XCV: Eye movements from vestibular nuclei stimulation in monkeys

The vestibular nuclei were electrically stimulated in alert or lightly-sleeping cervically-transected monkeys. Upward rotatory, horizontal and downward rotatory eye movements and nystagmus were induced. The planes of these eye movements were roughly similar to those of eye movements induced by stimulation of the semicircular canal nerves or of the cerebellum. The rotatorv components of these movements were counter-clockwise when the left side was stimulated with pulse trains. The slow phases of nystagmus induced by left-sided stimulation were also counter-clockwise and the quick phases were clockwise. There was some overlap in regions of the vestibular nuclei from which various types of eye movements were induced. However, when the stimulating electrode was in the region of the superior vestibular nucleus, mostly upward rotatory eye movements and nystagmus with upward rotatory slow phases and downward rotatory quick phases were evoked. Some horizontal eye movements but no downward eye movements were induced by superior vestibular nucleus stimulation. Upward rotatory, horizontal and downward rotatory eye movements followed stimulation of the midportion of the vestibular complex. Stimulation at the junction of the rostral poles of the medial and descending vestibular nuclei induced mostly downward rotatory eye movements and nystagmus with downward rotatory slow phases and upward rotatory quick phases. The question of whether there is a topographic separation of neural organizations in the vestibular nuclei based on various planes of space was considered. Insofar as results of stimulation reflect the underlying neural organization, there is some indication that this might be so, but further evidence is necessary before this hypothesis can be accepted or rejected. Outside the vestibular nuclei it seems likely that pathways which connect the fastigial nuclei and the pontine reticular formation at least in part carry activity responsible for horizontal eye movements.