Response of pursuit cells in MST after eye position perturbation by microstimulation of the Superior Colliculus (SC)

Abstract

Primates use the fovea to maintain high quality central vision of the world. When a target is moving, smooth pursuit eye movements (SEM) keep the fovea of both eyes on target. A large proportion of neurons in area MST discharge during SEM. These smooth pursuit cells (SPC) carry visual and extraretinal signals. We recorded 27 SPC in MST of a macaque monkey during SEM trials interrupted with a saccade that was elicited by electrical microstimulation (MS) in the SC. The MS consisted of a train of pulses (0.1ms, 400Hz, 40ms) at low currents (< 40mA). The tracking behavior was characterized by 1) an evoked saccade that brought the eye outside the target path, 2) a SEM following this abrupt change in eye position and 3) a corrective saccade. We quantified the ratio of activity after the evoked saccade, before the corrective saccade and during the corrective saccade. A majority of neurons (63%) presented ratios between 0.8 and 1.2 after the evoked saccade but ratios less than 0.8 during the corrective saccade. Among this subpopulation, 59% actually had a decrease of their firing rate before the corrective saccade. On average, this drop of activity occurred 54 (±11ms) after the evoked saccade offset or 109 (±9ms) after its onset. Interestingly, in 90% of cases the latency of this drop of activity was sensitive to the delay between the evoked and corrective saccades. Finally, 22% neurons did not present a drop of activity during these intervals while 15% presented a decrease from the evoked saccade offset. This eye position perturbation showed that the activity of a majority of MST smooth pursuit cells was not interrupted by a direct corollary discharge from the saccadic system. However this activity seems inhibited by the occurrence of a corrective saccade even though not always time-locked to it. Meeting abstract presented at VSS 2017