Reduced hemispheric asymmetry of the auditory N260m in dyslexia.

Dyslexia seems to be related to a lack of planum temporale (PT) asymmetry that is accompanied by functional differences to control subjects in both left and right hemispheric temporal regions during language tasks. PT asymmetry has been found to correlate with phonological and verbal skills. In accordance, reduced asymmetry of the auditory N100m sources in dyslexic adults and P100m sources in dyslexic children has been reported. These results might also be related to an atypical PT symmetry or the recruitment of other structures than the PT for speech processing in dyslexia. In the present study we tried to replicate and extend previous findings by examining a sample of 64 dyslexic and 22 control children in the MEG. We measured cortical activity during a passive auditory oddball-paradigm and localised ERF sources evoked by the standard stimulus /ba/. Reduced hemispheric asymmetry in the localisation of the auditory N260m was revealed. While control children displayed a typical asymmetrical pattern with more anterior sources in the right hemisphere, this asymmetry was not present for the dyslexic children. Further, a correlation between N260m asymmetry and spelling test performance was found. Our results suggest that localisation of ERF components is indeed an applicative tool for investigating cortical deviances in dyslexia. A lack of source localisation asymmetry in dyslexia appears to be a robust finding across different samples of dyslexic children and adults. It appears that cortical auditory (language) processing is organised differently in dyslexic subjects than in controls. This might be the consequence of a more symmetrical PT organisation, which in turn might be the result of maturational delay.