CORRELATION OF UNIT ACTIVITY IN HYPOTHALAMUS WITH EEG PATTERNS ASSOCIATED WITH SLEEP CYCLE

Unit activity in the hypothalamus and frontal cortex was assessed in relation to the three principal EEG patterns exhibited by the rat under the influence of the hypnotic drug, urethane. For a large part of each recording session, the EEG alternated between periods of large-amplitude waves at 1-2/sec (synchronized) and low-amplitude waves at 6-12/sec (desynchronized). Each phase lasted anywhere from a few seconds to an hour or more. The changes in activity did not appear to be related to any environmental stimulus and did not represent a noticeable change in the level of anesthesia. In the hypothalamus, 50% of units displayed substantially different firing rates between periods of synchronized and desynchronized EEG; 27% firing fastest during the synchronized phase and 23% fastest during the desynchronized phase. The mean firing rate of all units during synchronized EEG which subsequently slowed with an EEG change was 3.39/sec and the mean rate of those which accelerated was 1.02/sec. After the change to desynchronized EEG, the firing rates of these two types of cell were reversed and were 1.32 and 3.21, respectively. The total activity of the hypothalamus was therefore approximately the same during both EEG states. In the frontal cortex, 78% of units showed EEG-correlated changes in activity; 49% firing fastest during desynchronized EEG and 29% fastest during synchronized EEG. However, units fired at the same mean rate during synchronized EEG irrespective of whether they subsequently accelerated or decelerated when the EEG reverted to the other form. Thus, the total activity of the cell population was highest during desynchronized EEG. Occasionally a third EEG phase was intercalated between a period of synchronized and desynchronized EEG. This period lasted for about 2 min and consisted of large-amplitude waves at 6-9/sec. Many hypothalamic units displayed distinctive firing patterns only associated with this third EEG state, whereas cortical units displayed activity patterns which were simply intermediate between those associated with the other EEG conditions. © 1969.