THE EFFECT OF CEREBELLAR MIDLINE LESIONS ON EYE-MOVEMENTS

The oculomotor vermis (lobulus VI, VII) and its immediate output struc ture, the caudal part of the fastigial nucleus (fastigial oculomotor r egion, FOR), participate in the generation of saccades and smooth purs uit eye movements. Lesions to these cerebellar midline structures lead to step-size error dysmetria, with saccades to visual targets being e ither too large (hypermetric) or too small (hypometric). Smooth pursui t eye movements can have a reduced gain (cogwheel smooth pursuit). An analysis of lesion data in humans and monkeys reveals: 1. Lesions to t he oculomotor vermis have the opposite effect to lesions of the FOR, W hereas a unilateral oculomotor vermis lesion causes hypometric ipsilat eral and hypermetric contralateral saccades, FOR lesions lead to hypom etric contralateral and hypermetric ipsilateral saccades. Furthermore, bilateral oculomotor vermis lesions lead to hypometric saccades, and bilateral FOR lesions to hypermetric saccades. 2. Saccade and smooth p ursuit disorders are related. Hypometric saccades are always associate d with a reduced smooth pursuit gain, and hypermetric saccades are fou nd with a normal smooth pursuit gain during sinusoidal stimulation. In the latter condition a smooth pursuit gain of more than 1 ('hypermetr ic smooth pursuit') might be expected, but for reasons unknown the smo oth pursuit system seems to be able to prevent this even after bilater al deep cerebellar nuclei lesions. Lesions to the oculomotor vermis an d the FOR have no effect on gaze-holding, the gain of the vestibulo-oc ular reflex (VOR), and the velocity storage mechanism. A third type of cerebellar midline related oculomotor deficit (saccadic contrapulsion ) is found after infarction in the territory of the superior cerebella r artery. Here, the ipsilateral saccadic hypo- and contralateral hyper metria is due to a lesion of the crossed FOR efferents. With saccadic contrapulsion smooth pursuit is affected in both horizontal directions . This may indicate that lesions not only affect FOR efferents but als o pathways to and from the floccular region.