Target selection for saccadic eye movements: prelude activity in the superior colliculus during a direction-discrimination task.

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Abstract
We investigated the role of the superior colliculus (SC) in saccade target selection while macaque monkeys performed a direction-discrimination task. The monkeys selected one of two possible saccade targets based on the direction of motion in a stochastic random-dot display; the difficulty of the task was varied by adjusting the strength of the motion signal in the display. One of the two saccade targets was positioned within the movement field of the SC neuron under study while the other target was positioned well outside the movement field. Approximately 30% of the neurons in the intermediate and deep layers of the SC discharged target-specific preludes of activity that "predicted" target choices well before execution of the saccadic eye movement. Across the population of neurons, the strength of the motion signal in the display influenced the intensity of this "predictive" prelude activity: SC activity signaled the impending saccade more reliably when the motion signal was strong than when it was weak. The dependence of neural activity on motion strength could not be explained by small variations in the metrics of the saccadic eye movements. Predictive activity was particularly strong in a subpopulation of neurons with directional visual responses that we have described previously. For a subset of SC neurons, therefore, prelude activity reflects the difficulty of the direction discrimination in addition to the target of the impending saccade. These results are consistent with the notion that a restricted network of SC neurons plays a role in the process of saccade target selection.