Sensory-to-motor processing of the ocular-following response.

The ocular-following response is a slow tracking eye movement that is elicited by sudden drifting movements of a large-field visual stimulus in primates. It helps to stabilize the eyes on the visual scene. Previous single unit recordings and chemical lesion studies have reported that the ocular-following response is mediated by a pathway that includes the medial superior temporal (MST) area of the cortex and the ventral paraflocculus (VPFL) of the cerebellum. Using a linear regression model, we systematically analyzed the quantitative relationships between the complex temporal patterns of neural activity at each level with sensory input and motor output signals (acceleration, velocity, and position) during ocular-following. The results revealed the following: (1) the temporal firing pattern of the MST neurons locally encodes the dynamic properties of the visual stimulus within a limited range. On the other hand, (2) the temporal firing pattern of the Purkinje cells in the cerebellum globally encodes almost the entire motor command for the ocular-following response. We conclude that the cerebellum is the major site of the sensory-to-motor transformation necessary for ocular-following, where population coding is integrated into rate coding.