Psychological factors have been implicated in the induction or exacerbation of itch in various forms. In the medical community, it has been observed that a discourse about scabies (or pruritus in general) may produce the intriguing effect that listeners start to itch and scratch. Exposure to images suggestive of itch during a lecture has been reported to increase scratching in the audience3
. Although the “visual transmission” or contagion of itch was occasionally reported in daily life, it has not been investigated in a systematic manner previously.
In this study we examined the effect of visual cues of itch on the perception of a local itch induced experimentally and in the induction of scratching, in a controlled setting. Atopic dermatitis participants reported a significant increase in the perception of itch while watching itch videos, even when only a control solution was delivered to their forearm. A possible explanation of this phenomenon is the central mediation of “contagious”” itching within superior relays of CNS involving the “mirror neuron” system in the prefrontal cortex4–6
. It is conceivable that the neuronal networks or mechanisms underlying contagious itching may be similar with the ones involved in contagious yawning, a phenomenon that is still intensely studied, but not exactly elucidated 7
. Brain imaging studies directed at deciphering the neuronal networks involved in the contagious induction of itch by visual means would be of interest to pursue in the near future, expanding on these observations. It is equally important to note that the amplification of itch perception upon visual exposure to itch cues was manifest in the healthy group, albeit at a lower magnitude. It is therefore possible that the acute response of AD subjects to visual stimuli may represent an amplification of a built-in mechanism also present in the general, healthy population. The mechanisms involved in the amplification of itch perception observed in atopic dermatitis patients may shed light onto central neural pathways that aggravate chronic pruritus. Interestingly, AD patients were reported to be more susceptible to suggestion, in a manner than can aggravate itch: if atopic dermatitis patients were informed prior to a histamine prick test that histamine-induced itch is “uncontrollable and unpredictable”, 90% of them reported an increased itch or developed an amplified urticaria 8
In our study, in the presence of a real itch stimulus (histamine), atopic subjects shifted to scratching areas widely distributed over their body, well beyond the site of histamine application, when an itch video was presented (). The ratio of scratching in extended / local sites increased 3.8 times in atopics when the video was switched from neutral to itch video (respectively). Importantly, the skin areas targeted for scratching by atopics were not eczematous. Video-induced scratching extended bilaterally and was consistently stronger in the atopic population than in the healthy group. This scratching behavior in AD displayed vigorous attempts to quench a surreptitious, widespread itch sensation that apparently “gained” a larger distribution upon exposure to “itch” videos. Scratching decreased at the local site, therefore the effect of the video appeared to induce mostly a scattered itch sensation manifested in other areas of the body. We infer that the scratching response in extended bilateral areas mostly represents a “contagious” form of itch primarily induced by visual means. It appears that the main mechanism for the induction of the scattered itch by visual cues has to operate within the higher structures of the central nervous system. The lack of spatial specificity of scratching behavior suggests that viewing of another individual experiencing itch may potentially result in an attentional tuning of somatosensory processing mechanisms such that ongoing or spontaneous itch-related afferent activity is amplified sufficiently to reach subjective awareness. Alternatively, this effect may parallel a nocebo-type process (pruricebo
), where strong top-down attentional factors result in the mis-interpretation of non-itch related afferent input as being related to itch. We mention in this context that brain areas described in fMRI studies as significantly activated in atopic dermatitis9
such as bilateral insula, dorsal anterior cingulate cortex and putamen, were also implicated in the nocebo effect10
Contagious itch may represent another expression of humans’ ability to emulate others and it could also be seen as a form of empathy11
. It is possible that the mechanism by which humans subconsciously respond by developing itch and an urge to scratch upon seeing a fellow human scratch, is a remnant of collective-social behavioral conditioning that was more pronounced in earlier primates, which was somewhat conserved. Alternatively, contagious itch can represent a manifestation of a high degree of susceptibility to mental or visual suggestions that is inherently human.
These observations may lead to further findings that could help identify and target the central nervous mechanisms involved in the induction or amplification of itch in humans, for the benefit millions of itch sufferers.