title: Effect of discharge desynchronization on the size of motor evoked potentials: an analysis.
abstract: OBJECTIVE
  Motor evoked potentials (MEPs) after transcranial magnetic brain stimulation (TMS) are smaller than CMAPs after peripheral nerve stimulation, because desynchronization of the TMS-induced motor neurone discharges occurs (i.e. MEP desynchronization). This desynchronization effect can be eliminated by use of the triple stimulation technique (TST; Brain 121 (1998) 437). The objective of this paper is to study the effect of discharge desynchronization on MEPs by comparing the size of MEP and TST responses.
  
  
  METHODS
  MEP and TST responses were obtained in 10 healthy subjects during isometric contractions of the abductor digiti minimi, during voluntary background contractions between 0% and 20% of maximal force, and using 3 different stimulus intensities. Additional data from other normals and from multiple sclerosis (MS) patients were obtained from previous studies.
  
  
  RESULTS
  MEPs were smaller than TST responses in all subjects and under all stimulating conditions, confirming the marked influence of desynchronization on MEPs. There was a linear relation between the amplitudes of MEPs vs. TST responses, independent of the degree of voluntary contraction and stimulus intensity. The slope of the regression equation was 0.66 on average, indicating that desynchronization reduced the MEP amplitude on average by one third, with marked inter-individual variations. A similar average proportion was found in MS patients.
  
  
  CONCLUSIONS
  The MEP size reduction induced by desynchronization is not influenced by the intensity of TMS and by the level of facilitatory voluntary background contractions. It is similar in healthy subjects and in MS patients, in whom increased desynchronization of central conduction was previously suggested to occur. Thus, the MEP size reduction observed may not parallel the actual amount of desynchronization.
authors.affiliation: Department of Neurology, Inselspital, University of Berne, CH-3010 Berne, Switzerland. kai.roesler@insel.ch
authors.forename: Kai M
authors.initials: KM
authors.lastname: Rösler
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Petrow
authors.forename: Johannes
authors.initials: J
authors.lastname: Mathis
authors.forename: Zsuzsanna
authors.initials: Z
authors.lastname: Arányi
authors.forename: Christian W
authors.initials: CW
authors.lastname: Hess
authors.forename: Michel R
authors.initials: MR
authors.lastname: Magistris
mesh_terms: D000328:Adult; D003344:Cortical Synchronization; D004574:Electromagnetic Fields; D004576:Electromyography; D004594:Electrophysiology; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology; D009431:Neural Conduction / Q000502:physiology