Cerebellar Vermis Involvement in Monkey Saccadic Eye Movements: Microstimulation

The cerebellar vermis (lobules V to VII) was focally stimulated (maximum current = 300 microA) through microelectrodes in alert monkeys trained to fixate on visual targets located at different positions in the eye movement field. Microstimulation of this area evoked saccades whose direction and amplitude were dependent on the spatial locus of the vermal point stimulated and on the position of the eye at the time of stimulation. Stimulation evoked saccades on most (70%) of the electrode penetrations. By alternatively stimulating and recording through the electrode as it advanced through the depth of the vermis, it was possible to map the threshold for evoking saccades along a penetration as well as to ascertain the type of tissue (white matter or Purkinje cell layer) situated at the stimulation points. On penetrations where saccades were evoked, there was generally one (90%) and sometimes two (10%) low-threshold region(s). These low-threshold regions were located in fiber tracks and not in the Purkinje or other cellular layers of the cerebellar cortex. The direction and size of the evoked saccades were dependent on position of the eye in the orbit. At a few sites, even the presence or the absence of an evoked saccade depended on the initial eye position. Postsaccadic drifts after termination of evoked saccades were also a common feature (50% of the tracks) associated with vermal microstimulation. The presence or absence as well as the direction of the postsaccadic drift were also dependent on initial eye position. These observations suggest that the vermal stimulations evoked saccades by the antidromic activation of mossy fiber afferent inputs that emanate from the brain stem saccadic burst generator. Furthermore, stimulation in this manner would actually bypass the cerebellar circuitry and produce saccades without the usual modifying influence of the cerebellum.