Abnormal Oscillatory Brain Responses in Dyslexic children with Poor Categorical Perception

Abstract

The condition of dyslexia has been associated with problems in speech perception, particularly in the processing of speech signals (e.g., phonemes). Speech signals contain information on different time scales. For instance, while rapid spectral changes as formant transitions may occur on time scale (20-40ms), syllabic and prosodic information occur on a time scale of (150-300ms). Therefore, the segmentation of the auditory stream into discrete representations is a prerequisite for speech perception, According to Peoppel (2003); the input speech signal has a neural representation that is bilaterally symmetric at an early representational level; however it is elaborated asymmetrically in the time domain. Consequently, it has suggested that temporal integration in different windows is reflected as oscillatory neuronal activity in different frequency bands. In this study, we are testing if the temporal integration is reflected as oscillatory activity in different frequency bands by mapping focal slow waves in the delta (1.5-4Hz) frequency band. We used magnetoencephalographic (MEG) source imaging in a sample of 19 dyslexic children with poor categorical perception and 14 controls while listening passively to syllables /ba/ and /da/. Dyslexic children with poor categorical perception differ significantly in the density of magnetic slow waves produced by the two hemispheres. To illustrate, dyslexic children with poor categorical perception showed elevated production of focally generated slow waves (1-4Hz), predominately in the left hemisphere as compared to controls. The results suggest altered segmentation processes of speech sounds in a subset of children with dyslexia.