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Intracisternal injections of 6-hydroxydopamine were used to deplete brain catecholamines in cats. These animals were then subjected to a prolonged period of lateral head rotations while viewing a visual scene that always rotated with them. In contrast to their predepletion behavior, these cats showed very little decrease in the gain of their vestibuloocular reflex. These results suggest that brain catecholamines may play a role in regulating neural plasticity in the vestibuloocular system just as they appear to do in the visual cortex.

The cerebellar vermal cortex in monkey was electrically stimulated through a microelectrode with brief pulse strains synchronized to the onset of selected saccadic eye movements. The delivery of these intrasaccadic microstimuli were randomly interposed in groups of saccadic eye movements made by trained monkeys to known target locations. The trajectory of the saccades with intrasaccadic stimuli were then compared to those made to the same target without stimulation. Contralaterally directed saccades were consistently slowed from their normal trajectory (normally at 12 ms after stimulus onset) and were made hypometric. Ipsilaterally directed saccades were not affected. Vertical eye movement components were also affected at some stimulating sites. Changing the stimulus train duration from 10 to 80 ms had no effect on the change in velocity or amount of dysmetria produced in contralateral saccades. Taken together these results suggest that a different set of cerebellar elements were being stimulated by the intrasaccadic mode as contrasted to stimulation at the same site at random times. Furthermore, the short period of time during an ongoing saccade presents a unique instant in which to test the relationship of various central structures to the saccadic system. Based on current concepts of the organization of the brainstem saccadic pulse generators, these data are interpreted to suggest that: (1) the cerebellar vermis has a rather direct influence on the contralateral saccade generator, (2) this influence can act to recode (diminish) the generator's representation of the size of motor error, (3) the effect occurs at a point in the generator where motor error is spatially coded.