Early behavior of optokinetic responses elicited by transparent motion stimuli during depth-based attention.

When two visual patterns moving in different directions are superimposed on the same depth plane (transparent motion stimulus), observers perceive transparent surfaces sliding over each other on different depth planes. Simultaneously, an optokinetic response (OKR) occurs so that one of the visual patterns is stabilized on the retina. In this study, we investigated the early behavior of the OKR elicited by transparent motion stimuli while subjects focused their attention on either the near or far surface. Two random dot patterns were superimposed and moved in orthogonal or opposite directions. Subjects were instructed to report the motion direction of the surface on which their attention was focused. The mean latency of initiation of OKR in the case of motion in opposite directions (150 ms) was significantly longer than that in the case of motion in orthogonal directions (100 ms). In the case of motion in orthogonal directions, the distribution of directions of OKR during the initial period, from 100 to 150 ms, was biased toward the mean direction of the two stimulus motions. After 160 ms, the eyes started to pursue a particular motion pattern of which the direction agreed with the far-perceived motion regardless of depth-based attention. Depth-based attention changed the direction of eye movements after 200 ms and eventually made the eyes follow a pattern on which the attention was focused. These results suggest that pursuit eye movement immediately after 160 ms may determine perceptual depth order through change of retinal image motion, because the slow-moving retinal image may be perceived in the far depth plane. Following this process of determination of perceptual depth order, depth-based attention starts to affect OKR.