Visual Processing In The Primate Brain

National Eye Institute

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Abstract

The superior colliculus (SC) on the roof of the brainstem is an integral part of the system within the brain for the generation of rapid or saccadic eye movements. The visual world presents multiple potential targets that can be brought to the fovea by these saccades, and these targets in turn produce activity at multiple points on a movement map in the superior colliculus. In our experiment, we recorded the activity of neurons in the SC while the awake behaving monkeys made saccades to one of two targets that were flashed almost simultaneously onto the screen in front of it. We recorded with microelectrodes at two sites on the SC map simultaneously to see how the interaction of activity at multiple sites influences saccades. Our study concentrated on strongly curved saccades because they activate multiple sites on the SC map as they veer from one target to another. A striking sequence of activity was present during curved saccades, and the timing of activity within the sequence predicted the timing of the curvature. A second related experiment demonstrated that the SC activity did not predict saccade trajectory and was not the result of two concurrent saccades. Our experiments, which are the first simultaneous recording made on the SC map, support several points. First, it is not just the magnitude of activity in the populations of active neuron that is important but the timing of that activity as well; a shift in timing will produce different behavior, even though the total activity is the same. Second, our results suggest a closer relationship between SC activity and the desired target of a saccade than between SC activity and the resulting saccade. Third, the results are consistent with a role of the SC in the selection of the target as well as the execution of the saccade to the target; the curved saccades may reflect a change in that selection even after the eye has started to move. Study of the activity on the SC map can therefore address issues related to target selection as well as saccade generation.