Temporary lesions of the caudal deep cerebellar nucleus in nonhuman primates. Gain, offset, and ocular alignment

The caudal fastigial nucleus (cFN) is one of the main cerebellar areas involved in the control of eye movements. Lesions of this area cause hypermetria of ipsilateral saccades and hypometria of contralateral saccades, as well as cogwheel smooth pursuit. Most studies with nonhuman primates described this dysmetria with a change of the ratio target amplitude to saccade amplitude, whereas in cats this dysmetria was attributed to a static fixation error. The targeting of visually-guided saccades before and after 11 unilateral injections of the inhibitory transmitter muscimol into the caudal fastigial nucleus in 5 monkeys was analyzed. In one monkey, who had eye coils implanted in both eyes, the postsaccadic interocular alignment was also analyzed. The main result is that the dysmetria observed in nonhuman primates can be explained as a combination of (1) hypermetria of the ipsilateral horizontal saccade component, and (2) a horizontal static fixation error of about 1.2 deg toward the side of the lesion. The static postsaccadic alignment of both eyes was only slightly disturbed after unilateral injection. The results suggest that the dysmetria seen after muscimol inactivation of the cFN is due to two distinct processes: one causes a gain change, and the other causes the static fixation error. Unilateral lesions have only a slight influence on binocular postsaccadic alignment.