The portion of the brain devoted to vision may play a prominent role in processing the spoken word in blind people. Research conducted by the NIH's National Institute of Neurological Disorders and Stroke (NINDS) shows that the "sight" region of the brain is essentially reorganized in blind individuals to help them process spoken words more effectively.
The findings yield important information about the brain's ability to compensate for lost function. The study appears in the October 3, 2004 online issue of Nature Neuroscience1 and in the journal's November issue.
The research involved nine people who are either congenitally blind or who lost their sight before age 4 and nine sighted individuals who served as controls. NINDS investigators Amir Amedi, Ph.D., and Agnes Floel, M.D., spoke a noun out loud and then instructed each study subject to respond with an appropriate verb as quickly as possible, before the next noun was spoken 5 seconds later. Short bursts of repetitive transcranial magnetic stimulation (rTMS)-which uses a pulsed magnetic field directed by a special coil to influence electrical activity in the brain-were delivered to three sites in the brain's sight region, plus one non-sight control region, immediately after the noun was spoken. The rTMS pulses were designed to create a transient "virtual lesion" in the stimulated areas that would disturb the subject's ability to come up with an appropriate verb.
The investigators found that rTMS of a region in the brain called the "occipital pole" affected the ability of blind patients-but not sighted patients-to come up with a verb during the language task. The stimulation particularly affected the blind subject's ability to understand and respond to the spoken noun. The finding is the first of its kind to show the relevance of the brain's visual cortex in language processing in blind individuals, who must process words without any visual clues. This research demonstrates that the visual cortex, normally responsible predominantly for sight, can help the brain to process other senses and types of activities.
"Verbal processing in the blind is mediated by the interplay of a network of areas, including the occipital pole and the prefrontal cortex," said Leonardo G. Cohen, M.D., chief of the NINDS Human Cortical Physiology Section. "Our finding shows that the occipital cortex of blind subjects is recruited to be part of the network involved in performing high-level cognitive functions such as speech processing."
"This study provides an exciting, new insight into how the brain reorganizes following loss of nerve function," said Story C. Landis, Ph.D., NINDS director. "Additional research on brain plasticity may eventually lead to treatments promoting functional recovery following stroke or other neurological diseases."
Future studies are planned to determine if this form of plasticity is also possible in people who become blind later in life.
The NINDS is a component of the National Institutes of Health within the Department of Health and Human Services and is the nation's primary supporter of biomedical research on the brain and nervous system.