Asymmetrical auditory probe evoked potentials during REM and NREM sleep

Two experiments were conducted to evaluate interhemispheric differences in late auditory evoked potentials (AEPs) during rapid-eye-movement (REM), Stage 2, and Stage 4 sleep. In the first study, 1,000-Hz stimuli [80 db (SPL)] were presented binaurally at a rate of 1/1.1 s. Analyses of variance were computed on the absolute difference in the amplitude of right and left evoked responses. N1 AEPs were significantly more asymmetrical in Stage 4 sleep compared to either REM or Stage 2 sleep. A second study was conducted with a wider topographical electrode distribution. This study used both a long and a short interstimulus interval with 500-Hz 80-db SPL tone pips. The absolute difference in the amplitude of right and left AEPs was compared across sleep stage. Asymmetries were larger in Stage 4 sleep than in either REM or Stage 2. Examination of these data indicated relative hemispheric balance in REM and Stage 2 sleep with largest asymmetries in Stage 4. The results do not support the view that REM and non-REM sleep stages are associated with differential activation of the two cerebral hemispheres. Rather, they suggest that the sleep cycle is characterized by variations in the degree of asymmetry. Asymmetries in Stage 4 sleep were not consistently in favor of the left hemisphere.