When stroke or brain injury damages a part of the brain controlling movement or sensation or language, other parts of the brain can learn to compensate for this damage. It’s been thought that this is a case of one region taking over the lost function. Two new studies show us the story is not so simple, and help us understand the limits of this plasticity.
In the first study, six stroke patients who have lost partial function in their prefrontal cortex, and six controls, were briefly shown a series of pictures to test the ability to remember images for a brief time (visual working memory) while electrodes recorded their EEGs. When the images were shown to the eye connected to the damaged hemisphere, the intact prefrontal cortex (that is, the one not in the hemisphere directly receiving that visual input) responded within 300 to 600 milliseconds.
Visual working memory involves a network of brain regions, of which the prefrontal cortex is one important element, and the basal ganglia, deep within the brain, are another. In the second study, the researchers extended the experiment to patients with damage not only to the prefrontal cortex, but also to the basal ganglia. Those with basal ganglia damage had problems with visual working memory no matter which part of the visual field was shown the image.
In other words, basal ganglia lesions caused a more broad network deficit, while prefrontal cortex lesions resulted in a more limited, and recoverable, deficit. The findings help us understand the different roles these brain regions play in attention, and emphasize how memory and attention are held in networks. They also show us that the plasticity compensating for brain damage is more dynamic and flexible than we realized, with intact regions stepping in on a case by case basis, very quickly, but only when the usual region fails.
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(2010). Prefrontal cortex and basal ganglia contributions to visual working memory.
Proceedings of the National Academy of Sciences. 107(42), 18167 - 18172.