PERCEPTION AND ACTION ARE BASED ON THE SAME VISUAL INFORMATION - DISTINCTION BETWEEN POSITION AND VELOCITY

Ss were presented with spiders running from left to right at various velocities over a structured background. Motion of the background influenced the perceived velocity of the spider: Motion of the background in the opposite direction than the spider increased the perceived velocity. The perceived position of the spider was not influenced by background motion. Ss were asked to hit the spiders as quickly as possible. Fast spiders were hit with a higher velocity than slow spiders. The same effect was found if the spiders only differed in apparent velocity, induced by motion of the background. The trajectory of the hit was not influenced by motion of the background. The authors concluded that although velocity is nothing but the change of position in time, velocity and position are processed independently. Furthermore, these two separately processed sources of information are used in both perception and action.