Abstract
Making rapid saccadic eye movements is the most common strategy used for visual exploration. The superior colliculus (SC) integrates information from various cortical structures like the frontal eye fields (FEF), lateral inter-parietal area, etc., and is critical in generating saccades. The nature of the visual information sent from these cortical regions and its effect on the SC neural activity is not well understood. This circuit mechanism for saccade generation can be tested if specific cortical outputs to SC can be independently modulated, which is theoretically plausible with optogenetics. Here we test the feasibility of this method by injecting a retrograde viral vector (RetroAAV2) in the SC of rhesus macaques and verifying it by optically activating retrogradely tagged projection neurons from FEF. We delivered 473nm laser pulses via a custom-made optrode while the animal freely viewed a video or performed an instructed ocular fixation task. We found that 1) Optical stimulation reliably activated various neuronal populations in SC, which resulted in triggering contralateral saccades. This confirmed the successful expression of the viral vector in the injection sites in SC. 2) Successful retrograde tagging was confirmed by the activation of multiple neurons when FEF was optically stimulated. More interestingly, the three functional subtypes of FEF visual, visuomotor, and motor neurons were optically activated, suggesting that these classes of neurons independently project to SC. Next, we compared the efficacy of optical stimulations in FEF and SC for generating contralateral saccades. Optical stimulations triggered saccades with comparable latencies in SC and FEF with a higher probability of triggering saccades during free viewing than instructed fixation. Taken together, we have successfully validated a technique that can selectively activate cortical projection neurons to SC. This will be a powerful tool to dissect the relative contribution of each cortical node in the neural circuits for saccade generation.