Occlusion rate of ball texture as a source of velocity information.

When a ball is rolling on the ground toward an observer, its elements of texture are successively occluded. The number of the elements of texture occluded per unit of time determines the occlusion rate. The aim of this study was to examine the role of the occlusion rate of ball texture and the velocity of the ball in the perception of the time remaining before the arrival. If the occlusion rate is used to perceive the time to arrival, then timing the initiation of movement should depend on occlusion rate. On the other hand, if the optical variable tau is exclusively used, then no variation is expected. 20 subjects were required to avoid balls rolling directly toward them. Three different ball textures and five ball velocities were used, leading to 10 different occlusion rates. The results showed that the timing of the initiation of the movement was not modified by variations of occlusion rate. However, the velocity of the avoidance movement increased with occlusion rate. The role of timing initiation and movement velocity in the control of the action are discussed, and it is suggested that occlusion rate is perceived and taken into account in the control of avoidance movements.