Evidence for the existence of neural mechanisms selectively sensitive to the direction of movement in space.

1. Visual sensitivity to movement in depth was measured as a function of the relative distances through which the left and right retinal images moved. This relative distance (left:right ratio) provides a sensitive cue to the direction along which a target moves in three-dimensional space.2. Gazing at a target which moved along a fixed direction in space produced a gross reduction of visual sensitivity to movements in depth along that direction. For other directions of movement, visual sensitivity was not affected.3. Sensitivity to depth movement rapidly rose almost to the preadaptation level within the first 20-60 sec after removing the adapting target, but recovery was not complete until 100-300 min had elapsed.4. Any adapting target whose left:right ratio fell within a definite range gave similar reductions of visual sensitivity to movements in depth. There were five such ranges.5. The effects of adapting to movement in depth suggest that eight mechanisms underlie depth perception. These mechanisms are ;tuned' to the direction of movement in three-dimensional space. Four mechanisms are wholly concerned with movements along directions very close to the line which cuts midway between the eyes, and do not respond to movements whose direction departs by more than 1.5 degrees from the preferred direction.6. Neural mechanisms ;tuned' to different left:right ratios could provide a physiological basis for sensing the direction in which an object moves in three-dimensional space.