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Developmental dyslexia (DD) is a learning disorder characterized by specific difficulty in learning to read accurately and fluently. It has been argued that the source of the disorder in DD is phonological in nature (Snowling 2000; Ramus et al. 2003). However, this theory does not account for fine and gross motor problems frequently attested in DD (Lam et al., 2011; Nicolson & Fawcett, 1990). In addition, dyslexics often suffer from subtle deficits in the processing of morphosyntactic features and of complex syntactic structures (e.g., Cantiani et al. 2013; Robertson & Joanisse 2010). These facts could be accounted for in terms of comorbidity. But, the frequency of co-occurrence of these disorders can also hint at something deeper. In this thesis, I take up the following questions: which is the nature of the impairment in individuals with DD? What do language and motor activities have in common? To answer to these queries, I propose a framework according to which dyslexics struggle in exploiting temporal regularities to efficiently anticipate linguistic and motor events. I provide evidence for this hypothesis with 5 studies on Italian children and adults with DD. Study 1 shows that handwriting (a motor activity) and reading (which is based on language) follow a similar pathway in pupils in the first years of school. Study 2 shows that children with DD are less able to comply with the rhythmic principles of handwriting (RPH) as compared to controls. Moreover, the presence of correlations among handwriting and reading/language measures suggests that the language and the motor systems are potentially linked in the brain. Study 3 shows that RHP are at work from the age of 6 and that all groups of children (range 6-10 yo) comply with them in equal terms, thus disconfirming the possibility that the RHP emerge after some amounts of handwriting and reading experience. Study 4 shows that adults with DD display a greater error and are more variable than controls in high predictable rhythmic stimuli. They also display a poorer performance in the test for reception of grammar and inserted fewer pegs in the fine motor skill task. Study 5 is an extension of Study 4 to children with DD. It shows that children with DD over-anticipate the occurrence of the beat and are very variable in their response. They are also impaired in morphosyntax as compared to controls. In Study 4 and 5, participants with good predictive skills are also faster in reading and performed better in the language tasks. Overall, the results show that the language and the motor system are more linked than has been previously suggested. Language and motor acts have a rhythmic structure that allows humans to generate timing predictions about an upcoming sensory input. The ability to predict efficiently future events reduces memory load through the pre-activation of the sensory system. In the light of the present results, it seems that dyslexics are unable to exploit temporal regularities to efficiently anticipate the next sensory event. Therefore, the predictive timing system of dyslexics appears impaired, thus affecting both the reading/language and the motor domain. |
