Jamie Ward is a Professor of Cognitive Neuroscience at the University of Sussex. You can find his webpage here and he is @jamiewardsussex on Twitter.
If you see someone having a vigorous scratch then does this make you feel itchy too? There have been many anecdotes to suggest that observations or thoughts of itchiness make people feel itchy themselves. For instance, it can be quite hard for medical students to sit through dermatology classes without engaging in scratching at some point! However a new study published in PNAS this month explores this systematically for the first time and provides the first evidence for the neural basis of socially contagious itch. It also considers why some people may be more prone to it than others.
The study used movie clips of people scratching their arm or chest for 20 seconds. One of the first challenges for designing this study was to find control stimuli that were approximately matched to these stimuli but would not induce feelings of itchiness. Our pilot research led us to use movies of tapping of the same body parts: this involves both observation of touch, and observations of self-generated actions, but without any connotations of itchiness. The study itself involved presenting these two sets of stimuli to two groups of participants. At the end of each movie they were simply asked to rate how itchy it made them feel. One group of participants observed the movies whilst undergoing fMRI (functional magnetic resonance imaging) scanning. Another group took the test in the lab and we recorded any spontaneous scratching actions. (The imaging group were not allowed to scratch). The movies depicting scratching tended to elicit itchiness in virtually all participants and around two-thirds of participants scratched themselves at some point in this condition (when free to do so).
Still images from the stimuli used in the study to induce itchiness: left shows scratching and right shows the control condition of tapping
Previous research has emphasised the importance of individual differences in empathy (measured via questionnaire) in predicting individual differences in social contagion of pain, touch, and action (e.g. by correlating levels of brain activity when watching movies depicting those stimuli). However, empathy was not a mediating factor in the present study: if anything it tended to be negatively correlated. Instead the personality trait of neuroticism – the tendency to experience negative emotions – was the only significant predictor of individual differences in itch contagion. Itch relative to, say, pain may be linked to different social motivations: itch may be linked to social distancing (hence neuroticism-related), whereas pain may be linked to helping behaviour (hence empathy-related).
Previous reviews on the neural basis of itch have speculated that the human ‘mirror system’ may play a key role in social contagion of itch. The mirror system has shared neural resources for both action observation with action execution and is associated with regions such as premotor cortex and adjacent inferior frontal cortex, as well as certain parietal regions. Contrasting brain activity to watching the scratching movies, relative to the control movies, did indeed activate a premotor region but only as part of a wider network including the primary somatosensory cortex, anterior insula, and the inferior frontal cortex. This wider network is also activated by other studies that have induced ‘real’ itch (induced by sub-cutaneous histamine) and, in our study, was correlated with magnitude of subjective itchiness. As such, we can conclude that there is far more to social contagion of itch than merely a motor contagion of scratching actions (although that is likely to be part of the story). The somatosensory regions may be linked to the sensory (rather than motor) aspects of itching and scratching. The anterior insula is linked to bodily sensations (interoception), including both pain and itch, and has also been linked to coding the affective properties of such sensations. The insula also differed in its response properties relative to the other regions we observed. Whereas premotor and somatosensory cortex were particularly activated during the early phase of the stimulus (suggesting an involvement in perception of itch), the insular cortex showed a sustained response to the itch-inducing stimuli. We hope to explore this network further using connectivity analyses.
Brain regions that are activated when watching movies of scratching relative to control stimuli (tapping)
These results have implications for understanding psychological factors relating to itching. It is very likely that the same network is activated by the mere thought of itchiness (in addition to watching scratching). So patients with conditions such as atopic dermatitis can potentially magnify their symptoms (at least at the brain-level, if not the skin-level) if they have a natural tendency to be pre-occupied with thoughts of itching. Similarly patients who have delusional beliefs of skin infestation may have a tendency to habitually activate this same network and hence they may experience ‘real’ subjective itch sensations (supported by this brain network) despite appearing to be dermatologically normal.
The research was funded by a Research Grant from the Economic and Social Research Council (ESRC) and the Sackler Centre for Consciousness Science at the University of Sussex. The results are reported in Proceedings of the National Academy of Sciences of the USA, 12th November 2012, and can be viewed at https://www.pnas.org/content/early/2012/11/07/1216160109