Role of the cat substantia nigra pars reticulata in eye and head movements. I. Neural activity.

Single unit activity was recorded in the Substantia Nigra pars reticulata (SNpr) of cats trained to orient their gaze toward visual and/or auditory targets. Cells in the SNpr have a steady high rate of spontaneous activity ranging from 35 to 120 spikes per second. The neurons respond to sensory stimuli or in relation to saccadic eye movements with a decrease or a cut-off of the spontaneous discharge. Among 109 cells recorded in the SNPR 60 were responsive to visual stimuli (mean latency = 118 ms). Most of the receptive fields which were plotted were large encompassing part of the ipsilateral field. Thirty nine (39) cells were responsive to auditory stimuli (mean latency = 81 ms). A majority of these cells showed a better response for stimuli located in the contralateral hemifield. In a few cells, the sensory responses were modulated by the subsequent orienting behavior of the animals. Thirty one (31) cells showed a response in relation to saccades. These units typically stopped discharging between 50 and 300 ms prior to the onset of the saccade. 39% of these units also responded in relation to spontaneous saccades in the dark. 61% of the saccadic cells also responded to sensory stimuli in the absence of saccades. Six (6) cells were found to respond to active head movements. These results are discussed in the framework of the role that the basal ganglia might have in the selection of the sensory stimuli that trigger orienting behaviors.