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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.
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Dysfunctional Network in Brain's Left Hemisphere
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