Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 135-145).

published_or_final_version / abstract / Psychology / Master / Master of Philosophy

published_or_final_version / Public Health / Master / Master of Public Health

Experiments presented challenge theories on their ability to provide causal explanations of the pattern of performance in dyslexia. Studies la and 1 b employed a prism adaptation paradigm to investigate the Cerebellar Deficit Hypothesis (CDH). No group differences were found, although unfortunately it was concluded that the paradigm could not satisfactorily isolate cerebellar function from other compensation mechanisms. Studies 2a and 2b exploited a sequential stereopsis technique to test the visual deficit hypothesis. No group differences were found, although the dyslexic group did exhibit a fatigue effect on one condition. Using an attention shifting paradigm, Study 3 found a dissociation between focus and shift attention conditions in dyslexic children, but that they sustained their attention as well as controls. In Study 4, supporting the Dyslexia Automatisation Deficit (DAD) as opposed to a general resources deficit, control performance suffered most under visually degraded conditions of the same attention paradigm. Study 5 further investigated attention on a test thought to be sensitive to attentional lapses; dyslexic children did make more errors, although conclusions were limited by their qualitatively normal performance. Deficits in dyslexia were found to be wider reaching than many theories of dyslexia would suggest. At a cognitive level of explanation the DAD was able to account successfully for many of the findings. However, like the Phonological Deficit Theory the DAD specifies no neurological mechanism for the deficit; this is provided by the CDH (for which no evidence was found here). Analyses do point towards the need for either a very general explanation, or the identification of a smaller number of core deficits, for the apparently disparate deficits found. The fatigue effect found only in the dyslexic group on part of the vision experiment has further direct and immediate implications for future research.

This study tested the prediction that a group of dyslexic children (n = 70) would be less sensitive to auditory and visual temporal stimuli than a control group (n = 52). In the auditory domain, detection thresholds for 2 Hz FM, 2 Hz AM and 20 Hz AM were assessed. The modulations in these stimuli are detected on the basis of temporal cues. In contrast, the modulations in the control stimulus 240 Hz FM modulate too rapidly to be detected with temporal cues. The dyslexic group were significantly less sensitive than the control group to the temporal and non-temporal measures at initial testing (Phase 1) and again nine months later (Phase 4). These data demonstrated that the auditory deficit in the dyslexic group was more general in nature than had previously been suggested. In the visual domain, sensitivity to global coherent motion was assessed. The dyslexic group were significantly less sensitive than the control group on this measure at both phases of the study. Despite the overall between group differences, the magnitude of the effects were low to moderate. There was also substantial overlap between the performance of the two groups on the sensory processing measures. A deviance analysis was conducted to determine the proportion of dyslexic individuals who had sensory processing deficits. When data from each phase was examined separately, the incidence of sensory processing deficits in the dyslexic group was comparable to previous studies. However, when the data from both phases was combined, only 5-18% of the dyslexic group had impairments on any of the sensory tasks that were stable across time. Nevertheless, these results do not preclude sensory processing making a contribution to reading difficulties in some children. When the relationship between sensory processing thresholds and reading ability was considered, sensitivity to auditory and visual temporal measures accounted for significant unique variance in phonological processing, orthographic coding and overall reading skill, even after accounting for IQ and vigilance. This study was also tested the prediction that visual attention can explain the link between visual temporal processing and reading. Vidyasagar (1999) proposed that the magnocellular (M) system, which processes temporal stimuli (e.g., motion), is also important for efficient functioning of an attentional spotlight. This spotlight is proposed to arise in parietal cortex (a major endpoint of the M system), and is involved in highlighting areas for detailed visual processing when performing visual tasks, such as visual search or reading. It was predicted that only those dyslexic participants with motion detection impairments would also be impaired on a serial search task that required the attentional spotlight. On average, the dyslexic group had significantly slower serial search than the control group. However, the magnitude of effect was small and a deviance analysis demonstrated that only 8.5% of the dyslexic group had stable impairments relative to the control group. Furthermore, only one of the six dyslexic participants with a visual attention impairment had a co-existing deficit in detecting coherent motion. Thus, visual attention deficits of this type appear to exist independently of coherent motion deficits. This study also provided important evidence on the reliability of measurement for the sensory processing tasks. The data showed that the test-retest reliability of the sensory measures was only moderate over a nine month period. Test-retest for other cognitive measures over the same time frame was high - including that for an orthographic coding task, which had similar procedure and task demands to the sensory measures. The results also demonstrated that a high proportion of participants in both groups performed inconsistently across time (i.e., they had a threshold indicative of a deficit at one phase and performance within normal limits at the other). Up to 32% of the dyslexic group and 19% of the control group had inconsistent performance on the sensory measures across time. The importance of developing more reliable methods of estimating sensory sensitivity is discussed, as is the need for normative data on sensory processing tasks in order to more accurately make decisions about the incidence of sensory deficits. In summary, this study provided evidence for a relationship between sensory processing and reading. However, the current data demonstrated that sensory processing deficits are not characteristic of all dyslexic individuals. Future research should focus on explaining why only a sub-group of dyslexics have sensory deficits, and also why some control participants have deficits.

published_or_final_version / abstract / toc / Educational Psychology / Master / Master of Social Sciences

published_or_final_version / abstract / toc / Educational Psychology / Master / Master of Social Sciences

Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves 132-143) Also available in print.

Thesis (B.Sc)--University of Hong Kong, 2005. / "A dissertation submitted in partial fulfilment of the requirements for the Bachelor of Science (Speech and Hearing Sciences), The University of Hong Kong, June 30, 2005." Also available in print.

Thesis (M.Ed.)--Kutztown University of Pennsylvania, 1980. / Source: Masters Abstracts International, Volume: 45-06, page: 2801.