Biological Basis for Reading Disability Discovered

New research has found convincing evidence that dyslexia, a language-based learning disability, is caused by a functional disruption in the brain. The research, led by Dr. Sally Shaywitz, a Professor of Pediatrics at the Yale University School of Medicine, is published in the March 3, 1998 Proceedings of the National Academy of Sciences. These findings represent a critical new piece of evidence that builds on the already solid research in the area of reading disability.

The researchers used a relatively new technology, called functional magnetic resonance imaging (fMRI), which enables researchers to look into the brain as it is working. The research used fMRI to image the brains of 32 nondyslexic and 29 dyslexic adults while they attempted to perform a progressively complex series of reading tasks. The tasks included letter recognition, rhyming letters and words, and finally, categorizing words. The findings showed that brain activation patterns of dyslexic readers were significantly different from those of nondyslexic readers.


Reading requires an ability to recognize that spoken words can be segmented into smaller units of sound (phonological awareness) and that the letters in the printed word represent these sounds. Dyslexic readers do not recognize these smaller sounds and have difficulty mapping alphabetic characters onto the spoken word.

The new study shows that nondyslexic readers systematically increased their brain activation as the difficulty of mapping print into phonological structures increased. The readers with dyslexia failed to systematically increase their brain activity. The demonstrated disruption in brain function among dyslexic readers occurs in a part of the brain involving traditional visual and language regions. During reading, those with dyslexia showed a pattern of underactivation in a large posterior brain region, an area which connects the visual areas with the language areas.

These new findings reconcile seemingly contradictory evidence from previous imaging studies which were not able to map out the full extent of the disruption. Of particular importance is the finding that the angular gyrus, a brain region considered pivotal in carrying out cross-modal (e.g., vision and language) associations necessary for reading, is involved. The current findings of underactivation in the angular gyrus of dyslexic readers coincide with earlier studies of those who lost the ability to read due to brain damage centered in that same area of the brain. According to the authors, "it is no coincidence that both the acquired and developmental disorders affecting reading have in common a disruption within the neural systems serving to link the visual representation of the letters to the phonological structures they represent."

"These findings have important implications for the large numbers of intelligent men, women and children with dyslexia," said the study authors. "If you have a broken arm, you can hold up an x-ray as evidence. Up to now, individuals with dyslexia were often doubted and there was little concrete evidence they could show to support the neurobiologic nature of their reading difficulty. These brain activation patterns, by revealing a functional disruption in those neural systems responsible for reading, now provide neurobiologic evidence for what, up to now, has been a hidden disability."

According to the National Institutes of Health, 1 in 7 Americans has a learning disability; and 80 percent of those identified with a learning disability suffer from dyslexia.

Findings released on March 3, 1998 Proceedings of the National Academy of Sciences