Eye movements evoked by cerebellar stimulation in the alert monkey.

CEREBELLAR STIMULATION has long been known to evoke eye movements (I 1, 20). Such evoked movements have been reinvestigated by many others (e.g., 8, 19, 21, 28) and most recently by Cohen et al. (5). There is not much agreement among the many studies concerning which areas of the cerebellum produce eye movements when stimulated and the nature and direction of those movements. The only agreement is that stimulation of the vermis, lobes VI and VII, evoke ipsilateral, horizontal eye movements (5, X7, 18, 20, 21). The most controversial results relate to stimulation of the hemispheres. A variety of eye movements have been reported that included ipsilateral, horizontal movements (20), nystagmus, contralateral or up movements (8), and rotatory up movements (11). Controversial results were also obtained from cerebellar nuclei stimulation. Stimulation of some areas of the cerebellum were considered to evoke eye movements without any apparent correlation between the site stimulated and the direction of the evoked movement (5, 8, 9, 18, 19). Often, the results from only one or two sites in a given cerebellar subdivision were reported rather than a thorough local exploration. The fact that there is not a single cerebellar structure from which stimulation by one or another investigator did not evoke eye movements would imply that they are represented everywhere in the cerebellum. Eye movements were not the primary interest of most investigators (11, 18, 19, 21, 25, 28); the eye movements were not studied in detail and descriptions of them were qualititative and subjective. Studies done on anesthetized animals are difficult to inter-