Brain damage, though tragic, often provides unique insights into how the normal brain functions. A prime example can be found in hemispatial neglect, a neurological syndrome that commonly results from damage to one of the parietal lobes. Such damage, most often the result of stroke, impairs the orienting of attention to stimuli on the side of space opposite the damaged side of the brain. Neglect patients frequently fail to notice aspects of the affected side of space, particularly when competing stimuli are present on the unaffected side. Neuropsychologists have long puzzled over what neglect reveals about attention mechanisms in the normal brain, but have been hindered by two difficulties: brain damage is normally irreversible, making it impossible to compare performance before and after the damage, and recovery from brain damage is presumed to result in further changes to the brain, rendering the undamaged parts different from what would be found in normal people.
In this issue of Nature Neuroscience (Vol. 4, No. 9, Sep 01), Hilgetag and colleagues use a novel method to introduce temporary brain lesions in the parietal lobes of normal subjects, thus making it possible to compare behavior before and after the lesion for the first time. Using a technique known as transcranial magnetic stimulation, they repeatedly stimulated the brains of their subjects with a transient, focal magnetic field, in a way that is known to reduce cortical excitability. After 10 minutes of stimulation, their subjects exhibited symptoms of neglect for a few minutes before returning to normal, presumably because their parietal lobes were inactivated. Hilgetag and colleagues measured the ability of their subjects to detect stimuli presented on either side of space during the period of impairment. Just like neurological neglect patients, their subjects were worse at detecting stimuli opposite the damaged hemisphere, particularly when a competing stimulus was present at the same time on the other side of space. Remarkably, though, performance was actually better than normal for stimuli on the same side as the damaged hemisphere. This finding suggests that the left and right sides of the brain inhibit each other. When one is damaged, the other is apparently set free, boosting performance.
The technique used by Hilgetag and colleagues could have widespread importance for investigating brain function. The ability to reversibly inactivate part of the brain should allow researchers to investigate the function of many other brain regions, without the difficulties inherent in studying patients.
Robert Rafal discusses these findings in an accompanying News & Views article.
Dr. Claus C. Hilgetag
Department of Anatomy and Neurobiology
Boston University School of Medicine
700 Albany Street W746
Boston, Massachusetts 02118
tel: 617 638 4532
fax: 617 638 4102
Dr. Robert D. Rafal
University of Wales, Bangor
School of Psychology
Brigantia Building, Penrallt Road
Bangor, Gwynedd, Wales LL57 2AS
tel: +44 1248 38 3603
fax: +44 1248 382599
(C) Nature Neuroscience press release.
Message posted by: Trevor M. D'Souza
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