ABSTRACT
In both humans and monkeys rapid or saccadic eye movements move the eye from one part of the visual field to another, frequently in association with shifts of attention. The neural mechanisms related to stimulus selection for such shifts of attention or initiation of saccadic eye movements have been investigated in awake, behaving monkeys. When the monkey uses a stimulus as the target for a saccadic eye movement, the response of the cells to that stimulus is enhanced. The change in discharge of single cells in one structure within the basal ganglia of monkeys, the pars reticulata of the substantia nigra, illustrates the characteristics of the enhanced visual response, particularly in relation to the initiation of movement. These experiments on the substantia nigra also show that one overt response, the saccadic eye movement, can involve different neural circuitry depending on the conditions under which the saccade is made. Some cells in this structure discharge in relation to saccades to a target that must be remembered but not in relation to saccades to visual targets or to saccades made spontaneously in either the light or the dark. Selective alteration of the neural transmitter that conveys the signal from this structure to the next, the superior colliculus, alters the monkey’s ability to make saccades to remembered targets more than to visual targets.
