Types of eye movements evoked by thalamic microstimulation in the alert cat

Abstract

We characterized the type of eye movements evoked by electrical stimulation of the thalamic internal medullary lamina in alert cats. For comparison, other thalamic nuclei were studied: the lateral posterior, lateral dorsal, pulvinar complex, and ventral lateral nuclei. Three types of eye movements were distinguished: (i) Direction and amplitude-specific saccades were the most common type of eye movements evoked by electrical stimulation of the majority of the explored nuclei with the exception of the ventral lateral nucleus. Mean latency and threshold were 45 ms and 50 to 100 μA, compared with 60 ms and 200 to 300 μA for the same type of saccades evoked by electrical stimulation of the posterior thalamus. (ii) Saccades which appeared goal directed were evoked in 25% of the stimulations in the region of the internal medullary lamina. Their latency and threshold depended on the initial position of the eyes: The more deviated the eyes were from the “goal area,” the shorter the latency (minimum, 35 ms) and the lower the threshold (minimum, 16 μA). (iii) Centering saccades were evoked by stimulation of the ventral lateral nucleus. They were always accompanied by other movements. The minimum latency and threshold were, respectively, 90 ms and 200 μA. The finding that goal-directed saccades were selectively produced from certain internal medullary lamina sites is consistent with the view that a translation of spatial coordinates occurs at this level in the programming of eye movements.