1. Single units were recorded extracellularly in the turtle's cerebellar cortex from an isolated brain preparation during visual stimulation. Only a small fraction of the isolated units responded to visual stimuli. For these visually responsive units, the most effective visual stimulus was a moving check pattern that covered the entire surface of the retinal eyecup. The visually responsive units had little or no spontaneous spike activity, nor were they driven by flashes of diffuse light or stationary patterns. 2. All the visually responsive units were direction sensitive and were driven exclusively by the contralateral eye. This direction tuning was well fit by a limacon model (mean correlation coefficient, 0.89). The distribution of the entire sample indicates a slight preponderance of upward preferred directions. 3. The direction tuning of these cerebellar units was independent of stimulus contrast or the pattern's configuration (such as checkerboards or random check or dot patterns). In the preferred direction, a unit's spike frequency increased monotonically as a function of stimulus velocity until ~10°/s, but remained direction sensitive (relative to the opposite direction) at speeds as fast as 100°/s. 4. In some experiments the ventrocaudal brain stem was transected in the frontal plane just caudal to the cerebellar peduncles. Although this lesion presumably removes climbing fiber input from the inferior olivary nuclei, the visual-response properties in the cerebellar cortex were unaffected. 5. The response properties of these units indicate that they encode retinal slip information in the cerebellum. Unlike studies in rabbit for which visual responses in cerebellum are binocular and appear to encode visual field rotation along the three canal planes, these units in turtle are monocular and encode many directions of motion. The possible role of these units in oculomotor control are discussed.
