Short-latency ocular following responses of monkey. III. Plasticity.

The adaptability of the ocular following responses elicited by brief movements of the visual scene was studied in five rhesus monkeys. Adapting stimuli were double-ramp sequences, designed to initiate ocular following and then induce consistent visual errors that would challenge any adaptive mechanism regulating the performance of such tracking. Both ramps lasted 150 ms and could differ from one another in speed (speed steps) or direction (direction steps), depending on the experiment. Monkeys were repeatedly exposed to these adapting stimuli over a period of 3 days with overnight rest periods in darkness. Ocular following responses were calibrated with 100-ms test ramps that covered a range of speeds (10-100 degrees/s) and directions (right, left, up, down) and were applied 50 ms after spontaneous saccades. Repeated exposure to speed steps involving abrupt increases or decreases in speed resulted in clear increases or decreases, respectively, in the ocular following responses elicited by the standard test ramps. Changes were greatest during the first part of the day, with partial recovery during the overnight periods. Changes were specific to the direction and, to a lesser extent, the speed of the adapting stimuli. Repeated exposure to direction steps involving abrupt 90 degrees counterclockwise changes in the direction of movement resulted in the emergence of a counterclockwise orthogonal component of ocular following. Orthogonal responses differed from isogonal ones in having smoother response profiles and in showing no overnight recovery. All of the recorded changes were adaptive insofar as they would result in improved tracking of the second ramp in the double-ramp sequence. It is concluded that the ocular following responses of the monkey are subject to extensive visually mediated adaptive regulation that influences both the magnitude and direction of tracking.