A structural–functional basis for dyslexia
Tuesday, April 8th, 2008From PNAS …
Developmental dyslexia is a neurobiologically based disorder that affects {approx}5–17% of school children and is characterized by a severe impairment in reading skill acquisition. For readers of alphabetic (e.g., English) languages, recent neuroimaging studies have demonstrated that dyslexia is associated with weak reading-related activity in left temporoparietal and occipitotemporal regions, and this activity difference may reflect reductions in gray matter volume in these areas. Here, we find different structural and functional abnormalities in dyslexic readers of Chinese, a nonalphabetic language. Compared with normally developing controls, children with impaired reading in logographic Chinese exhibited reduced gray matter volume in a left middle frontal gyrus region previously shown to be important for Chinese reading and writing. Using functional MRI to study language-related activation of cortical regions in dyslexics, we found reduced activation in this same left middle frontal gyrus region in Chinese dyslexics versus controls, and there was a significant correlation between gray matter volume and activation in the language task in this same area. By contrast, Chinese dyslexics did not show functional or structural (i.e., volumetric gray matter) differences from normal subjects in the more posterior brain systems that have been shown to be abnormal in alphabetic-language dyslexics. The results suggest that the structural and functional basis for dyslexia varies between alphabetic and nonalphabetic languages.
From the AP …
Chinese- and English-speaking dyslexics have different neurological deficits, according to a study released Monday which suggests that dyslexia may be different brain disorders in the two cultures.
English speakers with the reading disability typically have functional abnormalities in posterior parts of the brain associated with reading and possibly less gray matter in these areas also.
In Chinese dyslexics, on the other hand, the functional and structural brain abnormalities related to reading correspond with the left middle frontal region of the brain, according to new research.
The new research is based on brain scans performed on 16 dyslexic Chinese speakers and 16 of their peers with normal reading ability during the course of a couple of tests.
Researchers first asked the 32 Beijing primary school students to look at two Chinese characters in different size font to see if they could identify the difference in size.
Having used this question to establish which part of their brains was involved with reading, the investigators then presented the students with two more Chinese characters and asked them if the two characters rhymed.
The second question was designed to test the students’ phonological awareness, their sensitivity to the sound structure of language, which is considered an important and reliable predictor of reading ability.
The scans revealed that the students with the reading disability had less activity in the left middle frontal gyrus on the second task than the children without the disability.
They also had less gray matter in this brain region than the children with normal reading skills.
Further, the Chinese dyslexic children did not have any abnormalities in the parts of the brain that have been shown to be problematic in alphabetic-language dyslexics.
While surprising, the contrast can be explained by the fact that the Chinese language uses characters, while English uses a letter alphabet, one of the researchers said.
“At the functional level, it’s easy to understand why Chinese and English speakers use different parts of the brain to read language,” said Li-Hai Tan, a professor of linguistics and neuroscience at the University of Hong Kong, and author on the paper.
“The different brain networks accommodate the different features of English and Chinese. The two systems are dramatically different. Chinese is pictographic and English is more phonological, or sound-based.”
However, he said that it was striking that the Chinese dyslexic children had less gray matter in the middle frontal gyrus, and that was probably a function of genetics, since this phenomenon is thought to be largely genetic.
This would suggest that the genetic makeup of Chinese speaking dyslexics is different from that of English speakers with the same disorder since they have reductions of gray matter in different sites of the brain.
… and again …
Dyslexia affects different parts of children’s brains depending on whether they are raised reading English or Chinese. That finding, reported in Monday’s online edition of Proceedings of the National Academy of Sciences, means that therapists may need to seek different methods of assisting dyslexic children from different cultures.
“This finding was very surprising to us. We had not ever thought that dyslexics’ brains are different for children who read in English and Chinese,” said lead author Li-Hai Tan, a professor of linguistics and brain and cognitive sciences at the University of Hong Kong. “Our finding yields neurobiological clues to the cause of dyslexia.”
Millions of children worldwide are affected by dyslexia, a language-based learning disability that can include problems in reading, spelling, writing and pronouncing words. The International Dyslexia Association says there is no consensus on the exact number because not all children are screened, but estimates range from 8 percent to 15 percent of students.
Reading an alphabetic language like English requires different skills than reading Chinese, which relies less on sound representation, instead using symbols to represent words.
Past studies have suggested that the brain may use different networks of neurons in different languages, but none has suggested a difference in the structural parts of the brain involved, Tan explained.
Tan’s research group studied the brains of students raised reading Chinese, using functional magnetic resonance imaging. They then compared those findings with similar studies of the brains of students raised reading English.
Guinevere F. Eden, director of the Center for the Study of Learning at Georgetown University in Washington, said the process of becoming a skilled reader changes the brain.
“Becoming a reader is a fairly dramatic process for the brain,” explained Eden, who was not part of Tan’s research team on this paper.
For children, learning to read is culturally important but is not really natural, Eden said, so when the brain orients toward a different writing system it copes with it differently.
For example, English-speaking children learn the sounds of letters and how to combine them into words, while Chinese youngsters memorize hundreds of symbols which represent words.
“The implication here is that when we see a reading disability, we see it in different parts of the brain depending on the writing system that the child is born into,” Eden said.
That means, “we cannot just assume that any dyslexic child is going to be helped by the same kind of intervention,” she said in a telephone interview.
Tan said the new findings suggest that treating Chinese speakers with dyslexia may use working memory tasks and tests relating to sensor-motor skills, while current treatments of English dyslexia focus on letter-sound conversions and sound awareness.
He said the underlying cause of brain structure abnormalities in dyslexia is currently unknown.
“Previous genetic studies suggest that malformations of brain development are associated with mutations of several genes and that developmental dyslexia has a genetic basis,” he said in an interview via e-mail.
“We speculate that different genes may be involved in dyslexia in Chinese and English readers. In this respect, our brain-mapping findings can assist in the search for candidate genes that cause dyslexia,” Tan said.
In their paper, the researchers noted that imaging studies of the brains of dyslexic children using alphabetic languages like English have identified unusual function and structure in the left temporo-parietal areas, thought to be involved in letter-to-sound conversions in reading; left middle-superior temporal cortex, thought to be involved in speech sound analysis, and the left inferior temporo-occipital gyrus, which may function as a quick word-form recognition system.
When they performed similar imaging studies on dyslexic Chinese youngsters, on the other hand, they found disruption in a different area, the left middle frontal gyrus region.
In a separate paper, published two years ago, University of Michigan researchers reported that Asians and North Americans see the world differently.
Shown a photograph, North American students of European background paid more attention to the object in the foreground of a scene, while students from China spent more time studying the background and taking in the whole scene.
