The visual control of movement involves both control of where a movement is to be made and when it is to be made. In the oculomotor system, these two steps appear to be performed by different sets of neurons. In the superior colliculus, these neurons are segregated into different regions. Neurons in the caudal superior colliculus discharge before the onset of rapid or saccadic eye movements that move the eye to one part of the visual field; these neurons are related to where the movement is directed. These neurons can be further divided into those that give a discrete burst of activity before each saccade (burst neurons) and those that have a long lead-sustained increase in firing frequency prior to the initiation of saccades (buildup neurons). Neurons in the rostral pole of the colliculus discharge vigorously while the monkey is fixating between the generation of saccades and pause at the time of the saccade; these fixation neurons are related to controlling when the eye movement occurs. Throughout selection of a target for the next movement, preparation to make the movement, and the actual execution of the movement, both the where and when of the movement must be coordinated. We investigated this coordination among the neurons of the superior colliculus (SC). We assessed the activity of burst, buildup, and fixation cells of the SC in a task that temporally isolated events leading up to and including saccade initiation. In the task, the monkey was required to fixate for a random period of time after which 1, 2, 4, or 8 possible targets were presented. After a random period of time, one of the targets dimmed, and then after removal of the fixation point, the monkey had to initiate a saccade to the dimmed target. This task has three temporally distinct periods: when the array is illuminated, the monkey is unaware of the goal of the impending saccade (period of uncertainty); the dimming of the target specifies the saccade goal (period of selection); the removal of the fixation point serves as a """"""""go"""""""" signal (period of saccade initiation). During the period of uncertainty, buildup neurons showed an increase in activity in the 1 and 2 target conditions but showed relatively less activity in the 4 and 8 target conditions. In the 4 and 8 target conditions, buildup activity occurred only after the saccadic target was selected (when the target dimmed). During this selection period, little additional buildup occurred in the 1 and 2 target conditions. Some fixation neurons tended to show the reverse of buildup neurons; they increased activity with increasing target uncertainty. The majority, however, remained unchanged. Burst neurons remained inactive until the signal to initiate a saccade was presented irrespective of the number of possible targets. Thus, in the preparation to make a saccade, the predictability of the saccadic target determines the extent of activity in the buildup neurons and has no influence on the activity of the burst neurons; the former appear to be closely related to the selection of the target, the latter to the generation of the saccade. Fixation neuron activity follows the time course of the target selection. Thus, neurons that control when the movement is to be made as well as a subset of the neurons that control where the movement is to be directed are closely related to the predictability of the movement. Other neurons related to where the movement is to be directed are more closely related just to the generation of the movement itself.
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