We have shown, for the first time, that the brains of
developmental dyslexics differ depending on the language they
speak. Developmental dyslexia affects 7% to 9% of children in
Hong Kong, and up to 17% throughout the world. It results in a
severe learning disability in acquiring reading skills. Previous
studies using magnetic resonance imaging (MRI) and functional
MRI (fMRI) have revealed that dyslexic readers of alphabetic
languages have decreased gray-matter volume and weak
reading-related activity in posterior brain systems. These brain
regions are responsible for letter-to-sound conversion and are
spatially close to the auditory cortex. For alphabetic readers,
reading and listening are very closely related.
In order to assess whether these
abnormalities are universal, or culture-dependent, our research
team studied dyslexic Chinese children in this RGC GRF research
project. Unlike alphabetic languages such as English which are
learnt using letter-to-sound conversion rules, written Chinese
is a non-alphabetic writing system which is composed of
square-shaped or picture-like characters. A Chinese character
cannot be pronounced by any letter-to-sound conversion rules and
must be memorized by rote. For Chinese readers, reading and
writing may be more closely related.
We used two brain imaging
techniques. Firstly, we used MRI to investigate brain structure
(gray matter volume) of 16 11-year-old Chinese dyslexic subjects
and 16 age-matched normal children. The children, who were
studying in Beijing primary schools, were all native speakers of
Putonghua. We found that the gray-matter volume in a left
anterior brain region (the left middle frontal region) was
significantly smaller in dyslexic children than in normal
subjects. This brain region has been shown to play a part in
Chinese reading and writing, and is also important for working
memory. Their posterior gray matter, however, was unaffected. |
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Chinese and Western
dyslexics show structural and functional abnormalities
in different brain regions, suggesting that dyslexia may
be two different brain disorders in the two streams of
culture. Figure 1: Normal Chinese subjects have stronger
brain activity during reading tasks and have bigger
gray-matter volume in the left middle frontal gyrus
region than the dyslexia children. Figure 2: When
compared with normal children, dyslexic readers of
alphabetic languages have decreased gray-matter volume
in posterior brain systems, and have weak
reading-related activity in the left temporoparietal and
occipitotemporal regions of the brain. |
Secondly, an fMRI experiment was
conducted on a subset of 12 of each of the dyslexics and
normal-reading groups. They were asked to decide whether two
Chinese characters viewed simultaneously rhymed with each other.
The rhyme judgment task involves reading and speech sound
processing. It was found that the normal subjects had much
stronger activation of the left middle frontal gyrus region
during the task than the dyslexic group. The posterior brain
systems, again, did not show differences between the
normal-reading and dyslexic groups.
The fact that Chinese and Western
dyslexics show brain abnormalities in different brain regions
suggests that dyslexia may even be two different brain disorders
in the two streams of culture. This research, published in the
Proceedings of the National Academy of Sciences, could have
important implications for teaching and learning. One
implication suggests we should use different treatment methods
for Chinese children but cannot simply adopt western treatment
methods for alphabetic reading. Our research may help
tailor-making therapies for children who grow up in different
cultures and may well provide useful clues for further genetic
studies in dyslexia. |