The purpose of this study is to investigate how the cable-driven arm exoskeleton (CAREX) can assist task performance during 3D arm movement tasks under various experimental conditions in healthy individuals and patients with stroke. This study is designed to test motor learning with the robotic rehabilitative device CAREX under three conditions in healthy subjects and subjects with post-stroke hemiparesis.

Inclusion Criteria for stroke population:

• Radiologically verified unilateral stroke at least 4 months previously.
• Right-handed subjects with or without right hemiparesis (as robot is built for right arm alone).
• Preserved passive range of motion at all upper limb joints.
• Spasticity <3 on the modified Ashworth Scale (MAS).
• Ability to follow study instructions and likely to complete the protocol; ability to comply with the protocol as assessed by the investigator; must be English speaking since the assessments are in English.

Exclusion Criteria for stroke population.

• History of surgery or other significant injury to either upper extremity causing mechanical limitations that preclude task performance.
• Previous neurological illness such as head trauma, prior stroke ( as we are recruiting subjects with first unilateral stroke), epilepsy, or demyelinating disease.
• Complicating medical problems such as uncontrolled hypertension, diabetes with signs of polyneuropathy, severe renal, cardiac or pulmonary disease, or evidence of other concurrent neurologic or orthopedic conditions precluding the subject from complying with the study protocol.
• Severe upper extremity spasticity suggested by an Ashworth score of >3 at any joint, or restriction of full passive range of motion.
• Evidence of alcohol, drug abuse or other relevant neuropsychiatric condition such as psychotic illness or severe depression.

Inclusion Criteria for control population.

• No previous stroke or any other previous neurological injury.
• Right-handed subjects without any weakness or hemiparesis (as robot is built for right arm alone).
• Preserved passive and active range of motion at all upper limb joints.
• No increase in muscle tone in response to passive range of motion. A score of zero on Ashworth Scale (MAS).
• Able to achieve full score on upper extremity Fugl-Meyer Scale (66/66)
• Ability to follow study instructions and likely to complete the protocol; ability to comply with the protocol as assessed by the investigator; must be English speaking since the assessments are in English.

Exclusion Criteria for control population.

• History of surgery or other significant injury to either upper extremity causing mechanical limitations that preclude task performance.
• Previous neurological illness such as head trauma, prior stroke, epilepsy, or demyelinating disease.
• Complicating medical problems such as uncontrolled hypertension, diabetes with signs of polyneuropathy, severe renal, cardiac or pulmonary disease, or evidence of other concurrent neurologic or orthopedic conditions precluding the subject from complying with the study protocol.
• Any condition or situation that, in the investigator's opinion, may put the subject at significant risk, confound the study results, or interfere significantly with the subject's participation in the study.
• Severe upper extremity spasticity suggested by an Ashworth score of >3 at any joint, or restriction of full passive range of motion.
• Evidence of alcohol, drug abuse or other relevant neuropsychiatric condition such as psychotic illness or severe depression.