An investigation into enlarging and reducing the size of mirror reflections of the hand on experimentally-induced cold-pressor pain in healthy human participants

Background and aims: Mirror visual feedback may be a useful clinical tool for reducing pain. Research suggests that reducing the size of a non-painful reflected hand can alleviate complex regional pain syndrome in the affected hand that is out of view. In contrast, research on healthy humans exposed to experimentally induced pain suggests that reducing the appearance of the size of a reflected body part can increase pain. The aim of this study was to investigate the effect of enlarging and reducing the visual appearance of the size of a hand using mirror visual feedback on pain threshold, intensity and tolerance in healthy human participants exposed to cold-pressor pain. Methods: Participants were a convenience sample of 20 unpaid, healthy pain free volunteers aged 18 years or above. Each participant took part in one experiment where they completed cold-pressor pain tests whilst observing normal, enlarged and reduced size reflections of a hand congruent to a hand immersed in the ice cold water. A 4 × 2 factorial repeated measures analysis of variance (ANOVA) was performed on time to pain threshold and pain tolerance, and pain intensity with Condition (four levels: no reflection, reduced reflection, normal reflection, enlarged reflection) being the within-subject factors and Sex (two levels: female, male) between-subject factors. Results: There were no significant effects for Condition, Sex, or Condition × Sex interaction for pain threshold, intensity or tolerance (p > 0.05). There were no significant differences between the 3 mirror reflection conditions for agreement with the statements: “It felt like I was looking directly at my hand rather than at a mirror image”; “It felt like the hand I was looking at was my hand”; and “Did it seem like the hand you saw was a right hand or a left hand?”. Conclusion: Enlarging or reducing the size of a hand using mirror visual feedback did not alter pain perception in healthy human participants exposed to cold-pressor pain. The different sizes of hands generated by mirror visual feedback created an illusion of looking at their own hand but this was not as strong as looking directly at the hand. Implications: In future, investigators and clinicians using mirror visual feedback may consider including an adaptive phase to ensure the reflection has been perceptually embodied. Reasons for the lack of effects are explored to inspire further research in the field