Visual and oculomotor functions of monkey substantia nigra pars reticulata. III. Memory-contingent visual and saccade responses

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Abstract
The 3 types of responses described were all related to either saccades to remembered targets or to the targets to be remembered. All the responses were a decrease in discharge rate. The new paradigm frequently used required the monkey to remember the location of a stimulus presented briefly while it was fixating; a later saccade was rewarded if it was made to the position of the no longer present stimulus. The 3 types of responses were revealed by the use of this paradigm; they were less obvious or undetectable in conventional paradigms in which the monkey responded to the stimulus that was still present. The 1st type of response was to the visual stimulus that the monkey had to use as the target for a subsequent saccade (memory-contingent visual response); a minimal response occurred if the monkey made a saccade to the stimulus while it was still present or if the monkey continued to fixate. Latencies and receptive fields for this response were similar to those for simple visual responses (17). Of 93 substantia nigra cells with some sensory-oculomotor response, 29 cells (31%) showed this type of response. The 2nd type of response was temporally correlated with a saccade made to the point where a visual stimulus was once present (memory-contingent saccade response). Nearly half of these cells showed no significant response if the saccade was made to the stimulus while it was still present, whereas others showed a comparable response in both conditions. None of them showed a change in activity in relation to spontaneous saccades in darkness. The onset of a memory-contingent saccade response usually preceded the saccade onset by up to 280 ms (most frequently by 70 to 240 ms). The response was usually spatially selective; for most responses, contralateral saccades were associated with an exclusive or greater response compared with ipsilateral saccades. Movement fields were demonstrated for some cells. Of 128 cells tested, 41 cells (32%) showed this second type of response. The 3rd type of response began after the briefly presented stimulus and continued until the saccade made to the stimulus position (memory-contingent sustained response). Of 94 cells tested, 15 cells (16%) showed this type of response. These cells frequently also showed a memory-contingent saccade response. These three types of substantia nigra cell activity are related to the special type of visuooculomotor behavior in which a visual input, particularly its spatial location, must be stored and then used as a target for a saccadic eye movement. One of their efferent connections, the nigrocollicular pathway, may act as a channel for the stored visual-spatial information to be executed as a saccadic eye movement.