Introduction

Movement, like orientation or shape, is a fundamental property in perception. For that reason, being able to detect and determine the direction in which objects are moving confers a great deal of adaptive power on organisms. Indeed, almost all everyday situations contain dynamic information, and, accordingly, the failure to perceive motion (the clinical syndrome of cerebral akinetopsia) renders everyday life tasks difficult and even dangerous (e.g., Zihl, von Cramon, & Mai, 1983). The issue addressed in this chapter concerns the integration of motion information across sensory modalities. The advantage provided by multisensory integration of motion signals becomes obvious in naturalistic environments, where the combination of auditory and visual motion cues can provide more accurate information than either modality alone. For example, an animal rushing across a cluttered forest floor will provide a potential observer with neither a clear visual signal nor an unambiguous auditory one about the direction of motion, but together the two sensory modalities can help the observer determine directional information more accurately.

Because dynamic information pervades everyday life environments, it may be surprising to learn that past research on multisensory integration rarely included moving events for investigation. For instance, the classic example to illustrate the behavioral consequences of multisensory integration is the ventriloquist illusion, which consists of mislocalizing a sound toward a concurrent visual event presented at a different location (e.g., Howard & Templeton, 1966). In the laboratory this illusion is often studied with a single stationary sound and a single stationary light, although real-world experiences of ventriloquism typically involve movement, such as movement of the mouth of a puppet or of items on a cinema screen.

In this chapter, we review research that has addressed multisensory interactions in the domain of motion perception. We consider both the influence of static events in one sensory modality on the perception of motion by another sensory modality, and the interactions between two moving events in different modalities. We also present data from our laboratory to support the argument that, as with other perceptual dimensions, the representation of dynamic properties of external objects is achieved by integration that occurs, at least in part, at an early level of processing. We also discuss potential dominance relationships between sensory modalities in the motion integration process.