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Biologic and Linguistic Bases for Dyslexia

The prevalence of dyslexia appears to vary across cultures (for example, it is diagnosed much less frequently in Italy than in Britain or France). Because of this, some have questioned whether dyslexia is a specific biological disorder. In a new study, researchers provide strong evidence that it is.

A multinational team compared dyslexic patients and normal controls (matched for age, IQ, and education) from the U.K., France, and Italy. On formal testing, dyslexic subjects in all 3 countries showed similar cognitive deficits and exhibited equally poor performance in comparison to controls from their own country. The dyslexic subjects (and controls) were challenged with reading and pronunciation tasks while cerebral blood flow was measured using positron emission tomography (PET) scans. The dyslexic patients from all 3 countries showed identical reductions in activation of the left middle temporal, inferior, superior temporal, and middle occipital gyri -- all important language areas.

Comment: Using both sophisticated cognitive testing and PET scanning, these researchers found similar deficits in dyslexic subjects from 3 countries. The authors argue that the lower prevalence of dyslexia in Italy is explained by an important feature of the Italian language: In a nutshell, Italian is easier than English or French because it has fewer speech sounds (phonemes) and those sounds can be spelled by many fewer combinations of letters. Although these study results do not prove the language hypothesis or suggest ways to prevent or treat dyslexia, they do underscore the biologic basis of dyslexia. For dyslexic patients who have been told that their condition is psychological or nonexistent, this will be welcome news.

— AL Komaroff

Published in Journal Watch General Medicine April 10, 2001

Citation(s):

Paulesu E et al. Dyslexia: Cultural diversity and biological unity. Science 2001 Mar 16 291 2165-2167.

• Original article (Subscription may be required)
• Medline abstract (Free)

Helmuth L. Dyslexia: Same brains, different languages. Science 2001 Mar 16 291 2064-2065.

• Original article (Subscription may be required)
• Medline abstract (Free)