title: Constitutive spectral EEG peaks in the gamma range: suppressed by sleep, reduced by mental activity and resistant to sensory stimulation.
abstract: OBJECTIVE
  In a systematic study of gamma activity in neuro-psychiatric disease, we unexpectedly observed distinctive, apparently persistent, electroencephalogram (EEG) spectral peaks in the gamma range (25-100 Hz). Our objective, therefore, was to examine the incidence, distribution and some of the characteristics of these peaks.
  
  
  METHODS
  High sample-rate, 128-channel, EEG was recorded in 603 volunteers (510 with neuropsychiatric disorders, 93 controls), whilst performing cognitive tasks, and converted to power spectra. Peaks of spectral power, including in the gamma range, were determined algorithmically for all electrodes. To determine if peaks were stable, 24-h ambulatory recordings were obtained from 16 subjects with peaks. In 10 subjects, steady-state responses to stimuli at peak frequency were compared with off-peak-frequency stimulation to determine if peaks were a feature of underlying network resonances and peaks were evaluated with easy and hard versions of oddball tasks to determine if peaks might be influenced by mental effort.
  
  
  RESULTS
  57% of 603 subjects exhibited peaks >2 dB above trough power at or above 25 Hz. Larger peaks (>5 dB) were present in 13% of subjects. Peaks were distributed widely over the scalp, more frequent centrally. Peaks were present through the day and were suppressed by slow-wave-sleep. Steady-state responses were the same with on- or off-peak sensory stimulation. In contrast, mental effort resulted in reductions in power and frequency of gamma peaks, although the suppression did not correlate with level of effort.
  
  
  CONCLUSIONS
  Gamma EEG can be expressed constitutively as concentrations of power in narrow or wide frequency bands that play an, as yet, unknown role in cognitive activity.
  
  
  CONCLUSIONS
  These findings expand the described range of rhythmic EEG phenomena. In particular, in addition to evoked, induced and sustained gamma band activity, gamma activity can be present constitutively in spectral peaks.
authors.affiliation: School of Computer Science, Engineering and Mathematics, and Medical Device Research Institute, Flinders University Adelaide, SA, Australia ; School of Medicine, and Centre for Neuroscience, Flinders University Adelaide, SA, Australia.
authors.forename: Tyler S
authors.initials: TS
authors.lastname: Grummett
authors.affiliation: School of Computer Science, Engineering and Mathematics, and Medical Device Research Institute, Flinders University Adelaide, SA, Australia ; School of Medicine, and Centre for Neuroscience, Flinders University Adelaide, SA, Australia.
authors.forename: Sean P
authors.initials: SP
authors.lastname: Fitzgibbon
authors.affiliation: School of Computer Science, Engineering and Mathematics, and Medical Device Research Institute, Flinders University Adelaide, SA, Australia.
authors.forename: Trent W
authors.initials: TW
authors.lastname: Lewis
authors.affiliation: School of Computer Science, Engineering and Mathematics, and Medical Device Research Institute, Flinders University Adelaide, SA, Australia ; School of Medicine, and Centre for Neuroscience, Flinders University Adelaide, SA, Australia.
authors.forename: Dylan
authors.initials: D
authors.lastname: DeLosAngeles
authors.affiliation: School of Medicine, and Centre for Neuroscience, Flinders University Adelaide, SA, Australia.
authors.forename: Emma M
authors.initials: EM
authors.lastname: Whitham
authors.affiliation: School of Computer Science, Engineering and Mathematics, and Medical Device Research Institute, Flinders University Adelaide, SA, Australia.
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Pope
authors.affiliation: School of Medicine, and Centre for Neuroscience, Flinders University Adelaide, SA, Australia.
authors.forename: John O
authors.initials: JO
authors.lastname: Willoughby
keywords: EEG bands; electroencephalogram; mental difficulty; neuro-psychiatric disorder; oddball task; steady-state responses