Altered hemispheric asymmetry of auditory P100m in dyslexia

In various studies, deviances of hemispheric laterality in the organization of the perisylvian region in dyslexia have been suggested. Although associated with impaired language functioning, the clinical significance of atypical cerebral lateralization remains unclear. The present study examined interhemispheric source differences of magnetic responses to the German synthetic syllable [ba:] in the auditory cortex of 14 dyslexic children and 12 normally literate controls aged 8-15 years. In all subjects, two main deflections, P100m and N260m, were evident in the responses over each hemisphere. While in the control group the right P100m dipole was located more anterior than the corresponding dipole of the left hemisphere, the dyslexic group displayed a rather symmetrical source configuration between the hemispheres. This symmetry reflected a deviance in the right perisylvian region for the dyslexic subjects' P100m, which was generated approximately 1 cm more posterior than the response in controls. The deviation was also obvious relative to the source location of the later component, N260m, which did not systematically differ between the participant groups. Our results suggest that the altered hemispheric asymmetry reflects an atypical organization of the right hemisphere in children and adolescents with dyslexia.