title: Changes in thresholds for intracortical excitability in chronic stroke: more than just altered intracortical inhibition.
abstract: OBJECTIVE
  The purpose of the present study was to assess changes in thresholds for the onset of short intracortical inhibition (SICI) and intracortical facilitation (ICF) in individuals with chronic stroke compared to age-matched healthy adults and evaluate the relationship between these thresholds and motor function in the chronic stroke group.
  
  
  METHODS
  Paired-pulse transcranial magnetic stimulation was used to derive thresholds for the onset of SICI and ICF in 12 neurologically healthy and 12 individuals with chronic stroke. Motor evoked potentials were elicited by a test stimulus of fixed intensity preceded by a conditioning stimulus ranging from 0%-125% of active motor threshold to generate recruitment curves. Regression functions were fit to these recruitment curves to identify thresholds for the onset of SICI and ICF. Mixed measures analysis of variance was used to compare thresholds for each hemisphere within and between groups.
  
  
  RESULTS
  Results showed a significant three-way interaction between Group (stroke, healthy), Hemisphere (ipsilesional, contralesional) and Stimulus interval (2 ms, 12 ms). Significant differences in the thresholds for the onset of both SICI and ICF were present in individuals with chronic stroke, with no between hemisphere differences for the control group. When compared to age-matched controls, comparisons revealed significant reductions in ipsilesional, but not contralesional thresholds for the onset of ICF, and significant reductions in contralesional, but not ipsilesional, thresholds for the onset of SICI in individuals with chronic stroke. In addition, as thresholds for ICF and SICI in stroke patients approached the level of healthy adults, higher function on the Wolf Motor Function Test was observed.
  
  
  CONCLUSIONS
  Reduced thresholds for the onset of SICI and ICF observed in the present study indicate that both inhibitory and facilitatory systems mediate changes in cortical excitability in chronic stroke patients. The association between higher onset thresholds and motor function in the stroke group also suggests that these thresholds have potential utility for tracking functional motor improvements in patients with chronic stroke. This study provides new insights to further characterize changes in intracortical neurotransmission that play an important role in modulating neuroplasticity and the potential relationship between inhibitory and facilitatory networks and motor function post-stroke.
authors: School of Population and Public Health, University of British Columbia, BC, Canada.
authors: Jodi D
authors: JD
authors: Edwards
authors: Sean K
authors: SK
authors: Meehan
authors: Meghan A
authors: MA
authors: Linsdell
authors: Michael R
authors: MR
authors: Borich
authors: Keivan
authors: K
authors: Anbarani
authors: Paul W
authors: PW
authors: Jones
authors: Jennifer
authors: J
authors: Ferris
authors: Lara A
authors: LA
authors: Boyd
mesh_terms: D000368:Aged; D000704:Analysis of Variance; D001132:Arm / Q000503:physiopathology; D002540:Cerebral Cortex / Q000503:physiopathology*; D002908:Chronic Disease; D003627:Data Interpretation, Statistical; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000503:physiopathology; D009433:Neural Inhibition / Q000502:physiology*; D009483:Neuropsychological Tests; D010291:Paresis / Q000209:etiology / Q000503:physiopathology / Q000534:rehabilitation; D020127:Recovery of Function; D011999:Recruitment, Neurophysiological; D012044:Regression Analysis; D020521:Stroke / Q000503:physiopathology*; D000071939:Stroke Rehabilitation; D050781:Transcranial Magnetic Stimulation / Q000379:methods*
keywords: Intracortical inhibition; chronic stroke; intracortical facilitation; transcranial magnetic stimulation