Assessment of mirror movements during the behavioural task

Mirror movements for each participant (patients and controls) were assessed over five longitudinal measurement sessions following recruitment (Table 1); Weeks 2, 4, 12, 24 and 52 post-stroke.

Patient information and measurement schedules for the behavioural and functional MRI experiments

A total of 53 patients and 14 age-matched controls were recruited for the study and measured at five different time points over the course of a year. For the behavioural experiment, each participant in the study was on average measured over at least three sessions (patients, 3.5 ± 1.5 sessions; controls, 4.3 ± 1.4), with the overall experimental data being 70.1% complete for patients and 85.7% complete for controls. For the functional MRI experiment, a subset of participants from the cohort were measured (n = 12 controls and n = 35 patients), with the experimental data being 73.7% complete for patients and 90% for controls.

During each measurement session, participants performed individuated force presses in the flexion direction with the instructed finger, while mirrored forces in the fingers of the passive hand were recorded. A visual representation of all 10 fingers was presented on a screen (Fig. 1A). The experiment began by estimating the strength of each finger, measuring two repetitions of the maximum voluntary force of each digit on both hands.

All subsequent trials required the production of isometric fingertip forces at a fraction of the maximum voluntary force for the instructed digit (at 20%, 40%, 60%, 80%). At the start of every trial, a force target-zone (target-force ± 25%) on a single finger was highlighted in green. This was the cue for participants to make a short force press with the instructed finger to match and maintain the target-force for 0.5 s while keeping the uninstructed fingers in either hand as motionless as possible. The trial was stopped if force on the instructed digit did not exceed 2.5 N in the 2 s following stimulus onset. Trials were presented in sequential order, starting from the left thumb to the left little finger, and ending with the right thumb to the right little finger. Trials were grouped as blocks, with each block consisting of one measurement each for the four target-force levels across the 10 fingers (four target-force levels × 10 fingers = 40 trials/block). Participants performed four such blocks during each measurement session.