Distinct Neural Signatures of Multimodal Resizing Illusions: Implications for Chronic Pain Treatment

Illusory body resizing typically uses multisensory integration to change the perceived size of a body part. Previous studies associate such multisensory body illusions with frontal theta oscillations and parietal gamma oscillations for dis-integration and integration of multisensory signals, respectively. However, recent studies support illusory changes of embodiment from visual-only stimuli. Multisensory resizing illusions can also reduce chronic pain, potentially through modulation of cortical body representations. This preregistered study (N=48) investigated differences between multisensory visuo-tactile and uni-modal visual resizing illusions using EEG. We hypothesised (1) stronger illusion in multisensory compared to uni-modal, and uni-modal compared to asynchronous (dis-integration) conditions, (2) greater parietal gamma during multisensory compared to uni-modal, and (3) greater frontal theta during asynchronous compared to baseline conditions. Results partially supported EEG hypotheses, finding increased parietal gamma activity comparing multisensory to unimodal visual conditions, whilst finding increased parietal theta activity when comparing asynchronous to non-illusion conditions. While results demonstrated that only 27% of participants experienced the illusion with visual-only stimuli, further analysis suggested that those who experience visual-only illusions exhibit a different neural signature to those who do not. Our results support the importance of multisensory integration for illusory changes in perceived body size. However, we also suggest that visual-only illusions can influence cortical body representations for a significant proportion of participants, which may have implications for the development of accessible visual-only chronic pain treatments. ### Competing Interest Statement The authors have declared no competing interest.