Hippocampal theta rhythm persists in the permanently isolated forebrain of the cat.

In cats with high mesencephalic transections surviving for up to 90 days, the hippocampal EEG shows exaggerated percentages of 3-5 Hz theta activity. The theta rhythm has similar waveform and frequency characteristics to that seen in the intact cat and can be selectively enhanced or abolished by systemic physostigmine or atropine respectively. These data demonstrate (1) that the caudal brainstem is not essential for the spontaneous appearance of the hippocampal theta rhythm, and (2) the integrity of a major cholinergic system in the chronically isolated forebrain.