Infatigability of pupillary constriction evoked by hypothalamic stimulation in monkeys

Abstract

In 5 rhesus monkeys, electrodes were permanently implanted in the hypothalamus to investigate the effects of electrical stimulations on pupillary size. Results were as follows: Ipsllateral constriction of the pupil in the absence of any other visible response, including eye movements, was evoked by electrical stimulation of the inferior part of the posterior hypothalamus in the neighborhood of the origin d the oculomotor nerve and also of more rostrally located structures. Dilatation of the ipsilateral pupil was evoked by stimulation of more dorsally located points. The most efficient parameters of stimulation were around 0.5 msec, and 50 to 100 cps, contradicting the general opinion that low-frequency stimulation and long-pulse duration are always preferable in order to induce autonomic responses. Within some limits there was a precise dose-response relation between intensity of electrical stimulation and pupillary size. The result of electrical stimulation of different constrictor points was similar and their simultaneous stimulation produced summation of effects while simultaneous stimulation of constrictor and dilator points produced a dynamic equilibrium. Luminous and electrical stimuli could substitute for and could summate with each other. Within some limits, the relation between luminance and pupillary size was logarithmic, while the relation between electrical intensity and pupillary size was linear. The pupillary effects of hypothalamic stimulation were blocked by instillation of atro-pine in the eye and by injection of Nupercaine within the hypothalamus. Constriction of the pupil was maintained without fatigue by continuous hypothalamic excitation. The longest tested time was 21 days. In free monkeys forming part of a colony, constriction of the pupil was evoked by radio stimulation of the hypothalamus without modifying individual or social behavior as recorded and quantified by time-lapse photography. Long-term continuous stimulation of the hypothalamus did not modify excitability, spontaneous electrical activity, or neuronal morphology, demonstrating that bilogical responses can be maintained for a long time by cerebral stimulation without producing detrimental effects.