Paradoxical facilitation alongside interhemispheric inhibition

• Published in: Experimental brain research Vol. 239; no. 11; pp. 3303 - 3313
• Main Authors: Belyk, Michel, Banks, Russell, Tendera, Anna, Chen, Robert, Beal, Deryk S.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.11.2021; Springer; Springer Nature B.V
• Subjects: Biomedical and Life Sciences; Biomedicine; Brain research; Cerebral hemispheres; Conditioned stimulus; Corpus Callosum; Cortex (motor); Electromyography; Evoked Potentials, Motor; Experiments; Hands; Humans; Inhibition (Neurophysiology); Magnetic fields; Medical research; Motor Cortex; Motor evoked potentials; Neural Inhibition; Neurology; Neurosciences; Physiological aspects; Research Article; Research centers; Transcranial Magnetic Stimulation; United Kingdom; Facilitation; Interhemispheric inhibition; Transcranial magnetic stimulation; Experimental design; Corpus callosum; Optimization
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s00221-021-06183-9

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.