The following experiment will be conducted with 15 chronic stroke survivors and 15 control subjects. Subjects will perform hand movements in the virtual reality environment using a head-mounted display. Quest's onboard cameras allow real-time tracking of hand motion, estimating joint angles of the finger and the thumb. For each feedback manipulation condition (i.e., movement amplification or reduction), the estimated joint angles will be multiplied by a factor greater than 1 (amplification) or a factor smaller than 1 (reduction). Then the newly-estimated hand posture will be displayed in the VR environment. Subjects will be randomly assigned into two groups (A and B), who will receive the assistance in a different order. Subjects will perform finger extension under three conditions: no sensory modulation, movement visual feedback (VF) amplification, and movement VF reduction (error augmentation), while their movement muscle activation patterns are recorded.

Inclusion Criteria for stroke group:

• Age: 20 - 70 years old
• First-ever unilateral cortical stroke with resultant hemiparesis at least 6 months prior to the experimental testing
• Fugl-Meyer Upper Extremity Section 31 - 55

Exclusion Criteria for both groups:

• Presence of concurrent severe medical illness
• Pain in hand or arm with movements
• Loss of voluntary control of fingers and thumb
• Inability to provide informed consent
• Musculoskeletal injuries (e.g., fracture) or medical complications (e.g., severe cardiovascular disease).

The investigators do not plan to share IPD but the de-identified data will be shared.