| Summary: | Recent studies about mirror neurons suggest that many cognitive processes are based on information about body control extracted while viewing the motion of others. Although biological motion has been well studied in the field of vision science, the focus to date has not been on the extraction of information directly related to body control. In this study, we examined how efficiently people detect “focal points” of hierarchical motion, which seems closely related to body control. Focal points are the loci of the cause of shape change. For example, when we see an arm bending at the elbow, the focal point is the elbow although other parts, e.g., the wrist and fingers, actually move. Focal points directly indicate which joint or muscles you should control when you mimic the movement. In our experiment, each stimulus was composed of five points of light representing a four folded arm. We set two conditions of motion: reaching and bending. Participants were asked to detect the focal point of the motion. Angle noise was introduced to each joint movement. Results showed that the focal points were detected more correctly when they were closer to the root of the arm (the highest level of the hierarchical structure) for both conditions. Further, focal point detection was more vulnerable to angle noise during bending than during reaching. Variation in the degree of local motion is one of the possibilities to explain the observed differences between the two conditions.
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