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.