What causes dyslexia?
As mentioned in the definitions of dyslexia, the condition is neurobiological or brain-based. In other words, the brains of children and adults with dyslexia do not function in the same way as the brains of children and adults without dyslexia.
This statement is now absolutely indisputable. Modern powerful techniques of brain-imaging, which allow us not only to take ‘pictures’ of any part of the brain, but also to locate which regions of the brain are activated during reading or other activities, have complemented information from studies from more than 50 years ago which detected different aspects of growth and organization in brains of people with dyslexia compared to people without dyslexia.
Taken together, this ever-growing body of evidence shows without any doubt that children and adults with dyslexia activate other regions of their brain when reading words compared to children and adults without dyslexia when reading the same words in the same circumstances.
The causes as well as the exact nature of these differences, however, are still unclear.
What we can say with certainty is that, because ‘Dyslexia is not a single category but a disorder found within a spectrum of deficits’ (Professor Maggie Snowling, University of York, UK), and because the combination of these deficits varies from one person with dyslexia to another, there is no single ‘cause’ of dyslexia. Accordingly, studies are showing diverse patterns of development and function amongst people with dyslexia.
There are numerous hypotheses about why the brains of people with dyslexia develop and function differently.
One is that one or several of the numerous connections and paths that are necessary for reading are not functioning optimally or have not been stimulated optimally. To get an idea of how many complex paths are needed for reading even a single word, look at this diagram from Professor Stanislas Dehaene, which, as he stresses himself, is very simplified!
If any one of these paths, or brain areas connected by these paths, is not working efficiently, there may be problems.
If you would like to look at the whole presentation (4.4Mb ) from Professor Dehaene, given during an online conference organised by Dyslexia International and illustrated with fascinating neuroscientific data:
Another hypothesis, which is not incompatible with the previous one, is that the neurons which form the paths between the brain areas involved in reading have not developed and navigated to their exact normal destination because of a faulty genetic coding.
Shortcomings in development could arise from insufficient nutrition, auto-immunity, environmental impact, or other causes.
In sum, here is no question that significant differences exist between the development and functioning of the brains of people with dyslexia and people without dyslexia, even if the exact causes of these differences are questioned.
Although these differences involve difficulties with the development and mastery of literacy skills, and often with other abilities, there is another side to the picture. The fact that the brains of children and adults with dyslexia work differently endows some, if not many of them, with other ways of perceiving, understanding, and thinking. When referring to people with dyslexia, many use the expression ‘thinking outside the box’. This will often be reflected in creative, unconventional, and sometimes superior abilities.
Following this line of thought, some researchers and practitioners prefer to say that the differences between the brains of people with dyslexia and the brains of people without dyslexia merely represent different facets of the neurodiversity which is inherent in human nature.
More about causes
Surveys involving many hundreds of families show that some forms of dyslexia are inherited. Studies of identical twins have shown that these people display much closer similarities on their scores for reading and writing than non-identical twins of the same sex.
In the same family, if one of the members has dyslexia, there is a 50 % likelihood that one of his or her close relatives will also be dyslexic. However, this does not mean that the two persons will display the same traits of dyslexia, nor that their dyslexia will have the same degree of mildness or severity.
Experts disagree about the relative prevalence of dyslexia amongst boys and girls. Some say that dyslexia occurs more often in boys than in girls; others think that dyslexia is simply more often diagnosed in boys than in girls but that its incidence in the two sexes is the same.
Modern brain-imaging studies show, beyond any doubt, that the brains of children with dyslexia must have developed in different ways to those of children without dyslexia. Most striking and observed practically in all studies, there is relatively less activity in a specialized part of the brain, usually in the left hemisphere, when the child or adult with dyslexia is attempting to read. Therefore the marked asymmetry between the cerebral hemispheres seen in people without dyslexia is not observed in people with dyslexia.
More precisely, most of these studies show under-activity in the left temporal area in two regions:
- the lateral temporal (side-middle) cortex which deals with the management of the representation of the spoken language (sounds and words), called phonological representations
- the temporal area lower down that is part of a “lower” visual pathway and which is close to the occipital cortex at the back of the brain which deals with vision
These observations support the theory of a double deficit: visual and phonological. In brief, in the brains of people with dyslexia, several key areas may not be sufficiently activated at both the level of visual analysis and at the level of phonological processing.
To observe the abnormal under-activity of this part of the brain in people with dyslexia, researchers have also examined anatomical or physical organization. They use a technique which images in microscopic detail layers of parts of the cortex. It shows that in the case of people with dyslexia there appears to be some disorganization in the left temporal region. This disorganization arises because some neurons are not in their right place. Either they have stopped migrating to their correct final destination during the development of the brain, or they have partially migrated to the wrong places. These sites are called “ectopias”.
A very recent technique which can give pictures of activity in the long-distance connections, called the fascicles, between areas of the brain, shows there is also a degree of disorganization in some of these tracts.
Nutritional factors during the pregnancy of the mother, and in the early childhood of the infant, are implicated as well as immunological resistance in the foetus. With regard to nutritional factors in the growing child, there may be a lack of a key fatty acid which is essential for the formation of the membranes of the nerves.
Problems do not always lie solely in the cerebral cortex. The brain has to control the fine tracking of the central most sensitive part of the eye when reading. One of the several pathways from eye to cerebral cortex, composed of the “magnocellular” nerves which deal fast with the detection of light and movement, and hence motor response, may also be impaired. Similarly, magnocellular auditory paths may be compromised and this would affect sensitivity to sound. Ultimately these factors would affect the speed of the processing which is instrumental in integrating the information from our senses and coordinating behaviour.
There is evidence that other subcortical structures like the cerebellum (“little brain”, part of the hindbrain) are implicated because it is instrumental in the fine control of movement, automaticity, and even memory. Another subcortical set of structures called the basal ganglia could also be implicated because they deal with the initiation and suppression of movement.
Some children are excessively sensitive to certain wavelengths of light. Reading is therefore physically distressing.
For an overview of many of these factors, and the magnocellular theory in particular, do a search for Professor John Stein of Oxford University, who is Chairman of Dyslexia International’s Scientific Advisory Committee.
Fawcett and Nicolson, on the (de-activated) site of the Learning Skills and Improvement Service (UK) said:
“Future research may reveal a magnocellular sub-type, a cerebellar sub-type and various mixed sub-types”.
This site is no longer available. For more info, see: https://en.wikipedia.org/wiki/Learning_and_Skills_Improvement_Service
These brief notes reveal the complexity of the issue.
A person may suffer from a few or many of the deficits to a greater or lesser extent. Dyslexia will then present itself in different ways and in different degrees.
There are two important messages:
- for the time being, an open mind on the causes of the many types of dyslexia is prudent
- more practically, during your teaching experience, you will come to appreciate the neurodiversity of all your children and the particular types of diversity displayed in children with dyslexia