Role of the monkey substantia nigra pars reticulata in orienting behaviour and visually triggered arm movements

The role of the substantia nigra pars reticulata (SNpr) has been studied in the head-free monkey during orienting behaviour in response to visual instruction signals triggering head positioning and conditioned arm movement. During the behavioural responses we recorded the electromyographic activities of neck muscles and triceps brachii, head movement, horizontal electrooculogram and single unit activity of SNpr neurons. Activity of 38 neurons located in the medial part of SNpr were analysed during the visuo-motor task. Forty percent of these units showed a moderate decrease in tonic firing rate during postural preparation preceding the orientation toward eccentric visual signal. This decrease, unrelated with saccadic eye movements per se, was followed by a marked pause observed when the rewarded stimulus was switched on and the conditioned arm movement was executed to get the reward. These data suggest that the pause in discharge of these SNpr neurons are time locked with behaviourally relevant visual stimuli and/or appropriate motor responses.