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Junzhu Su, Hongjing Lu; Action prediction and the flash-lag effect. Journal of Vision 2015;15(12):501. doi: 10.1167/15.12.501.
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The flash-lag effect, a misperception of spatial relations between a moving object and a briefly flashed object, has been empirically well-established. However, it remains unknown whether high-level visual representations, such as biological motion, influence this illusion. To address this question, observers viewed a biological motion stimulus (a walker shown in a skeleton display). A stationary dot was briefly flashed (33ms) adjacent to a reference joint (either a hand or a foot joint) of the walker. Participants were asked to judge the relative spatial relation between the flashed dot and the reference limb. We found a strong flash-lag illusion for biological motion: observers perceived a lag between the reference limb and the flashed dot even when they were exactly aligned in space and time. Furthermore, when the actor walked in a natural direction (i.e., forward walking), the flash-lag effect was enhanced compared to the effect for a backward walker. This asymmetry suggests that updating future postures for familiar actions plays an important role in perceiving the relative spatial relation between a moving agent and other objects. When the walker was inverted, the flash-lag difference between the forward and backward walking directions was maintained for hand joints. However, when the flashed dot was adjacent to foot joints in an inverted walker, the flash-lag difference between the forward and backward walking directions was eliminated. This result suggests that action prediction may maintain different degrees of precision for different joints, depending on the functional importance of the joints for a particular action (e.g., feet are the most critical joints for a walking action, hence prediction of its future location may be more sensitive to the change of the body orientation). Overall, our findings indicate that the flash-lag effect can provide a powerful tool for studying action prediction.
Meeting abstract presented at VSS 2015
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