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

Abstract

<jats:title>Abstract</jats:title><jats:sec id="j_j.sjpain.2015.07.003_s_007_w2aab3b7c12b1b6b1aab1c17b1Aa"><jats:title>Background and aims</jats:title><jats:p>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.</jats:p></jats:sec><jats:sec id="j_j.sjpain.2015.07.003_s_008_w2aab3b7c12b1b6b1aab1c17b2Aa"><jats:title>Methods</jats:title><jats:p>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.</jats:p></jats:sec><jats:sec id="j_j.sjpain.2015.07.003_s_009_w2aab3b7c12b1b6b1aab1c17b3Aa"><jats:title>Results</jats:title><jats:p>There were no significant effects for Condition, Sex, or Condition × Sex interaction for pain threshold, intensity or tolerance (<jats:italic>p</jats:italic> &gt; 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?”. </jats:p></jats:sec><jats:sec id="j_j.sjpain.2015.07.003_s_010_w2aab3b7c12b1b6b1aab1c17b4Aa"><jats:title>Conclusion</jats:title><jats:p>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.</jats:p></jats:sec><jats:sec id="j_j.sjpain.2015.07.003_s_011_w2aab3b7c12b1b6b1aab1c17b5Aa"><jats:title>Implications</jats:title><jats:p>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.</jats:p></jats:sec>

Changing the size of a mirror‐reflected hand does not affect pain perception: A repeated measures study on healthy human participants

“…() found that magnifying a mirror reflection of the hand reduced contact‐heat pain, whereas minifying the reflected hand increased pain. In contrast, Johnson and Gohil () used cold‐pressor pain and found no difference in pain measures when magnifying and minifying reflections of the hand.…”

The effect of visual feedback of body parts on pain perception: A systematic review of clinical and experimental studies

“…(), there was a significant moderate effect in favour of a magnified reflection compared with a normal‐sized reflection (SMD = −0.34, 95% CI = −0.63, −0.05), and a small effect in favour of mirror VF using a normal‐sized reflection compared with a minified reflection (SMD = 0.18, 95% CI = 0.01, 0.37). There were no significant differences in the other seven comparisons (Osumi et al., ; Johnson and Gohil, ).…”

“…Studies included in our systematic review used a variety of VF protocols ranging from a session of 1 min to 1 h 9 5 days a week 9 6 weeks. Visually distorting the size of painful body parts is another component of optimal technique that has aroused interest, despite few available studies on which to judge efficacy (Wittkopf and Johnson, 2016). Likewise, embodiment of the viewed body part, which describes the subjective experience of having a sense of one's own body, including a sense of ownership of body parts (Longo et al, 2008;de Vignemont, 2011), is considered an important determinant of outcome but rarely assessed in trials (McCabe, 2011;Foell et al, 2014;Wittkopf et al, 2017).…”

“…There was no significant effect of mirror VF compared with an object view in four comparisons (Supporting Information Table S5, (Torta et al, 2015). There was no significant effect of mirror VF compared with direct view of the body part (Torta et al, 2015;Johnson and Gohil, 2016). There was a significant moderate effect size in favour of mirror VF compared with the reflection of the hand of the experimenter (SMD = À0.26; 95% CI = À0.40, À0.12 (Longo et al, 2009)).…”

Managing Pain by Visually Distorting the Size of Painful Body Parts: Is There Any Therapeutic Value?

“…The investigators have suggested that magnifying the body part increased anticipatory reactions associated with expectancies about the intensity of the experimentally-induced noxious stimuli and that this prepares individuals for exposure to the noxious event [12,13]. However, a study from our laboratory failed to detect differences in coldpressor pain by reducing and enlarging the appearance of the size of a hand using mirror visual feedback in 20 healthy human participants [14]. Furthermore, Osumi et al [15] found that on healthy participants with low thresholds to experimental pain reported unpleasant emotions towards a magnified hand and that two-point discrimination threshold decreased when the hand was magnified in participants with higher pain threshold.…”