Contralesional motor cortex is causally engaged during more dexterous actions of the paretic hand after stroke-A Preliminary report

Bilateral activation in motor cortex is observed during paretic hand performance after stroke; however the functional significance of contralesional motor cortex (C-M1) activation is highly debated. Particularly, it is not known if task characteristics such as dexterity influence the causal engagement of C-M1 during paretic hand performance. Transcranial magnetic stimulation (TMS) was used to quantify motor corticospinal physiology of the CM1 projecting to the contralateral resting extensor carpi radialis brevis (ECRB) and first dorsal interosseous (FDI) while eleven participants with unilateral stroke performed unimanual tasks of differing dexterity with their paretic hand. The novel finding was that compared to rest and less dexterous task (LDT), more dexterous task (MDT) performance led to increased corticospinal excitability and decreased intracortical inhibition of the C-Ml projecting to the resting FDI, but not resting ECRB. Further, using trains of repetitive TMS during MDT and LDT, we tested the behavioral relevance of C-Ml for paretic hand performance. Online rTMS perturbation to C-M1, but not to the vertex or sham stimulation led to significantly more movement errors during MDT without consistently affecting LDT performance. The present results argue for a beneficial role of C-Ml for accurate performance during dexterous motor actions with the paretic hand after stroke.