Properties of saccadic responses in monkey when multiple competing visual stimuli are present

Important insights into the neural organization of the saccadic system have been gained when the usually stereotyped movement trajectories of saccades have been altered by experimental manipulation. In the present study we produced trajectory variability in monkeys by using a visual search task in which both the location and color of an odd-colored target were changed randomly trial by trial, and the number of distractors was varied on each trial. We wished to determine whether increasing the number of distractors also increased the movement trajectory variation, i.e., the amount of initial directional deviation, endpoint deviation (averaging), and curvature of saccades. Overall, saccade latencies and the proportion of saccades directed to distractors decreased as the number of homogenous distractors increased. We also found that saccades have much more dispersion in their initial direction when distractors are present in comparison to the case when only a single target without distractors appears. However, initial dispersion decreases systematically as the number of distractors increases. The percentage of averaging saccades produced in the search task was not consistently dependent on the number of distractors. A significant fraction of averaging saccades still occurred for much wider spatial separations of stimuli than in previous studies using two visual stimuli with no specified target. The curvature of saccade trajectories increased dramatically when distractors were present, but the amount of curvature was not systematically affected by the number of distractors. Errors present in saccade trajectory in the search task were only poorly compensated. We conclude that these variable saccade trajectories result from incomplete or inaccurate specification of the target when competing stimuli are present and that a smaller number of more widely spread distractors facilitate saccade variability, perhaps due to the greater difficulty of target selection.