Human parahippocampal activity: non-REM and REM elements in wake-sleep transition.
Brain Research Bulletin March 15, 2005 Róbert Bódizs, Melinda Sverteczki, Alpár Sándor Lázár et al.
During the wake-sleep transition, a REM-like 1.5-3.0 Hz rhythmic activity in the parahippocampal cortex increases, peaking after about 30 seconds of sleep onset and reaching 82% of the level seen during REM sleep. This activity follows the dropout of alpha waves but is not linked to short-term changes in alpha or sleep spindles. Non-REM slow activity below 1.25 Hz steadily increases in both scalp and parahippocampal recordings. The findings suggest that this REM-like parahippocampal slow activity results from the switching off of hypothalamic wake-promoting centers at sleep onset, which inhibits hippocampal function and may contribute to hypnagogic hallucinations.