Learning to associate letters and speech sounds is an important milestone in literacy acquisition. We used pediatric functional magnetic resonance imaging (fMRI) to investigate letter-speech sound integration in 8-12 year old children with and without reading impairment.
Dyslexic and control children (N=20) were scanned at 3T while viewing and/or listening to letters and speech sounds and performing a simple attention control task. Speech sounds and letters were presented in isolation or in congruent or incongruent combinations.
Dyslexic readers activated early auditory cortex (Heschl Sulcus/Planum Temporale) less than fluent readers in response to congruent versus incongruent letter-speech sound pairs. Moreover, dyslexic readers show less pronounced activation for speech sounds in isolation. Finally, dyslexic readers but not fluent readers activate left-hemispheric pre-central motor regions in response to visual and bimodal stimuli.
These findings suggest that during reading acquisition fluent readers but not dyslexic readers sufficiently automate letter-speech sound associations. Moreover, the data suggest that the cause of this insufficient letter-speech sound binding reside in the phonological domain. On the basis of the present findings, we propose that the impaired neural binding of letters and speech sounds might act as mediator between phonological processing deficits and reading problems in developmental dyslexia.