Balancing the Senses: Electrophysiological Responses Reveal the Interplay between Somatosensory and Visual Processing During Body-Related Multisensory Conflict.

In the study of bodily awareness, the predictive coding theory has revealed that our brain continuously modulates sensory experiences to integrate them into a unitary body representation. Indeed, during multisensory illusions (e.g., the rubber hand illusion, RHI), the synchronous stroking of the participant's concealed hand and a fake visible one creates a visuotactile conflict, generating a prediction error. Within the predictive coding framework, through sensory processing modulation, prediction errors are solved, inducing participants to feel as if touches originated from the fake hand, thus ascribing the fake hand to their own body. Here, we aimed to address sensory processing modulation under multisensory conflict, by disentangling somatosensory and visual stimuli processing that are intrinsically associated during the illusion induction. To this aim, we designed two EEG experiments, in which somatosensory- (SEPs; Experiment 1; N = 18; F = 10) and visual-evoked potentials (VEPs; Experiment 2; N = 18; F = 9) were recorded in human males and females following the RHI. Our results show that, in both experiments, ERP amplitude is significantly modulated in the illusion as compared with both control and baseline conditions, with a modality-dependent diametrical pattern showing decreased SEP amplitude and increased VEP amplitude. Importantly, both somatosensory and visual modulations occur in long-latency time windows previously associated with tactile and visual awareness, thus explaining the illusion of perceiving touch at the sight location. In conclusion, we describe a diametrical modulation of somatosensory and visual processing as the neural mechanism that allows maintaining a stable body representation, by restoring visuotactile congruency under the occurrence of multisensory conflicts.

Multisensory-driven facilitation within the peripersonal space is modulated by the expectations about stimulus location on the body.

Compelling evidence from human and non-human studies suggests that responses to multisensory events are fastened when stimuli occur within the space surrounding the bodily self (i.e., peripersonal space; PPS). However, some human studies did not find such effect. We propose that these dissonant voices might actually uncover a specific mechanism, modulating PPS boundaries according to sensory regularities. We exploited a visuo-tactile paradigm, wherein participants provided speeded responses to tactile stimuli and rated their perceived intensity while ignoring simultaneous visual stimuli, appearing near the stimulated hand (VTNear) or far from it (VTFar; near the non-stimulated hand). Tactile stimuli could be delivered only to one hand (unilateral task) or to both hands randomly (bilateral task). Results revealed that a space-dependent multisensory enhancement (i.e., faster responses and higher perceived intensity in VTNear than VTFar) was present when highly predictable tactile stimulation induced PPS to be circumscribed around the stimulated hand (unilateral task). Conversely, when stimulus location was unpredictable (bilateral task), participants showed a comparable multisensory enhancement in both bimodal conditions, suggesting a PPS widening to include both hands. We propose that the detection of environmental regularities actively shapes PPS boundaries, thus optimizing the detection and reaction to incoming sensory stimuli.

Reach planning with someone else's hand.

To investigate the relationship between the sense of body ownership and motor control, we capitalized on a rare bizarre disorder wherein another person's hand is misattributed to their own body, i.e., a pathological form of embodiment (E+). Importantly, despite E+ is usually associated with motor deficits, we had the opportunity to test two E+ patients with spared motor function, thus able to perform a reaching task. Crucially, these patients had proprioceptive deafferentation, allowing us to purely isolate the embodiment-dependent effect from proprioception-dependent ones that are usually associated in experimental manipulations of body ownership in healthy participants. Previous evidence suggests that the reaching movement vector is attracted towards an embodied hand during the rubber hand illusion (RHI). However, these results are confounded by the spared proprioception, whose modulation alone could explain the effects on reach planning. The neuropsychological approach employed here provides unambiguous evidence about the role of body ownership in reach planning. Indeed, three brain-damaged patients with proprioceptive deafferentation, two E+ and a well-matched control patient without pathological embodiment (E-), and 10 age-matched healthy controls underwent a reaching task wherein they had to reach for a target from a fixed starting point, while an alien hand (the co-experimenter's) was placed on the table. Irrespective of proprioception, damaged in all patients, only in E+ patients reaching errors were significantly more shifted consistently with the pathological belief, i.e., as if they planned movements from the position of the alien (embodied) hand, as compared to controls. Furthermore, with an additional experiment on healthy participants, we demonstrated that reaching errors observed during the RHI correlate with the changes in ownership. In conclusion, our neuropsychological approach suggests that when planning a reach, we do so from where our owned hand is and not from its physical location.