Eye movements evoked by microstimulation in the flocculus of the alert macaque.

We have used microstimulation in the floccular cortex of alert monkeys to study the role of the cerebellar flocculus in the control of slow eye movements and the representation of eye movement direction in the flocculus. Eye movements having peak velocities of less than 15 deg/s, sometimes accompanied by rapid eye movements, were evoked using trains of current pulses (22 microA or less). The initial slow eye movement was designated Phase I. By restricting current spread to specific layers of the flocculus, evidence was obtained that Phase I was caused by the activation of molecular layer elements; it appeared to reflect the suppression of Purkinje cell activity. Phase I, therefore, probably represents the best estimate of the effect of changing Purkinje cell activity in a limited region. Phase I ended either during the pulse train or after the end of the pulse train; following the end of Phase I, the eye movement reversed its direction. Again by stimulating various layers, the eye movement following the reversal (Phase II and the off-response) was attributed to activation of Purkinje cell axons and other elements, including floccular afferents, in the granular layer and white matter. Phase I eye movements in all directions were evoked from the flocculus. Homogeneous zones with respect to eye movement direction were approximately 300 micron in diameter. When the cortex was represented as a two-dimensional sheet, a large number of zones corresponding to different directions of Phase I were observed.