Auditory Steady State Cortical Responses Indicate Deviant Phonemic-Rate Processing in Adults With Dyslexia

Objectives: Speech intelligibility is strongly influenced by the ability to process temporal modulations. It is hypothesized that in dyslexia, deficient processing of rapidly changing auditory information underlies a deficient development of phonological representations, causing reading and spelling problems. Low-frequency modulations between 4 and 20 Hz correspond to the processing rate of important phonological segments (syllables and phonemes, respectively) in speech and therefore provide a bridge between low-level auditory and phonological processing. In the present study, temporal modulation processing was investigated by auditory steady state responses (ASSRs) in normal-reading and dyslexic adults. Design: Multichannel ASSRs were recorded in normal-reading and dyslexic adults in response to speech-weighted noise stimuli amplitude modulated at 80, 20, and 4 Hz. The 80 Hz modulation is known to be primarily generated by the brainstem, whereas the 20 and 4 Hz modulations are mainly generated in the cortex. Furthermore, the 20 and 4 Hz modulations provide an objective auditory performance measure related to phonemic- and syllabic-rate processing. In addition to neurophysiological measures, psychophysical tests of speech-innoise perception and phonological awareness were assessed. Results: On the basis of response strength and phase coherence measures, normal-reading and dyslexic participants showed similar processing at the brainstem level. At the cortical level of the auditory system, dyslexic subjects demonstrated deviant phonemic- rate responses compared with normal readers, whereas no group differences were found for the syllabic rate. Furthermore, a relationship between phonemic- rate ASSRs and psychophysical tests of speech-in-noise perception and phonological awareness was obtained. Conclusions: The results suggest reduced cortical processing for phonemic- rate modulations in dyslexic adults, presumably resulting in limited integration of temporal information in the dorsal phonological pathway.