Paradoxical facilitation alongside interhemispheric inhibition

MICHEL BELYK, Russell Banks, Anna Tendera, Robert Chen, Deryk S Beal

Research output: Contribution to journalArticle (journal)peer-review

4 Citations (Scopus)
46 Downloads (Pure)


Neurophysiological experiments using transcranial magnetic stimulation (TMS) have sought to probe the function of the motor division of the corpus callosum. Primary motor cortex sends projections via the corpus callosum with a net inhibitory influence on the homologous region of the opposite hemisphere. Interhemispheric inhibition (IHI) experiments probe this inhibitory pathway. A test stimulus (TS) delivered to the motor cortex in one hemisphere elicits motor evoked potentials (MEPs) in a target muscle, while a conditioning stimulus (CS) applied to the homologous region of the opposite hemisphere modulates the effect of the TS. We predicted that large CS MEPs would be associated with increased IHI since they should be a reliable index of how effectively contralateral motor cortex was stimulated and therefore of the magnitude of interhemispheric inhibition. However, we observed a strong tendency for larger CS MEPs to be associated with reduced interhemispheric inhibition which in the extreme lead to a net effect of facilitation. This surprising effect was large, systematic, and observed in nearly all participants. We outline several hypotheses for mechanisms which may underlie this phenomenon to guide future research.
Original languageEnglish
Pages (from-to)3303-3313
Number of pages11
JournalExperimental Brain Research
Issue number11
Early online date2 Sept 2021
Publication statusPublished - 1 Nov 2021


  • Transcranial magnetic stimulation
  • Interhemispheric inhibition
  • Corpus callosum
  • Experimental design
  • Optimization
  • Facilitation


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