Adaptive modifications of post-saccadic smooth pursuit eye movements and their interaction with saccades and the vestibulo-ocular reflex in the primate.

Adaptation of the horizontal smooth pursuit eye movement was examined using step-ramp moving target paradigms in chronically prepared Macaca fuscata. Monkeys were trained to pursue a small target which moved in the horizontal plane in a 3 degrees step-10 degrees/s ramp or a 0 degrees step-10 degrees/s ramp mode for 300-400 ms. When the target moved from central fixation point in a step-ramp mode, the monkeys usually responded with an initial pursuit eye movement (latency, 100-120 ms) which reached to a nearly constant velocity of 4 degrees/s in 50 100 ms, followed by a 1-3.5 degrees catch-up saccade (latency, 170-230 ms). The catch-up saccade was followed by a 7-8 degrees/s post-saccadic pursuit. The post-saccadic pursuit velocity was measured 40-90 ms after the end of the catch-up saccade. When the target velocity was doubled (20 degrees/s) for 100-200 ms immediately after the onset of the catch-up saccade, the post-saccadic pursuit velocity increased by 40%. When the target velocity was decreased by half (5 degrees/s) immediately after the onset of the catch-up saccade for 150 ms, the post-saccadic pursuit velocity decreased by 30%. These increases or decreases of post-saccadic pursuit were observable within just 50-150 trials. The adaptation of post-saccadic pursuit occurred independent of the position of the target. The amplitude and latency of the catch-up saccade also increased correspondingly when the post-saccadic pursuit velocity was adaptively increased. Adaptation of smooth pursuit did not affect the dynamics of reflex eye movements, including the horizontal vestibulo-ocular reflex (HVOR) gain and phase measured by 0.33 Hz-10 degrees (peak-to-peak) turntable oscillations in darkness. Conversely, adaptation of the HVOR gain induced by a 2 h sustained oscillation of the turntable and screen in reversed direction at 0.33 Hz-10 degrees affected little the velocity of post-saccadic pursuit or the amplitude of the catch-up saccade. These results suggest that different neural mechanisms are respectively involved for the adaptation of horizontal smooth pursuit and HVOR in the primate.