Microsaccade control signals in the cerebellum.

Microsaccades, the small saccades made when we try to keep the eyes still, were once believed to be inconsequential for vision, but recent studies suggest that they can precisely relocate gaze to tiny visual targets. Because the cerebellum is necessary for motor precision, we investigated whether microsaccades may exploit this neural machinery in monkeys. Almost all vermal Purkinje cells, which provide the eye-related output of the cerebellar cortex, were found to increase or decrease their simple spike firing rate during microsaccades. At both the single-cell and population level, microsaccade-related activity was highly similar to macrosaccade-related activity and we observed a continuous representation of saccade amplitude that spanned both the macrosaccade and microsaccade domains. Our results suggest that the cerebellum's role in fine-tuning eye movements extends even to the oculomotor system's smallest saccades and add to a growing list of observations that call into question the classical categorical distinction between microsaccades and macrosaccades.