Local invariants for biological motion perception
Chang, Dorita Hue Fung
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Observers can retrieve the facing direction of a walker from point-light displays that are devoid of structure-from-motion information and retain solely local motion signals. This ability is orientation-dependent and relies on the motions representing the feet of the agent. The experiments described here were designed to investigate visual sensitivity to local cues contained in biological motion. Initial experiments revealed that local biological motion carries information about animacy in addition to the agent’s facing direction in an orientation-dependent manner (Chapter 2). The mechanism underlying the perception of local biological motion can be dissociated from that underlying the retrieval of global structure-from-motion information according to characteristics such as sensitivity to learning and noise (Chapter 3). Further experiments revealed that the orientation-dependency for perceiving local biological motion is carried by vertical acceleration in the foot’s motion (Chapter 4). The importance of acceleration for biological motion perception raises the need to achieve a better understanding of acceleration sensitivity across various parameters such as stimulus size. To this end, Chapter 5 showed that acceleration thresholds for perceiving a linearly accelerating stimulus scale according to mean velocity as predicted by size invariance and are inversely proportional to stimulus duration. An important role for acceleration for the perception of biological motion was further corroborated by findings in an evolutionarily guided psychophysical search for the adequate local motion, defined as one that carries maximal directional information and a large inversion effect (Chapter 6). Finally, although orientation-dependency is a pervasively demonstrated characteristic of biological motion perception, the reference systems in which the stimuli are encoded are unclear. The experiments in Chapter 7 revealed that both global structure and local motion aspects of biological motion, like faces, are primarily coded in an egocentric frame of reference. Unlike faces however, there is an additional contribution of non-visual information about gravity for the perception of biological motion. These findings are finally discussed in the context of emerging behavioural, neuroimaging, and electrophysiological work that further characterize a local motion mechanism that is proposed to serve as a fundamental first stage towards interpreting animate motion patterns.