Synaptic Plasticity Underlying the Cerebellar Motor Learning Investigated in Rabbit’s Flocculus

An old concept that the cerebellum is equipped with capabilities of motor learning has been substantiated by recent theoretical exploration of neuronal network models of the cerebellum and by experimental investigation of adaptive motor phenomena which involve the cerebellum. The cerebellar flocculus is inserted into the oculomotor system as a sidepath of the vestibulo-ocular reflex (VOR) arc, and it also receives visual information. It has thus been hypothesized that the flocculus is the site of adaptive modification of the VOR which takes place under visual-vestibular interaction. This flocculus hypothesis of the VOR control has been supported by lesion experiments and by recording impulse signals from flocculus Purkinje cells. Marr-Albus’ modifiable neuronal network model of the cerebellum predicts that this flocculus action is effected through plastic modifiability of synaptic transmission from granule cells to Purkinje cells in the flocculus cortex. The prediction has been substatiated by recent demonstration of a long-lasting depression at granule cell-Purkinje cell synapses after conjunctive stimulation of visual and vestibular inputs to the flocculus.