Rapid eye movement density shows trends across REM periods but is uncorrelated with NREM delta in young and elderly human subjects

Saccade-like eye movements are the most prominent phasic component of rapid eye movement (REM) sleep. Eye movement density (EMD) appears to be negatively related to sleep depth. Thus, EMD is depressed by sleep deprivation. We sought to determine in 19 young normal (YN) and 19 elderly normal (EN) subjects: (a) whether EMD is correlated with delta EEG in baseline sleep; (b) whether EMD is increased by daytime naps; and (c) whether EMD patterns across sleep cycles differ in the two age groups. Subjects participated in four separate 2-day recording sessions, each consisting of a baseline night, a daytime nap, and post nap night. EMD was measured as 0.3-2 Hz integrated amplitude (IA)/20 s stage REM. EMD was not correlated with rate of non rapid eye movement (NREM) delta production (power/min) in the baseline sleep of either group. Changes in EMD and delta power/min on post nap nights also were uncorrelated. These data indicate that very strong changes in sleep depth (state) are required to overcome the individual stability (traits) of NREM delta and eye movement density. ANOVA for EMD across REM periods 1-4 showed a significant cycle effect and a significant age x cycle interaction. These effects were mainly due to YNs having depressed EMD in the first REM period, likely due to the low arousal level early in sleep in these subjects. Compared with waking saccades the saccade eye movements of REM sleep have received little investigation. Further study of these movements could shed new light on neurophysiology of REM sleep. Such studies might also be clinically useful because the density of these movements appears to be related to depression and (independently) to cognitive function in individuals with brain impairment.