Optimizing audiovisual itch induction: the role of attention and expectancy.

In this issue of the BJD, Marzell and colleagues1 show for the first time that the level of itch induced by audiovisual itch stimuli is not inferior to histaminergic itch after dermal priming. New insights into the underlying mechanisms of audiovisual itch induction can further optimize its effectiveness.

The itch‐inducing property of audiovisual material has been described previously.2 Itch contagion may serve a nocifensive function (i.e. signalling potential bodily threat),3 and it probably involves activation of an affective mirror neuron system.1, 4 Audiovisual itch contagion has been described for both humans and nonhuman primates, but it does not seem effective in rodents.5 This underlines the role of higher‐order cognitive processes, of which attention and expectancies will be highlighted below.

Focusing attention on itch cues is evolutionarily advantageous because it enables a protective response, for example removing a mosquito from your skin. Marzell et al. showed that audiovisual effects on itch are particularly strong after dermal priming (i.e. showing a nonitch‐inducing skin‐related video). They plausibly state that dermal priming would lead to attention being shifted in a way that it ‘potentiates mental processes’.1 Priming prioritizes subsequent stimuli presented within the same modality,6 arguably resulting in facilitation of the audiovisual material (whether somatosensory priming would result in prioritization of somatosensory input remains to be investigated). This focusing was further enhanced because the participants were instructed to report their bodily sensations and emotions. At the same time, showing neutral audiovisual material during the histamine provocation may have distracted participants from the histaminergic itch, similarly to the approximately 50% reduction in itch we previously observed during a simple visual task.7

Negative expectancies are known powerful itch amplifiers.8 In the present study, it is not unlikely that placebo iontophoresis induced nocebo effects on itch, amplifying the itch‐inducing effects of the audiovisual itch induction. Marzell and colleagues’ statement that somatosensory provocations induce anxiety1 – which plays a key role in nocebo effects9 – is consistent with this hypothesis.

From this perspective, it is not surprising that audiovisual stimuli are more effective in patients with chronic itch.2 The persistent clinical itch of these patients may induce a tendency to be attentive to itch stimuli, to expect itch and to interpret stimuli in the context of itch.8, 10

To conclude, advantages of audiovisual itch induction over histamine iontophoresis are noninvasiveness, more widespread distribution of audiovisual itch (representative of patients’ symptoms) and less contamination by painful sensations.1 Limitations of audiovisual itch include its inability to target specific body locations, and less control over induced scratching and the onset and duration of induced itch. The effectiveness of the method can be further enhanced by increasing the relevance (e.g. dermal priming) and inducing negative expectations (e.g. informing participants that the audiovisual stimuli induce quite some itch). Audiovisual itch stimuli may even be used as a short‐lived human model of widespread chronic itch, for example by repetitively combining the presentation of the material together with a unique cue (i.e. conditioning), under ethical conditions. In summary, we agree with Marzell and colleagues1 that audiovisual itch material can be very powerful.