Dysfunctional Network in Brain's Left Hemisphere Linked to Dyslexia

Using a novel technique to examine how different brain regions interact during reading, National Institutes of Health (NIH) scientists have found that people with dyslexia do not use the same neural networks as normal readers. For millions who struggle with dyslexia, a disorder that causes problems with reading, writing, and spelling, this controlled study shows a lack of communication between regions of the brain involved in reading.

The scientists have discovered an absence of the strong functional links between the left angular gyrus and other left hemisphere regions of the brain. Their finding suggests a functional disconnection of the left angular gyrus—part of the brain thought to play a critical role in relating letters to speech—from the occipital and temporal lobes, brain areas involved in visual and language processing. In normal readers, these regions interact strongly during reading. "These results explicitly demonstrate that the brain circuits mediating reading in dyslexia are abnormal," reports Barry Horwitz, Ph.D., of the National Institute on Aging.

Horwitz and National Institute of Mental Health colleagues Judith Rumsey, Ph.D., and Brian Donohue used positron emission tomography (PET) to find that in the good readers, regional cerebral blood flow (rCBF) in the left angular gyrus shows strong correlations with rCBF in occipital and temporal lobes during single word reading. In the men with dyslexia, the PET data show an absence of these relationships, indicating that as a group they were not using the same reading networks as the normal readers. "A major strength of this study," explains Rumsey, "is that it examined dyslexic men while they were actually reading aloud, a task that activates posterior brain circuits critical to reading." Because written language is a relatively recent addition to human communication, and because most of a typical individual’s day is not spent reading, it is unlikely that a neural network evolved that is solely dedicated to reading, according to Horwitz.

Neurons that are activated during reading are also engaged by other cognitive tasks. While previous studies have compared scans obtained during reading to those during performance of other tasks, such as visual matching, the researchers say it is not surprising that individual functional neuroimaging studies have shown activation of some but not all of the crucial nodes of the neural network. Because this study examined correlations within tasks, its results are not confounded by what neurons in the angular gyrus might be doing in other tasks used for the comparison. To examine the functional connectivity of the angular gyrus during single word reading, the researchers measured rCBF using PET in 14 good readers and in 17 men with persistent developmental dyslexia.

Each subject read two types of material, nonsense words that need to be sounded out using phonological rules (e.g., "phalbap," "chirl") and words that do not follow the rules (e.g., "choir," pharaoh"), requiring the reader to rely on experience. Two scans during each type of reading were used in the analysis. All subjects read continuously, although those with dyslexia read less accurately and more slowly. If men with dyslexia who have learned to read to some degree are not using the normal circuits, they may have developed pathways to compensate for their impairment. Improved understanding of what various parts of the brain normally do during reading may lead to neural-based diagnosis of the nature of each individual’s dyslexia.

Determining alternate pathways that are successfully used may provide clues for better treatment. Horwitz, Rumsey, and Donohue report their findings in the July 21 issue of the Proceedings of the National Academy of Science.

top | next

Dysfunctional Network in Brain's Left Hemisphere | Atypical Brain Activity Detected in People with Dyslexia

home | adhd | autism | depression | learning disabilities | articles
violence | send page