Multisensory resizing illusions as a non-pharmaceutical treatment for chronic hand-based pain

Resizing illusions use augmented reality to alter the perceived size of a body part, typically combining
visual and tactile inputs. These illusions have shown analgesic effects when applied to painful areas in people
with chronic pain. This thesis explored the subjective experiences, neural underpinnings, and analgesic
potential of resizing illusions through various sensory modalities, including visuotactile, unimodal visual,
and visuo-auditory presentations.

Before inviting participants with chronic pain to take part in illusory resizing projects, Chapter 2
examined barriers and facilitators to involvement in non-pharmaceutical research for people with chronic
pain. Findings revealed that addressing participant distrust and improving accessibility could improve the
chances of people wanting to take part and their comfort levels when participating. Chapter 3 introduced
hand-based resizing illusions in participants without chronic pain and used electroencephalography (EEG)
to assess the associated neural underpinnings. Gamma-band oscillations related to multisensory integration
were found during visuotactile illusions and theta-band oscillations indicative of increased cognitive load were
found during incongruent illusory presentations. Chapter 4 assessed whether non-naturalistic auditory input
enhances visual presentation of resizing illusions. Results showed that auditory input increased subjective
experiences but was less effective than visuotactile conditions. Chapter 5 explored somatosensory steady state
responses in participants without chronic pain, finding no significant differences between illusory and non
illusory conditions. Chapter 6 used the same approach as Chapter 5 for participants with chronic hand pain,
and whilst finding no changes in somatosensory representations, showed meaningful pain reductions following
illusory resizing for several participants. These findings extend previous research on illusory analgesia and
suggest resizing illusions could offer a non-pharmaceutical treatment for chronic primary and secondary pain
conditions.

The studies presented within this thesis highlight the potential of resizing illusions to be used both as a
tool for understanding multisensory integration and as a potential non-pharmaceutical treatment for chronic
hand-based pain.