Overall materials and methods: The proposed experiments will follow the general pattern we have used in previous behavioral psychophysical studies in individuals with and without stroke. A controlled virtual environment that records kinematics of the arm and allows real-time movement interaction with a virtual motor task will be used. A program integrating motion capture system (Acsention Technology TrakSTAR) and MATLAB-based GUI-gaming environment allows tracking of the two arms, experimenter-control for specific manipulation such as position of the virtual brick, target gap, relative contribution of the arms and mapping of each arm movement to the movement of the virtual brick to provide real-time and post-response feedback. Participants will be seated in an adjustable chair facing a computer monitor with their trunk constrained to the chair. Their arms will be completely supported on a low friction table top and free to move in the horizontal (X-Y) plane with minimal resistance. An opaque screen will occlude participants' direct vision of their arms. Magnetic markers will be secured to their hands just proximal to the wrist joint and the position of the markers mapped to a virtual brick shown on the computer screen. The motor task is to move the virtual brick(s) to a virtual target gap(s) on the computer screen by moving both arms in 2D (X-Y) plane on the low-friction table. While real-time visual feedback of the brick is available, the brick cannot be felt haptically. From a perceptual perspective, in independent goal condition, each arm moves its own virtual brick to the target gap. In common-goal condition, a common brick was moved on the computer screen to a target window by predetermined weighting of each arm movement.

Prior to beginning the experiments, participants will reach with the paretic arm in three different directions (135º, 90º and 45º relative to the horizontal) to record the maximum reaching distance for two trials in each direction. The minimum reach distance across the three directions will be used to calibrate the start and end position of the target gaps. The target window position will be placed at 70 and 90% of the maximum reach distance, oriented at 45º, 90º and 135º relative to the horizontal. From a motor execution perspective, bimanual reaching to the 90º target will require mirror movements of the two arms in a forward direction. Reaching to 45º and 135º targets will require parallel movements of the two arms in those directions. Mirror and parallel movements have been shown to require distinct motor control strategies, as well as different levels of transcallosal inhibition, hence have been included in this study.

Procedures: Participants will come to the laboratory for a baseline evaluation to determine eligibility to participate in the experimental protocol. During this baseline evaluation, we will perform the following tests: (1) Fugl-Meyer Examination, (2) Mini-mental scale, (3) Tests for hemineglect using a line-bisection test, (4) Western Aphasia Battery for patients with Aphasia (5) TMS safety questionnaire, (6) MRI safety questionnaire, (7) Box and Block test as well as (8) Penn Neurocognitive assessment.

Once the patients qualify for the study, they will come to the lab for three separate sessions. The first two sessions will be behavioral sessions while the third session will be a neuroimaging session.

During the first behavioral session, participants will be tested for bimanual coordination in two different tasks- virtual-reality (VR) based task and real-world tasks. In the VR-based task, we will test a total of 240 trials under different conditions to obtain a baseline performance data. We will also have participants perform a battery of bimanual real-world tasks. During the second session, we will test the effect of two different perceptual cues on bimanual coordination. During Aim 2 experiments, participants will be instructed to "move" a common virtual brick with both arms to the target windows in three directions (mirror- 90º; parallel- 45º and135º) without tilting the brick within a target MT of 800 milliseconds- 1.2 seconds. Participants will complete four 60-trial blocks a pseudorandom order. Each block will consist of a distinct task conditiondepending upon the nature of perceptual cues provided.

Condition 1 will be similar to the common-goal bimanual condition in Aim 1 where they will transport a common virtual block fixed in a horizontal position to three targets in pseudorandom order. The movement of the bar will be an unweighted average of the two arm movements; i.e., each arm will contribute to 50% of the virtual bar movement (50-50 weighting). For Condition 2, the weighting coefficients of the two arms will be equal (i.e. 50-50); and the virtual brick will tilt in the direction of the lagging arm proportional to the relative time-lag between arms.Operational definition of relative time-lag: Relative time-lag is different than absolute time lag. Relative time-lagis the time-lag between the relative timing of each arm within its trajectory. To illustrate, if the left and right arm are contributing to 70 and 30% of the brick movement, relative time-lag at mid-movement will be zero if the left and right arm have covered half of their respective trajectories. Therefore, relative time-lag is influenced by temporal as well as spatial component of the movement of each arm. Concurrent and post-response feedback about the tilt and path of the virtual brick will be provided after each trial.

Condition 3 will be similar to block 1 (block fixed in horizontal position), but the arm weighting will be 70-30. Condition 4: In addition to the "tilt" feedback about the relative time-lag, each arm will be differentially weighted- i.e., the paretic arm will have a higher weighting coefficient compared to the nonparetic arm. Specifically, the paretic arm will contribute to 70% of the virtual brick movement while the nonparetic arm will contribute to 30% of the virtual brick movement. 20 trials of bimanual reaching in each direction (90º; 45º and 135º) will be completed in pseudorandom order.