Mechanisms of static and dynamic stereopsis in foveal cortex of the rhesus monkey.

1. The sensation of stereoscopic depth rests on the central neural processing of signals evoked by the two retinal images of a single object in space. It was our purpose in this study to investigate in the behaving monkey the binocular cortical mechanisms that might underlie the ability to recognize the relative position and motion of objects in three-dimensional space. 2. The large majority of neurones studied in A17 (n = 245), and all neurones studied in A18 (n = 21), were functionally connected to both eyes, and a substantial proportion (75%) of these neurones were sensitive to positional binocular disparity. On the basis of their depth sensitivity profile, four types of stereoscopic neurones were recognized, each type characteristically sensitive to visual contours appearing in depth farther than, at, or nearer than the point of binocular fixation. 3. Tuned excitatory and tuned inhibitory neurones display binocular facilitation and binocular suppression respectively, to stimuli over a narrow range of small disparities, including zero disparity, with more or less pronounced reciprocal responses to stimuli with larger disparities. These neurones, the tuned excitatory in particular, may be considered to be the substrate for central fusion of slightly disparate retinal images, and to provide the basis for the neural mechanisms leading to three-dimensional perception of objects with high stereoacuity (fine stereopsis). 4. Two other sets of reciprocally organized neurones, near and far neurones, respond differentially to wider ranges of crossed and uncrossed disparities. The near neurones are activated by stimuli in front of and inhibited by stimuli behind fixation. The far neurones have the reciprocal depth sensitivity. These neural elements may be regarded as active in the processing of binocular information leading to qualitative depth estimates in the presence of double vision (coarse stereopsis). 5. Binocular response selectivity for the direction of object motion-in-depth depends chiefly upon monocular sensitivity to the direction of retinal image motion, a property we observed in about one half of the foveal neurones. Cortical neurones with the same directional sensitivity for monocular stimuli in both eyes display coarse binocular selectivity for the trajectory of object motion but provide unambiguous signals for the direction of motion, towards the right or towards the left within the depth domain of the neurone. A small group of neurones (3%) displays opposite and opponent directional sensitivity for stimuli in the two eyes. Their binocular response, therefore, is best when the two retinal images move in opposite directions at the same time, a condition that obtains with motion directly towards or away from the animal with little or no lateral movement. These directionally dual-opponent cells usually have coarse or no selectivity for position-in-depth. 6. The results of this study indicate that basic mechanisms for the stereoscopic analysis of the position (static) and motion (dynamic) of objects in space relative to one another are present at early stages of binocular interaction in the visual cortex of primates, and that they are in effective action during normal binocular vision.