70 CHAPTER 4 rotated away from the body midsagittal plane). Medially oriented hands are in more comfortable positions, making medial trials biomechanically easy to perform, whereas lateral orientations are biomechanically complex. This results in laterally oriented stimuli eliciting longer reaction times than medially oriented stimuli known as the “biomechanical complexity effect”. 21, 25, 29, 58, 86 Second, the compatibility of the presented hand stimulus with the posture of the participant’s own hand. More specifically, the inclusion of the two different views (palm or back) enabled us to assess the effect of postural manipulation (Figure 1B). With this manipulation, we could test whether participants incorporated their own body posture during the task. At the start of each block, participants were instructed to place their own limbs in one of four possible positions: both hands with palms facing up, both hands palms facing down, one hand palm up (left/right) and the other hand palm down (right/left). The view of the stimulus on the screen could either be congruent or incongruent with the current position of the corresponding limb. Typically, participants are slower when their corresponding limb is in an incongruent compared to a congruent position, which indicates that this task comprises embodied components. 58, 69, 70 Each trial started with a white fixation cross in the centre of the screen, followed by the stimulus. The stimulus was presented until a response was registered, with a maximum of 4 seconds. Participants responded by pressing a foot button with the corresponding left or right great toe. The inter-trial interval varied randomly between 2000 and 3000ms. The task consisted of 32 blocks of 8 trials (totalling 256 trials; duration 20-30 minutes). The trial order was pseudo-randomized, ensuring that the different trial types were spread evenly across blocks. We recorded reaction times (RT) and error rates (ER) to evaluate behavioural performance. Throughout the task we additionally monitored muscle activity, using EMG over both thumbs (thenar eminence) to rule out that participants made hand movements during the trails. Alertness was monitored through an online eye monitor. Figure 1 Experimental design A. Stimulus overview. Stimuli differed in laterality (left, right), view (dorsal, uneven rows and palmar, even rows) and were rotated from −135° to 135°, in 30° increments. Hands with a medial orientation were associated with biomechanically easy movement, whereas hands with a lateral orientation were associated with biomechanically complex movement. B. Postural manipulation. Participants were instructed to assume a limb position (with the palm of their hand up or down) at the start of each block. The current position of their own limb could either be congruent with the view (dorsal or palmar) of the stimulus on the screen (see congruent example), or it could be incongruent (see incongruent example).
RkJQdWJsaXNoZXIy MjY0ODMw