VISUAL-RESPONSE PROPERTIES OF UNITS IN THE TURTLE CEREBELLAR GRANULARLAYER INVITRO

1. Single units were recorded extracellularly in the turtle's cerebell ar cortex from an isolated brain preparation during visual stimulation . Only a small fraction of the isolated units responded to visual stim uli. For these visually responsive units, the most effective visual st imulus was a moving check pattern that covered the entire surface of t he retinal eyecup. The visually responsive units had little or no spon taneous spike activity, nor were they driven by flashes of diffuse lig ht or stationary patterns. 2. All the visually responsive units were d irection sensitive and were driven exclusively by the contralateral ey e. This direction tuning was well fit by a limacon model (mean correla tion coefficient, 0. 89). The distribution of the entire sample indica tes a slight preponderance of upward preferred directions. 3. The dire ction tuning of these cerebellar units was independent of stimulus con trast or the pattern's configuration (such as checkerboards or random check or dot patterns). In the preferred direction, a unit's spike fre quency increased monotonically as a function of stimulus velocity unti l approximately 10-degrees/s, but remained direction sensitive (relati ve to the opposite direction) at speeds as fast as 100-degrees/s. 4. I n some experiments the ventrocaudal brain stem was transected in the f rontal plane just caudal to the cerebellar peduncles. Although this le sion 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 th ey encode retinal slip information in the cerebellum. Unlike studies i n rabbit for which visual responses in cerebellum are binocular and ap pear to encode visual field rotation along the three canal planes, the se units in turtle are monocular and encode many directions of motion. The possible role of these units in oculomotor control are discussed.