69 ALTERED VISUOMOTOR PROCESSING IN NA 4 screening, one patient was excluded due to movement during the MRI scan (mean framewise displacement > 0.5 mm) and one patient and one healthy participant due to high behavioural error rates on the hand laterality judgment task (> group mean plus 3 standard deviations). Sample size calculation The RCT was powered to demonstrate clinical effects of a specialized rehabilitation program on functional capability of the upper limb in NA patients. 124 The current sample (39 NA patients, 23 healthy participants) suffices to replicate our previous behavioural finding of a significant interaction effect of GROUP x LATERALITY on error rate on the same task in an independent sample of NA patients122 (with power at .90 and α at .05, the required sample size is 14) 125. Previous fMRI studies using the same task have found differences in brain activity between patient populations and healthy participants with sample sizes similar to the current sample. 30, 59 Procedures All participants were briefed on the nature of the study and gave written informed consent prior to participation according to the Declaration of Helsinki. The study included multiple assessments (see Lustenhouwer et al. 124). All participants started with the MRI session consisting of a structural scan, a resting-state fMRI scan (not included in this study) and the task-fMRI scan (described in detail below). Following the MRI session, we collected several objective and subjective clinical measures to quantify NA-related symptoms: the serratus anterior muscle strength on both the right (affected), and left (unaffected) side, the Disabilities of the Arm, Shoulder and Hand (DASH) questionnaire, 126 and pain. We estimated the maximal force exerted with the serratus anterior muscle, which is often affected in NA patients, 7 with a manual digital dynamometer (MicroFET2®).75 The DASH is a validated questionnaire that measures the functional capability of the upper extremity. Scores range from 1-100 with higher scores reflecting more impairment. Patients were asked to indicate how much pain they currently experienced on a visual analogue scale (VAS), ranging from no pain (0) to unbearable pain (100). 127 Experimental design All individuals performed the hand laterality judgment task29 in the fMRI scanner. Participants were presented with white line drawings of hands on a black background (the stimulus). Their task was to judge whether the hand on display represented a left or a right hand. They were instructed to respond as quickly and accurately as possible and to use their own hands as reference (such that they could imagine making a limb movement to match the hand on the screen), but that they were not allowed to overtly move their own limbs (this was confirmed using EMG, see below). Participants could not rely on visual input, since they were not able to see their own hands. Stimuli varied in laterality (left or right), degree of rotation (rotated −135°, −105°, −75°, −45°, 45°, 75°, 105°, 135° from the upright position), and view (palmar or dorsal), amounting to 32 different stimuli (Figure 1A). Different rotations and views were included as engagement of sensorimotor processes critically relies on these factors. 29, 76 Moreover, they enable the assessment of two factors of interest. First, biomechanical complexity: stimuli either have a medial (i.e. hand rotated towards the body midsagittal plane), or a lateral orientation (i.e. hand