Neurones in cat parastriate cortex sensitive to the direction of motion in three‐dimensional space

Abstract

On psychophysical grounds, Beverley and Regan suggested that in man different neural mechanisms mediate the binocular perception of movement in depth and the binocular perception of positional (static) depth. They proposed that the human visual pathway contains several neural mechanisms, each sensitive to a different direction of motion in space. These mechanisms compute the direction of motion from the relative speeds and directions of movement of the left and right retinal images. Units (101) in area 18 of cat visual cortex were recorded, searching for neurons tuned to the direction of motion in 3 dimensions, with properties that could account for the proposed directionally tuned binocular motion detectors in man. The cat''s left eye viewed 1 bar, while its right eye simultaneously viewed a 2nd bar. Single units were stimulated by independently oscillating the bars from side to side. The apparent direction of movement in 3 dimensions was altered by varying the relative speeds of the bars and their relative directions of motion. The mean (positional) disparity of the bars could also be varied. For 1 class of neuron (20 cells), binocular stimulation inhibited firing for trajectories parallel to the frontoparallel plane over a large volume of space. Strong firing was produced by oppositely directed bar movements. Some of these neurons were especially narrowly tuned to the direction of movement in depth, responding only to a range of 2-3.degree., i.e., to moving bodies that would hit or only narrowly miss the cat. These cells emphasized the direction of movement at the expense of positional information. These units occurred in clusters. On the perpendicular penetrations in which they were found, they comprised a substantial majority of all cells encountered. For a 2nd class of neuron (9 cells), binocular facilitation produced selective responses to objects moving along trajectories that missed the head. The 2 classes of neuron provide a basis for 4 proposed directionally tuned binocular motion detectors. A 3rd class of neuron (17 cells) was selectively sensitive to movements parallel to the frontoparallel plane. There was strong binocular facilitation when the bars moved at the same speeds in the same directions: oppositely directed movements might be more than 100 times less effective. These neurons may signal positional disparity. These 3 classes of neuron cut across established categories. Only when both eyes were stimulated simultaneously with targets moving in different speeds and directions was it possible to demonstrate the binocular interactions described here.