Developmental dyslexia is at the low end of a spectrum in reading and writing abilities, and may arise despite normal intelligence and education. It often is accompanied by difficulties in domains important for reading, such as phonological processing and verbal working memory. Neural impairments in speech processing are evident in the majority of dyslexic individuals and could be linked to phonological and temporal sampling problems. This thesis integrates four studies for which neuropsychological assessments, magnetoencephalography (MEG), electroencephalography (EEG), and magnetic resonance imaging (MRI) were conducted. The first study examined the influence of familial dyslexia risk on neural speech-sound discrimination in newborn infants (Study I). The second and third study investigated neural processing of speech-sound changes (Study II) and natural speech (Study III) in adult dyslexic and typical readers. The fourth study analyzed anatomical brain abnormalities in dyslexia (Study IV). In addition, the associations of neural measures to reading and related phonological-processing and working-memory skills were investigated (Studies II–IV). The main findings of this thesis were neural speech-processing impairments in newborns at risk of and adults with dyslexia, neuroanatomical abnormalities in adults with dyslexia, and links between the neural measures and skills relevant for reading. Specifically, newborns at risk of dyslexia compared to a group of low risk showed atypical neural speech discrimination responses that may be precursors of phonological deficits in dyslexia (Study I). However, neuromagnetic discrimination responses elicited by the same speech-sound changes suggested no abnormalities in adults with dyslexia, yet, the responses were associated with reading and working memory functions (Study II). Inter-subject correlation (ISC) to natural speech was weaker between dyslexic than typically-reading adults in delta- and high gamma-frequency bands, and stronger in the theta, beta, and low gamma bands, possibly reflecting temporal sampling deficits of natural speech features (Study III). The ISC strength was related to all three reading-relevant skills of interest. Structural abnormalities were observed in dyslexic adults as decreases in grey- and white-matter volumes in temporal, frontal, and subcortical structures important for reading (Study IV). Furthermore, grey- and white-matter volumes were associated with reading and working memory functions. Taken together, this thesis illuminates neural speech processing deficits in dyslexia and its risk at birth and pinpoints associations between reading skills and neurofunctional and -anatomical measures.
|Tila||Julkaistu - 2020|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|
LisätietojaM1 - 121 s. + liitteet
- 6162 Kognitiotiede