title: Characterizing changes in the excitability of corticospinal projections to proximal muscles of the upper limb.
abstract: BACKGROUND
  There has been an explosion of interest in methods of exogenous brain stimulation that induce changes in the excitability of human cerebral cortex. The expectation is that these methods may promote recovery of function following brain injury. To assess their effects on motor output, it is typical to assess the state of corticospinal projections from primary motor cortex to muscles of the hand, via electromyographic responses to transcranial magnetic stimulation. If a range of stimulation intensities is employed, the recruitment curves (RCs) obtained can, at least for intrinsic hand muscles, be fitted by a sigmoid function.
  
  
  OBJECTIVE
  To establish whether sigmoid fits provide a reliable basis upon which to characterize the input-output properties of the corticospinal pathway for muscles proximal to the hand, and to assess as an alternative the area under the (recruitment) curve (AURC).
  
  
  METHODS
  A comparison of the reliability of these measures, using RCs obtained for muscles that are frequently the targets of rehabilitation.
  
  
  RESULTS
  The AURC is an extremely reliable measure of the state of corticospinal projections to hand and forearm muscles, which has both face and concurrent validity. Construct validity is demonstrated by detection of widely distributed (across muscles) changes in corticospinal excitability induced by paired associative stimulation (PAS).
  
  
  CONCLUSIONS
  The parameters derived from sigmoid fits are unlikely to provide an adequate means to assess the effectiveness of therapeutic regimes. The AURC can be employed to characterize corticospinal projections to a range of muscles, and gauge the efficacy of longitudinal interventions in clinical rehabilitation.
authors.affiliation: Trinity College Institute of Neuroscience and School of Psychology, Trinity College Dublin, Dublin 2, Ireland; School of Psychology, Queen's University Belfast, Northern Ireland, UK. Electronic address: richard.carson@tcd.ie.
authors.forename: Richard G
authors.initials: RG
authors.lastname: Carson
authors.forename: Barry D
authors.initials: BD
authors.lastname: Nelson
authors.forename: Alison R
authors.initials: AR
authors.lastname: Buick
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Carroll
authors.forename: Niamh C
authors.initials: NC
authors.lastname: Kennedy
authors.forename: Rachel Mac
authors.initials: RM
authors.lastname: Cann
mesh_terms: D000328:Adult; D019540:Area Under Curve; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000294:innervation*; D011712:Pyramidal Tracts / Q000502:physiology*; D015203:Reproducibility of Results; D050781:Transcranial Magnetic Stimulation; D034941:Upper Extremity / Q000294:innervation; D055815:Young Adult
keywords: Afferent stimulation; Arm; Goodness of fit; Motor evoked potentials; Plasticity