Scans have revealed how the brain develops previously underused areas to help it repair stroke damage. A stroke happens when the blood flow to vital brain cells is interrupted and some die. The degree of disability suffered by stroke survivors depends on exactly where in the brain the damage occurs, although doctors are still unsure about exactly which roles are played by different parts of the brain. Patients in whom certain parts of the left side of the brain is damaged often experience problems with speech and language - although many manage to recover their abilities within six to 12 months.

Now scientists from Washington University School of Medicine in St Louis may have uncovered just how this recovery is possible, even though dead brain cells do not regenerate. Instead, the brain appears to start using the corresponding area on the other side, or hemisphere, of the brain. The research project involved using MRI scans to determine which part of the brain was being employed in different mental tasks.

Left to right

They identified six stroke patients whose damage was restricted an area of the left hemisphere called the inferior frontal gyrus. These patients performed far worse at complex language tasks than healthy volunteers. About six months after the stroke, recovery rates had slowed in these patients and scan images revealed that the area on the right side of the brain corresponding to the inferior frontal gyrus, while little used in the healthy volunteers, was heavily used among the stroke patients.

It suggests that language processing is transferred across the brain to make up for the damage. Eoin Redahan, from the Stroke Association, said: "We know more about the surface of the Moon than the brain and these are potentially exciting results. The more specific we can be about the affected area of the brain, the more we can act on it in future. We may one day be able to use stem cells to try to restore function in these areas."