This study consists of experiments on two non-consecutive days in the Human Neuromuscular Physiology Laboratory located in the Biological Sciences/Applied Physiology Building of Georgia Tech. If subjects qualify and agree to take part in this study, subjects will be asked to read and sign the consent form. The whole procedure will last about 3 hours per day, including preparation. Subjects will come to the lab on two separate days on the same tasks. The first day will serve as familiarization with the same tasks as the second day. Subjects will perform the following experiment. Individuals will sit upright and relax their hands and shoulders. The right hand and forearm will be rested on a plate hidden below a table. The robotic hand will be placed on the table as one would normally place own arm on the table. There will be no physical connection between the hand and the robotic hand. Subjects will activate the abdominal muscles for making a grasping movement of the robotic hand. For making a releasing movement of the robotic hand, subjects will activate the back muscles. Muscle activity will be recorded with two small sensors on each muscle. Subjects will practice activation of each muscle while watching the signal on a monitor. In each muscle, a clearer signal will be used for controlling the robotic digits. Healthy subjects only: Healthy subjects will perform the following tasks A-F. \[A: Rest\] Subjects will relax the muscles without physical or cognitive effort while gazing at the turned-off monitor. \[B: Robotic action observation\] Subjects will relax the muscles and focus on observing the computer-controlled grasp and release actions (2 s in each movement) of the robotic prosthesis. \[C: Visual MI\] Subjects will relax the muscles and perform conventional visual motor imagery (MI). With the guidance of audio instruction, the subjects will image the grasp and release motions with the right arm for 2 s in each motion in their mind. There will be no proximal muscle contraction. \[D: Kinesthetic MI\] The same MI procedure as Task C will be performed, except that the subjects will focus on the kinesthetic sensation that was felt with the imaged motions. \[E: Robotic-Hand Interaction with MI\] Subjects will perform robotically augmented mental practice (see below detailed description) for grasp and release motions with the activation control of the proximal muscles. During this task, subjects will also image the kinesthetic sensation that was felt with the corresponding motions with the right arm. \[F: Virtual-Hand Interaction robot\] Subjects will perform Task E with visual feedback of virtual robot actions in 2D on a monitor. \[G: Robotic-Hand Interaction without MI\] Subjects will perform Task E without MI. As a reaction task, subjects will flex the index finger as soon as they hear an auditory cue. Post-stroke subjects only: Tasks C \[Visual MI\], E \[Robotic-Hand Interaction with MI\], and A \[Rest\] above will be performed by post-stroke subjects with right-side hemiparesis who can volitionally activate fingers and proximal muscles. As a reaction task, subjects will flex the index finger as soon as an auditory cue is heard. TMS (both healthy and post-stroke subjects): During the above-mentioned tasks, brain stimulation (called TMS) will be applied over the motor cortex in the left hemisphere. The TMS procedure will follow the one used in the previous studies by the investigators. A TMS coil will be placed over a precise point on the scalp where the investigators will stimulate the brain to make the muscle move. The investigators will tell the subject when the stimulation portion of the procedure is about to begin. The first part of the procedure will be to find the area of the brain that controls muscles. These investigators will position the TMS coil on the head and will give the subject a series of stimulations (called magnetic pulses). Once the spot that controls the muscles is identified, the investigators will find the least amount of stimulation needed to activate the resting muscles. The second part of the procedure will be to evoke muscle contraction with TMS at rest and during the tasks described above. The investigators will apply TMS with 5-second intervals or more. Robotically augmented practice (Task E and F above): An individual will use proximal muscle contractions to flex and extend the robotic prosthesis digits for performing a grasp-release task. 1) Grasp. The individual will activate the proximal muscles as if for a retrieving motion (the back muscles) for 2 s. This muscle activation controls the robotic hand to flex the digits to grasp a cylinder-shaped object. 2) Release. The individual will relax the above-activated proximal muscles and activate the proximal muscles as if for a reaching motion (the abdominal muscles) for 2 s. This muscle activation controls the robotic hand to extend the digits to release the object. After releasing the object, the individual will relax the proximal muscles. The individual will repeat this sequence while observing and hearing the actions of the robotic hand.