Developmental dyslexia and the dual route model of reading: simulating individual differences and subtypes.

Developmental dyslexia was investigated within a well-understood and fully specified computational model of reading aloud: the dual route cascaded model (DRC [Coltheart, M., Rastle, K., Perry, C., Langdon, R., & Ziegler, J.C. (2001). DRC: A dual route cascaded model of visual word recognition and reading aloud. Psychological Review, 108, 204-256.]). Four tasks were designed to assess each representational level of the DRC: letter level, orthographic lexicon, phonological lexicon, and phoneme system. The data showed no single cause of dyslexia, but rather a complex pattern of phonological, phonemic, and letter processing deficits. Importantly, most dyslexics had deficits in more than one domain. Subtyping analyses also suggested that both the phonological and surface dyslexics almost always had more than a single underlying deficit. To simulate the reading performance for each individual with the DRC, we added noise to the model at a level proportional to the underlying deficit(s) of each individual. The simulations not only accounted fairly well for individual reading patterns but also captured the different dyslexia profiles discussed in the literature (i.e., surface, phonological, mixed, and mild dyslexia). Thus, taking into account the multiplicity of underlying deficits on an individual basis provides a parsimonious and accurate description of developmental dyslexia. The present work highlights the necessity and merits of investigating dyslexia at the level of each individual rather than as a unitary disorder.