Phase-amplitude coupling (PAC) between slow waves (0.5-0.6 Hz) and gamma activities occurs not only during light sleep and slow-wave sleep but also during wakefulness and rapid eye movement (REM) sleep, as shown by electrocorticography in 11 patients with epilepsy. During slow-wave sleep, PAC is high over a large region, whereas during REM sleep it is stronger in the posterior cortical region around the temporoparietal junction than in the frontal region. PAC also tends to be higher posteriorly during wakefulness. The findings suggest the posterior cortical region has a functional role in REM sleep and may contribute to maintaining the dreaming experience.
The sleep/wake cycle alters how neural activity spreads across large-scale brain networks. Using single-pulse electrical stimulation in patients with intracranial EEG electrodes, the study found that patterns of neural propagation at high-gamma frequencies (80-150 Hz) differ between wakefulness and sleep stages. During slow-wave sleep, frontal lobe stimulation produced greater propagation toward the parietal lobe than during wakefulness. During REM sleep, propagation decreased within the frontal lobe and increased within the parietal lobe. These directional biases in large-scale cortical network dynamics during REM sleep may help explain some unique experiential aspects of that sleep stage. The findings suggest that conscious awareness and sleep regulation involve changes in the balance of neural propagation across frontal-parietal networks.
Cortical excitability to external input differs between phasic and tonic rapid eye movement (REM) sleep. Single-pulse electrical stimulation applied directly to the cortex of seven patients with epilepsy elicited a transient increase in high gamma activity (100–200 Hz), a proxy for cortical excitability, followed by suppression. Suppression was stronger during both phasic and tonic REM than during wakefulness, but suppression during phasic REM was weaker than during tonic REM. Corticocortical evoked potentials, a measure of connectivity, did not differ between the two REM periods. The shift in cortical excitability during phasic REM toward wakefulness may produce incomplete short bursts of consciousness, contributing to dreams.