This post has been contributed by Max Coltheart
I am not starting at the behavioural level to define the reading deficits in dyslexia. I would get lost in a maze where cause and effect can hardly be distinguished.
Behaviour is determined by so many factors that a glitch in just one of these factors is incredibly hard to discover. It’s like being in the midst of climate change and generally rising temperatures, while being confounded by a cool summer. Like the observed cool temperatures, observed behaviour is potentially misleading if we want to learn something about what causes the behaviour.
Therefore I am starting at the cognitive level. I will only later go to the behavioural level when I know what signs to look out for. At the cognitive level I can let my thoughts range freely around the imagined mental machinery. I would like to poke into different bits and take them apart. I would like to see what would happen if a particular piece were missing or not working properly. Would the projected outcome resemble the real reading problems that are experienced by dyslexic people?
What parts? What mental machinery? Well, it is just a metaphor, and it may be better to talk of apps, perhaps. But here are some ideas. I believe learning to read involves acquiring a number of gadgets and these enable us to become the skilled readers that many adults are. When any of them goes wrong, reading deficits should result (acquired dyslexia). When any of them are never acquired properly, reading deficits should also result (developmental dyslexia).
If we can find clues to the gadgets and what can go wrong, we are on the way to understanding what the gadgets that make up the reading system are. Progress! But how many are there? Can they each go wrong, separately or together? Here I am speculating and limit myself to 7 hypothetical mechanisms.
Perhaps any of the 7 mechanisms can be broken in dyslexia. This leads to different forms of dyslexias and hence different cognitive phenotypes and explains the heterogeneity of the dyslexia condition. It can also explain different degrees of severity – the more the worse, obviously. Testing different mechanisms separately has led us to form subtypes of the dyslexia condition.
How then do you identify cognitive phenotypes? You need to devise behavioural tests to capture the hypothesised cognitive deficit. We have tests for identifying how well each of the seven components in the Figure are working.
It has been beautiful to discover reading specific architecture, a whole city of familiar, yet different, patterns of impairments and preservations of the components of this architecture.