Simultaneous simulations of pure, surface and phonological acquired dyslexia within a full computational model of the primary systems hypothesis

According to the primary systems hypothesis, reading requires interactions of visual-orthographic, phonological and semantic systems. Damage to each primary system generates very different types of acquired dyslexia. Variants of the connectionist ‘triangle’ models of reading have been developed to investigate individual acquired dyslexia. However, only a few studies have investigated multiple acquired alexia within one framework. Importantly, there are no studies that simultaneously simulate both central dyslexia (e.g. surface and phonological dyslexia) and peripheral dyslexia (e.g. pure alexia). That is largely due to the lack of a visual component in the traditional reading models. To verify the predictions made by the primary systems hypothesis, we developed a connectionist ‘deep’ multi-layer triangle model of reading including visual, orthographic, phonological and semantic processing layers. We investigated whether damage to the model could produce the general behavioural patterns of impaired performance observed in patients with the corresponding reading deficits. Crucially, damage to the visual-orthographic, phonological or semantic components of the model resulted in the expected reading impairments associated with pure alexia, phonological dyslexia and surface dyslexia, respectively. The simulation results demonstrated for the first time that neurologically-impaired reading including both central and peripheral dyslexia could be addressed within a single triangle model of reading. The findings are consistent with the predictions made by the primary systems hypothesis.