Analysis of Purkinje Cell Responses in the Oculomotor Vermis during the Execution of Smooth Pursuit Eye Movements
Smooth pursuit eye movements are movements of the eyes that are used to foveate moving objects. Their precision and adaptation is believed to depend on a constellation of sites across the cerebellum, but only one region’s contribution is well characterized, the floccular complex. Here, I characterize the response properties of neurons in the oculomotor vermis, another major division of the oculomotor cerebellum whose role in pursuit remains unknown. I recorded Purkinje cells, the output neurons of this region, in two monkeys as they executed pursuit eye movements in response to step ramp target motion. The responses of these Purkinje cells in the oculomotor vermis were very different from responses that have been documented in the floccular complex. The simple spikes of these cells encoded movement direction in retinal, as opposed to muscle coordinates. They were less related to movement kinematics, and had smaller values of trial-by-trial correlations with pursuit speed, latency, and direction than their floccular complex counterparts. Unlike Purkinje cells in the floccular complex, simple spike firing rates in the oculomotor vermis remained unchanged over the course of pursuit adaptation, likely excluding the oculomotor vermis as a site of directional plasticity. Complex spikes of these Purkinje cells were only partially responsive to target motion, and did not fall into any clear opponent directional organization with simple spikes, as has been found in the floccular complex. In general, Purkinje cells in the oculomotor vermis were responsive to both pursuit and to saccadic eye movements, but maintained tuning for the direction of these movements along separate directions at a population level. Predictions of caudal fastigial nucleus activity, generated on the basis of our population of oculomotor vermal Purkinje cells, faithfully tracked moment-by-movement changes in pursuit kinematics. By contrast, these responses did not faithfully track moment-by-moments changes in saccade kinematics. These results suggest that the oculomotor vermis is likely to play a smaller role in influencing pursuit eye movements by comparison to the floccular complex.
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