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

[摘要] 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.

[学科分类] 眼科学