Phase Synchronization Analysis of Natural Wake and Sleep States in Healthy Individuals Using a Novel Ensemble Phase Synchronization Measure.

PURPOSE: Electrical activity in the brain is presumed to arise from a combination of tonic asynchronous neuronal firing during wake and a synchronized, burst-pause firing of large number of neurons during sleep. This study aims to compare the phase synchronization index (SI) across multiple channels during wake and various sleep stages on scalp electroencephalographic recordings.

METHODS: Forty healthy subjects were subjected to overnight polysomnography using 8-channel electroencephalography. Electroencephalographic phase synchronization during awake, non-rapid eye movement (N1, N2, N3), and rapid eye movement sleep states was studied using ensemble measure (multichannel measure across all the eight channels based on Hilbert transformation between any two pairs).

RESULTS: With the progression of states of wakefulness to non-rapid eye movement sleep, there was progressive increase in phase SI in delta band while SI decreased in alpha band (P < 0.001). The SI in delta band during rapid eye movement was comparable with that of awake state (P < 0.001). In theta band, SI tends to decrease in N2 and increase in N3 (P < 0.001). In beta band, there was progressive increase in SI from awake to non-rapid eye movement stages that decreased in rapid eye movement stage (P < 0.001).

CONCLUSIONS: This is the first study that has used an ensemble measure to assess the long-range cortical phase synchronization during awake and various sleep stages. The findings support the previous view of increased delta synchrony during non-rapid eye movement sleep and alpha synchrony during wakefulness. Rapid eye movement stage was characterized by marked desynchrony in all frequency bands. These findings suggest the possible role of cortical synchronization in influencing the occurrence of epileptic activity during sleep and awake states.