Risk and Protective Factors of Dyslexia: A study of auditory temporal processing and morphological awareness

Dyslexia is a neurological condition affecting 5-7% of the population, which impacts an individual’s ability learning to read and write despite adequate intelligence, education and remediation. The predominant etiological view postulates that dyslexia results from a deficit in the phonological domain, specifically in the quality and accuracy of phonological representations. A vital component in the development of phonological representations is the awareness of individual speech sounds. Recent findings have suggested the existence of an underlying deficit in low-level auditory temporal processing within the dyslexic population. This auditory temporal processing deficit theory hypothesizes that a disruption in the processing of dynamic changes in frequency and amplitude of sounds causes speech perception problems, leading to deviant phoneme representations, ultimately disrupting phoneme-grapheme mappings, which, in turn, is manifest as reading and spelling problems. Such single cognitive deficit models of dyslexia, however, are incapable of explaining all of the expressed behavioral traits observed in a dyslexic population. Additionally, not all individuals with phonological impairments develop dyslexia. This leads researchers to explore a multifactorial aetiology which accounts for multiple risk or protective factors that act probabilistically together to produce the expressed behavioral symptoms of dyslexia. 

The aim of this study was to investigate whether children and adults with dyslexia have auditory temporal processing deficits and how these deficits can be characterized. Additionally, within a context of the multiple cognitive deficit model of dyslexia, we aimed to examine the nature and role of morphological awareness (MA) as a unique risk and/or protective factor in literacy outcomes of children and adults with dyslexia. To evaluate developmental and causal influences of both MA and auditory measures, pre-reading children with (HR) and without (LR) a family risk of dyslexia were longitudinally followed up through their early literacy development. Additionally, we examined the nature and expression of these relationships as represented within an adult population with dyslexia. 

In an initial study, we investigated whether auditory processing, speech perception and phonological skills contribute to adult reading ability, either independently or conjunctively. Results showed phonological and slow-rate dynamic auditory deficits are related to literacy. Yet, at the individual level the theorized cascading effects of problems in auditory temporal processing could not explain literacy problems. In the same population, we conducted a second study to examine the compensatory role of MA in literacy of adults with dyslexia. MA was found to significantly predict a greater proportion of word reading and spelling within the dyslexic group compared to the controls. While MA deficits were found in adults with dyslexia, compensated dyslexics were found not to differ from controls on measures of MA, implicating intact MA skills being utilized in their achieved reading compensation. 

Our remaining studies investigated the nature and development of the auditory temporal processing deficit and early MA of children with dyslexia. In a group of HR and LR pre-reading children, study 3 examined the relation of phonological awareness (PA) with auditory processing and MA prior to formal reading instruction. Results demonstrated an MA deficit in HR children prior to reading instruction. In addition, a trend for lower rise time discrimination (RT) thresholds in HR children was also found. Further comparison provided evidence supporting the notion that pre-reading MA is a function of an individual’s pre-reading PA. 

Study 4 addressed questions concerning auditory temporal processing deficits in the early stages of reading acquisition based on a retrospective examination of the longitudinal data. Results indicated atypical performance of children who developed dyslexia in auditory processing of RT and PA at each of the three time points (kindergarten, first, and second grade). Additionally, results showed that RT and FM sensitivity in kindergarten uniquely contribute to growth in reading ability in grades one and two, even after controlling for letter knowledge and PA. Additional evidence was provided suggesting the possibility of a causal relationship where kindergarten RT significantly predicts later PA. 

Study 5 focused on the association of MA growth with PA in early childhood development. Results demonstrated that children with dyslexia have MA deficits at all time points. Additionally, PA was found to contribute to MA development prior to the onset of formal reading instruction. After the start of formal reading instruction, decoding skills were found to be the major variable contributing to MA growth. 

