Distinct neural signatures of cognitive subtypes of dyslexia: effects of lexicality during phonological processing.
The present study investigates the neurobiological basis of two subtypes of dyslexia with either a double deficit (concerning phonological awareness and rapid naming) or a single rapid naming deficit. We compared such groups of German dyslexic primary school children to each other and with good reading children in a phoneme deletion task performed during fMRI scanning. Children heard German words or pseudowords and repeated the remainder of the stimulus while deleting the initial phoneme (e.g. tear - _ear). In four conditions, the input stimulus (word or pseudoword) could either become another word or pseudoword as output. The word-word condition stuck out against all other conditions involving pseudowords: Dyslexics with a double deficit showed a strong response in left areas 44 and 45 in Boca's region, whereas dyslexics with rapid naming difficulties revealed a contralateral effect in right areas 44 and 45. These findings, which were obtained without presenting written or pictorial stimuli, reveal that a double deficit in dyslexia is not the sum of single deficits, but rather involves the interaction of lexical and phonological processing, making strong demands on the left inferior frontal cortex. In general, the results stress the importance of considering subtypes of dyslexia differentially in order to obtain better insights in the neurocognitive mechanisms of impaired and successful reading.