Ocular oscillations on eccentric gaze

Many normal individuals show ocular oscillations on eccentric gaze. This study was designed to investigate the effect of visual disengagement and visual feedback on the nature of these end point oscillations. Three test conditions were examined: target present, target absent and when the target position was determined by the subject's eye position via a variable feedback control system. Feedback gains (i.e. target velocity/eye velocity) ranged from 0, where the target position was decoupled from the subject's eye movements (i.e. the target is stationary on the screen), to +1.0 where the retinal image was stabilised (i.e. the target is driven by the subject's eye movements). Only subjects who exhibited sustained end-point oscillations with no latency were included in the study (n=6). Seven different oscillations including square-wave jerks were recorded in the abducting eye during eccentric gaze of a stationary target. The three most common oscillations were the jerk oscillations, with decelerating, linear or pendular slow phases. A number of additional previously unreported waveforms were also recorded. On removal of the target, the mean drift velocity of the slow phase was greatly reduced. The response to the introduction of a change in the visual feedback was specific to each subject, although in all cases, the end-point oscillations generally were of a lower velocity, and gaze was shifted by up to 8 deg in the direction of the slow phase within the first two seconds. The important role of slow eye movement control for maintaining gaze holding is discussed.