Correlated changes in perceptions of the gender and orientation of ambiguous biological motion figures

The sensitivity of the mammalian visual system to biological motion cues has been shown to be general and acute [1–3]. Human observers, in particular, can deduce higher-order information, such as the orientation of a figure (which way it is facing), its gender, emotional state, and even personality traits, on the basis only of sparse motion cues. Even when the stimulus information is confined to point lights attached to the major joints of an actor (so-called pointlight figures), observers can use information about the way the actor is moving to tell what they are doing, whether they are a male or female, and how they are feeling [4–6]. Here we report the novel finding that stimulus manipulations that made such walkers appear more female also had the effect of making the walkers appear more often as if they were walking away from rather than towards observers. Using frontal-view (or rear-view) pointlight displays of human walkers, we asked observers to judge whether they seemed to be walking towards or away from the viewing position. Independent of their own gender, observers reliably reported those figures they perceived to be male as looking like they were approaching (as reported in [7]), but those they perceived to be female as walking away. Furthermore, figures perceived to be gender-neutral also appeared more often, although not exclusively, to be walking towards observers. In Experiment 1a, observers judged the gender, male or female, of each walker. Mean ratings across Correspondences observers are illustrated in Figure 1 (right ordinate, solid circles). A cumulative Gaussian was fitted to the ratings of each figure. Female walkers uniformly were judged as being female, and male walkers as male. The point of subjective ambiguity corresponded to the middle walker (indicated in Figure 1 as 0) of the 13 stimuli used. In Experiment 1b, the same observers were asked, when viewing the same stimuli, to judge whether or not each figure was walking towards them. Means were calculated, for each walker, as a proportion of the number of times observers judged that walker to be facing (walking towards) them and a cumulative Gaussian was fitted to the function (Figure 1, solid triangles, left ordinate). The point of subjective ambiguity — that is, the point at which observers were equally likely to judge the figure as walking towards them as away from them — corresponded to a female walker. In other words, all walkers perceived to be male were judged by observers to be facing towards them. Only when the walkers had characteristics consistent with being female did observers begin to perceive them more often as facing away. While not unequivocal these data do suggest that perceived direction-of-facing is related to the perceived gender of a point-light walker. In Experiment 2, we examined the effect of perspective information on perceptions of orientation. Three point-light walkers used in Experiment 1, those judged as most female (–6), most male (+6), and gender-neutral (0), were presented again on a treadmill but this time with perspective information. The three walkers were presented to observers, this time incorporating visual cues consistent with the walker’s approach or retreat. The addition of perspective cues was expected to standardise observers’ perceptions across the three walkers if perspective affects perceived orientation more strongly than gender. Conversely, if perceived orientation is affected more strongly by the gender than the perspective of the walker the gender differences in Experiment 1b will persist. The gender effects reported for Experiment 1 were confirmed. Although perspective cues modified facing judgements for each figure (female, neutral, male) from convincingly looming to convincingly receding, Figure 2 shows orientation biases that were consistent with the findings from Experiment 1b: male walkers were reported as facing towards observers on more than 50% of trials for all perspective cues except for the two walkers with perspective cues most strongly suggesting retreat. A similar pattern of results was observed for the neutral walkers. This was different for female walker stimuli which frequently were judged as facing away from observers even when perspective cues unambiguously signalled looming. Figure 2 (insert) illustrates those points by plotting the means of the fitted functions against the perspective cues. Veridical (non gender-biased) use of the perspective cues would place the mean for each of the walker stimuli at the perspective-cue neutral point (broken horizontal grey line). Instead, the means of the functions arising from judgements of the male and gender-neutral walkers are above the neutral point, consistent with those walkers appearing to