DIRECTIONAL ORGANIZATION OF EYE-MOVEMENT AND VISUAL SIGNALS IN THE FLOCCULAR LOBE OF THE MONKEY CEREBELLUM

The floccular lobe of the monkey is critical for the generation of vis ually-guided smooth eye movements. The present experiments reveal phys iological correlates of the directional organization in the primate fl occular lobe by examining the selectivity for direction of eye motion and visual stimulation in the firing of individual Purkinje cells (PCs ) and messy fibers. During tracking of sinusoidal target motion along different axes in the frontoparallel plane, PCs fell into two classes based on the axis that caused the largest modulation of simple-spike f iring rate. For ''horizontal'' PCs, the response was maximal during ho rizontal eye movements, with increases in firing rate during pursuit t oward the side of recording (ipsiversive). For ''vertical'' PCs, the r esponse was maximal during eye movement along an axis just off pure ve rtical, with increases in firing rate during pursuit directed downward and slightly contraversive. During pursuit of target motion at consta nt velocity, PCs again fell into horizontal and vertical classes that matched the results from sinusoidal tracking. In addition, the directi onal tuning of the sustained ''eye velocity'' and transient ''visual'' components of the neural responses obtained during constant velocity tracking were very similar. PCs displayed very broad tuning approximat ing a cosine tuning curve; the mean half-maximum bandwidth of their tu ning curves was 170-180 degrees. Other cerebellar elements, related pu rely to eye movement and presumed to be messy fibers, exhibited tuning approximately 40 degrees narrower than PCs and had best directions th at clustered around the four cardinal directions. Our data indicate th at the motion signals encoded by PCs in the monkey floccular lobe are segregated into channels that are consistent with a coordinate system defined by the vestibular apparatus and eye muscles. The differences b etween the tuning properties exhibited by PCs compared with messy fibe rs indicate that a spatial transformation occurs within the floccular lobe.