EVOLUTION OF DELTA-ACTIVITY WITHIN THE NONREM SLEEP EPISODE - A BIPHASIC HYPOTHESIS

The time course of delta activity within nonREM (NREM) episodes is mea sured for 24 healthy subjects with normal REM latencies. The first two NREM episodes in particular, show two very clearly separated peaks fo r about 35% of the subjects. Another 25% show two less well separated peaks. These double peak patterns are also prevalent in the literature , but there has been a tendency to dismiss them as a skipped REM effec t. They are, however, still evident even when the data are averaged ov er the 24 subjects, indicating a systematic phenomenon. These averaged data are well fitted by an analytic function given by the sum of two consecutive overlapping Gaussian curves. The well-behaved residuals al so, are an indication that a biphasic model of this kind is statistica lly appropriate. The model proposed is simple, with parameters related to physiological phenomena, and it suggests that there may be an unde rlying process with delta waves emanating from two separate signal sou rces. Recent neurophysiological findings suggest that delta oscillatio ns are generated both in the thalamus and in the cortex and show that excessive synchronization of slow oscillations may lead to seizures. H ence the speculation that the biphasic process may emanate from cortic al and thalamic sources and be protective in; the sense that it permit s smaller delta amplitudes at each source while retaining the integral delta energy necessary to satisfy sleep pressure. It is significant t hat the two peaks are most evident in the first two NREM episodes wher e delta power is high. (C) 1997 Elsevier Science Inc.