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Understanding the Causes of Dyslexia

By Elaine Cassel

Dyslexia is one of the most common forms of learning disability. For years, researchers have suggested that dyslexia may reflect problems in the structure or function of the brain, rather than deficits in cognitive functioning. Now there is solid proof that the roots of dyslexia are biological.

The origins of dyslexia have long puzzled researchers. The results of brain studies presented at this year's meeting of the American Association for the Advancement of Science (AAAS) have unequivocally placed the origins of dyslexia in both the language and visual areas of the brain. The findings can be summarized as follows:
  • There are subtle differences in brain anatomy in both language areas of the brain, Broca's area and Wernicke's area, as well as in the brain's visual pathways.
  • The brain's electrical activity in people with dyslexia is different from that of people without the disorder.
  • Dyslexics have a visual deficit that is unrelated to reading.
  • Genetic research is identifying the source of the brain anomalies on several chromosomes.

Broca's and Wernicke's areas are located on the left side of the cerebral cortex. The cortex consists of six layers of cells. In people with dyslexia, several hundred nerve cells have formed a small fold, known as a microgyrus, which has the effect of reducing the number of cellular layers from six to four. The visual pathways of dyslexics contain smaller-than-normal cells in the layers of the lateral geniculate nucleus, along the pathway that transmits information from the eyes to the visual cortex at the back of the brain. Researchers believe that because both of these brain areas have been completely developed by the fifth month of gestation, the root cause of dyslexia must lie in genetics and/or prenatal development.

The researchers scanned the brains of people with dyslexia while they attempted to perform a task in which they were presented with a word, such as "cat," and instructed to remove the first phoneme, "c," and pronounce the remaining word ("at"). The scans provided evidence that compared with non-dyslexic controls, dyslexic subjects showed lower activity in the area of the brain that processes visual and auditory information in close proximity.

In an effort to ascertain if brain activity in dyslexics was compromised in other than a reading task, subjects' ability to detect visual motion was tested by having them look at a field of small spots on a computer screen and indicate which way the spots were drifting. Brain scans revealed that dyslexic subjects had less activity in the area of the brain where this processing took place compared to normal controls. This finding could lead to the development of a test to assess preschool-age children for incipient signs of dyslexia before they are of reading age.

Advances in the sequencing of the human genome have found DNA markers in dyslexic subjects that indicate genetic mutations on several chromosomes, including six, one, two, and fifteen. Researchers are working to refine their understanding of the genetic underpinnings of the disorder and to discern just how the genetic mutations lead to the abnormal brain structure and function.

With politicians and educators placing increased emphasis on reading, it is important that research into dyslexia--which affects ten percent of the population--lead to the development of interventions that can help dyslexics improve visual and language processing abilities and maximize their reading skills.

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