The itch-scratch cycle is a well-documented problem for people itchy skin conditions. Itching prompts scratching, which creates skin damage, which causes further itching. This physical exacerbation also has a psychological component, which this thesis aimed to examine using Visually-Evoked Itch (VEI). VEI is the phenomenon whereby itch-related images create sensations of itch in the absence of a physical pruritic stimulus. This effect was manipulated and combined with other methods to study the relationship between itch inducers, itches and scratches, and to elucidate how an itch develops across time.
This thesis comprises four experimental chapters. Chapter 3 investigated VEI directly, measuring itchiness, located itches, and observed scratches. This approach revealed complex interactions between patterns of itch and scratch responses. Chapter 4 isolated the visual element of VEI by removing additional itch cues from the procedure. Without this priming, participants scratched less than those who were asked to report their itch experiences. Chapter 5 examined VEI using psychophysics by combining it with the Somatic Signal Detection Task. This demonstrated that VEI corresponds with a slightly lowered response criterion and decreased overall perceptual thresholds. Chapter 6 compared whether VEI differences between healthy and clinical itch participants is reflected in an attentional bias to VEI-inducing images. Clinical participants showed implicit and explicit biases towards itch images, whereas healthy participants showed an implicit aversion and explicit indifference.
Knowledge from these studies and the wider literature has been synthesised into a new theoretical model. The Threshold Model proposes that the process of entering into the itch-scratch cycle via VEI consists of a set of perceptual thresholds, which use input from visual attention and interoceptive processes, and are modulated by threat detection mechanisms. It conceptualises itch as an interpreted experience, for which VEI provides the context and manipulates the interpretation. Developing our understanding of this will elucidate how psychological triggers can affect the development and persistence of pruritic skin conditions.
