Exploring interneuron conduction time in the motor cortex and the link to motor function in individuals with chronic...

Interneurons in the motor cortex (M1) contribute to key motor functions, which are often altered after stroke. Changing the direction of transcranial magnetic stimulation (TMS) alters the onset time of motor evoked potentials (MEPs). Yet, how stroke changes motor evoked potential (MEP) onset latency in association with directional TMS is not understood. The goals of the current study were to 1) investigate directional MEP onset in the contralesional and ipsilesional hemispheres and 2) characterize the relationship between MEP onset latency and motor function poststroke. Twenty individuals with chronic stroke received single-pulse, monophasic TMS over M1 in the lateral-medial (LM), posterior-anterior (PA), and anterior-posterior (AP) directions. Latency of activation of the corticospinal tract was indexed by subtracting MEP onset times from the paretic and nonparetic extensor carpi radialis (ECR) muscle in the LM direction from the AP and PA directions in both hemispheres. Upper limb motor function was quantified using the wolf-motor function test (WMFT). There was a significant difference between AP-LM and PA-LM MEP onset latencies within and between hemispheres. AP-LM MEP onset latency was longer in the contralesional than the ipsilesional hemisphere. Hemispheric differences in AP-LM MEP onset latency explained a significant proportion of variance in WMFT performance between limbs. AP-LM MEP onset latency is altered poststroke, and larger hemispheric imbalances in AP-LM MEP onset times related to worse motor function. These data highlight a new relationship between connectivity of interneuron pools in the human motor cortex and motor function after stroke.NEW & NOTEWORTHY AP-LM was greater than PA-LM MEP onset latency after stroke in both the ipsilesional and contralesional hemispheres, following patterns previously noted in healthy brains. However, AP-LM MEP onset latency was shorter in the ipsilesional hemisphere than the contralesional hemisphere. AP-LM, but not PA-LM, MEP onset differences between hemispheres were related to upper limb motor function after stroke. These data suggest that interneuron onset timing could be a motor function recovery biomarker after stroke.