Wednesday, 13 October 2010

Reading, Dyslexia and the Eyes

One of my patients told me recently that her son had been referred to have some tests on his eye movements because he sometimes makes spelling mistakes. He can read perfectly well, and actually does OK in spelling tests, but just makes mistakes when writing. It struck me that if you referred every child who made spelling mistakes for special tests, you’d probably need to refer 98% of them. So how do these ideas come about, and what’s the story of eyes, reading and dyslexia?

Why reading – especially English – is difficult
Reading is a complicated task that requires the involvement of several different parts of the brain. The image of words falls on the retina and is transferred to the visual cortex of the brain. Here, the image that we see is formed. As part of the processing of that image, shapes are recognised. This information has to link up with the language centres, which recognise sound and meaning.
The written word is, in evolutionary terms, a relatively modern invention. Our brains are really set up to learn language through sound, as we nearly all manage to do easily when we learn to understand and speak our native language at a young age. So when we read, we still have to link the words with how they sound. Even expert readers will say the words in their head when they read.
English is a particularly difficult language to learn to read because there are so many words like “eye” or “cough” whose pronunciation isn’t obvious from the way they’re spelt. These words are generally learned through their meaning, and efficient readers go straight to the meaning when they read, so they don’t have to piece words together from the sounds. Because Italian is a much more phonetic language than English, there are only half as many diagnosed dyslexics in Italy than in England. It seems likely, however, that there are many more Italians whose dyslexia is too mild to be significant.

Reading and the brain
Cutting & Rimrodt (2010) found used a form of MRI scanning called Diffusion Tensor Imaging to study part of a branch of nerves that runs from the visual cortex to the areas at the front of the brain that are responsible for articulation and speech. They found that some of these nerve fibres were less well organised in dyslexics than in controls.
There have been many reports of differences in nerve structure in dyslexics, but these have not been consistent. Differences have been found in the temporal or parietal or occipital parts of the brain, as well as in the cerebellum. It may be that a well organised nerve structure is a sign of proficiency in that particular task, and that, for a mixture of genetic and environmental reasons, this is achieved more easily in some than in others. Italian dyslexics have been found to show the same kind of nerve activation when reading as English and French dyslexics (Paulesu et al 2001). They also had the same difficulty in tests that looked at their ability to recognise the sounds of words. It’s just that in Italian, there’s only one way to represent a sound, whereas in English there are lots, such as “their” or “there”.

Dyslexia and eye movements
Because anomalies have sometimes been found in the nerve pathways that control eye movements, and because dyslexics often have irregular eye movements when reading, it’s been suggested that poor control of eye movements is a cause of dyslexia.
Hutzler et al (2006) investigated this idea by showing dyslexics and controls groups of consonants, such as DSB, LQWB, ZBB and VPLL. The subjects had to say whether each group of consonants had a double letter in them or not. This meant that the subjects had to look at each “word” in a similar manner to reading, but no higher order skills of language and sound recognition were required. No difference in the eye movement patterns between dyslexics and non-dyslexics were found.
The researchers then changed things, so that the subjects had to try and pronounce “words” such as ZIB, VULL and CRUF. Now, the dyslexic subjects showed the same irregular eye movement patterns as had been found in previous research. Patients with diseases such as Parkinson’s and Huntingdon’s often have trouble with reading because of poor control of eye movements. However, they have a lot of other problems for the same reason, and this would not appear to be the case for the vast majority of dyslexics.

Dyslexia and the cerebellum
Similarly, because scans have sometimes shown anomalies in the cerebellum of dyslexics, it’s been suggested that the cerebellum holds the key to dyslexia. The cerebellum is in control of the things we do without thinking, such as brushing our teeth, and it also plays a crucial role in maintaining our balance. And it’s certainly true that dyspraxia (a disability affecting movement and coordination) and attention deficit hyperactivity disorder (ADHD) are more frequently found in dyslexics than non-dyslexics. On this basis, it’s suggested that specific exercises aimed at improving balance and coordination will improve reading.
One of the many problems with his idea is that there are plenty of elite sportsmen and sportswomen, who would appear to have excellent balance and coordination, but who have suffered from dyslexia. Another problem, and one shared with several other “miracle cures”, is that the statistics used to promote it are unreliable. One of the main reasons for this is a statistical phenomenon called “regression to the mean”.
Imagine 100 people took a multiple choice test in a subject about which they knew absolutely nothing, so had to guess every time. If each question had 2 choices, you’d expect the overall mark to be around 50%. Some people, however, would do rather better, and some worse. If you got the top 10% do another similar guessing test, their mean score would be about 50%, so on average, will have done worse on the second test than on the first. Similarly, a repeat test on the bottom 10% would see their average score improve. This is regression to the mean, and it’s why it’s easy to show that people who weren’t very good at something when you first tested them, can show a dramatic improvement when you test them again after their “treatment” compared with those who did well first time round. The only way round this is to have a genuine control group, who are as similar as possible to the treated group, and who have the same tests at the same time. So you would need to compare dyslexics of the same level of disability, with and without the treatment, to make a valid comparison. This seems to be rarely done.

Dyslexia and eye dominance
I recently read the claim by an optometrist offering diplomas in “Schoolvision” that the “predominant visual skill in reading is aiming”, and that this can be undermined by unstable eye dominance. The role of aiming in reading has been largely neglected up until now (for good reason, you might think), although the importance of eye dominance in aiming sports is widely accepted, including by myself.
But the idea that eye dominance is relevant to dyslexia is not a new one. Stein & Fowler (1982) claimed that more than half of the dyslexic population had an unfixed reference eye on a binocular vision test, compared with 1% of controls. The test used was the Dunlop Test, in which the eyes diverge until double vision occurs, and the child has to say in which direction an object has moved. The test is repeated ten times, and if they give inconsistent answers they’re said to have an unstable response. This doesn’t seem to bear much relation to reading, and, anyway, others (Newman et al 1985; Bishop et al 1979) found that excellent readers did just as badly as poor readers on this test.
Stein & Fowler went on to suggest that some dyslexic children might be helped by the occlusion of one eye. But when Bishop (1989) re-analysed the data, she found no evidence that occluding one eye improved reading scores, and described the original findings as “methodologically flawed”. And whilst some have claimed that cross-dominance (e.g. right eye dominant + left handedness) is especially prevalent in dyslexia, many studies of large numbers of dyslexics have failed to find this link.

What’s the answer?
Most dyslexics clearly don’t have something fundamentally wrong with them, otherwise they’d have a lot more problems than being slower than others in learning to read. Also, we wouldn’t see the differences between languages if vision were really at the heart of dyslexia.
Having said that, there are quite a lot of children who have difficulty when reading because of their vision, and some of them will be dyslexic. Their difficulty often arises because they have either a weakness of accommodation (ability to focus on near objects) or convergence (ability to bring the eyes together for near objects), or both. After a while, they find words go blurred and/or double.
These children generally respond very well to simple exercises, and rarely need glasses. So if your child is having reading difficulties, it’s worth getting their eyes examined to rule out this kind of problem.
Whatever the cause of dyslexia, the answer seems to be one-to-one tuition in reading. Whether this should be phonics or something else, I leave to others to debate.

David Donner