Illusory finger stretching and somatosensory responses

Resizing illusions, delivered using augmented reality, resize a body part through stretching or shrinking manipulations. These resizing illusions have been investigated in visuotactile, visual-only, and visuo-auditory presentations. However, the neural underpinnings of these resizing illusions remain undefined. This study sought to understand the neural mechanisms behind these illusions by using somatosensory steady state evoked potentials (SSEPs) in addition to subjective self-report questionnaires, to enhance knowledge of what drives the subjective embodiment during resizing illusions. Since these Illusions have been shown to provide analgesic effects for individuals with chronic pain conditions, this study also aimed to provide an empirical basis for future investigations in chronic pain samples. Confirmatory analyses (N = 46) demonstrated significant differences in subjective experience between non-illusion and multisensory illusion conditions, while electroencephalography (EEG) data measuring SSEP response across electrodes of interest (F1 & FC1) to 26Hz stimulation of the resized digit showed no significant effects of condition. However, further exploratory non-parametric SSEP analyses revealed a significant effect of condition, with reduced amplitudes in illusion conditions compared to non-illusion conditions, but no significant differences in exploratory post hoc tests. While confirmatory findings demonstrated no clear effect of resizing illusions on SSEP amplitudes for participants without chronic pain, exploratory findings could be interpreted as a potential "sharpening" of neural representations resulting from illusory stretching. These findings therefore provide a basis for investigations of comparable subjective and steady state illusion responses in a chronic pain population, who are thought to have more diffuse neural representations of their affected body parts.