MSense Bounce or stream? Sound changes what you see
Two identical discs move towards each other, overlap and move apart. Do they stream through each other or bounce off each other? Watch a few passes in silence, then switch on the click.
What to notice. In silence, most people see the discs stream through each other. Add a brief click at the moment they overlap, and many now see them bounce (Sekuler, Sekuler & Lau, 1997). Move the click a few hundred milliseconds away from the overlap and the bounce fades again: a sound only counts as evidence of a collision when it arrives together with the visual event.
Why? The display is ambiguous: two objects passing, or two objects colliding. The brain picks the interpretation that explains sight and sound together, so a well-timed sound tips the balance towards a collision. This is causal inference at work: do these signals come from one event or from two?
Use the computer's own or wired speakers. Bluetooth audio can lag by 100–200 ms, enough to weaken the effect. Inspired by Michael Bach's audio-visual bounce demo.
Reference: Sekuler, R., Sekuler, A. B., & Lau, R. (1997). Sound alters visual motion perception. Nature, 385(6614), 308. doi:10.1038/385308a0