The influence of day-night cycles and the additive effects of methamphetamine and illumination on the spontaneous activity in rhesus monkeys (Macaca mulatta)

The spontaneous activity of rhesus monkeys in a cage situation was measured under 12 hr. day-night cycles in Exp. I. Without exception, all the subjects (n=10) proved to be more active in the daytime than at night. In Exp. II the effect of methamphetamine upon spontaneous activity was investigated in light and dark conditions. It is supposed in the present study that methamphetamine and illumination raise the arousal level of the subjects and increase their spontaneous activity. The results show that the spontaneous activity of rhesus monkeys, which are thought to be visually dominant, greatly depends on illumination. That is, rhesus monkeys move around well in light conditions but little in dark conditions. Methamphetamine increases spontaneous activity, however, in dark as well as in light conditions. Interaction between methamphetamine and illumination effects was not significant. It would appear from these results that both internal and external factors—methamphetamine and illumination—feed into a common pool of the arousal level which controls spontaneous activity.