54 CHAPTER 3 30 blocks). The stimuli varied in laterality (left or right), degree of rotation (rotated-135°, -105°,-75°,-45°, 45°, 75°, 105°, or 135° from upright position), and view (palmar or dorsal); totalling 32 different stimuli types (see Figure 1A). The inclusion of several rotations along multiple rotational axes is important as the engagement of sensorimotor representations, through first person motor imagery or other cognitive embodied processes, critically depends on these factors. 29, 76 We manipulated the participant’s own hand position in a block-wise manner during the experiment. Before each block, participants were instructed to place their own hands in one of four possible positions: both palms facing up, both palms facing down, one palm facing up (left/right) and the contralateral palm facing down (right/left). Thus, for each stimulus, the subject’s own hand position was either congruent (e.g. posture: palm up; stimulus: palmar view) or incongruent (e.g. posture: palm up; stimulus: dorsal view) with respect to the stimulus on the screen (see Figure 1B). This allowed us to test whether participants used a first-person imagery perspective to perform the task. It additionally enables the assessment of the influence of disease-related somatosensory changes. We were interested in somatosensory changes, as sensory involvement is common in NA. 7 The stimulus order was pseudo-randomized, ensuring that the different stimuli types were spread evenly across blocks. The inter-trial interval varied randomly between 500-1500ms. In this inter-trial interval, participants were presented with a fixation cross. Stimuli were presented until a button was pressed, with a maximum of 5s. We used reaction times and error rates to evaluate behavioral performance. In addition, we monitored muscle activity using electromyography over both thumbs (i.e. thenar eminence) to rule out that subjects made hand movements during the task. Figure 1 Experimental design A. Stimulus set Overview of all stimuli. Stimuli differed in Laterality (left, right), Rotation (-135°, -105°, -75°, -45°, 45°, 75°, 105°, or 135°), and view (dorsal, palmar). Laterality and Rotation are entered as factors in the statistical analyses for error rate and reaction time. The two different views (i.e. dorsal, upper row and palmar, bottom row) allow for assessing the effect of the manipulation of posture (see also B.). B. Postural manipulation Participants are seated in front of a computer screen with their hands on a table in front of them. A cover obscures the participants’ hands from view. At the start of each task block, participants are asked to place their hands in one of four possible positions: both hands with the palms facing down, both hands with palms facing up, one palm facing up (left/right) and the contralateral palm facing down (right/left). The view (dorsal, or palmar) of the stimulus on the screen can either be congruent with their current hand position (see Congruent example on the left), or it can be incongruent (see Incongruent example on the right).
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