Audiovisual bounce-inducing effect: when sound congruence affects grouping in vision.

Two disks moving from opposite points in space, overlapping, and stopping at one another's starting point can be seen as either bouncing off one another or streaming through one another. With silent displays, observers report streaming, whereas, if a sound is played when the disks are in the overlap region, observers report bouncing. The change in perception is thought to be modulated by a lack of attention that inhibits the integration of the motion signal when disks overlap and by the sound that increases the congruence of the display, in comparison with a real elastic bounce. Here, we accompanied the disks' motion with either a bounce-congruent sound (a billiard ball) or with bounce-incongruent sounds (a water drop, a firework). When the sound was switched on 200 msec before the disks' overlap, (1) all the audiovisual displays induced more bounce responses than did the silent display, but (2) the bounce-congruent sound induced more bounce responses than did the bounce-incongruent sounds. However, when the sound was switched on at the disks' overlap, only the first result was observed. These results highlight both the role of attention and that of sound congruence.