Visual and oculomotor functions of monkey substantia nigra pars reticulata. II. Visual responses related to fixation of gaze

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Abstract
Two types of visual responses that are dependent on the monkey''s fixation of gaze were described. To demonstrate these responses, responses of substantia nigra cells were compared to the same visual stimulus with the same retinal location under 2 different conditions: one in which the monkey was or had been fixating on a spot of light when the stimulus came on (fixation condition), and the other in which no spot of light was present for the monkey to fixate on when the stimulus came on (nonfixation condition). The 1st type of response occurred to a spot of light when the monkey was not fixating on another spot of light (nonfixation-contingent visual response). Of 155 substantia nigra cells with some sensory or oculomotor responses, 45 cells (29%) showed this type of response. The response was a decrease in discharge rate that was time-locked to the onset of the stimulus (mean latency, 126 ms). Visual receptive fields of these substantia nigra cells determined under the nonfixation condition were large, extending both into the contralateral and ipsilateral sides frequently by 40.degree. of visual angle. A stimulus in the central part of the visual field was always the most effective; a stimulus in the contralateral visual field was usually more effective than 1 in the ipsilateral field. The presence of an overt stimulus on the fovea, which was usually associated with visual fixation, was important for the suppression of the visual response. The shorter the intertrial interval, the weaker the visual response of these substantia nigra cells under the nonfixation condition. The response reduction built up over several trials. The 2nd type of response related to the monkey''s fixation of gaze was a response to the offset of a spot of light (mean latency, 131 ms) that the monkey was fixating (fixation-contingent visual off-response). Of 154 substantia nigra cells with some sensory or oculomotor responses, 36 cells (23%) showed this type of response. Many of them also had a nonfixation contingent visual on-response. This contingent off-response was largely suppressed if another spot went off; the response was present only when the fixation spot went off leaving no other overt stimulus. The response became greater as the period from the offset of the fixation spot to the onset of a visual target increased. These substantia nigra cells respond to the appearance of a light spot that the monkey is going to fixate or to the disappearance of a light spot that it was fixating. They may signal the initiation or the termination of visually guided oculomotor behavior.