This program of research aims at an understanding of the operational mechanisms in the cerebral cortex of primates which underlie stereoscopic depth perception. Experiments will be conducted in alert rhesus monkeys behaviorally trained to perform repeatedly a visual task, while single neuron activity will be recorded with one or more microelectrodes from striate and prestriate cortex. Stimuli will be presented stereoscopically on two color displays separately seen by the left and right eyes, and generated with a high-speed graphics system. The position of one (or both) eyes will be monitored continuously in order to control the monkey's direction of gaze. The principal objective is to obtain experimental evidence for or against the hypothesis that disparity-selective responses of cortical visual neurons are directly linked to stereoscopic depth perception, and to ascertain the significance of the different types of the neural disparity selectivity in the stereoscopic process. Three lines of investigation will be followed: First, experiments will be done to assess the responses of disparity selective neurons in visual areas V1, V2 and V3- V3A to solid figure and dynamic random-dot stereograms of different luminance contrast. Stereopsis is degraded or lost when viewing isoluminant stereopatterns: changes in the responses of stereoscopic neurons of monkey with stimuli of low luminance contrast would be a strong indication in favor of a direct role of these neurons in stereopsis. Moreover, the results of these experiments would provide information about which anatomo-physiological visual subsystem (magno- or parvo-cellular) neurons with stero properties are part. An associated series of experiments will be designed to investigate the mechanisms underlying the responses of cortical neurons to random-dot stereograms, that is of the mechanisms providing the substrate for the correct matching of corresponding elements of the images in the two eyes, a task that human subjects perform effortlessly and most accurately. Finally, I will attempt to assess the influences of attention and motivation on the neural responses of depth neurons in different cortical areas. The basic strategy of thee experiments will be to train the monkey to attend to the very stimulus that activates the cortical neuron under study, and to identify differences in the neuron's responses when that stimulus is or is not of behavioral relevance to the monkey.