PCMS Task After Moderate-to-severe Stroke

120 pairs of TMS and PNS volleys will be administered during electromyography-triggered functional electrical stimulation (FES)-assisted task-specific training (task-specific PCMS). In the task-specific PCMS, participants will first grasp a spherical ball when prompted by an auditory 'Ready' cue, and upon seeing a visual 'Go' cue will voluntarily extend the wrist and fingers to release the ball. After the 'Go cue', our customized stimulation delivery algorithm triggers the TMS and PNS only when the extensor digitorum communis (EDC) muscle activity exceeds a pre-determined threshold. The algorithm will then trigger FES to EDC after detecting EDC activity (\~2 ms after PNS) for the next 2 seconds, allowing the pairing of PCMS (TMS and PNS) with the voluntary movement of EDC without the confounding effects of FES.

120 pairs of sham-PCMS stimuli will be administered during task-specific practice. The TMS coil will be placed \~10 cm behind the participant's head, and PNS electrodes will be placed in the same position as for task-specific PCMS, but no stimulation will be delivered. Like the task-specific PCMS condition, participants will perform the same grasp and release a ball task in an identical manner. FES will be delivered after detecting voluntary EDC activity, similar to the task-specific PCMS condition.

120 pairs of TMS and PNS volleys will be administered with the EDC muscle at rest every 10 s (\~20 min, 0.1 Hz). TMS will be used to elicit motor-evoked potentials (MEPs). Antidromic activation of spinal motor neurons will be elicited by supramaximal peripheral nerve stimulation (PNS) applied to the radial nerve near the elbow. Volleys will be timed to arrive in the spinal cord based on central and peripheral conduction times calculated for each subject so that the pre-synaptic terminal is depolarized via TMS \~1-2 ms before spinal motor neurons are depolarized via PNS at a pulse duration of 200us. We will also perform stimulation of the cervical roots (C-root) by placing the coil behind the neck and stimulating the C-root (C6 and C7 vertebrae), which innervate the finger extensor muscles. Conduction times will be calculated from latencies of the primary motor area (M1) MEP, C-root, and M-wave.