Those with dyslexia have a poorer reading ability than expected from their intelligence. However, there is so much natural variation in how fast children learn to read and write that it is often not properly diagnosed, and many children are unfairly labelled slow learners.

People with dyslexia have no problems understanding the meaning of words, recognising individual letter shapes, and seeing anything other than text. So why does it leave some 18-year olds with the reading ability of a 12-year old and the spelling ability of a 10-year old?

What causes dyslexia?

For many years it has been believed that dyslexia results from a problem with the language centres of the brain. However, research does suggest a completely different explanation.

John Stein, a scientist at Oxford University in Britain believes that dyslexia is a widespread neurological problem. He believes that during development in the womb, something damages the vulnerable young nerve cells as they grow and form connections inside the brain.

It appears that this damage is selective, affecting only those nerves which relay information about fast-changing events in the environment. This belief arises from the fact that Stein thinks that dyslexia is actually a problem with sensing the most rapid changes in our world, particularly those things which we see and hear.

One of the most interesting aspects of Stein's theory is that this attack may come from the mother through her immune system.

Many children with dyslexia also suffer from poor co-ordination and balance and are labelled "clumsy" children. They may also find it difficult to remember instructions, days of the week and months of the year. If Stein is correct about the immune system attack then these other subtle aspects of dyslexia would be easier to understand.

Through his research Stein has discovered that dyslexics have several specific problems with the way in which they physically see the world. This theory is proving very unpopular, particularly among psychologists.

Experiments were carried out with dyslexics in which they were asked to watch two continuous TV displays of tiny dots moving at random. At various times, some dots on the screen were made to move together. Dyslexics needed more of the dots to move together before they spotted the change. The worse a dyslexic's ability to spot the change in movement of these TV screen dots, the worse their reading and spelling.

Visual system important

This has convinced Stein that the visual system is important in dyslexia.

He believes that a particular part of the visual system is involved. It is called the magnocellular pathway, or M-pathway, which contains very large, fast-conducting nerve cells which are essential in telling the visual system of the brain about rapid changes or movement. Several other groups of researchers have found that the M-pathway of dyslexics is slower to send impulses from the eye to the visual centre of the brain that that of normal people.

Another group has looked at the M-pathway tissue of the brain at postmortem in dyslexics who have died and found that their cells in this area look abnormal.

Yet another centre has reported that people with dyslexia have less brain activity than normal in the part of the visual system which detects motion.

How does all this translate into dyslexia?

So why should these problems cause the reading difficulties which dyslexics have?

Text on a page is not moving, but reading is not a smooth process and defects in the M-pathway could easily account for difficulties with reading and writing.

When you read, the brain lines up each image next to the previous, slightly different one, in order to get a smooth impression. Normally the M-pathway controls these movements. However, many dyslexics find it difficult to hold their eyes steady between eye movements. If the eyes are not steady, they will send images which are moving slightly, which could explain why many dyslexics complain that words blur and dance around the page.

But how can a visual defect as severe as this not be noticed in other ways? A dyslexic has normal colour, shape and texture perception. These senses are controlled by the P-pathway or parvocellular pathway. It is the combined activity of the M and P-pathways which create the visual scene. So, any small defect in seeing one aspect of a visual scene is usually compensated for. However, reading and writing needs the sort of attention to detail which looking at a landscape does not.

Other aspects of dyslexia for many are slow learning of speech, muddling of words and confusion of sounds when reading out aloud. This is the reason why dyslexia has been regarded as a language defect. A neuroscientist who deals with learning has questioned Stein's suggestion that the visual system is crucial in the development of dyslexia. She has shown that dyslexics have a problem with different sounds. They often cannot hear that two pure tones are separate if they are presented too close together.

Stein's group says that the problems with hearing and vision are linked. In a paper published last year from Stein's group, people who have trouble detecting rapidly changing visual signals also have problems detecting fast sound changes. The worse the person's ability to detect visual and auditory changes, the worse their ability to read and spell.

Stein has suggested that there might be an area of the hearing system similar to the M-pathway in the visual system and there is some research to support this.

Other research from Stein's group suggests that there are problems in the area of the brain involved in co-ordination in dyslexics which accounts for their clumsiness.

All this is suggesting that dyslexia is a sensory problem, rather than a language problem. However, the only way that Stein will convince sceptical members of the scientific community is if he can show how the problems in the M-pathways arise in the first place. He believes that the only way in which these defects could be as bad as they are, is if the damage was caused in the womb, and that the answer lies in the way in which dyslexia runs in families.

He and a team of geneticists are studying families in which at least two members have dyslexia. It appears that members of these families share a group of genes found on chromosome six. This region is very close to a group of genes which control the human immune system. They code for the proteins which white blood cells use to recognise differences between our own body and invading viruses.

However, the geneticists are not saying that it is the genes controlling the immune system which cause dyslexia, because they may be next to the genes shared by dyslexics by chance.

However, there is some evidence that the immune system could be influencing the M-pathway and this could be happening between the mother's immune system and that of her unborn child.

Where does this leave us?

Whatever the underlying cause of dyslexia, it should be seen as something more than a simple language problem. It may be possible to use Stein's test of visual perception to lead to routine ways of identifying the more subtle differences between individuals with dyslexia. This will allow teachers to work on a child's particular weakness. It could also lead to earlier diagnosis of dyslexia which would prevent the damaging label of "slow learner" and allow dyslexic children to reach their full potential.

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