Visual and oculomotor functions of monkey substantia nigra pars reticulata. I. Relation of visual and auditory responses to saccades

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Abstract
The substantia nigra pars reticulata represents a major output of the basal ganglia. The efferent projection of the substantia nigra to the visual and oculomotor layers of the superior colliculus suggests some visual or oculomotor role for the substantia nigra as well. The sensory and oculomotor properties of cells in the monkey substantia nigra pars reticulata were studied by recording extracellular unitary spikes of these cells while the monkey was performing behavioral paradigms that allowed the presentation of visual and auditory stimuli and the initiation of saccadic eye movements under different behavioral conditions. Substantia nigra pars reticulata cells typically showed high background discharge rates (usually 50-100 spikes/s). The cells that responded in relation to sensory (visual or auditory) stimuli or saccadic eye movements were located primarily in the lateral part of the pars reticulata. This response was almost always a decrease in discharge rate. It was found that substantia nigra cells may respond in different ways under different experimental conditions. Response types were classified rather than cell types and were able to identify 9 response types based on their temporal correlation with sensory stimulation or eye movement and their behavioral dependency. Many cells (58% of all cells that showed some type of response) showed a simple visual response: they decreased their discharge rate in response to visual stimuli while the monkey was fixating on a spot of light. Latency of visual response ranged from 70 to 167 ms (mean, 120 ms). Spots of light were usually more effective than larger stimuli. No preference for moving stimuli or stimuli with particular shapes as present. A fraction of the cells (9%) responded to auditory stimuli. Cell activity in the substantia nigra may be related to the monkey''s orienting behavior, including shift of gaze of novel stimuli. This activity may be dependent on input directly from the caudate nucleus and indirectly from the frontal eye field.