Parallel motion processing for the initiation of short-latency ocular following in humans.

With the scleral search coil technique, we recorded ocular following responses elicited by either grating or plaid pattern motions. Grating motion elicited tracking responses at short latencies ( approximately 85 msec). Type I plaid motion made by summing two orthogonal moving gratings elicited ocular following with identical short latencies. Trial-by-trial vector decomposition showed that plaid-driven responses were best predicted by a vector average of the component-driven responses. Similar results were found with micropatterns made of 16 Gabor patches with drifting carriers of two different orientations. "Unikinetic" plaids were constructed by summing a moving and stationary grating, with a 45 degrees orientation difference, so that component and pattern motion directions were separated by 45 degrees. Eye movements exhibited two components. Ocular following was first initiated in the grating motion direction, at ultra-short latency. A second component was initiated approximately 20 msec later, curving the responses toward the pattern motion direction. The later component was specifically, and independently, affected by both relative spatial frequency and contrast between component gratings. The early response components showed a much steeper contrast response function than the late component. These results suggest that initial ocular following is underpinned by parallel processing of component- and pattern-related velocities followed by an integrative stage that computes the two-dimensional surface motion.