Abstract
Dorsal stream areas provide motion information used by the oculomotor system to generate pursuit eye movements. Neurons in these areas saturate at low levels of luminance contrast. We therefore hypothesized that during the early phase of pursuit, eye velocity would exhibit an oculomotor gain function that saturates at low luminance contrast. To test this, we recorded eye movements in two macaques trained to saccade to an aperture in which a pattern of dots moved left or right. Shortly after the end of the saccade, the eyes followed the direction of motion with an oculomotor gain that increased with contrast before saturating. The addition of a second pattern of dots, moving in the opposite direction and superimposed on the first, resulted in a rightward shift of the contrast-dependent oculomotor gain function. The magnitude of this shift increased with the contrast of the second pattern of dots. Motion was nulled when the two patterns were equal in contrast. Next, we varied contrast over time. Contrast differences that disappeared before saccade onset biased post-saccadic eye movements at short latency. Changes in contrast occurring during or after saccade termination did not influence eye movements for approximately 150 ms. Earlier studies found that eye movements can be explained by a vector average computation when both targets are equal in contrast. We suggest that this averaging computation may reflect a special case of divisive normalization, yielding saturating contrast response functions that shift to the right with opposed motion, averaging motions when targets are equated in contrast.
Highlights
When two moving targets appear in the visual field, the early phase of an ensuing pursuit eye movement can be fit by a weighted average of the motions of the two stimuli [1,2]
In Experiment 1, we characterized the gain of smooth eye movements following a saccade to an aperture containing a moving field of dots, as a function of root-mean-square (RMS) luminance contrast [6]
We find that smooth pursuit gain increases with the luminance contrast of the moving stimulus, but saturates, with asymptotic gain differing across the two animals tested
Summary
When two moving targets appear in the visual field, the early phase of an ensuing pursuit eye movement can be fit by a weighted average of the motions of the two stimuli [1,2]. This is thought to be indicative of how the motions of multiple objects are processed prior to selection mechanisms, such as attention or the pursuit system. In these studies, the moving targets were equal in size, shape, and contrast. The results will constrain models of the mechanisms underlying motion processing and smooth pursuit
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