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People who stutter show unusual brain activity not only when they speak, but also when they are listening to words and silently reading, a new study has found.

The work is part of an increased effort to map the brain circuitry involved in stuttering. A complex picture is emerging, one that may eventually explain why more men than women stutter and could lead to new therapies for the disorder.

An estimated one in 20 children stutter, and of those, one in five don't grow out of it. About 1 per cent of the population suffers from chronic, developmental stuttering.

"I think it can help people who stutter to know there is a reason in the brain for their stuttering, that it is not what people call pure psychology - nervousness or anxiety," said Kate Watkins, a researcher at the University of Oxford in Britain, part of the team that used magnetic resonance imaging to peer inside the brains of 12 adults who stutter.

She asked them to silently read sentences and to listen to sentences as they were read. When she compared their brain activity to 12 volunteers with normal, fluent speech, she found significant differences.

When they were reading, the people who stuttered had less activity in the brain circuitry that helps orchestrate sequences of movement, for example, playing a bar of music.

"We suspected very strongly that this network would be involved based on some of the features of stuttered speech, a difficulty in moving on with the sequence."

She said some people who stutter say they do it "in their heads," when they aren't actually speaking out loud.

There were also differences in brain activity when the volunteers were listening to sentences being read. The people who stuttered had more activity in the areas that process sound, suggesting they were paying more attention than the control group. This may be because they use the speech of others as a cue to help with their fluency, Dr. Watkins said.

She added it is well documented that people who stutter are less likely to do so when they are speaking in unison with others.

The sample size in the study is small, as is often the case in experiments that involve brain imaging, which is expensive. But the results are statistically significant.

Dr. Watkins' work is with adults, whose brains may have been changed by years of stuttering. The next step is to see if there are similar differences in children.

At Michigan State University, Soo-Eun Chang is starting a five-year, $1.8-million (U.S.) study to look for clues to explain how stuttering differs between males and females.

Girls start to recover from stuttering between the ages of four and six, she said, but boys often don't. In adults, the disorder affects about five times more men than women.

She wants to see if boys' and girls' brains develop differently in a way that makes it easier for girls to stop stuttering as they mature.

Both Dr. Watkins and Dr. Chang said their goal is to find new therapies.

"This work will hopefully change the face of stuttering diagnosis and treatment," Dr. Chang said. "It is the first of a series of studies to identify neural reasons for early childhood stuttering, and sexual differences that lead to recovery or persistence of stuttering."

Speech therapy can be very effective for many people, she added, but they have to make an effort all the time, and relapse is very common.

If scientists can figure out the circuitry involved, they might be able to modulate it, said Dr. Chang, perhaps with therapies that deliver a weak electrical current to specific brain regions through the skull.

Dr. Watkins said they might eventually be able to tailor therapies for individual patients based on brain imaging.

Both Dr. Chang and Dr. Watkins presented their findings last week at the Society for Neuroscience conference in San Diego.

Dr. Chang reported on a study that compared the brains of men and women who stutter. They have some brain differences in common, including increased connectivity between the two hemispheres of the brain and fewer connections in the left hemisphere, usually the dominant side for language.

But there were some differences. Women who stutter are less likely than men to make up for weakness in the left side of the brain by using equivalent areas on the right side.