The organization of visual processing in the pigeon cerebellum.

1. Visual responses were sought in the cerebella of decerebrate pigeons using extracellular micro-electrodes, and were found in folia VIc-IXb, especially folia VII and VIII. The responses were mainly, but not exclusively, from the ipsilateral eye. Four binocular units were recorded.2. In the anterior and posterior walls of folium VII the organization was clearly, though rather crudely, retinotopic. The temporal field was represented laterally, on the ipsilateral side, and the nasal field medially; the superior field was represented superficially, and the inferior field towards the base of the folium. In the lateral wall of folium VII there was a small anomalous region innervated by the contralateral eye.3. The visual input arrived via the mossy fibre system.4. Units exhibited a strong preference for moving targets, 20-60 degrees /sec being the range of optimal speeds. About three quarters of the units responded most strongly to a particular direction of target motion. The preferred direction was most frequently upwards or backwards.5. Units in the granule layer were to some extent clustered according to their direction-preference, which tended to change gradually as the electrode advanced along the granule layer.6. Units were classified as Gr-units (granule cells or mossy fibre rosettes) or P-units (Purkinje cells) or were left unclassified. Receptive fields of Gr-units were usually 5-30 degrees across; they were much larger for P-units in folium VIII, but not obviously so for those in folium VII. Gr-units were more frequently direction-sensitive than were P-units.