Visual cortical projections to the paraflocculus in the rat. An electrophysiologic study.

The visual cortex of the rat was electrically stimulated with monopolar tungsten electrodes to determine the response properties of single neurons in the parafloccular lobule of the cerebellum. Following stimulation of the contralateral visual cortex, mossy fiber (MF)-evoked responses were indicated in 80 of 91 cells (88%) by activation of single or double spike potentials at latencies of 6-16 ms followed by an inhibition in simple spike frequency. MF activations were further identified by the demonstration of graded responses with increasing stimulus intensity and by following stimulus frequencies up to 20 Hz. Fifteen percent (9/61) of identified Purkinje cells in the parafloccular lobule demonstrated activation by climbing fiber excitations, as identified by the characteristic all-or-none burst response, which were evoked at variable latencies between 15-31 ms and 68-115 ms. Experiments involving stimulation of the superior colliculus and lesioning of the cerebral peduncle suggest that the visual cortical projection to the paraflocculus is mediated by the cortico-ponto-parafloccular pathway rather than relayed through midbrain nuclei. The electrophysiologic evidence suggests that the paraflocculus is a major receiving lobule of the cerebellum for inputs from the visual cortices.