Dyslexics' phonological processing in relation to speech perception

Sammanfattning: The general aim of this thesis was to investigate phonological processing skills in dyslexic children and adults and their relation to speech perception. Dyslexia can be studied at various levels: at a biological, cognitive and an environmental level. This thesis mainly looks at environmental and cognitive factors. It is a commonly held view that dyslexia is related to problems with phonological processing, that is, dyslexics have problems dealing with the sound structure of language. The problem is for example seen in tasks where the individual has to manipulate sound segments in the spoken language, read non-words, rapidly name pictures and digits, keep verbal material in short-term memory, and categorize and discriminate sound contrasts in speech perception. To fully understand the dyslexic’s problems we have to investigate both children and adults since the problems might change during the lifespan as a result of changes in the language system and compensatory mechanisms in the poor reader. Research indicates that adult dyslexics can reach functional reading proficiency but still perform poorly on tasks of phonological processing. Even though they can manage many everyday reading situations problems often arise when adult dyslexics enter higher education. The phonological problems of dyslexics are believed to be related to the underlying phonological representations of the language. The phonological representations have been hypothesized to be weakly specified or indistinct and/or not enough segmented. Deviant phonological representations are believed to cause problems when the mapping of written language is to be made to the phonological representations of spoken language during reading acquisition. In Paper 1 adults’ phonological processing and reading habits were investigated in order to increase our understanding of how the reading problems develop into adulthood and what the social consequences are. The results showed that adult dyslexics remained impaired in their phonological processing and that they differed substantially from controls in their choices regarding higher education and also regarding reading habits. Paper 2 reviews research that has used the sine wave speech paradigm in studies of speech perception. The paper also gives a detailed description of how sine wave speech is made and how it can be characterized. Sine wave speech is a course grained description of natural speech lacking phonetic detail. In Paper 3 sine wave speech varying with regard to how much suprasegmental information it contains is employed. Results showed that dyslexics were poorer at identifying monosyllabic words but not disyllabic words and a sentence, plausibly because the dyslexics had problems identifying the phonetic information in monosyllabic words. Paper 4 tested dyslexics’ categorization performance of fricative-vowel syllables and the results showed that dyslexics were less consistent than controls in their categorization indicating poorer sensitivity to phonetic detail. In all the results of the thesis are in line with the phonological deficit hypothesis as revealed by adult data and the performance on task of speech perception. It is concluded that dyslexic children and adults seem to have less well specified phonological representations.