Saccadic dysmetria and adaptation after lesions of the cerebellar cortex

We studied the effects of small lesions of the oculomotor vermis of the cer ebellar cortex on the ability of monkeys to execute and adapt saccadic eye movements. For saccades in one horizontal direction, the lesions led to an initial gross hypometria and a permanent abolition of the capacity for rapi d adaptation. Mean saccade amplitude recovered from the initial hypometria, although variability remained high. A series of hundreds of repetitive sac cades in the same direction resulted in gradual decrement of amplitude. Sac cades in other directions were less strongly affected by the lesions. We su ggest the following. (1) The cerebellar cortex is constantly recalibrating the saccadic system, thus compensating for rapid biomechanical changes such as might be caused by muscle fatigue. (2) A mechanism capable of slow reco very from dysmetria is revealed despite the permanent absence of rapid adap tation.