Neuronal Mechanism Of Contagious Itch

Contagious itch is an itchy sensation elicited when watching someone scratching without peripheral pruritogen. This study sought to investigate the neural processes of contagious itch in chronic itch patients with atopic dermatitis (AD) who tend to be more prone to contagious itch, using functional magnetic resonance imaging (fMRI).In our experiments, scratching videos were used to elicit contagious itch. Subjects were asked to rate itch intensities that they felt when watching the videos. There were17patients in behavior experiments and11patients in fMRI experiments. We found that the intensity of contagious itch was lower (P<0.004) in atopic patients (0.5±1.0) than healthy subjects (1.4±1.6) using a VAS rating scale of1-10.Our results indicated that, when the patients were watching scratching videos, the brain areas which constitute the core network for affective-perceptual empathy were activated, including bilateral insular, anterior cingulate cortex, right thalamus and midbrain(P<0.005), suggesting the involvement of emotional empathy in AD patients’contagious itch. This is consistent with the observation in healthy subjects, but the extent of the involved brain areas was much greater. Thus no significant activation was found in midbrain and right thalamus in healthy subjects. In addition, a subset of brain areas within the mirror neuron system was activated in AD patients, including inferior frontal gyrus, left inferior parietal lobe and supplementary motor area(P<0.005). Similar results were obtained from the healthy subjects with an exception that no activation in left inferior parietal lobe was found. We also detected activities in anterior and posterior cerebellum (P<0.005).Our results demonstrated that contagious itch in AD patients share a similar mechanism with the healthy subjects, which involves the activations of the core network for affective-perceptual empathy and mirror neuron system. We also detected some difference in the extent of activated areas in these two systems between AD and healthy subjects. Moreover, AD patients reported significant lower score of VAS rating on contagious itch than healthy subjects. All these results indicate that there exist differences in neuronal process of contagious itch between AD patients and healthy people and further studies are needed for fully understanding of the observations in the current work.