THE EFFECT OF CEREBELLAR MIDLINE LESIONS ON EYE-MOVEMENTS

The oculomotor vermis (lobulus VI, VII) and its immediate output structure, the caudal part of the fastigial nucleus (fastigial oculomotor region, FOR), participate in the generation of saccades and smooth pursuit eye movements. Lesions to these cerebellar midline structures lead to step-size error dysmetria, with saccades to visual targets being either too large (hypermetric) or too small (hypometric). Smooth pursuit eye movements can have a reduced gain (cogwheel smooth pursuit). An analysis of lesion data in humans and monkeys reveals: 1. Lesions to the oculomotor vermis have the opposite effect to lesions of the FOR, Whereas a unilateral oculomotor vermis lesion causes hypometric ipsilateral and hypermetric contralateral saccades, FOR lesions lead to hypometric 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 pursuit disorders are related. Hypometric saccades are always associated with a reduced smooth pursuit gain, and hypermetric saccades are found with a normal smooth pursuit gain during sinusoidal stimulation. In the latter condition a smooth pursuit gain of more than 1 ('hypermetric smooth pursuit') might be expected, but for reasons unknown the smooth pursuit system seems to be able to prevent this even after bilateral deep cerebellar nuclei lesions. Lesions to the oculomotor vermis and the FOR have no effect on gaze-holding, the gain of the vestibulo-ocular 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 cerebellar artery. Here, the ipsilateral saccadic hypo- and contralateral hypermetria 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 also pathways to and from the floccular region.