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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 parsympathetic reactivity of 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.