Neuromodulation of the Cortex and Spinal Cord

Primary Outcome Measure-Clinical Assessment

A stimulus intensity will be incrementally increased or decreased from 0.1 to 10 mA as the subject verbally reports their first sensation of the stimulus or perceived loss of the stimulus sensation. The ascending and descending perception tests will be repeated 3 times. The mean of the intensities will be used as the primary outcome variable. The KINARM robot will passively move the participant's hand-arm to various positions in the workspace as they hold the graspable robotic arm. The participant can see the position of their hand as a white dot in the virtual reality display. Once the robot returns home, the participant repeats the same movement that the robot performed. The difference between the final position of the hand when the robot passively moved the participant's arm and the final position when the participant actively moved their arm will be the primary outcome variable.

Primary Outcome Measure- fMRI

1. Resting State: Functional brain imaging will be performed while the participant looks at a fixation cross for 5 minutes. 2. Brain-Spinal Cord Activity: Functional imaging of the activity in the brain and spinal cord will be acquired concurrently while the participant performs a grip force target matching task. This task will consist of squeezing a force transducer with the hand to animate a box that ascends vertically based on the amount of force applied to the transducer. Target forces will be 10% and 30% of the participant's maximum grip force. The target forces will be randomized. The participant is instructed to match the target forces as quickly and as accurately as possible. The experiment will be conducted separately for the left and right hands and will take 10 minutes each to complete. 3. Spinal Cord Microstructure: Structural images of the spinal cord will be acquired while the participant watches a move. The total scan time will be 20 minutes.

Primary Outcome Measure - Hoffman Reflex Assessments

A percutaneous nerve stimulation will be used to determine the participant's maximal H-wave (Hmax) of the flexor carpi radialis muscle. This will be determined through 20-50 stimulations of increasing intensity. Next, the participant will sit quietly as 10 stimulations will be delivered at the participant's Hmax with an interstimulus interval of 15 seconds. Stimulations at the participant's Hmax will be applied as the participants produce a contraction of 10-20% of their maximum voluntary contraction. The participant will be instructed to maintain their EMG activity at the target level for 6.5 seconds as 10 stimulations are delivered at the participants Hmax. The ratio between the mean H-reflex amplitude in the Active Condition and the mean H-reflex amplitude in the Resting Condition will be used as the primary outcome variable. This task will be completed with the non-dominant hand, which will be determined from the clinical assessments.

Primary Outcome Measure - MEG Brain Imaging

1. Resting state: The participants will sit quietly in the MEG with their eyes closed for 5 minutes. 2. Somatosensory: An electric percutaneous stimulator will be used to deliver a low-level electrical stimulation to the median nerve at a level that is below the motor threshold (i.e., doesn't generate a twitch in the thumb). 3. Isometric Force Matching: Participants will grasp a force transducer that will animate a box vertically to place it inside a target force box that represents 10 and 30% of their maximal grip force. The participants will be instructed to position and hold the animated box inside the target box for 300 ms until the target box disappears. The disappearance of the target box will signify when to stop the sustained force production. Tasks 2 \& 3 will be completed with the non-dominant hand, which will be determined from the clinical assessments.