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Title: Auditory processing in dyslexia: electrophysiology and psychophysics
Other Titles: Auditieve verwerking bij mensen met dyslexie: elektrofysiologie en psychofysica
Authors: Poelmans, Hanne
Issue Date: 14-May-2012
Abstract: Developmental dyslexia is known as a failure to acquire adequate reading and spelling skills despite normal intelligence, education and intense remedial effort. Until now, the exact origin of this disorder remains unknown. One theory suggests that the reading and spelling problems in dyslexia are caused by a fundamental deficit in low-level auditory temporal processing. That is, reduced sensitivity to temporal acoustic cues during the crucial phase of reading acquisition may impair the accuracy of which speech sounds are mapped onto their corresponding written symbols and this may cause reading and spelling difficulties in individuals with dyslexia.In the brain, temporal information is encoded by neural oscillations that phase-lock to the rhythm of the temporal variation. Spontaneous neural oscillations in certain frequency bands in the EEG coincide with the time windows in which important acoustic information about syllables (~4 Hz) and phonemes (~20 Hz) is processed. It has been suggested that temporal information within these time windows is processed asymmetrically in the brain, with a right hemispheric preference for processing syllabic-rate acoustic information and a left hemispheric preference for processing phonemic-rate acoustic information. In this context, it has recently been proposed that neural oscillatory activity in response to syllabic- and phonemic-rate information deviates in individuals with dyslexia.The aim of this PhD project was to investigate the nature of the auditory temporal processing deficit in dyslexia by means of electrophysiological and psychophysical techniques. In the first part of this project, we aimed to determine an objective electrophysiological marker for the auditory temporal processing deficit in dyslexia by means of auditory steady-state responses (ASSRs). Moreover, we focused on three research questions. First, we examined whether adults with dyslexia have different electrophysiological responses to phonemic- (20 Hz) and/or syllabic-rate (4 Hz) modulations, or to a modulation rate that is less relevant for speech perception (80 Hz). Results demonstrated that adults with dyslexia display deviant responses specifically to the phonemic-rate modulation, but not to syllabic- or higher modulation rates. Moreover, phonemic-rate group differences reflected in lower responses at electrodes over the left hemisphere as well as in lower coherence between and within hemispheres. Furthermore, a relation between phonemic-rate ASSRs and psychophysical tests of speech-in-noise perception and phonological awareness was obtained. These results suggest that in adults with dyslexia, phonemic-rate acoustic information is less accurately sampled into phonemic representations and that this problem may result in difficulties perceiving speech and processing phonological information.Second, because not much is known about phonemic- and syllabic-rate ASSRs, we examined whether these phonemic- and syllabic-rate ASSRs are influenced by the manner by which stimuli are presented or whether slow-rate ASSRs are rather influenced by a functional hemispheric specialization to process phonemic- or syllabic-rate modulations. Results demonstrated that for 80 and 20 Hz ASSRs the highest responses could be recorded at electrodes over the same hemisphere as the stimulus was presented. In contrast, for 4 Hz ASSRs functional lateralization was observed. ASSRs were higher in the right hemisphere, independent of which ear was stimulated. These results may imply that both hemispheres are sensitive to process phonemic-rate information whereas the right hemisphere displays the strongest activity to syllabic-rate modulations. Third, the relation between functional phonemic- and syllabic-rate processing and anatomical cortical white matter pathways was examined. Functional processing of phonemic-rate modulations related to structural white matter properties of regions that are involved in speech perception and this relation was different for normal-reading and dyslexic adults. These results indicate that in individuals with dyslexia functional phonemic-rate processing problems may relate to anatomical differences in the brain. The second part of this project aimed to elucidate the auditory temporal processing deficit in 12 year old children with dyslexia. We demonstrated that children with dyslexia are less sensitive than normal-reading children to slowly varying temporal auditory information, speech-in-noise perception, phonological awareness and reading. Furthermore, correlations were found between auditory temporal processing and phonological awareness, and between speech-in-noise perception and reading. Together, these results confirm that children with dyslexia have difficulties at levels of the auditory temporal processing deficit theory and that these problems persist until sixth grade.In general, this project demonstrates that individuals with dyslexia process temporal acoustic information that occurs in important time windows for phonological processing less accurately compared to normal-reading persons, and this implies that in individuals with dyslexia phoneme-level information is insufficiently integrated in the phonological pathway involved in speech perception.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Research Group Experimental Oto-rhino-laryngology
Parenting and Special Education

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