Researchers have discovered why scratching an itch may be so addictive.
Scratching impacts of various areas of the brain
A team from Wake Forest University in North Carolina used imaging technology to follow the changes that take place in the brain when we scratch.
Activity was reduced in areas associated with unpleasant emotions, and memories, but increased in an area associated with compulsive behaviour.
The Journal of Investigative Dermatology study raises hopes of new treatments for itching disorders.
For some people the irritation of an itch can be so intense that they scratch so hard they draw blood.
Among those who are most badly affected are people with eczema and those on kidney dialysis.
The researchers used functional magnetic resonance imaging (MRI) technology to monitor the brains of 13 volunteers while they were scratched on the lower leg with a small brush.
The scratching went on for 30 seconds and was then stopped for 30 seconds - for a total of about five minutes.
The results showed reduced activity in an area of the brain called the anterior cingulate cortex, which is linked to unpleasant sensory experiences, and in the posterior cingulate cortex, which is associated with memory.
When the volunteers reported the scratching was most intense, activity in these areas was at its lowest.
Lead researcher Dr Gil Yosipovitch said: "This is the first real scientific evidence showing that itch may be inhibited by scratching.
"We know scratching is pleasurable, but we haven't known why.
"It is possible that scratching may suppress the emotional components of itch and bring about its relief.
"Of course, scratching is not recommended because it can damage the skin.
"But understanding how the process works could lead to new treatments. For example, drugs that deactivate this part of the brain might be effective."
The study also found some areas of the brain were made more active by the scratching.
These included the secondary somatosensory cortex, a sensory area involved in pain, and the prefrontal cortex, which is associated with compulsive behaviour.
One drawback to the study is that the scratching occurred in the absence of itch. The researchers now plan to examine whether the same pattern of brain activity is produced when a chronic itch is scratched.
Dr Irene Tracey, an expert in pain at the University of Oxford, said effective new treatments for itching would be a significant advance, as the problem could be a debilitating for some people.
"I know of people who have been given morphine for pain relief, and have developed a facial itch that was so severe they have opted to come off the morphine and put up with the pain," she said.
Dr Tracey said identifying the areas of the brain affected by itching and scratching could potentially lead to new treatments.
However, she said cognitive behaviour therapy designed to impact on the relevant areas was likely to be the most productive approach.