Neural basis of contagious itch and why some people are more prone to it

Abstract

Watching someone scratch himself can induce feelings of itchiness in the perceiver. This provides a unique opportunity to characterize the neural basis of subjective experiences of itch, independent of changes in peripheral inputs. In this study, we first established that the social contagion of itch is essentially a normative response (experienced by most people), and that the degree of contagion is related to trait differences in neuroticism (i.e., the tendency to experience negative emotions), but not to empathy. Watching video clips of someone scratching (relative to control videos of tapping) activated, as indicated by functional neuroimaging, many of the neural regions linked to the physical perception of itch, including anterior insular, primary somatosensory, and prefrontal (BA44) and premotor cortices. Moreover, activity in the left BA44, BA6, and primary somatosensory cortex was correlated with subjective ratings of itchiness, and the responsivity of the left BA44 reflected individual differences in neuroticism. Our findings highlight the central neural generation of the subjective experience of somatosensory perception in the absence of somatosensory stimulation. We speculate that the habitual activation of this central "itch matrix" may give rise to psychogenic itch disorders.functional MRI | pruritus | visual induction | insula | touch

Are sex and history of pruritic skin conditions factors which affect the phenomenon of visually evoked itch? An exploratory study

“…This partially agrees with a previous study where participants rated videos depicting scratching of the upper arm significantly itchier than the forearm and chest [3] . Of the 21 (64%) participants who produced spontaneous scratches in response to these videos, the majority were directed to the face and hair [7] , but there were no differences between males and females.…”

“…We also replicated our previous finding that self and other itch ratings correlated but there were no correlations between these and the scratch response. While it seems logical to assume that people scratch themselves when they feel "itchy" previous studies have identified only a weak correlation between itch perception and the scratch response [3] (see revised definition of itch proposed by Savin [11] in which the "threshold to scratch" is implemented), which suggests that the itch-scratch cycle is not absolute in that not all scratches are triggered by feelings of itch and not all feelings of itch result in scratching behavior. Although it seems subjective feeling states can be shared between the self and others, this occurs without obligatory motor stimulation [12,13] , reflecting the automatic and autonomous nature of scratching.…”

“…While some studies have found no effect of sex on itch perception [1,[3][4] , others have suggested that females may be more prone to itch-related skin conditions because of unique epithelial/ hormonal shifts leading to cyclical changes across the age spectrum in the skin's basic composition [14] . Women also have more neuropathic and somatic symptom disorders (DSM V), underlying their chronic pruritus than males [6] .…”

“…Although recent studies have shown sexspecific differences in the quality, localization and triggering of chronic pruritus, and in the underlying disease and scratching behavior [6] , studies from the experimental literature have typically found no sex differences in itch contagion [3] , itch ratings or scratch behavior [1] . However, this may be due to limited sample sizes (eg, only 3 males and 8 females with AD were tested in Papoiu et al [1] ) or biased samples that recruited twice as many females to males [3] . To address this bias, similar numbers of males and females were recruited in the current study.…”

“…This cycle can be triggered by "contagious itch," that is an induced feeling of itch typically followed by a scratch response, resulting from viewing another person scratching [1] . While several studies have explored this effect using different methods for triggering the phenomenon [1][2][3][4] , only 1 study has thus far compared the effectiveness of different visual stimuli to evoke itch and scratch [5] . However, it did not assess the impact of individual differences such as sex or history of pruritic dermatological conditions on visually evoked itch (VEI).…”