Temporal dynamics of the flash-induced bouncing effect.

Two identical visual disks moving toward each other on a two-dimensional (2D) display are more likely to be perceived as "streaming through" than "bouncing off" each other after their coincidence. However, either a brief auditory tone or visual flash presented at the coincident moment of the disks can strikingly increase the incidence of the bouncing percept. Despite the neural substrates underlying the sound-induced bouncing effect have been widely investigated, little is known about the neural mechanisms underlying the flash-induced bouncing effect. The present study used event-related potential recordings to explore the temporal dynamics of the flash-induced bouncing effect. The results showed that the amplitude of the postcoincidence parietooccipital P2 component (190-230 ms after coincidence) elicited by the visual motion was significantly smaller on bouncing relative to streaming trials only when the flash was presented but not when absent. In addition, the parietal P3 component (330-430 ms) was found to be larger on bouncing than streaming trials when the flash was presented, but the opposite was true when no flash was presented. These electrophysiological findings suggest that the flash-induced bouncing effect may occur at both perceptual and postperceptual stages of processing.