Contagious scratching: shared feelings but not shared body locations

Listening to a lecture on “itching—what’s behind it?” can induce observable scratching behavior and self-reported itchiness in the audience (Niemeier and Gieler, 2000). In another study, Papoiu et al. (2011) showed 5min movies of scratching or rest (either with or without an itchinducing histamine injection) and noted that watching scratching can increase selfreported itchiness and scratching although the effects tended to be small in participants without a pre-existing dermatological condition. Previous speculations concerning the neural basis of socially contagious itching have centered on the action-based mirror system (e.g., Ikoma et al., 2006). Recently, Holle et al. (2012) attempted to explore this using fMRI. The stimuli consisted of brief (20 s) movies depicting scratching to the arm or upper chest, and the control movies consisted of tapping the same body part (i.e., the control stimuli involve both a motor act and selfdirected touch but imply quite different bodily states). The movies were cropped at the neck to avoid facial expression. The movies depicting scratching were effective inducers of self-reported itch. Participants tested outside the scanner were videotaped and the scratch movies tended to induce scratching behavior (participants in the scanner were instructed not to scratch). The movies depicting scratching (minus tapping) activated many of the regions associated with physically induced itch (via histamine administration) including the premotor cortex, inferior frontal lobe, anterior insula, and primary somatosensory cortex. Thus, contagious scratching is by no means limited to motor-related regions of the brain. In this commentary, we carry out an additional analysis of the gestures of the videotaped participants in Holle et al. (2012) to examine which aspects of the scratching gesture were reproduced. Two independent raters were asked to determine: (A) whether the participants scratched themselves vs. performed some other body-directed action (e.g., touching); (B) to note the bodily location acted upon; and (C) the hand used. The second rater was blind as to the nature of the visual stimulus presented to the participants and a third rater (again blind) was used to adjudicate between disagreements. Figure 1A shows that when participants observed a movie depicting scratching they weremore likely to scratch themselves (χ2 = 3.81, P < 0.05). That is, both the quality of itchiness (self-reported) and the action of scratching (as observed) is vicariously shared—as already noted by Holle et al. (2012). However, our new analysis shows that other features of the event are not vicariously shared. Figure 1B) shows the hand used to perform the scratching action in relation to the hand observed to perform the action. It can be seen that participants use their left and right hands equally often to scratch themselves and this is independent of the hand used in the visual stimulus (χ2 = 0.14). Similarly, we coded the part of the body that was