Dyslexia is a neurological condition affecting 5-7% of the population, which impacts an individual’s ability learning to read and write despite adequate intelligence, education and remediation. The predominant etiological view postulates that dyslexia results from a deficit in the phonological domain, specifically in the quality and accuracy of phonological representations. A vital component in the development of phonological representations is the awareness of individual speech sounds. Recent findings have suggested the existence of an underlying deficit in low-level auditory temporal processing within the dyslexic population. This auditory temporal processing deficit theory hypothesizes that a disruption in the processing of dynamic changes in frequency and amplitude of sounds causes speech perception problems, leading to deviant phoneme representations, ultimately disrupting phoneme-grapheme mappings, which, in turn, is manifest as reading and spelling problems. Such single cognitive deficit models of dyslexia, however, are incapable of explaining all of the expressed behavioral traits observed in a dyslexic population. Additionally, not all individuals with phonological impairments develop dyslexia. This leads researchers to explore a multifactorial aetiology which accounts for multiple risk or protective factors that act probabilistically together to produce the expressed behavioral symptoms of dyslexia. 

The aim of this study was to investigate whether children and adults with dyslexia have auditory temporal processing deficits and how these deficits can be characterized. Additionally, within a context of the multiple cognitive deficit model of dyslexia, we aimed to examine the nature and role of morphological awareness (MA) as a unique risk and/or protective factor in literacy outcomes of children and adults with dyslexia. To evaluate developmental and causal influences of both MA and auditory measures, pre-reading children with (HR) and without (LR) a family risk of dyslexia were longitudinally followed up through their early literacy development. Additionally, we examined the nature and expression of these relationships as represented within an adult population with dyslexia. 

In an initial study, we investigated whether auditory processing, speech perception and phonological skills contribute to adult reading ability, either independently or conjunctively. Results showed phonological and slow-rate dynamic auditory deficits are related to literacy. Yet, at the individual level the theorized cascading effects of problems in auditory temporal processing could not explain literacy problems. In the same population, we conducted a second study to examine the compensatory role of MA in literacy of adults with dyslexia. MA was found to significantly predict a greater proportion of word reading and spelling within the dyslexic group compared to the controls. While MA deficits were found in adults with dyslexia, compensated dyslexics were found not to differ from controls on measures of MA, implicating intact MA skills being utilized in their achieved reading compensation. 

Our remaining studies investigated the nature and development of the auditory temporal processing deficit and early MA of children with dyslexia. In a group of HR and LR pre-reading children, study 3 examined the relation of phonological awareness (PA) with auditory processing and MA prior to formal reading instruction. Results demonstrated an MA deficit in HR children prior to reading instruction. In addition, a trend for lower rise time discrimination (RT) thresholds in HR children was also found. Further comparison provided evidence supporting the notion that pre-reading MA is a function of an individual’s pre-reading PA. 

Study 4 addressed questions concerning auditory temporal processing deficits in the early stages of reading acquisition based on a retrospective examination of the longitudinal data. Results indicated atypical performance of children who developed dyslexia in auditory processing of RT and PA at each of the three time points (kindergarten, first, and second grade). Additionally, results showed that RT and FM sensitivity in kindergarten uniquely contribute to growth in reading ability in grades one and two, even after controlling for letter knowledge and PA. Additional evidence was provided suggesting the possibility of a causal relationship where kindergarten RT significantly predicts later PA. 

Study 5 focused on the association of MA growth with PA in early childhood development. Results demonstrated that children with dyslexia have MA deficits at all time points. Additionally, PA was found to contribute to MA development prior to the onset of formal reading instruction. After the start of formal reading instruction, decoding skills were found to be the major variable contributing to MA growth. 

The final chapter summarizes the conclusions of this work, highlights its limitations and provides a critical discussion of its theoretical and practical relevance as well as suggestions for future research. 

The final chapter summarizes the conclusions of this work, highlights its limitations and provides a critical discussion of its theoretical and practical relevance as well as suggestions for future research.