Abstract
ObjectiveThe goal of this study was to test whether central mechanisms of scratching-induced 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. MethodsHealthy 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 (right) forearm was being scratched while in fact the non-itching (left) forearm was scratched. To control visibility of the left (non-mirrored) 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 only, on both forearms, or no scratching at all.ResultsIn 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. DiscussionThese data provide evidence that the visual illusion that an itching limb is being scratched while in fact the non-itching limb contralateral to the itching limb is scratched, can lead to significant itch relief. This effect might be due to a transient illusionary intersensory perceptual congruency of visual, tactile and pruriceptive signals. “Mirror scratching” might provide an alternative treatment to reduce itch perception in focal skin diseases with persistent pruritus without causing additional harm to the affected skin and might therefore have significant clinical impact.
Highlights
Itch can be defined as an unpleasant sensation that provokes the desire to scratch the itching site
Histamine is physiologically released by mast cells and activates unmyelinated peripheral C-fibers and spinothalamic lamina I neurons [2,3,4]
The level of significance was set at α= 0.05 for all tests. For both studies ANOVAs on rank-transformed ratings were performed in order to estimate the impact of trial repetition. In both experiments ANOVAS with factors condition and repetition on visual analogue scale (VAS) and non-itching (left) forearm [S-V-] (NRS) revealed no significant main effect of repetition and no significant condition-by-repetition interaction
Summary
Itch can be defined as an unpleasant sensation that provokes the desire to scratch the itching site. E.g. atopic eczema, elicit an itch sensation [1] but patients must not scratch the itching skin rashes as skin inflammation might deteriorate. Histamine is physiologically released by mast cells and activates unmyelinated peripheral C-fibers and spinothalamic lamina I neurons [2,3,4]. Via spinothalamic afferents these signals are transmitted to brain regions that encode location and intensity of somatosensory sensations , i.e., the primary and secondary somatosensory cortex [5], and valence, i.e., insula and anterior cingulate cortex (ACC) [6]
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