Learned stimulation in space and motion perception.

Abstract

In the perception of distance, depth, and visual motion, a single property is often repre- sented by two or more stimuli. Two instances of such redundant stimulation are discussed. (a) the various stimuli that represent visual motion and (b) the two forms of stimulation by which binocular parallax evokes stereoscopic depth perception. In the case of visual motion, simultaneous operation of redundant stimulation has unexpected consequences and raises interesting problems. Experiments are briefly de- scribed that suggest that some redundant stimuli owe their existence to learning. Evidence is reviewed that shows that binocular parallax causes stereoscopic depth by means of two different perceptual processes. During the last 30 years evidence that space percep- tion can be rapidly altered by perceptual learning has been accumulating. Adaptation to displaced visual direction involves, under certain conditions, a visual change, and there are other ways in which the relation between the position of the eyes and perceived visual direction can be altered. When subjects turn their heads from side to side while they observe a visual environment that moves left and right dependent on those head movements, an adaptation develops within minutes. It involves changes in the evaluation of eye movements that compensate for head movements and in the evalua- tion of eye positions as measured by pointing tests (Wallach & Bacon, 1977). In stereoscopic depth perception, the relation between retinal disparity and the extent of perceived depth can be rapidly changed. In such experiments, tridimensional shapes are viewed through a mirror arrangement that enhances the disparities with which their depth is given so that they are perceived with greater than normal depth. When these shapes are rotated, information about their true depth is pro- vided, which disagrees with the enhanced stereoscopic depth that the mirror arrangement causes. Exposure to this conflict between two different kinds of stim- ulation that both represented the depth of the same object resulted in adaptation; stereoscopic depth perception was temporarily altered so that it partly compensated for the enhanced depth that the mirror