception of Biological Motion

Altogether, these findings suggest that there is a general, positive, association between the importance or value of an object to the viewer and its perceived size. Animate motion patterns are rapidly perceivable 15 , 16 , 17 , visually salient 18 and carry numerous types of information that are readily retrievable by human observers 19 , 20 , The perception of biological motion is arguably of high ecological importance, making preferential processing by the visual system for such stimuli likely, even though direct evidence is scarce.

The most commonly used tool to explore this question is point-light figures. They eliminate all visual information obtainable from the surface of the body by only showing the movements of a few important articulations depicted as dots. With the help of point-light stimuli, biological motion has been shown to yield to several perceptual benefits as compared to similar non-biological motion.

For example, coherent and upright point-light walkers PLWs are processed incidentally in a flanker paradigm, as opposed to static, scrambled 22 or inverted 23 walkers.

Biological motion distorts size perception

Upright, scrambled biological motion stimuli lead to faster hits in a search task than similar, but inverted figures 24 , which means that local cues of biological motion act on a preattentive level of visual processing. Upright human or terrestrial animal PLWs induce reflexive attentional orienting in a central cueing paradigm, while inverted or static figures do not 25 , showing that incidental effects are not specific to stimuli presenting configural information that is typical of humans.

Biological-motion stimuli, therefore, seem to be of special importance in visual processing. Also, important objects tend to look larger to the viewer. We thus hypothesize that stimuli carrying ecologically valid biological motion cues appear larger to observers, as compared to similar motion stimuli lacking ecological validity. We tested this hypothesis using human PLWs in three experiments, where we compared coherent, upright, PLW figures to inverted ones. In the inverted displays, both local and global biological motion cues lack ecological validity, while all other aspects of the stimulus remain equal to those in the upright figures.

Hence, seeing a difference in perceived size between the two conditions can only be due to the effect of the ecological importance of biological motion. Perceived sizes of upright and inverted PLWs see Fig. All participants had normal or corrected-to-normal vision, provided written informed consent and received monetary compensation. One participant dropped out after reporting problems with larger stimulus sizes. Walker size was varied in 10 steps between 2. Each PLW presentation started at a random frame of the stride, resulting in slightly varying sizes for each trial.

All displays were gray on black background, with a red fixation point continuously shown in the center of the screen. After the mask, participants had to move the mouse in order to adjust a rectangle to frame the area occupied by the previously seen walker as tightly as possible Fig. The rectangle stayed centrally symmetrical at all times. That way, the width and height of the walker were set independently, albeit in a single response. Each participant completed trials. For each trial, the percentage of overestimation area of the response rectangle divided by the area of the smallest frame containing all dots at any time was calculated.

This confirms our hypothesis that upright walkers are perceived to be larger than inverted walkers.

Maggie Shiffrar

If the effect is caused by configural information alone instead of biological motion, static figures should elicit the same pattern of results as dynamic PLWs. Stimuli were presented on a Each participant completed a total of trials split over four blocks.

Two blocks contained dynamic PLWs while the other two contained static point-light figures. Static and dynamic trials were otherwise identical. The order of the four blocks was counterbalanced across observers. Perceived sizes of upright and inverted PLWs were compared indirectly, with a size judgment task on targets presented subsequently to PLWs. As upright PLWs are perceived as larger, we expect that contrast effects will lead to a subsequent target to appear as smaller. Since participants react to simple disc targets and they are instructed to ignore the preceding figures, this experiment further ensures that our previous findings are caused by a perceptual distortion of size and not by any unexplored bias related to PLWs.

Generation and presentation of stimuli were as described for Experiment I. PLWs both walkers, in all conditions: 5. All displays were gray on black background, and a fixation point was continuously shown in the center of the screen. Participants were instructed to ignore these displays. Targets were either identical or differed in size Participants gave a non-speeded forced choice response by pressing one of two buttons, indicating which of the targets was larger than the other. Participants 1—8 also completed trials for a temporal judgment task in separate blocks, which are not reported here.

For participants 1—4, no trials with identical targets were presented. For participants 5—8, eye tracking was used to validate that observers maintained fixation throughout stimulus presentation. Participants 1—4 each performed trials, participants 5—8 each performed trials and participants 9—16 each performed trials in total. For each participant, the point of subjective equality PSE between targets preceded by upright and inverted walkers was calculated. A one-sample t-test was then used to determine whether PSEs were significantly different from zero.

Percent of responses indicating that the target preceded by an upright walker was larger plotted against the difference between target disc sizes. Means per condition with fitted psychometric function. Error bars show s. Note that the psychometric function for illustration is a fit to the average data, while for statistical analysis each individual was fitted with a separate psychometric function and analysis was based on the distribution of the individual PSEs. This is in line with our hypothesis and shows that targets preceded by an upright walker are perceived as smaller than targets preceded by an inverted walker Fig. The findings presented here show that stimuli with ecologically valid biological motion cues appear larger than similar motion stimuli without ecological validity. Experiment I demonstrates this phenomenon. Experiment II replicates the findings and shows that static point-light displays do not lead to a similar distortion in perceived size. Experiment III shows that the effect can also be measured indirectly, as it extends through a contrast mechanism to subsequently presented, neutral, stimuli.

Prior studies have demonstrated that discrimination of biological motion stimuli takes place at an early stage of visual processing 16 and induces reflexive attentional orienting This suggests that biological motion stimuli bear high importance, which is further supported by experiments demonstrating that humans 32 and other animals 33 have an innate sensitivity to visual invariants characteristic to biological motion.

Although a contrast effect seems the most likely mechanism transferring the distortion in perceived size from PLWs to the disc targets used in Experiment III, alternative causes are also possible. For example, spatial attention might be deployed asymmetrically between upright and inverted walkers, causing an inhibition of return 34 on responses to subsequent target discs.

While PLWs are useful in eliminating surface information from the body, they thus also take biological motion cues out of their natural context. If that happens more likely for upright than for inverted figures, a larger percept would be formed for the former.

Original Research ARTICLE

However, Experiment II offers some hint that this may not be the case in our experiments, as the human figure is also clearly recognizable from the frontal view of a static point-light display cf. We cannot exclude that sex differences might also play some role in the results, considering that our participant population was dominantly females and it is conceivable that women are more responsive to biological motion and its social implications Studies on the link between social power and size 6 , 7 , 8 , 9 , 10 , 11 suggest that the sex as well as the displayed power of the stimulus figure can likewise affect the outcome.

Studying sex differences of the reported effects might therefore be an interesting extension in further research.

As it has been shown with other stimuli already, importance to the viewer makes objects look larger. Our data show that biological motion is no exception. It clearly demonstrates a so far unknown example of distorted size perception.