Studies on developmental dyslexia in transparent orthographies have established that children learning to read in such languages hardly experience difficulties in word reading accuracy and phonological awareness tasks, but suffer from a reading speed deficit. On the other hand in the English orthography, where the mappings between graphemes and phonemes are largely inconsistent, children exhibit significant difficulties in both word reading accuracy and speed. Greek is characterized by a high degree of regularity for reading, but is inconsistent for spelling. The variability of phoneme-to-grapheme correspondences and the highly inflectional nature of the particular orthography constitute spelling in Greek a considerably demanding task. The present thesis comprises three studies that were concerned with understanding the reading and spelling difficulties that Greek children/participants with dyslexia have and their underlying cognitive deficits, in relation to typically developing children and English children/participants with dyslexia. The first study examined the reading and spelling difficulties in Greek- and English-speaking children/participants with dyslexia, each compared with two control groups. Greek children/participants with dyslexia outperformed their English counterparts on word/nonword phoneme deletion, word/nonword reading, and grammatical spelling. However the two language groups performed similarly on rapid digit naming, spoonerisms and on the choice tasks. Results are discussed in relation to the differences in orthographic consistency between the two languages. The second study examined the development of literacy skills in twenty-three Greek children/participants with dyslexia over a period of 18 months (10 years 5 months to 12 years 3 months). At Time 1 children/participants with dyslexia performed worse on literacy tests than chronological-age control children, but similarly to reading-age controls. At Time 2 children/participants with dyslexia performed worse on all the tasks than CA control children, and worse than RA controls on the tasks of phoneme deletion of nonwords, nonword reading and orthographic spelling. Moreover the concurrent and longitudinal predictors of children's/participants' with dyslexia and typically developing children's reading and spelling abilities were examined. The findings are discussed in relation to theories of normal and atypical reading and spelling development. The third study investigated the ability of twenty-three 10-13 year-old Greek children/participants with dyslexia, and their reading-level and age-level-matched children to spell derivational and inflectional suffixes. Children/participants with dyslexia performed significantly worse than CA controls and RA controls. When they spelled the inflectional ending of adjectives and nouns children/participants with dyslexia did not differ from RA controls. It is suggested that children/participants with dyslexia have weaknesses in grasping the morphological rules of the Greek orthographic system and applying this knowledge in the spelling of word suffixes. The thesis concludes with a discussion of findings in relation to previous literature, the limitations of the present studies and avenues for future research.

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For those engaged in trying to understand the cause of dyslexia, these are interesting times. There is increasing evidence that dyslexia may result from a deficit in the brain's ability to process general visual and auditory information, which may subsequently contribute to observed language difficulties. While some suggest that this processing deficit is confined to lower perceptual levels, others propose that it extends to higher cognitive levels of attention and learning. So far there is surprisingly little evidence of research wherein both modalities, both processing levels and various stimulus features have been tested in the same set of dyslexics using electrophysiological measures. This was the purpose of this research. In four studies, event related potentials were recorded from dyslexic and control brains during the non-attentive and attentive discrimination of various visual and auditory stimuli. Average dyslexic-control ERP comparisons were made for sensory N 1 and MMN waves in the passive, and cognitive P2, N2 and P3 waves in the active response conditions. Dyslexics had attenuated MMNs during the pre-attentive discrimination of changes in peripheral visual field, auditory frequency and rapid auditory sequences but not auditory duration. Moreover, dyslexics had abnormal P2 or P3 waves during the attentive discrimination of all visual and auditory stimuli. Finally, the previously attenuated MMN to frequency discrimination was enhanced after attentive practice. The feature-specific MMN abnormalities suggest a highly selective, multi-modal, perceptual dysfunction in dyslexics, as predicted by the pan-sensory deficit theory. However, the ubiquitous task-related P2 and P3 abnormalities suggest that their deficits also extend to higher cognitive domains, as predicted by the automatization/cerebellar deficit theory. The subsequent MMN enhancement suggests practice-induced improvements in their perceptual acuity. These findings suggest that dyslexia is a multilevel syndrome: the same dyslexics have problems in both domains: visual and auditory, and at both processing levels: sensory and cognitive.

Assessment of motor performance and motor learning in dyslexia is crucial because of its ability to shed light on the underlying biology of the disorder and to discriminate between theoretical approaches. It remains a controversial area due to existing discrepant research findings and interpretations. Three studies are described in this thesis. The first used three sets of experiments to test balance and postural control in single and dual-task conditions. The second study examined the production and timing of responses in a classical eyeblink conditioning paradigm. The final study investigated motor skill acquisition. The results of the three studies were similar in that in dual-task balance, conditioned response timing and motor skill consolidation around half of the dyslexic adults showed substantial deficits compared with a control group. The samples of participants in the three studies overlapped sufficiently for some cross-study comparisons of strengths and weaknesses to be conducted. These showed that it was rare for a participant with dyslexia to show motor impairment in just one of the three domains, with dual task balance and conditioned response timing seeming to be most closely associated. Overall the results provide strong evidence of enduring deficits outside the literacy domain in dyslexia and also highlight the considerable heterogeneity of the disorder. Consequently they lend particular weight to the notion of cerebellar causation. Further studies should be undertaken on a larger scale to scrutinize the consistency of motor impairments in dyslexia and the possibility that those showing motor problems might form a definite subgroup within dyslexia. In the longer term, this work points to a possibility of multiple, independently diagnosable sub-classes of dyslexia, based on specific neurological abnormalities, with their own specific remediation and objective early detection schemes.

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Despite recent extensive research in the field of dyslexia, the causal links between various behavioural symptoms and underlying neural mechanisms of this developmental disorder proposed by different theories are still hotly debated. In this project I aimed to combine behavioural and neurophysiological tests of global coherent motion (magnocellular), visual word from recognition and lexical decision (phonological), as well as attention deficits in English speaking dyslexic adolescents.

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