A virtual reality setup to investigate the self-attribution of the hand 43

In order to extend the investigation of the mechanisms underlying multisensory integration and the representation of the peripersonal space to encompass the psychological and neural phenomena associated with the self-attribution of the hand, we developed a setup based on virtual reality techniques that allowed the use of more complex experimental conditions. Despite the advantages associated with the simple, ecologically valid setups described above, the specific experimental manipulations in

Studies II and V required the introduction of an appropriate virtual reality platform.

Explicitly, the manipulation of the spatio-temporal congruence of the multisensory signals from the upper limb, as well as the introduction of unexpected sensory omissions that was crucial to the design of Study V, would not have been possible with the experimental setups we adopted in Studies I, III, and IV.

In order to overcome these limitations, we set out to develop a platform based on virtual reality tools that would deviate as little as possible from the ecologically valid setups described above. Specifically, we chose to adopt pre-recorded visual stimuli that featured the participants’ own right hand, a step that allowed us to gain insights on the link between multisensory integrative mechanisms and the self-attribution of one’s own hand, as opposed to an artificial body part or object (Botvinick, 2004; Moseley, 2011; Ehrsson, 2012). The visual stimuli were custom-made and consisted of high-quality stereoscopic videos that were presented to the participants in the MRI scanner via a pair of head-mounted displays placed right next to their eyes (Nordic Neuro Lab, Bergen, Norway; see also Petkova et al., 2011). All the other aspects of the experimental setups for Studies II and V were kept as similar as possible to the other studies. Namely, the participants had their head tilted forward and their right hand placed on the same inclined support, while looking at the three-dimensional videos through the head- mounted displays (Figure 5). Furthermore, the tactile stimuli associated with the videos were delivered manually by the experimenter following the same procedures described in the other studies. Special care was taken to ensure that the position of the participants’ hand in the scanner matched as closely as possible the position of the hand in the videos. As mentioned previously, this arrangement allowed us to introduce a number of experimental manipulations that would not have been feasible without the virtual reality platform.

In Study II, we significantly disrupted the perceptual binding of visual, tactile, and proprioceptive signals from the upper limb by introducing highly noticeable temporal delays, spatial incongruences, or mismatches between the seen and felt postures of the hand. Thus, we were able to relate the degree of the neural integration of the multisensory signals to the strength of the self-attribution of the hand, as indexed by subjective measures of visuo-tactile-proprioceptive perceptual binding and the sense of limb ownership. Furthermore, the use of pre-recorded videos permitted the introduction of effective threat stimuli (a kitchen knife sliding just above the pre-recorded video image of the hand), which allowed the recording of threat-evoked psychophysiological and neural responses. The latter measures served as complementary evidence for changes in the self-attribution of the hand as a function of the congruence between the multisensory signals. All the participants reported a vivid feeling of ownership toward the video image of their right hand only in the presence of congruence between visual, tactile, and proprioceptive signals. The participants’ subjective reports, corroborated by the complementary objective measures, confirmed the effectiveness of the experimental manipulations, particularly with respect to the marked reduction in the perceptual binding induced by the incongruences between the sensory signals in the control conditions. Taken together, these observations validated the virtual reality setup as a tool to induce changes in the self-attribution of the hand as a function of the congruence between the sensory signals from the upper limb.

The virtual reality setup was perhaps even more crucial to the design of Study V. In that study, we combined the manipulation of the multisensory congruence of signals from the hand employed in Study II with the introduction of unexpected omission trials. In the latter, one of the components of a multisensory event was unpredictably omitted, with the goal of unveiling the existence of multisensory predictions concerning self-attributed stimuli. The unpredictable omission of a visual or tactile stimulus, while maintaining the non-omitted stimulus intact, would have proven to be a severe methodological challenge without the use of the virtual reality setup. With the use of the latter, we were instead able to (1) change the self-attribution of the hand as a function of the congruence between the signals, and (2) introduce unexpected sensory omission events in multiple sensory modalities. In summary, we developed a setup based on virtual reality tools for use in fMRI studies that significantly extended the

how the integration of signals from multiple sensory modalities relates to the representation and self-attribution of one’s hand.

4.11 MONITORING THE PARTICIPANTS’ ALERTNESS: CATCH TRIALS