Closed-Loop Optogenetic Perturbation of Macaque Oculomotor Cerebellum: Evidence for an Internal Saccade Model

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This article examines the role of the cerebellum in saccade production using optogenetic techniques in macaque monkeys. The researchers delivered light pulses to Purkinje cells in the oculomotor vermis (OMV) during saccades and observed the effects on eye movements. They found that light pulses during ipsiversive saccades slowed the deceleration phase, indicating that the OMV is involved in predicting and adjusting for eye displacement. In contrast, light pulses during contraversive saccades reduced saccade velocity but were followed by a compensatory reacceleration to help the gaze land on or near the target. The study suggests that the OMV contributes to saccade production differently depending on the direction of the saccade, with the ipsilateral OMV involved in forward prediction and the contralateral OMV in force generation for optimal eye movement. This research contributes to the understanding of the internal models and mechanisms involved in accurate eye movements.

Internal models are essential for the production of accurate movements. The accuracy of saccadic eye movements is thought to be mediated by an internal model of oculomotor mechanics encoded in the cerebellum. The cerebellum may also be part of a feedback loop that predicts the displacement of the eyes and compares it to the desired displacement in real time to ensure that saccades land on target. To investigate the role of the cerebellum in these two aspects of saccade production, we delivered saccade-triggered light pulses to channelrhodopsin-2-expressing Purkinje cells in the oculomotor vermis (OMV) of two male macaque monkeys. Light pulses delivered during the acceleration phase of ipsiversive saccades slowed the deceleration phase. The long latency of these effects and their scaling with light pulse duration are consistent with an integration of neural signals at or downstream of the stimulation site. In contrast, light pulses delivered during contraversive saccades reduced saccade velocity at short latency and were followed by a compensatory reacceleration which caused gaze to land on or near the target. We conclude that the contribution of the OMV to saccade production depends on saccade direction; the ipsilateral OMV is part of a forward model that predicts eye displacement, whereas the contralateral OMV is part of an inverse model that creates the force required to move the eyes with optimal peak velocity for the intended displacement.

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