title: Attenuation of the 40-hertz auditory steady state response by propofol involves the cortical and subcortical generators.
abstract: BACKGROUND
  The 40-Hz auditory steady state response (40-Hz ASSR) provides a reliable marker of anesthetic-induced unconsciousness. Brain electric source analysis indicates that the 40-Hz ASSR arises from cortical and subcortical generators. The authors used source analysis to assess the effect of propofol anesthesia on the cerebral generators of the 40-Hz ASSR. They also examined the effect of propofol on two auditory evoked potentials of cortical origin: the N1 and the sustained potential.
  
  
  METHODS
  Eleven healthy human volunteers were anesthetized with propofol given in target-concentration mode at the minimal concentration causing unconsciousness. The 40-Hz ASSR was recorded before, during, and after anesthesia. The source model consisted of five concurrently active generator dipoles: two in the contralateral auditory cortex (one tangentially oriented, one radially oriented), two in the ipsilateral auditory cortex (same orientations), and one in the midline brainstem.
  
  
  RESULTS
  During anesthesia, the strength of the cortical and brainstem dipoles was reduced to the same extent (to 54% of baseline for the four cortical dipoles pooled vs. 53% for the brainstem dipole). Dipole strength during anesthesia was significantly less (P < 0.01) than during baseline and recovery for both cortical and brainstem dipoles. The N1 and sustained potential were no longer recordable during anesthesia.
  
  
  CONCLUSIONS
  The attenuation of the 40-Hz ASSR during propofol anesthesia results from a reduction of similar magnitude of the activity of the cortical and brainstem generators. The N1 and sustained potential are so profoundly attenuated during propofol anesthesia that they are no longer recordable from the scalp.
authors.affiliation: Department of Anesthesia, McGill University, Montreal, Quebec, Canada. gilles.plourde@mcgill.ca
authors.forename: Gilles
authors.initials: G
authors.lastname: Plourde
authors.forename: Alfonso
authors.initials: A
authors.lastname: Garcia-Asensi
authors.forename: Steven
authors.initials: S
authors.lastname: Backman
authors.forename: Alain
authors.initials: A
authors.lastname: Deschamps
authors.forename: Daniel
authors.initials: D
authors.lastname: Chartrand
authors.forename: Pierre
authors.initials: P
authors.lastname: Fiset
authors.forename: Terence W
authors.initials: TW
authors.lastname: Picton
mesh_terms: D000328:Adult; D001307:Auditory Perception / Q000187:drug effects*; D001933:Brain Stem / Q000187:drug effects / Q000502:physiology*; D002540:Cerebral Cortex / Q000187:drug effects / Q000502:physiology*; D004569:Electroencephalography; D005072:Evoked Potentials, Auditory / Q000187:drug effects / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D015742:Propofol / Q000494:pharmacology*; D012016:Reference Values; D013016:Sound
chemical_list: D015742:Propofol