Purpose. Sound can alter the perceived path of ambiguous 2D motion (Sekuler, Sekuler & Lau Nature 1997). We extended this result with two studies, examining (a) sound's influence on motion in depth, and (b) the additivity of various stimulus attributes, including sound, in resolving the perceived direction of ambiguous 2D motion.

Methods.

• Study One measured perceived motion in depth of computer-generated quadrilateral forms that expanded or contracted over time. These were sometimes accompanied by an amplitude modulate sound that seemed to move sagitally, toward or away from the observer. Visual stimuli were combined with sounds that were congruous (seemed to move in same direction as the visual stimuli), incongruous, or weakly directional.
• In Study Two, temporal transients, sounds, shape deformations and various combinations of these factors were introduced into a display in which objects' 2D trajectory was ambiguous. We measured the individual and combined effect of these factors on the perceived trajectory.

Results.

• In Study One, direction-congruent sounds enhanced the impression of visual movement in depth (p<.01). Some sounds, whose directionality was easily recognized but not compelling, had no impact on perception of the visual display.
• In Study Two, various factors, including sound, disambiguated the 2-D motion display to differing degrees (p<.01). The variables' joint effect was a non-linear combination of their individual effects. For example, one variable had no effect on its own, but exerted a considerable effect in combination with other variables.

Conclusion. Together, these results reveal unexpected, non-linear behaviors that emerge when diverse stimulus variables are combined, particularly across modalities.

Supported by NSERC and by the James S. McDonnell Foundation.