Cortical and behavioural characteristics of children with developmental disorders : does treatment change the picture?

This dissertation was motivated by two questions: 1) Do children with developmental disorders have reliable cortical characteristics that distinguish them from control children or that are unique for different disorder-subtypes? 2) Do these cortical characteristics reflect behavioural changes effectuated by an intervention? Five studies that contribute to answering these questions will be presented and discussed in detail. Study I was dedicated to source localisation of auditory syllable-processing in children with developmental dyslexia and control children. Study II investigated the influence of different training programs on a cortical measure of auditory sensitivity in dyslexia. Study III investigated visual word processing differences between dyslexic and control children. For more detailed insight, study IV investigated the same set of words in skilled adult readers. Study V examined the effect of a stimulant medication on cortical measures of attentional deficit hyperactivity disorder (ADHD) – a disorder that often occurs comorbid with dyslexia. The results indicate that dyslexia is characterised by (a) more posterior source localisations of speech sounds in right temporal regions possibly resulting from more symmetrical plana temporali and (b) reduced activation and enhanced spectral frequencies following words that require sublexical processing in occipital regions possibly reflecting reduced phonological awareness in dyslexia. Dyslexic children did not differ from control children in their sensitivity for sound changes. Nevertheless, the auditory sensitivity measure was differentially influenced by different intervention programs. It was found that a training of phonological awareness had most effects on cortical and behavioural measures. These findings favour a phonological awareness deficit as general aetiological factor in dyslexia rather than a more basic auditory perceptual deficit. The results on ADHD suggest different cortical profiles of ADHD subtypes. Children with ADHD of the combined type not only differed from children with ADHD of the predominantly inattentive type, they also responded differently to stimulant medication. The results further propose that good and poor medication responders have differing cortical characteristics. The overall medication effect supports frontocortical hypoarousal as a relevant aetiological factor in ADHD.