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 | Research in the News: Dyslexia Leaves Its Mark (Grades 9-12) |
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Dyslexia Leaves its Mark
Ruth Levy Guyer
For a long time people with dyslexia were called dumb or lazy. New research shows that specific brain abnormalities lie at the heart of this learning disability.
During the first four months of life, the human brain is a very busy place. In fact, says Guinevere Eden, a neuroscientist at Georgetown University in Washington, D.C., so much is happening -- so many cells growing, dividing, and firing, so many structures maturing and taking appropriate shapes, so many important connections being made -- that "it is almost surprising that you don't actually hear all the commotion inside the infant's head!"
An fMRI image, showing orientation in human head
All that brain development is what permits people eventually to read and dance, talk and throw a frisbee. But not everyone can perform all these activities correctly or with ease. Some people, for example, have difficulties reading. Many of them have a condition called developmental dyslexia. They can run into serious problems at school, at work, and in social situations, because reading is so important in everyday life.
Many people think that the only problem in dyslexia is that the affected person mistakes "was" for "saw" and "tip" for "pit." But dyslexia is more complicated than that. "There is a huge language component to dyslexia," in addition to troubles interpreting visual information says Eden. But, she adds, that is about the only thing that people interested in dyslexia agree upon.
Children with dyslexia describe how the letters and words on the printed page seem to jump around, superimpose themselves on one another, become indistinguishable (b looks like d, for example), or in other ways prove unmanageable. "It's like the words are walking," said one child with dyslexia.
At the end of the 19th century, two doctors in Britain -- a physician working in a school and an eye doctor -- described a condition that they called "congenital word blindness." Children with the condition couldn't read, even though they were of normal intelligence. Soon, another eye doctor proposed that word blindness arose when the area of the brain that was responsible for the "visual memory of words" had not developed properly.
In 1928, Samuel Orton, a neurologist in Iowa, described 15 children who shared some unusual quirky characteristics. In addition to confusing the letter b with d and the letter p with q, some could read more easily if they held pages up to a mirror, and a few were rapid mirror writers. Orton was optimistic that many of the children could be taught to read with new methods that exploited their other senses -- touch and hearing -- which were not impaired. He suggested that this condition might develop when needed connections in the brain do not get made.
The human brain consists of a patchwork of regions that carry out different activities. At least 32 regions (labeled with a "V") are thought to participate in vision. Region V5, for example, seems to be crucial for tracking moving objects; V1 and V2 recognize colors and patterns. A number of studies in recent years have targeted a visual pathway that includes V5 as a trouble zone in people with dyslexia.
fMRI images of people with (bottom) and without (top) dyslexia, taken while looking at stationary patterns (left) or moving patterns (right) of dots
In 1996, Eden and her coworkers at the National Institue of Mental Health (NIMH) in Bethesda, Md., (where she worked at the time) confirmed this association. They used a technique called functional magnetic resonance imaging (fMRI) to look at brain activities in men with dyslexia and in men with no known reading problems. (See sidebar "sb_fmri"). As test subjects watched moving dots march around a movie screen, V5 became active only in the brains of those who read normally; the movements did not trigger V5 activity in those with dyslexia. Next, subjects were shown motionless dots in various patterns. In this test, V1 and V2 glowed similarly in the brains of everyone, whether they were normal readers or had dyslexia.
Eden's dramatic pictures indicate that, indeed, V5 is not working the way it should in people with dyslexia. Can V5 inactivity account for the inability of these individuals to make sense of the flow of words on the written page, or are these independent phenomena?
V5 is part of a broader system that processes fast-moving objects. This system works in concert with systems that process patterns and colors to make vision possible. One interpretation of Eden's results is that a specific cell type in the movement tracking system develops abnormally in people with dyslexia. This might in turn cause the coordination of the tracking system with the rest of the visual system to be incomplete or offbeat in people with dyslexia, so that the handling of words is faulty. Whatever the cause of developmental dyslexia, the
|outcome is tortured or unsuccessful reading.
fMRI images showing decreased activity in region V5 in people with dyslexia (right, white arrows)
Developmental dyslexia is said to be an "unexpected" condition because the people with it are smart enough to read, and typically have had ample exposure to books and reading instruction. For some, the language difficulties are confined to reading. For others, writing, spelling, and speaking are also problematic. Most do not have wide-ranging developmental disabilities.
Lots of people -- kids and adults with dyslexia, their friends and family members (many of whom also have dyslexia), educators, psychiatrists, physiologists, behavioral scientists, policy makers, neuropsychologists -- are interested in dyslexia. They want to know where the problems lie, why they develop, and what can be done about them. Eden and her coworkers now have provided an answer to one of the "wheres."
Neuroscience researcher, Guinevere Eden
fMRI: Pictures of the Brain In Action
Functional magnetic resonance imaging (fMRI) provides researchers with pictures revealing which parts of the brain activate when a person does a certain task. The brightly colored hot spots on the images indicate where the brain is processing specific types of information -- sounds, sights, thoughts, actions. Best of all, the technique is completely "noninvasive," meaning that the subject is not harmed in any way. Because it is risk free, researchers can use fMRI to study anyone, whether they are ill or not.
An fMRI image. Red Spots indicate more active areas.
fMRI works by sensing changes in the natural magnetic properties of blood cells that carry oxygen. When brain cells are active and firing away -- when they are responding to visual or other stimuli -- they use more energy and they need more oxygen. In response to the increased energy demands of the active brain cells, the blood flow increases near active areas of the brain. fMRI senses this increased blood flow because the magnetic signals from blood cells containing hemoglobin molecules bound to oxygen are different from the signals of hemoglobin molecules from oxygen-depleted cells.
To get an fMRI brain image, the subject's head is placed in a strong magnetic field. The fMRI machine then uses radio waves to measure the concentration of oxy-hemoglobin molecules in each of thousands of small "volume elements" in the subject's head. By putting together all the data on the amount of oxy-hemoglobin in each volume element, the machine delivers a map of which parts of the brain are most active.
Now that the techniques for doing fMRI have been worked out, they will soon provide many more insights into both the normal activities of human brains and changes in their activities as they mature or are altered by injury and disease, says researcher Guinevere Eden of Georgetown University in Washington, D.C.. Part of her enthusiasm comes from the inherent safety of fMRI. Some older techniques for mapping brain activity required the patient to take a small dose of a radioactive isotope, which carried with it a small risk. This is not necessary with fMRI. Furthermore, many machines capable of doing fMRI are already in place. Doctors have used standard MRI (without the "f") for many years to look at normal brain structures and to detect tumors in the brain. These machines, which are now in most big hospitals, can be easily adapted to do fMRI studies, "just by adding a bit more equipment to the system." says Eden.
An fMRI scanner made by Surrey Medical Imaging Systems. |
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