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.

Fig. 1.

Degree to which watching videos induced feelings of itchiness in the participants, as indicated by ratings. The scale ranges from 0 (not at all) to 7 (extremely), with 4 as moderate. n = 51. Error bars indicate 1 SEM.

Fig. 2.

(A) Cross-sections showing regions significantly activated (P < 0.05, corrected) in the comparison of scratch and control. n = 18. The position of sagittal slices is indicated by the number above (x coordinate in the MNI system) and is also shown in the coronal section on the right. The scattergram shows a significant correlation between the magnitude of the categorical itch effect in left BA44 and neuroticism, as measured by the BFI (50). (B) Magnitude of the parametric itch effect in key regions. n = 15. The bold black line indicates the significance threshold (all r > 0.52 are significant at P < 0.05, two-tailed). LOC, lateral occipital complex. (C) t values of the categorical effect in key regions for the first half (1–10 s) and second half (11–20 s) of the 20-s video clips. n = 18. The critical t value (P < 0.05, two-tailed) is in bold black.