title: Phase Synchronization Analysis of Natural Wake and Sleep States in Healthy Individuals Using a Novel Ensemble Phase Synchronization Measure.
abstract: OBJECTIVE
  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.
authors.affiliation: Departments of *Neurology and †Clinical Neurosciences, National Institute of Mental Health and Neurosciences (NIMHANS), Bangalore, India; ‡Statistical Quality Control Unit, Indian Statistical Institute, Kolkata, India; §Systems Science and Informatics Unit, Indian Statistical Institute, Bangalore, India; ‖Department of Neurology, University of Alabama, Birmingham, Alabama, U.S.A.; Units of ¶Statistical Quality Control and **Systems Science and Informatics, Indian Statistical Institute, Bangalore, India.
authors.forename: Chetan S
authors.initials: CS
authors.lastname: Nayak
authors.forename: Aakash
authors.initials: A
authors.lastname: Bhowmik
authors.forename: Pradeep D
authors.initials: PD
authors.lastname: Prasad
authors.forename: Sandipan
authors.initials: S
authors.lastname: Pati
authors.forename: Kalyan K
authors.initials: KK
authors.lastname: Choudhury
authors.forename: Kaushik K
authors.initials: KK
authors.lastname: Majumdar
mesh_terms: D000513:Alpha Rhythm / Q000502:physiology; D000704:Analysis of Variance; D001611:Beta Rhythm / Q000502:physiology; D001921:Brain / Q000000981:diagnostic imaging / Q000502:physiology*; D003344:Cortical Synchronization / Q000502:physiology*; D003700:Delta Rhythm / Q000502:physiology; D004569:Electroencephalography / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D017286:Polysomnography; D011446:Prospective Studies; D012815:Signal Processing, Computer-Assisted; D012890:Sleep / Q000502:physiology*; D014851:Wakefulness / Q000502:physiology*; D055815:Young Adult