A cortical EEG frequency with a REM-specific increase in amplitude.

1. We used period/amplitude (PA) analysis to survey electroencephalography (EEG) across vigilance states in the Sprague-Dawley rat. We found a fast cortical EEG frequency band, which we designate rho, whose wave amplitude in rapid eye movement (REM) was elevated above the levels in both waking and nonrapid eye movement (NREM). Rho wave amplitudes in NREM and waking were similar. 2. Rho occurred between 20 and 30 Hz, which appears to be a transitional range. In the 5-Hz band below rho, EEG wave amplitudes are higher in both NREM and REM than in waking; above 30 Hz, amplitudes are higher in waking and REM than in NREM. The augmented amplitude of rho in REM may result from a unique point of interaction of EEG mechanisms of sleep and waking. 3. In addition to its theoretical interest, rho is of practical value for scoring REM sleep. It could obviate the need to record hippocampal theta EEG, eliminating electrode penetration of the brain and reducing the number of leads required to classify rat vigilance states.