VISUALLY AND MEMORY-GUIDED SACCADES IN A CASE OF CEREBELLAR SACCADIC DYSMETRIA

Saccades under four specific test conditions (visually guided, visuall y remembered, vestibular remembered, and cervical remembered) were stu died in a 38 year old man with ocular dysmetria due to an angioma of t he dorsal cerebellar vermis. The aim of the study was to investigate i f the saccadic disorder was specific to certain subsets of saccades el icited by different sensory modalities. The experiments showed that in itial saccades were equally hypermetric in all four conditions and tha t final eye position was normal in all memory guided saccade tests. Ey e movements differed after the initial saccade, however. Whereas corre ctive saccades were seen in most visually guided and visually remember ed experiments, postsaccadic centripetal drifts were documented in non -visual (vestibular and cervical) remembered saccades. These results i ndicate that the cerebellar vermis modulates the amplitude of the init ial saccade (pulse size of saccadic innervation) independently of the saccadic task. The finding that postsaccadic drift never occurred when saccades were programmed using visual positional information suggests that the dorsal vermis may participate in the process of pulse step i ntegration of saccades elicited by memorised vestibulocervical informa tion.