Definitions of dyslexia are arbitrary, because reading ability is a continuum. Some people are good at it, and some are less good. In this way, dyslexia is not like measles where you either have it or you don’t. So while we can say how many Australians have measles, it is harder to say how many Australians have dyslexia. Any definition of ”dyslexia“ is picking a point on the line of ability between poor readers and skilled readers.
Firstly there are a couple of things that need to be understood about reading. In evolutionary terms, the ability to read is something very, very recent. Our species has likely only used language to communicate for ~100,000 years. As a result the parts of our brain which are involved in reading are circuits that are relatively new, have had little evolutionary pressure applied to them and are not ”dedicated reading circuits“. This means that there are many potential disruptors of these circuits and many different potential underlying causes of reading difficulties.
Reading is a complex skill that involved coordinated visual, auditory, memory and retrieval tasks. The ability to remediate poor reading relies heavily on being able to identify the deficit(s) underlying the problem. Because reading is a complex task, there are many competing theories about what causes dyslexia. In general, a one-size-fits-all approach to treatment is not likely to be optimal.
We are interested in the brain circuitry underlying reading. For instance, you can read a word like RANE using phonological decoding, even though RANE isn’t really a proper word. But if you try this approach on a word like YACHT, you will not end up reading the word correctly, as YACHT is not a word that you can read correctly using phonological rules. Instead, you have an internal dictionary, or lexicon, that you look up to see if you recognise YACHT. If you’ve seen it before and if you remember it, you will be able to read it. However, both routes of reading are activated to some extent when one sees a word.
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