Phonetic and temporal aspects of speech perception in dyslexia and its relationship with neuroanatomy (invited lecture)

There is evidence that individuals with dyslexia exhibit problems in the representation of speech sounds, and that these may be rooted in a more fundamental auditory processing deficit. Despite the substantial effort in research, there is the central issue of debate concerning the nature of these auditory problems in dyslexics. In particular, whether these problems are speech- and/or temporal specific. By applying a balanced design with stimuli that are adequately controlled for acoustic complexity, we find evidence that dyslexic children and adults display a specific temporal processing deficit for both speech and non-speech sounds. In a next study, the same adult subjects were scanned using diffusion tensor imaging (DTI), which enables a non-invasive examination of the white matter bundles. As intact myelination of the white matter is important for rapid conduction of action potentials, we hypothesize that a disturbance of myelination would be extremely detrimental for temporal processing. Our preliminary results demonstrate a relationship between the performance on the temporal processing task and the degree of left lateralisation of white matter in the auditory cortex. In addition to searching for neural correlates of temporal processing, DTI was used to examine the reading network and its possible disconnections in dyslexics. We investigated therefore the left inferior fronto-occipital fasciculus and the left superior longitudinal fasciculus which are presumed to respectively represent the lexical and phoneme-to-grapheme reading route. Anatomical correlates for the dual route model are found, as well as specific group differences.status: publishe