Changes in thresholds for intracortical excitability in chronic stroke: More than just altered intracortical inhibition

Article type: Research Article

Authors: Edwards, Jodi D. | Meehan, Sean K. | Linsdell, Meghan A. | Borich, Michael R. | Anbarani, Keivan | Jones, Paul W. | Ferris, Jennifer | Boyd, Lara A.^{; ;}

Affiliations: School of Population and Public Health, University of British Columbia, BC, Canada | Department of Physical Therapy, University of British Columbia, BC, Canada | Graduate Program in Rehabilitation Science, University of British Columbia, BC, Canada | Department of Engineering, University of British Columbia, BC, Canada | Graduate Program in Neuroscience, University of British Columbia, BC, Canada

Note: [] Currently an Assistant Professor in the School of Kinesiology at the University of Michigan, Ann Arbor, Michigan, USA.

Note: [] Corresponding author: Lara A. Boyd, PT, Ph.D. Associate Professor and Canada Research Chair, Neurobiology of Motor Learning, University of British Columbia, 217-2277 Wesbrook Mall, Vancouver, BC V6T 2B5, Canada. Tel.: +1 604 822 7392; Fax: +1 604 822 1860; E-mail: lara.boyd@ubc.ca

Abstract: Purpose: 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.

Keywords: Intracortical inhibition, intracortical facilitation, transcranial magnetic stimulation, chronic stroke

DOI: 10.3233/RNN-120300

Journal: Restorative Neurology and Neuroscience, vol. 31, no. 6, pp. 693-705, 2013