title: Prefrontal TMS produces smaller EEG responses than motor-cortex TMS: implications for rTMS treatment in depression.
abstract: BACKGROUND
  The stimulus intensity of prefrontal repetitive transcranial magnetic stimulation (rTMS) during depression treatment is usually determined by adjusting it with respect to the motor threshold (MT). There is some evidence that reactivity of the prefrontal cortex to transcranial magnetic stimulation (TMS) is lower than that of the motor cortex at MT stimulation. However, it is unknown whether this is true when other stimulus intensities are used. We investigated whether the magnitude and shape of the overall TMS-evoked electroencephalographic (EEG) responses differ between prefrontal and motor cortices.
  
  
  METHODS
  Magnetic pulses to the left motor and prefrontal cortices (the middle frontal gyrus identified from magnetic resonance images) were delivered at four intensities (60, 80, 100, and 120% of MT of the right abductor digiti minimi muscle) for six subjects. Simultaneously, EEG was recorded with 60 scalp electrodes.
  
  
  RESULTS
  Global mean-field amplitudes (GMFAs) reflecting overall cortical activity were significantly smaller after prefrontal- than after motor-cortex TMS. A significant positive correlation (r (s)=0.84, p<0.01) was found between GMFAs of motor- and prefrontal-cortex TMS across the experiments. However, when correlation between the responses of motor and prefrontal cortices was examined, significant positive correlations were found at 80 and 100% intensities only.
  
  
  CONCLUSIONS
  This study provides further evidence that the prefrontal and motor cortices have different reactivity to TMS, but the MT may be used for determining the stimulus intensity of prefrontal rTMS treatment in depression, at least at motor threshold intensities or near to it.
authors.affiliation: BioMag Laboratory, Helsinki University Central Hospital, P.O. Box 340, FIN-00029 HUS, Helsinki, Finland. Seppo.Kahkonen@helsinki.fi
authors.forename: Seppo
authors.initials: S
authors.lastname: Kähkönen
authors.forename: Soile
authors.initials: S
authors.lastname: Komssi
authors.forename: Juha
authors.initials: J
authors.lastname: Wilenius
authors.forename: Risto J
authors.initials: RJ
authors.lastname: Ilmoniemi
mesh_terms: D000328:Adult; D003863:Depression / Q000503:physiopathology / Q000628:therapy; D004599:Electric Stimulation Therapy; D004569:Electroencephalography; D019054:Evoked Potentials, Motor / Q000502:physiology; D006801:Humans; D009044:Motor Cortex / Q000166:cytology / Q000502:physiology*; D017397:Prefrontal Cortex / Q000166:cytology / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*
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