Influence of successive catch-up saccades on the accuracy of tracking eye movements

Catch-up saccades and smooth pursuit are two components, quick and slow, of tracking eye movements. Their interaction has usually been studied during tracking a target moving only in the horizontal direction. In these experiments catch-up saccades were invoked by changing target velocity or introducing target jumps and occlusions. Our research was focused on ocular pursuit of two-dimensional non-predictive target trajectories without target jumps. Target trajectories presented to the subjects had the same shape but moved with slow, medium and high average speeds, and each trial had the duration of 60, 30 and 15 s, respectively. During the long tracking experiments, the sequences of successive catch-up saccades were obtained and their parameters were computed. Due to the contribution of catch-up saccades, elicited during ocular pursuit, the position errors and lagging times were reduced from 30 to 50 % and larger values were obtained for higher target velocities. Analyzing the sum of tracking distances which were overcome during slow phases and during catch-up saccades, we proposed new method to evaluate subject's pursuit gain.