Neural Basis Of The Streaming/Bouncing Bistable Perception

Two identical visual disks moving towards one another on a two-dimensional display can be perceived as either “streaming through” or “bouncing off” each other after their coincidence(i.e.overlapping),which was referred to as “streaming/bouncing bistable motion”.Although previous studies have observed a strong bias toward the streaming percept,a brief sound presented at the moment of the two disks' coincidence can strikingly bias the percept towards bouncing,which was termed “audiovisual bounce-inducing effect(ABE)”.Apart from the ABE,previous studies have also reported many other factors that can modulate the perceptual dominance between the streaming and bouncing percepts.However,the classic(visual)streaming/bouncing motion display was typically used only as a baseline in previous studies,and little is known about the neural basis underling the rivalry between the two perceptual representations in this bistable baseline itself.On the other hand,although the ABE was often considered as an audiovisual cross-modal phenomenon originating from early perceptual stage of processing,evidence based on the neural level about whether early cross-modal interaction contributes to the ABE is still scarce so far.Therefore,the present study used event-related potential(ERP)recordings to explore the brain dynamics underling the rivalry between two perceptual representations in the visual streaming/bouncing bistable display(Experiment 1).Surprisingly,the amplitude of the fronto-central P2 component that was elicited by the moving discs ~200 ms before their coincidence was found to be predictive of subsequent streaming or bouncing percept,with larger pre-coincidence P2 amplitude for subsequent streaming trials(i.e.trials on which streaming event was perceived)than bouncing trials.This P2 effect suggest that even before the streaming/bouncing event happens,unconscious rivalry between the two perceptual representations has already begun and could substantially influence the perceptual outcome formed after the streaming/bouncing event happens.The Experiment 2 introduced the audiovisual streaming/bouncing display(in which the ABE could be induced)on the basis of Experiment 1,and found that the pre-coincidence P2 effect occurred only in the visual-only display but not in the audiovisual display.These findings not only provide evidence that the pre-coincidence P2 effect reflect the unconscious rivalry between the two perceptual representations before coincidence,but also indicate that the unpredictable sound presented at the moment of thetwo disks' coincidence could strongly attenuate the influence of the pre-coincidence rivalry process on forming the final perceptual outcome.Moreover,cross-modal difference waveforms calculated separately for bouncing and streaming trials in the audiovisual display further revealed that two early cross-modal ERP components,the fronto-central positivity PD170(125–175 ms after sound onset),as well as the occipital positivity PD190(180–200 ms),were significantly larger on bouncing than streaming trials.These results provide direct electrophysiological evidence that cross-modal interactions occurring at perceptual stage of processing underlie the origin of the ABE phenomenon.