title: Slow-Wave Oscillations in Awake Healthy Subjects: Methodological and Physiological Considerations.
abstract: OBJECTIVE
  Detection of pathologic slow-wave oscillations (0.5-7 Hz) in awake subjects has gained increasing interest in clinical diagnostics. Their significance, however, is hampered by the occasional presence of slow waves in healthy subjects, as well as the abundance of artefactual signals at low measurement frequencies. The aim of this study was to assess the occurrence of slow-wave oscillations in healthy subjects and to sharpen the management of possible measurement artifacts, in order to create a normative database for neurological patients.
  
  
  METHODS
  The authors analyzed magnetoencephalography recordings of spontaneous brain oscillations in 139 awake healthy adults. Sources of artifacts were first identified and suppressed by temporal extension of signal space separation method, and the remaining artifact components were projected out using signal space projection. Individual amplitude spectra were compared with the channel-level average spectra over all subjects.
  
  
  RESULTS
  Slow-wave oscillations deviating ±2 standard deviations from the average spectrum were detected in 12 subjects (∼9%). In 10 subjects, the oscillations were considered as normal physiological phenomena. Only two subjects showed activity that could have been interpreted as pathological: one subject with widespread parietal bilateral polyrhythmic slow-wave activity and one with focal rolandic 2.7-Hz slow-wave activity.
  
  
  CONCLUSIONS
  The prevalence of slow-wave oscillations in a healthy adult population is low. Knowledge about their occurrence, however, is essential for interpreting their significance in brain diseases. Artifacts and benign oscillatory variants at slow frequencies have to be recognized.
authors.affiliation: *Department of Neuroscience and Biomedical Engineering, School of Science, Aalto University, Espoo, Finland; †Elekta Oy, Helsinki, Finland; ‡MEG Core, Aalto Neuroimaging, Aalto University, Espoo, Finland; and §Departments of Clinical Neurosciences and Neurology, University of Helsinki and Helsinki University Central Hospital, Helsinki, Finland.
authors.forename: Hanna-Leena
authors.initials: HL
authors.lastname: Kaltiainen
authors.forename: Liisa M
authors.initials: LM
authors.lastname: Helle
authors.forename: Hanna M-L
authors.initials: HM
authors.lastname: Renvall
authors.forename: Nina H
authors.initials: NH
authors.lastname: Forss
mesh_terms: D000293:Adolescent; D000328:Adult; D016477:Artifacts; D058256:Brain Waves / Q000502:physiology*; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D015225:Magnetoencephalography / Q000379:methods / Q000706:statistics & numerical data*; D008297:Male; D008875:Middle Aged; D014851:Wakefulness; D055815:Young Adult