Perhaps a clue to these and related problems lies in the fact that changes in the intensity of hypothalamic discharges which are associated with changes in its balance lead also to qualitative alterations in reactivity. A state of parasympathetic ``tuning'' of the hypothalamus induced experimentally causes not only an increase in the parasympathetic reactivity this structure to direct and reflexly induced stimuli, but leads also to an autonomic reversal: a stimulus acting sympathetically under control conditions elicits in this state of tuning a parasympathetic response! Furthermore, conditioned reactions are fundamentally altered when the hypothalamic sympathetic reactivity is augmented beyond a critical level, and several types of behavioral changes probably related to the degree of central autonomic ``tuning'' are observed. If, for instance, such a change is produced by one or a few insulin comas or electroshocks, previously inhibited conditioned reactions reappear. However, if these procedures are applied more often, conditioned emotional responses are temporarily abolished. In other studies, loss of differentiation in previously established conditioned reflexes resulted from repeated convulsive (metrazol) treatments, suggesting a fundamental disturbance in the balance between excitatory and inhibitory cerebral processes.

It has further been shown that: (1) an experimental neurosis in its initial stages is associated with a reversible shift in the central autonomic balance; (2) drugs altering the hypothalamic balance alter conditioned reactions; (3) in a state of depression, the positive conditioned stimulus may fail to elicit a conditioned reaction but cause an increased synchrony instead of the excitatory desynchronizing (alerting) effect on the EEG. These are few and seemingly disjointed data, but they illustrate the important fact that fundamental alterations in conditioned reactions occur in a variety of states in which the hypothalamic balance has been altered by physiological experimentation, pharmacological action, or clinical processes.