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Developmental dyslexia is a specific difficulty in acquiring literacy skills that manifests despite normal IQ, adequate educational opportunity and in the absence of any obvious sensory or neurological damage. According to the Phonological Representations Hypothesis a core deficit for individuals with dyslexia across languages is a brain-based difficulty in accurately storing the sound sequences that make up words, or 'phonological' representations. In this thesis the Phonological Representation Hypothesis (PRH) of dyslexia was tested and elaborated. Twenty-four dyslexic children alongside chronological age and reading age matched groups were assessed over a three-year period. Consistent with the PRH, associations were found between the quality of the dyslexic children's phonological representations, as indexed by picture naming, and their performance on related input and output phonological processing tasks based on the same lexical items. Possible reasons for the underspecificity of dyslexic phonological representations were also investigated at cognitive and perceptual levels. The sensitivity of dyslexic individuals to the presence of similar-sounding words within their mental lexicon, 'phonological neighbourhood density', was assessed. Across a range of phonological awareness tasks the dyslexic group were found to be as sensitive to this lexical factor as their age peers. Perception of amplitude envelope onsets (AEOs) was also investigated. AEOs are an auditory parameter associated with speech rhythm and were hypothesised here to be important for the establishment of well-specified phonological representations. Dyslexic insensitivity to AEO variation was seen longitudinally through both behavioural and neurophysiological assessment. These findings suggest that for some dyslexic children perception of basic rhythmic speech cues may play a role in their phonological representation deficit.

There is growing interest in children's sensitivity to orthographic probabilistic constraints governing well-formed letter sequences in print. A few studies have demonstrated that young children's spellings conform to some un taught orthographic restrictions and have postulated that distributional statistical learning processes may underlie this ability. However, there are no studies investigating whether similar patterns can be learnt under experimental conditions whereby participants are not instructed to learn. Therefore, an incidental learning task was used in this thesis to investigate whether novel constraints on letter positions and letter contexts can be exploited and used in a subsequent legality discrimination task by 7-year-old children and adults. Results indicated that (a) novel letter positions and contexts were reliably learnt by children and adults. (b) Adults were, by and large, superior learners. (c) Children's and adults' ability to learn was similarly affected by pattern complexity. These findings confirm the statistical nature of children's sensitivity to general properties of their orthography. The next question addressed was whether implicit sensitivity to frequency-based information that is widely embedded in written language (allowable letter positions; probable/improbable letter pairs/triplets and larger units) is impaired in dyslexia. To this aim, variants of an implicit artificial grammar learning task were used among groups of dyslexic adults and skilled adult readers. It was demonstrated that (a) in letter versions of the task, most aspects of dyslexics' performance were spared relative to skilled readers' performance. (b) Importantly, in a nonlinguistic version, dyslexics' performance matched that of their skilled reader counterparts in every aspect. These results contrast with some previous studies of implicit learning in dyslexia and challenge the claim that a general learning deficit contributes to reading and spelling disability.

In studies of dysgraphia, graphemic buffer disorder is considered a distinct syndrome since traditional models of spelling place the graphemic output buffer at the junction of both the lexical and sub-lexical processing paths. Damage to the graphemic buffer often results in well recognised symptoms, but there is evidence to suggest that damage presumed to exist prior to the graphemic buffer such as with deep dysgraphia may also produce errors qualitatively similar to graphemic buffer disorder. We build on an existing connectionist model of the graphemic output buffer and examine how damage inspired by physical impairment found in the nervous system may produce characteristics of both disorders. Since each has been observed to produce some common attributes, we investigate in more detail, and expand on, a claim made by others of a new putative functional syndrome. As part of our investigations, we suggest a methodical and rigorous approach to lesioning connectionist systems. We also critique a number of core design principles associated with the original model and augment its functionality to allow a broader theoretical examination of a number of new areas. These include the production of geminate errors, whether orthography affects a word's propensity for error, and how the model may explain a minimum complexity principle associated with repair strategies in the presence of damage. This thesis models disorders, which are known to produce subtly different, yet qualitatively similar behaviours in different patients, and we assume that a model must be able to produce comparable behaviour. In order to provide a rigorous and structured approach to analysing our results, we create a number of quasi-patients and examine the effect of damage across multiple lesion severities, lesion types, and lesion locations.

