Activity of substantia nigra pars reticulata neurons during smooth pursuit eye movements in monkeys

Abstract

Nuclei within the basal ganglia (BG), in particular the substantia nigra pars reticulata (SNr), subthalamic nucleus (STN) and caudate nucleus, are known to be involved in the generation of rapid or saccadic eye movements. Neurons in the SNr are active tonically and generally show a pause, but also increase, in discharge rate, for the appearance of visual stimuli and the generation of saccades. Recent experimental results in oculomotor regions of the brainstem reveal overlap in the neuronal pathways used for saccades and smooth pursuit, or slow tracking, eye movements. Whether the overlap of processing for saccades and pursuit extends to the oculomotor BG is unknown. In the present report, we were interested in whether the overlap between the pursuit and saccadic systems extends into the oculomotor BG. Using single-neuron recording and electrical stimulation techniques, we tested whether neurons within the saccade portion of the BG, the SNr, could be involved in smooth pursuit eye movements. Monkeys were required to follow visual targets with either a smooth eye movement or a saccade while we recorded from SNr neurons. We report here on SNr neuronal activity that was modulated during the performance of visually guided saccades and also during the initiation and the maintenance of smooth pursuit eye movements. Importantly, the modulation of neuronal activity during pursuit was present even when catch-up saccades were absent. The majority of SNr neurons was active tonically and their discharge ceased during pursuit, although some neurons also increased their discharge rate during smooth pursuit, similar to the behaviour reported for saccades. We also found that electrical stimulation of the SNr during the initiation of pursuit suppressed ipsiversive and, in some cases, enhanced contraversive pursuit. Our combined recording and stimulation results are consistent with the hypothesis that the overlap between the pursuit and saccadic systems extends, at least somewhat, into the BG and that the signal conveyed by the SNr can be used by the pursuit system. Like the signal for saccades, the SNr may provide a permissive disinhibition for pursuit eye movements. We hypothesize that alterations in this signal in BG diseased states such as Parkinson's may explain in part the deficits observed in smooth pursuit eye movements of these patients.