A new study of dyslexia provides what researchers say is the first physiological and anatomical evidence that the reading disorder may be linked to the sense of vision.
The study, reported this month in the Proceedings of the National Academy of Sciences, challenges a previously held notion that the disorder is exclusively a malfunction of the way people understand language. Rather, the researchers hypothesize, dyslexia, which is characterized by difficulty in reading, may also stem from abnormalities in the “fast and slow” systems in the body for perceiving sight and, possibly, for perceiving sounds and sensations as well.
“The linguistic theory holds that [dyslexia is] a high-level cognitive defect,” said Margaret S. Livingstone, the Harvard Medical School neurobiology professor who led the study. “But what we’re saying is that that’s secondary to the incorrect input a child may have been getting for 11 years of life.”
The visual system in humans and other primates comprises two pathways-the magnocellular system and the parvocellular system, Ms. Livingstone said.
Recent research suggests that the former pathway--made up of larger cells that carry out fast processes--aids in perceiving motion, depth, and high contrast, and in conveying positional information. It is color-blind.
The latter system handles slower processes and specializes in distinguishing detail, sustaining scrutiny of objects, and perceiving color.
The same “fast and slow” systems are thought to exist for the senses of hearing and touch. But, said Dr.
Livingstone, their precise functions are not yet known.
The researchers contend that people with dyslexia may suffer from a defect in the magnocellular system for vision.
Implications for Teaching
Previous behavioral studies have also pointed to delays in the visual systems of dyslexic individuals. But the new study is the first to measure that delay using physiological indicators and pinpoint its possible location.
In the study, the researchers measured the brain-wave responses of five dyslexic adults and seven nondisabled individuals to visual stimuli. They were shown a checkerboard on which the squares were reversed at different speeds under conditions of high and low contrast.
Under circumstances when both of the visual pathways should be working, the dyslexic subjects showed a normal response. But at low contrast, when only the magno system should be operating, their responses were much slower.
The researchers also did autopsies on the brains of five dyslexic and five nondyslexic individuals. They found that the magnocellular layers were more disorganized, and the cell bodies appeared smaller, in the dyslexic brains.
The researchers cautioned that because of the small number of subjects, their findings are preliminary. If their results are borne out by further research, however, the study could affect the kinds of treatments and teaching methods used for dyslexic children, Ms. Livingstone said.
She noted, for example, that other researchers have experimented with colored light filters to aid dyslexic individuals in reading.