Reading disability or developmental dyslexia is a specific disorder of reading that is associated with inconsistent evidence of deficits in motor control. Balance measures have revealed the most convincing evidence and several dyslexia screening batteries include tests of postural stability. A prominent causal theory of dyslexia links reading and balance difficulties through mild impairment of the cerebellum. Cerebellar abnormalities are associated with other developmental disorders including Attention Deficit Hyperactivity Disorder (ADHD), which co-occurs with both dyslexia and motor deficits (Chapter 1). A meta-analysis of dyslexia and balance studies (Chapter 3) yielded a robust (d = .64) combined between-groups effect size, however heterogeneity was revealed across studies. The strongest predictor of postural stability was whether samples had been screened for ADHD. Inconsistencies in sampling, procedure and measurement across studies were also revealed. The thesis aimed to introduce an objective, quantitative approach to the study of balance in dyslexia using theory and techniques from postural and balance research (Chapters 2 & 4). Unselected samples of adults and children from the dyslexia and normal reading populations were assessed on psychometric, literacy and behavioural measures. Postural stability was measured using digital optical motion capture (Qualisys Systems), obtained during a perturbation of balance caused by the release of a suspended weight equivalent to 5% body mass. The paradigm replicated the hitherto most frequently used balance measure with adults (Chapters 5 & 6) and children (Chapters 7 & 8). The findings suggest that balance difficulties in the dyslexia and general populations are correlates of reading difficulties but are related through hyperactivity or combined inattention and hyperactivity ratings. Tests of postural stability are therefore more likely to be useful predictors of risk of more general developmental disorder than specifically of reading failure. Postural control is therefore placed within the wider context of co-occurring developmental disorders (Chapter 9).

No description available.

It was hypothesized that the deficits underlying reading impairment may arise from supra-modal deficits in temporal and spatial attention, disrupting, on the one hand, the ability to segment the temporally ordered phonemes of language and thus the acquisition of decoding skills, and, on the other, the ability to integrate spatially and temporally ordered orthographic information acquired from the fluent visual scanning of written text. Temporal and spatial attentional deficits in dyslexia were investigated using a lateralized visual temporal order judgment (TOJ) paradigm that allowed both sensitivity to temporal order and spatial attentional bias to be measured. Dyslexic and non-dyslexic participants were required to report the temporal order of two simple visual stimuli presented in either the same or different lateral hemifields. Findings indicated that dyslexic participants showed markedly impaired sensitivity to temporal order, and that the degree of impairment was correlated with the severity of their dyslexia. Furthermore, the findings suggested that at least three partially dissociated deficits may underlie both impaired TOJ task performance and reading disorder. One is a deficit associated with difficulty in reporting the temporal order of two visual stimuli, particularly when the first is presented in right hemifield; with slow word recognition and non-word reading; and with deficits in spelling and phonological skill. This constellation of deficits was interpreted as reflecting deficits in networks in left cerebral hemisphere implicated in phoneme-grapheme mapping and visual orienting. The second is a deficit that is associated with a rightward attentional bias; with inaccurate non-word reading that is worse than predicted by phonological skill or by word recognition; and with poor sustained attention. This constellation of impairments was interpreted as evidence of a deficit in right-lateralised networks implicated in the modulation of arousal, and possibly reflecting a “developmental left-neglect” syndrome. A third deficit was associated with impaired temporal order sensitivity, regardless of hemifield presentation; with symptoms of Attentional Deficit and Hyperactivity Disorder (ADHD); and with increased interference from distractor stimuli. This constellation of deficits suggests that the impaired network is implicated in executive control of attention, including conflict resolution and working memory. The results of the investigation as a whole suggests that the reading impairments of dyslexia may arise from attentional deficits that have with substantial overlap with those of ADHD, and include deficits in attentional networks implicated in orienting attention to temporally presented stimuli.

Included in the challenges possibly faced by those with dyslexia, is the acquisition of literacy skills. Despite 21st century advancements made in technology, literacy skills remain central to education and day-to-day living. Therefore, it is essential to meet the learning needs of those with dyslexia, as failure herein could impact negatively on their lived experiences and on the attainment of their needs. The main aim of this transcendental phenomenological study was to explore, and describe the first-hand lifelong experiences of adults living with dyslexia. The study utilised the theoretical and conceptual frameworks of Bronfenbrenner’s Ecological Systems Theory and Maslow’s Hierarchy of Needs. Following a multistage procedure, a criterion-based sampling strategy was used to select study participants, comprising of eight South African adults between the ages of 18 and 40, who were formally diagnosed with dyslexia. Data collection included semi-structured one-on-one in-depth interviews and the researcher’s reflective journal. Data analysis, following the principles of phenomenology, resulted in the formation of five themes. Findings reveal that dyslexia is viewed positively by adults diagnosed with it, and used as a driving force to succeed and obtain set goals. Various professionals were consulted with the aim of obtaining a diagnosis; however, obtaining a diagnosis and hence an explanation for their learning challenges, was a stressful experience for both the participants and their parents. Families provide much assistance with school work, although all participants revealed negative schooling experiences caused by schools not acknowledging dyslexia and not meeting their specific learning needs. Although living with dyslexia presents challenges, coping mechanisms are in place for overcoming anticipated challenges. Whilst self-confidence is not negatively affected, dyslexia appears to negatively affect the self-esteem of some. This study found that the dyslexia label is preferred to that of having additional learning needs, as the dyslexia label is viewed as referring to specific challenges and not to entire learning abilities. Finally, this study hopes to provide a deeper understanding of the experiences of those living with dyslexia and fill the existing gap in South African literature. / Psychology / Ph. D. (Psychology)