Auditory Processing in Dyslexia: Evidence from Psychophysical Studies and Event-Related Magnetic Fields

Abstract

The present thesis sought to examine behavioral and neurophysiological aspects of auditory processing in people with dyslexia compared to normally literate controls. To this end, a series of psychophysical experiments and magnetoencephalographic recordings were conducted, involving stop consonant-vowel (CV) syllables. The mismatch field (MMF) and other cortical event-related field (ERF) components served as dependent variables. As an important result, these studies pointed to deviances in hemispheric asymmetry of temporal-lobe sites in children with dyslexia. Signal space analyses revealed an altered topographical distribution of the MMF, while source space analyses showed pronounced deviations in source positions of ERFs. These findings may reflect atypical organization of the primary auditory cortex and adjacent areas, in particular the planum temporale. Cross-sectional investigations indicate that a deviance in source configuration of magnetic waves around 100 ms after syllable presentation continue into adulthood. However, differences in hemispheric balance might be altered following linguistic training as suggested by a study in a small group of language-impaired children with symptoms of dyslexia. A behavioral experiment inspired by the hypothesis of a temporal processing deficit for dyslexia suggests that limitations in auditory processing might be compensated by a well-functioning visual sensory modality. Another study revealed two subgroups of dyslexic individuals differing in their discrimination ability of short and temporally extended CV syllables both at the behavioral and neuronal level. Taken together, the studies indicate predominantly right-hemisphere deviances in children and adults with dyslexia. These observations complement functional neuroimaging studies suggesting mainly a left-hemisphere deficit in dyslexia and characterize the temporal dynamic of the relevant brain processes.