Functional changes in the oculomotor system of the monkey at various stages of barbiturate anesthesia and alertness.

1. Single units in the III. and VI. nerve nuclei were continuously recorded together with vestibular stimuli and eye movements in macaques before, during, and after administration of barbiturate. 2. The visual input was functionally detached from the oculomotor system during the deeper stages of anesthesia, whereas some kind of vestibulo-ocular response could always be elicited. 3. The finding of various phase values between the maximum impulse rate IRmax of oculomotor units and the maximum stimulus velocity vmax during 1 Hz sinusoidal vestibular stimulation ranging from about 65 deg phase lead to 65 deg phase lag is suggested as important for the explanation of the phase shifts between head rotation and eye movement during anesthesia. 4. The phase relationship between IRmax and vmax was found to be unchanged, whereas the characteristic of IRmax versus vmax was highly sensitive to arousal stimuli for some oculomotor neurons. This sensitivity was represented exclusively by activation rather than inhibition.