CHICAGO - OH, it brings such blessed relief and now scientists can tell you why - scratching an itch temporarily shuts off areas in the brain linked with unpleasant feelings and memories.

'Our study shows for the first time how scratching may relieve itch,' Dr Gil Yosipovitch, a dermatologist at Wake Forest University Baptist Medical Center in Winston-Salem, North Carolina, said in a statement.

Prior studies have shown that pain, including vigorous scratching, inhibit the need to itch. Dr Yosipovitch and colleagues looked at what goes on in the brain when a person is scratched.

He and colleagues used a technique known as functional magnetic resonance imaging to see which areas of the brain are active during scratching. They scratched 13 healthy people with a soft brush on the lower leg on and off in 30-second intervals for a total of five minutes.

Scratching reduced activity in the anterior cingulate cortex and the posterior cingulate cortex - areas linked with pain aversion and memory.

And the more intensely a person was scratched, the less activity they found in these areas of the brain.

'It's possible that scratching may suppress the emotional components of itch and bring about relief,' Dr Yosipovitch said.

But they also found why one scratch often begets another.

Scratching increased activity in the secondary somatosensory cortex, a pain centre, and in the prefrontal cortex, which is linked with compulsive behaviour.

'This could explain the compulsion to continue scratching,' Dr Yosipovitch said.

The researchers noted that the study is limited because people were not scratching in response to an actual itch.

But they said understanding what goes on in the brain may lend clues about how to treat people tormented by chronic itch, including people with eczema and many kidney dialysis patients.

The study, which appears online in the Journal of Investigative Dermatology, was paid for by the National Institutes of Health. -- REUTERS