Auditory temporal processing in pre-readers at risk for dyslexia: neurophysiological markers and preventive intervention

Literacy contributes considerably to an individual’s educational and professional outcomes. Even though learning to read initially requires a great amount of effort, the majority of people eventually attains fluent reading skills. Nonetheless, approximately 7% of the population continues to experience severe difficulties with reading and/or spelling and is diagnosed with developmental dyslexia. Research highlights the importance of early identification and preventive intervention to promote reading and related skills. Yet, this stands in sharp contrast to clinical practice where interventions are typically only provided after years of reading difficulties. Additionally, dyslexia interventions largely target phonological skills.
Although the phonological deficit theory, which assigns dyslexia to underlying difficulties with phonological processing, has received a lot of support, a large body of research suggests that at least in a subgroup of individuals with dyslexia, these difficulties might be caused by an underlying deficit in auditory temporal processing. More specifically, it is proposed that an atypical processing of slow-rate dynamic information in speech, such as amplitude rise times, and an atypical neural synchronization to the speech signal, impede speech perception and consequently the quality of phonological representations. In this respect, interventions would benefit from addressing auditory temporal processing difficulties, by for example
enhancing the speech envelope.

The current doctoral research had two overarching aims. The first aim was to identify the neurophysiological markers underlying the auditory temporal processing deficit in dyslexia. Part of this first aim was to investigate acoustic stimuli with an increased potential to uncover these neurophysiological markers in a pediatric population. The second aim was to investigate the impact of a preventive intervention on auditory temporalprocessing. To evaluate auditory temporal processing, we measured (1) auditory steady-state responses (ASSRs) to syllable and phoneme rate modulations using both regular sinusoidal amplitude modulated stimuli and pulsatile stimuli with shortened rise times, and (2) rise time sensitivity. We evaluated auditory temporal processing before, immediately after, and approximately one year after a 12-week preventive home-based intervention in pre-readers at cognitive risk for dyslexia who received a phonics-based training, either with or without auditory enhanced speech training, or engaged in active control training. Reading and spelling data collected at the start of third grade allowed to redefine all children as either typically reading children or children with dyslexia. In the first study (Chapter 2), we examined the suitability of the pulsatile stimuli in a typically reading population of pre-reading to beginning reading children. We found that pulsatile stimuli greatly enhanced neural synchronization to syllable rate modulations, thereby considerably increasing data acquisition efficiency. Furthermore, the pulsatile stimuli also elicited a different hemispheric specialization at syllable rate. In our second study (Chapter 3), we demonstrated an atypical hemispheric specialization in children with dyslexia, even before the onset of formal reading instruction. This indicates a potential causal mechanism and supports the hypothesis that atypical neural synchronization is related to the development of dyslexia. We found no evidence for an atypical neural processing of rise times. Additionally, the phonics-based and enhanced speech intervention did not impact neural synchronization to either syllable or phoneme rate modulations. In the last study (Chapter 4), we showed that the enhanced speech intervention did improve rise time sensitivity. Therefore, it seems that the enhanced speech intervention directly trained rise time sensitivity, but did not yield transfer effects to indirectly trained neural synchronization. The phonics-based training did not improve rise time sensitivity.

In sum, the current research contributes to our understanding of the neurophysiological mechanisms underlying the development of dyslexia and the development of preventive interventions. Our results showed evidence of an auditory temporal processing deficit present already in pre-readers and therefore potentially causally related to dyslexia. Additionally, our findings highlight the importance of auditory trainings in dyslexia. The enhanced  speech training is certainly promising, but can still be improved. We believe that the present work represents a vital step towards a better understanding of dyslexia and the development of more effective interventions.