Why does scratching an itch feel so good?
| 26 Sep 2015Broad network of brain areas mediates the satisfaction of scratching
The act of scratching not only relieves an itch but also evokes pleasant feelings. These feelings of enjoyment can be traced to activation in regions of the brain involved in reward, or positive stimulation that reinforces certain behaviors. While the pleasantness of scratching an itch has been documented before, a 2014 study claims to be the first to identify the specific brain regions involved in this sensation.
Researchers used functional magnetic resonance imaging (fMRI), which measures changes in blood flow in the brain, to see which areas “light up” in response to scratching. The 16 participants in the study didn’t scratch themselves, though. The researchers carefully controlled the duration (five seconds) and location of scratching, which was done with a tool consisting of two copper plates that apparently delivered the same sensation as using fingernails. Itching was induced on participants’ wrists via electrical stimulation for four-and-a-half seconds. In the “pleasant” condition, the wrist was then scratched by the experimenter; in a control condition, part of the forearm that was not itchy was scratched instead. Participants also reported how pleasant the experience was 10 seconds after the scratching ended. These conditions were done both with and without brain imaging; in the latter case, the participants also reported the duration of pleasantness by pressing a button. An extra condition confirmed that there was no pleasant sensation derived from scratching without itch stimulation.
A bear scratching its forehead. Image by Tambako The Jaguar / CC BY-ND 2.0
Participants’ ratings showed that no pleasant feelings were evoked in the control condition. In the other condition, the pleasant feeling from scratching lasted for five seconds on average, the same duration as the scratching itself. Many brain regions were activated in both the pleasant and control conditions, as could be expected. Significantly higher activation in the pleasant condition was found in areas including the supplementary motor area, inferior frontal gyrus, striatum, midbrain, premotor and somatosensory cortices, and the insular cortex. The latter area is important for awareness of changes in the body and mind. Many of the same regions were active in response to the itch stimulation itself or when participants rated the pleasantness of scratching. It was not clear what the roles of inferior frontal gyrus or premotor cortex were in the feeling of pleasantness; activation in these areas had not been observed in previous studies. Reduced activation was seen in both conditions in primary motor cortex, occipital cortex, medial orbitofrontal cortex and hippocampus. This was similar to results of previous studies.
The numerous brain regions involved in the feeling of pleasure from scratching are also activated by other somatosensory stimuli such as touch or pain. This led the researchers to surmise that the core of the pleasant scratching feeling is the somatic sensation itself; the fact that scratching the itch (i.e. relief) was preferable to the control condition likely also contributed to increased reward activity. The activation seen in motor regions during the pleasant condition was also new, and was attributed to the desire to act (scratch) to alleviate the itch. Activation of the motor regions may also explain how scratching-induced pleasantness reinforces the scratching behavior. Midbrain and striatum dopaminergic neurons, part of the reward system activated during the pleasant sensation, may be responsible for recruiting the motor regions, as they innervate these higher areas.
The pleasant feeling of scratching an itch thus comes from activation in reward areas of the brain, such as the striatum, midbrain and medial frontal regions. The involvement of both motor and reward circuits is relevant to conditions like chronic itch and excessive non-pleasant scratching.
Citation:
Mochizuki, H., Tanaka, S., Morita, T., Wasaka, T., Sadato, N., & Kakigi, R. (2014). The cerebral representation of scratching-induced pleasantness. Journal of Neurophysiology, 111(3), 488-498. DOI: 10.1152/jn.00374.2013