Modulating systems of hippocampal EEG.

Hippocampal EEG changes in response to stimulation of several mesodiencephalic inputs (mesodiencephalic reticular formation, MRF; medial and lateral hypothalamus, MH and LH; medial septum, MS) were quantitatively investigated by means of spectral analysis in cats with chronically indwelling electrodes. The rhythmic activity of the hippocampus, in the theta frequency range, increased as a function of stimulus strength wherever the stimulation took place in MRF, LH and MH. Characteristic response patterns were found for each input area. These differed as regards the occurrence of shifts in theta peak frequency and the decrease in one or more theta frequency components below baseline levels at high stimulation voltage. Desynchronization (EEG flat spectrum) did not take place at low stimulation strength of the MRF, LH and MH except in a few cases, where the electrodes were placed in the neighbourhood of fibre tracts (fornix and medial forebrain bundle). Thus evidence for the existence of "synchronizing" and "desynchronizing" systems was not found. A model of the origin of the hippocampal EEG is discussed in the light of the results, by means of which several hypotheses concerning the mechanisms of desynchronization are considered.