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Yujia Peng, Hongjing Lu; Seeing illusory body movements in human causal interactions. Journal of Vision 2017;17(10):68. doi: 10.1167/17.10.68.
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Goal-directed actions entail causality. One person moving his limbs in a certain way causes another person to react, creating a meaningful interaction. For example, seeing a friend throwing a ball towards you causes you to raise your arm to catch it. If humans are sensitive to the causal relation between two individual actions, then one action may provide information about the causal history of a static frame of the other action: the causal actions that generated the posture change over time. The present study examined whether human causal interactions can induce a percept of gradual motion between two distinct postures. The stimuli involved an interactive action, with one actor throwing an object and another actor catching it. The object itself was not presented. On each trial, the thrower was shown first, followed by a brief presentation of the catcher, while the thrower continued his movements in the entire trial. During the brief presentation, the catcher demonstrated either a sudden posture change (two static posture frames) or a gradual posture change with smooth movements (multiple frames). The two actors either showed a meaningful interaction (i.e. they faced each other), or a non-interactive situation (i.e. they faced away), or were presented upside-down. Participants judged whether the catcher showed a sudden or gradual posture change. We found that in the interaction condition, the proportion of trials in which a sudden change was misidentified as a gradual change was significantly higher than in the non-interactive or the inverted conditions. This finding suggests that observers were more likely to perceive illusory gradual motions when body movements were consistent with a causal interpretation of two actors interacting to achieve a common goal. To account for the human results, a Bayesian model was developed that incorporated inferred expectations based on causal actions.
Meeting abstract presented at VSS 2017
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