There are a number of different theories about the causes of dyslexia which all tend to agree it is a genetic condition passed on through families.
It is estimated that if you have dyslexia, there is 40-60% likelihood your child will also develop the condition.
If one identical twin is born with the condition, it is very likely the other twin will also have it.
A number of genes that may contribute to dyslexia have been indentified by researchers, but exactly how these genes may affect development of the brain is still uncertain.
Phonological processing
The most widely supported theory of how dyslexia affects reading and writing is known as the phonological processing impairment theory. In order to better understand this theory, it is useful to distinguish between how spoken language and written language is understood.
The ability to understand spoken language seems to be a natural capacity of the human brain. This is why children as young as three years of age can often speak and understand relatively complicated sentences.
As a result of this natural ability, when we listen to spoken language, we do not register that a word is made up of phonemes (the smallest units of sound that make up words). We only hear the word itself.
For example, when you hear the word ‘crocodile’, you hear it as a seamless whole. You do not have to break up the word into its phonemes and then reassemble them to make sense of it (which in this case, would be the sounds ‘crok’, ‘er’, ‘dyle’).
The same is not true of reading and writing. Both of these skills require the ability to first recognise the letters in a word, then using the letters to identify the phonemes, and assembling them to make sense of the word.
This ability is known as phonological processing. It is thought that people with dyslexia find phonological processing much more difficult than other people.
Dyslexia and the brain
It is thought the reason people with dyslexia have problems with phonological processing is that some areas of their brain function in a different way than in people without the condition.
One body of research looked at three areas that are found in the left hemisphere (half) of the brain. These are known as:
- Broca’s area - thought to be involved in producing written words
- parietotemporal region - thought to be involved in analysing written words
- occipitotemporal region - thought to be involved in identifying written words
In research, special scans called functional magnetic resonance imaging (MRI) scans have shown activity levels in all three regions of the brain are much lower in people with dyslexia when they are trying to read. Another body of research has looked at an area of the brain called the cerebellum, which is located at the lower back of the brain. It is thought the cerebellum plays an important role in helping to process language.
The cerebellum is also responsible for coordination, and your ability to estimate how much time has passed. This may explain why many people with dyslexia have problems with coordination and time management.
It should be noted that there are no major differences seen with a standard magnetic resonance imaging (MRI) scan and this test is not helpful in diagnosing dyslexia.
Again, the results of MRI scans have suggested the cerebellum functions differently between people with dyslexia and those without the condition.
It should be emphasised that all areas of the brain discussed above are thought to have a high degree of 'plasticity'. Plasticity is a term that means an area of the brain and its function is not ‘set in stone’, but can be remodelled over time.
It may therefore be possible for people with dyslexia to use appropriate treatment to effectively ‘rewire’ areas of their brain over time and improve their phonological processing ability.
