Itch is an unpleasant sensation with the desire to scratch. Although it is well known that scratching itchy skin is pleasurable, the cerebral mechanisms underlying this phenomenon are poorly understood. We hypothesized that the reward system is associated with scratching-induced pleasantness. To investigate this hypothesis, a functional magnetic resonance imaging study was performed in 16 healthy subjects. Pleasantness was evoked by scratching the wrists where itch stimuli were applied, while scratching the dorsal forearms, far from itch stimuli, did not evoke pleasantness. Interestingly, pleasantness evoked by scratching activated not only the reward system (i.e., the striatum and midbrain) but also key regions of perception (i.e., the primary somatosensory cortex) and awareness of subjective feelings (i.e., the insular cortex), indicating that a broad network is involved in scratching-induced pleasantness. Moreover, although itch was suppressed by scratching, motor-related regions such as the supplementary motor area, premotor cortex, and cerebellum showed significant activation when pleasantness was evoked. This activation could explain why scratching-induced pleasantness potentially reinforces scratching behaviors. This study is the first to identify networks activated by scratching-induced pleasantness. The results of the present study provide important information on the cerebral mechanisms underlying why scratching itchy skin evokes pleasurable feelings that reinforce scratching behaviors.
Keywords: fMRI; itch; motor-related regions; pleasantness of scratching; reward system.