Auditory temporal processing in dyslexia: An ASSR study in young children

Background: Slow temporal modulations below 40Hz have been shown to be important for speech understanding. Modulation frequencies near 4 and 20Hz represent the rate by which syllables (±250ms) and phonemes (±50ms) appear in speech, respectively. It has been hypothesized that a neural deviance in the processing of these lower temporal modulations relates to the reading and spelling problems in dyslexia, a specific learning disorder. This deviance may cause subtle speech perception problems and could eventually lead to impaired literacy skills. The neural correlates of temporal auditory processing can be investigated by recording Auditory Steady-State Responses (ASSRs) in the electroencephalogram (EEG). ASSRs measure the ability of the auditory system to follow the rate of temporal information and may therefore provide an objective measure to determine the sensitivity for important acoustical-phonological elements in language. Methods: In the present longitudinal study, temporal auditory processing was investigated in a group of 45 children with and 45 children without an increased hereditary risk of dyslexia by means of ASSRs in combination with cognitive and psychophysical measures. All participating children attended last year of nursery school at the moment of testing. Multichannel ASSRs were recorded in a 64 electrode configuration using 100% amplitude-modulated speech-weighted noise stimuli at 4, 20 and 80Hz. The 80Hz ASSR is known to be evoked in the brainstem, while the 4Hz and 20Hz ASSRs are generated in the cortex. Response strength is examined with response amplitudes and signal-to-noise ratios. In addition, hemispheric response asymmetry is assessed with a laterality index. The underlying neural sources are fitted using brain source analysis methods. Results: Preliminary results indicate differences between syllable and phoneme level for children with and children without an increased risk of dyslexia. Previous research demonstrated deviant processing on phoneme level in adults with dyslexia. Analyses are ongoing. Further results and implications will be discussed at the conference.