Dyslexia is very common. As children who have it cannot understand why they find reading so difficult when they know they are as intelligent as friends who find it easy, dyslexia is a real cause of misery. Many children lose confidence, and this can lead to a downward spiral of frustration, rebellion, aggression and even delinquency. Yet many dyslexics go on to display great talents in other spheres – sport, science, computing, commerce or the arts – provided their early problems with reading have not caused them to lose all hope and self- esteem.
Hence understanding the biological basis of dyslexia is not only important in itself, but also a contribution to preventing a burden of misery. Understanding the process of reading better may lead us to a way of overcoming or treating the problem.
Learning to read
Reading depends on being able to recognize alphabetic visual symbols in their right order – the orthography of whatever language a child is learning – and to hear the separate sounds in words in their right order. This involves extracting what is called the phonemic structure, so that the symbols can be translated into the correct sounds. Unfortunately most dyslexics are slow and inaccurate at analyzing both the orthographic and phonological features of words.
The ability to sequence letters and sounds accurately depends on both visual and auditory mechanisms. For unfamiliar words, and all are unfamiliar to the beginning reader, each letter has to be identified and then to be put in the right order. This process is not as easy as it sounds, because the eyes make small movements flicking from one letter to the next.
The letters are identified during each fixation of the eye but their order is given by where the eye was pointing when each letter was seen. What the eyes see has to be integrated with motor signals from the eye movement system; and it is with this visuomotor integration that many dyslexics have problems.
Visual control of the eye movement system is dominated by a network of large neurons known as the magnocellular system. It gets this name because the neurons (cells) are very large (magno). This network can be traced right from the retina, through the pathway to the cerebral cortex and cerebellum, to the motor neurons of the eye-muscles. It is specialised to respond particularly well to moving stimuli and it is therefore important for tracking moving targets.
An important feature of this system is that it generates motion signals, during reading, when the eyes move off letters they are meant to be fixating. This motion error signalis fed back to the eye-movement system to bring the eyes back on target. The magnocellular system plays a crucial part in helping to point the eyes steadily at each letter in turn, and hence in determining their order.
Neuroscientists have found that the visual magnocellular system is mildly impaired in many dyslexics. Looking at brain tissue directly is one way to reveal this but, in addition, the sensitivity to visual motion of dyslexics is poorer than that of normal readers and their brain wave responses to moving stimuli are abnormal.
Brain imaging has also revealed altered patterns of functional activation in regions sensitive to visual motion. The control of the eye in dyslexics is less steady; hence they often complain that letters seem to move around and change places when they are trying to read. These visual confusions are probably the result of the visual magnocellular system failing to stabilise their eyes as well as it does in good readers.
Putting sounds into the right order
Many dyslexics also have problems putting the sounds of words in the right order so that they tend to mispronounce words (such as pronouncing lollypop as pollylop) and they are very bad at tongue twisters. When they come to reading, they are slower and more inaccurate at translating letters into the sounds they stand for. Like their visual problems, this phonological deficiency is probably rooted in a mild deficiency of basic auditory skills.
We distinguish letter sounds, called phonemes, by detecting the subtle differences in the sound frequency and intensity changes that characterise them. Detecting these acoustic modulations is carried out by a system of large auditory neurons that track changes in sound frequency and intensity. There is growing evidence that these neurons fail to develop as well in dyslexics as in good readers and that the categorical boundaries between similar sounds, such as ‘b’ and ‘d’, are harder for them to hear.
Many dyslexics show evidence of impaired development of brain cells, extending beyond the visual and auditory problems they have with reading. These are problems in neurons that form networks throughout the brain that seem to be specialised for tracking temporal changes. The cells all have the same surface molecules by which they recognise and form contacts with each other, but which may make them vulnerable to antibody attack.
What can be done?
There are a number of treatments for dyslexia, each indicated by the different hypotheses about its underlying cause. Sum focus on the magnocellular hypothesis, but other accounts distinguish different forms of the acquired condition, known as surface and deep dyslexia, which may require different kinds of treatment. All treatments rely on early diagnosis.
Scientists do not always agree on things and the best treatment for dyslexia is one such area of disagreement. It has been suggested recently that problems in sound processing result in some dyslexics going down the wrong path for learning about sounds using the brain’s normal mechanisms of plasticity.
The idea is that children can get back on the ‘straight and narrow’ if they are encouraged to play computer games in which they hear sounds that have been slowed down to the point where phonemic boundaries are much clearer. The sounds are then gradually speeded up.
It is claimed that this works very well, but independent tests are still being done. What is scientifically interesting about the idea is that perfectly normal brain processes interact with an early genetic abnormality to produce an exaggerated effect. It’s a striking example of how genes and the environment can interact.
It is important to stress that dyslexics may be slightly better than even good readers at some perceptual judgements such as colour distinctions and global, rather than local, shape discriminations. This hints at a possible explanation of why many dyslexics may be superior in seeing long-range associations, unexpected associations and at ‘holistic’ thinking in general. Remember that Leonardo da Vinci, Hans Christian Andersen, Edison and Einstein and many other creative artists and inventors were dyslexic.