title: Intrahemispheric dysfunction in primary motor cortex without corpus callosum: a transcranial magnetic stimulation study.
abstract: BACKGROUND
  The two human cerebral hemispheres are continuously interacting, through excitatory and inhibitory influences and one critical structure subserving this interhemispheric balance is the corpus callosum. Interhemispheric neurophysiological abnormalities and intrahemispheric behavioral impairments have been reported in individuals lacking the corpus callosum. The aim of this study was to examine intrahemispheric neurophysiological function in primary motor cortex devoid of callosal projections.
  
  
  METHODS
  Intracortical excitatory and inhibitory systems were tested in three individuals with complete agenesis of the corpus callosum and sixteen healthy individuals. These systems were assessed using transcranial magnetic stimulation (TMS) protocols: motor threshold at rest, paired-pulse curve, and cortical silent period.
  
  
  RESULTS
  TMS revealed no difference between the patient and control groups on the motor threshold measure, as well as intracortical facilitation and intracortical inhibition systems as tested by paired stimulation. However, intrahemispheric inhibitory function was found to be abnormal in participants without callosal projections, as the cortical silent period duration was significantly increased in the patient group.
  
  
  CONCLUSIONS
  These data suggest that in addition to previously reported impaired interhemispheric function, patients lacking the entire corpus callosum also display abnormal intrahemispheric excitability of the primary motor cortex.
authors.affiliation: Centre de Recherche en Neuropsychologie et Cognition, Département de Psychologie, Université de Montréal and Hôpital Sainte-Justine, Montreal, Canada. sfecteau@bidmc.harvard.edu
authors.forename: Shirley
authors.initials: S
authors.lastname: Fecteau
authors.forename: Maryse
authors.initials: M
authors.lastname: Lassonde
authors.forename: Hugo
authors.initials: H
authors.lastname: Théoret
mesh_terms: D000328:Adult; D061085:Agenesis of Corpus Callosum; D001927:Brain Diseases / Q000473:pathology / Q000503:physiopathology*; D003337:Corpus Callosum / Q000503:physiopathology; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000503:physiopathology*; D009433:Neural Inhibition / Q000502:physiology; D012684:Sensory Thresholds / Q000502:physiology; D050781:Transcranial Magnetic Stimulation