A psychophysical study of itch relief by mirror scratching

Objective: The goal of this study was to test whether itch attenuation can be activated by scratching the limb contralateral to the itching limb when the participant is made to visually perceive the non-itching limb as the itching limb by means of mirror images. Methods: Healthy participants were asked to assess the intensity of an experimentally induced itch at their right forearm while they observed externally guided scratch movements either at their right (itching) or left (non-itching) forearm which were either mirrored or not mirrored. In the first experiment, a mirror placed between the participant's forearms was used to create the visual illusion that the participant's itching forearm was being scratched while in fact the non-itching forearm was scratched. To control visibility of the left forearm, a second experiment was performed in which unflipped and flipped real-time video displays of the participant's forearms were used to create experimental conditions in which the participant visually perceived scratching either on one forearm, on both forearms, or no scratching. Results: In both experiments, scratching the non-itching limb attenuated perceived itch intensity significantly and selectively in the mirror condition, i.e., when the non-itching forearm was visually perceived as the itching limb. Discussion: These data provide evidence that the visual illusion that an itching limb is being scratched can lead to significant itch relief. This effect might be due to a transient illusionary perceptual congruency of visual, tactile and pruriceptive signals. “Mirror scratching” might provide a treatment to reduce itch perception in focal skin diseases.