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

BACKGROUND: Studies suggest that observing magnified and minified body parts using mirrors, lenses and virtual reality may affect pain perception. However, the direction of effect varies between studies. The aim of the present study was to evaluate the effect of observing a normal-sized, magnified and minified reflection of a hand on perceptual embodiment and contact-heat stimuli. METHODS: Participants (n = 46) observed a normal-sized, magnified and minified reflection of the hand and a no-reflection condition while performing synchronized finger movements for 3 min (adaptive phase). Measurements of embodiment were taken before adaptive phase, pre- and post-contact-heat stimuli. RESULTS: There were no differences in pain threshold nor tolerance between reflection and no-reflection conditions. Altering the size of the reflection of the hand did not affect estimates of pain threshold nor tolerance. The temperature for warm detection threshold was lower when participants were observing the magnified reflection of the hand compared with the no-reflection condition. Perceptual embodiment of the reflection of the hand was stronger after an adaptive phase with visuo-motor feedback, and the painful stimuli did not weaken the perceptual experience. CONCLUSION: Observing a reflection of the hand in front of a mirror did not alter heat pain threshold nor tolerance when compared with a no-reflection condition, and altering the size of the reflection did not affect pain perception. Researchers and clinicians using visual feedback techniques may consider including an adaptive phase with visuo-motor feedback to facilitate embodiment of the viewed body part. SIGNIFICANCE: An adaptive phase with visuo-motor feedback enhances the perceptual experience of embodiment of a reflection of a hand and a painful stimulus does not weaken the experience. This should be considered when using visual feedback techniques for pain management.