The findings, which appear in this month’s issue of the journal Stroke, are based on functional magnetic resonance imaging (fMRI) scans showing that brain function associated with language shifted away from the stroke-damaged area of the adult brain to the corresponding area on the undamaged side of the brain.

The findings show the healing that happens after a stroke occurs at a high level of organisation, demonstrating the plasticity of the human brain long into adulthood. Such plasticity was routinely credited to the brain in the first few years of life.

The research team consists of Dr. Keith Thulborn of the University of Pittsburgh Medical Center, and Marcel Just and Patricia Carpenter, co-directors of the Carnegie Mellon's Center for Cognitive Brain Imaging and professors of psychology.

"The new findings demonstrate extremely rapid adaptation in adult patients," Just said. "While no one looks forward to a stroke, there is some comfort in knowing that we all carry around a set of thinking spare parts that know how to install themselves if the need arises."

The results also indicate the organisational flexibility of the cortical systems that underlie higher level thinking processes. The researchers said this knowledge may be useful in designing future rehabilitation strategies that can exploit the flexibility.

Using non-invasive fMRI, the team looked at the brains of two stroke patients, 34 and 45 years old, as they read and indicated their comprehension of normal English sentences. Very soon after stroke, the cortical areas on the right sides of their brains, the right-hand homologues of Broca's area or of Wernick's area, showed increasing activation during the sentence comprehension, at about the same time as the patients' ability to process language was coming back to them.

In healthy brains, language functions are carried out by a network of mirror image brain areas in the left and right side of the brain, with one side, usually the left, being dominant. The subordinate side, usually the right, may spend most of its lifetime playing an understudy role, as well as developing its own specialisation’s. But if a stroke or some other neurological damage disables one of the network components on the dominant side, the corresponding left side component rapidly and spontaneously emerges from its understudy role and starts to activate to a normally high level during language processing.

The rapid recovery of the ability to use language after stroke damage to the language network was previously attributed to tissue healing functions, like reduction of swelling in the brain.