Multiple sources of information and time-to-contact judgments.

Four experiments examined how the visual system deals with multiple information sources for perceiving dynamic events. Two tau-type optical variables, one defined by the expanding object's image and the other defined by the expanding angular extent composed of the line of sight and the object's shadow, were manipulated in time-to-contact judgments. When the information specified by both variables was consistent, little perceptual accuracy was gained by having two information sources. When the two sources conflicted, perceptual accuracy deteriorated in proportion to the degree of conflict. Based on these results, we concluded that the visual system integrates multiple sources of event-specific information, and that a reliable source of information can be the shadows cast by moving objects.