Are there subtypes of developmental dyslexia?

Talcott, J. B.

Aston Brain Centre, Aston University, Birmingham UK

Although there has been significant progress in our understanding of the cognitive architecture that underpins the development of both normal and atypical literacy skills, our knowledge about the nature of developmental dyslexia remains much less advanced. As developmental dyslexia is characterised by a diverse constellation of symptoms, it seems natural to ask whether the underlying risk and protective factors aggregate into subtypes of individuals or not and this is currently an active area of research. To date, this pursuit has been most successful in identifying clusters of individuals with presumed dissociations between specific components of reading, consistent with two theoretical approaches: the dual route model and the double deficit hypotheses. Both models predict the occurrence of discrete subtypes of individuals, with relatively isolated deficits in a single component process and comparatively normal functioning in the other. Each approach has reported subtypes of dyslexia that are largely consistent with its own theoretical perspective. The evidence for these subtypes largely derives from measuring the proportion of individuals who score below some arbitrary threshold on one or both of two theoretically relevant cognitive tasks. However, defined in this way, any continuous bivariate distribution will reveal individuals with single or combined deficits and the proportion in each category that will arise by chance can be estimated through straightforward statistical modelling. Applying these estimates to published data on dyslexia subtypes suggests that the frequencies observed in each group in empirical studies often fail to differ significantly from what would be expected by chance. This provides a powerful challenge to the use of this simple dissociation logic in this context and suggests that stronger evidence is required in order to reify the existence of distinct diagnostic entities from such data.