Human smooth pursuit direction discrimination

The smooth pursuit system is usually studied using single moving objects as stimuli. However, the visual motion system can respond to stimuli that must be integrated spatially and temporally (Williams DG, Sekuler R. Vision Res 1984;24:55-62; Watamaniuk SNJ, Sekuler R, Williams DW. Vision Res 1989;29:47-59). For example, when each dot of a random-dot cinematogram (RDC) is assigned a new direction of motion each frame from a narrow distribution of directions, the whole field of dots appears to move in the average direction (Williams and Sekuler, 1984). We measured smooth pursuit eye movements generated in response to small (10 deg diameter) RDCs composed of 250 dynamic random dots. Smooth eye movements were assessed by analyzing only the first 130 ms of eye movements after pursuit initiation (open-loop period). Comparing smooth eye movements to RDCs and single spot targets, we find that both targets generate similar responses confirming that the signal supplied to the smooth pursuit system can result from a spatial integration of motion information. In addition, the change in directional precision of smooth eye movements to RDCs with different amounts of directional noise was similar to that found for psychophysical direction discrimination. These results imply that the motion processing system responsible for psychophysical performance may also provide input to the oculomotor system.