Effects of reversible lesions and stimulation of olivocerebellar system on vestibuloocular reflex plasticity.

Abstract

The effect of rapid, reversible lesions of the climbing fiber system on the gain (eye velocity/head velocity) of the vestibuloocular reflex (VOR) was studied in alert cats. Eye movements were measured by the eye coil-magnetic field technique. A device was implanted on the skull to fix the head, which included a chamber to allow a cannula and/or electrode to be placed in or near the inferior olive. The cannula was used to inject to lidocanine to effect reversible lesions of the climbing fibers. The gain of the horizontal VOR of cats was measured in darkness by sinusoidal rotation. Gain was plastically adapted by chronic wearing of visual reversing or 2x magnifyng spectacles. VOR gain was decreased to < 0.30 or increased to > 1.6 by these maneuvers after some 3-4 days of adaptation combined with forced rotation in the light. The inferior cerebellar peduncles of 11 alert cats were electrically stimulated bilaterally during rotation in darkness. In 6 animals the VOR gain decreased by 15-60%, depending on stimulus intensity; 4 exhibited no change; 1 had an increase in VOR gain of 15-75%. Infusions of lidocaine into the CF dec [climbing fiber decussation] of alert cats produced a large, reversible, prompt increase in VOR gain whether the initial state of gain adaptation was low, normal or high. The gain incease was always paralleled by a reduction or loss of conduction in the CF pathway. The gain decreased to its preinfusion value as CF pathway conduction recovered. Motor learning is stored in modifiable synapses in the cerebellum. When climbing fibers are silenced but not killed, a learned behavior (altered VOR gain) is rapidly abolished. The modifiable synapses-storing learned behavior are not in the cerebellum, since these would not be expected to change rapidly in the absence of climbing fiber activity.