Bounce or stream? Motion dynamics modulate the audiovisual bounce inducing effect

Abstract

In classic audiovisual bounce inducing effect (ABE) demonstrations, the perceptual interpretation of two identical objects moving along the azimuth with uniform motion and towards opposite directions is bistable and depends on whether a sound is presented in coincidence with the point of overlap of the two objects’ motion trajectories. The mechanistic basis of the ABE is poorly understood. Here, we sought to characterize the mechanisms underlying the ABE in two experiments. In Experiment 1, we tested the effects of altering visual motion dynamics on bounce vs stream perceptual interpretations by presenting observers with events where the two discs moved with uniform rectilinear motion, acceleration, or deceleration. Sound was presented in coincidence with the point of overlap of the two objects or was absent, and object motion was varied (i.e., upwards or downwards). We found that motion dynamics acted to shift perceptual interpretations such that events with downwards accelerating visual motion were more likely to be interpreted as a ‘bounce’ event relative to those with uniform motion and decelerating motion. Curiously, there were no differences in perceptual interpretations between the three motion dynamics when stimuli moved in the upwards direction. In Experiment 2, we presented identical stimuli (downwards only) to observers while measuring functional magnetic resonance imaging (fMRI) responses concurrently with behaviour in a 3T scanner. We found that events with accelerating- and to a lesser extent, decelerating motion, elicited stronger responses in a number of cortico-vestibular regions, including the posterior parietal cortex, premotor cortex, and the posterior insula, as compared to events containing uniform motion. Notably, responses in these regions also varied depending on perceptual interpretation (i.e., bounce vs stream). Our data suggest that the origins of the ABE extend beyond simple attentional mechanisms as commonly put forth and involves vestibular engagement.