No abstract

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.

Variable saccade trajectories are produced in visual search paradigms in which multiple potential target stimuli are present. These variable trajectories provide a rich source of information that may lead to a deeper understanding of the basic control mechanisms of the saccadic system. We have used published behavioral observations and neural recordings in the superior colliculus (SC), gathered in monkeys performing visual search paradigms, to guide the construction of a new distributed model of the saccadic system. The new model can account for many of the variations in saccade trajectory produced by the appearance of multiple visual stimuli in a search paradigm. The model uses distributed feedback about current eye motion from the brainstem to the SC to reduce activity there at physiologically realistic rates during saccades. The long-range lateral inhibitory connections between SC cells used in previous models have been eliminated to match recent physiological evidence. The model features interactions between visually activated multiple populations of cells in the SC and distributed and topologically organized inhibitory input to the SC from the SNr to produce some of the types of variable saccadic trajectories, including slightly curved and averaging saccades, observed in visual search tasks. The distributed perisaccadic disinhibition of SC from the substantia nigra (SNr) is assumed to have broad spatial tuning. In order to produce the strongly curved saccades occasionally recorded in visual search, the existence of a parallel input to the saccadic burst generators in addition to that provided by the distributed input from the SC is required. The spatiotemporal form of this additional parallel input is computed based on the assumption that the input from the model SC is realistic. In accordance with other recent models, it is assumed that the parallel input comes from the cerebellum, but our model predicts that the parallel input is delayed during highly curved saccadic trajectories.

No abstract

No abstract