Abstract:
Developmental dyslexia is an unexpected difficulty in reading that persists into adulthood. This study contains three experiments designed to test components of reading in adult dyslexic subtypes. The first experiment used decision tasks to compare adult dyslexic subtypes with matched non-impaired readers. Irrespective of subtype classification, adult dyslexics showed poor performance on the phonological-decoding task in comparison to normal controls. In the lexical-access decision task dyslexics also performed below normal controls, but within the dyslexic sample, dyseidetics (who spell with phonological accuracy) performed below dysphonetics (who are phonologically inaccurate spellers). The second experiment used high-density electroencephalography to measure topographic beta distributions between dyslexic subtypes and non-impaired readers. The ratio of anterior-posterior beta power separated dyslexic subtypes from each other and from normal controls. Dysphonetics showed a more anterior than posterior asymmetry, suggesting a focus on grapheme-to-phoneme conversion, whereas dyseidetics showed a more posterior asymmetry, suggesting a focus on lexical access. The third experiment was an fMRI case study that examined dyslexic activation as phonological demands were systematically increased. The dysphonetic subject showed strategic compensation in the left inferior frontal gyrus and ancillary compensation in right posterior areas for the component that demanded phonological assembly. Together, these results provide both behavioural and neural validation for the distinction between dyseidetic dysphonetic subtypes in adulthood.