Before we comment further on these pathological conditions, we should remember that changes in the state of the hypothalamus within physiological limits distinguish sleep from wakefulness. Thus, a low intensity of hypothalamic cortical discharges prevails in sleep and a high one during wakefulness, resulting in synchronous EEG potentials in the former and asynchrony in the latter condition. Moreover, the dominance in parasympathetic action (with reciprocal inhibition of the sympathetic) at the hypothalamic level induces, by its peripheral action, the autonomic symptoms of sleep and, by its action on the cortex, a lessening in the reactivity of the sensory and motor apparatus of the somatic nervous system. With the dominance of the sympathetic division of the hypothalamus, the opposite changes occur. Since electrical stimulation of the posterior hypothalamus produces the effects of wakefulness while stimulation of the anterior hypothalamus induces sleep, it may be said that the reactivity of the whole organism is altered by a change in the autonomic reactivity of the hypothalamus. Similar effects can be induced reflexly via the baroreceptor reflexes in man and animals.

Of particular importance is the study of the actions of drugs in this respect. Although no drugs act exclusively on the hypothalamus or a part of it, there is sufficient specificity to distinguish drugs which shift the hypothalamic balance to the sympathetic side from those which produce a parasympathetic dominance. The former comprise analeptic and psychoactive drugs, the latter the tranquilizers. Specific differences exist in the action of different drugs belonging to the same group as, for instance, between reserpine and chlorpromazine. Important as these differences are, they should not obscure the basic fact that by shifting the hypothalamic balance sufficiently to the parasympathetic side, we produce depressions, whereas a shift in the opposite direction causes excitatory effects and, eventually, maniclike changes. The emotional states produced by drugs influence the cortical potentials in a characteristic manner; synchrony prevails in the EEG of the experimental animal after administration of tranquilizers, but asynchrony after application of analeptic and psychoactive drugs.