Brain dysfunctions underlying developmental dyslexia

Our previous research established slow effortful word decoding as the main manifestation of dyslexia in German and found dyslexic abnormalities in the early ERP components in response to visual words. The present proposal will examine neurological dysfunctions which prevent that perceived letters get in rapid contact with stored orthographic word representations. It will follow-up exciting results from a recent small-scale study from our lab which combined electrophysiological ERP data (indicative of time course abnormalities) with hemodynamic fMRI data (indicative of abnormal brain localization). Preliminary results showed that dyslexic readers fail to exhibit the "normal" early differential brain response from about 170 ms onwards to correct and incorrect spellings of the very same words (e.g. TAKSI vs. TAXI) and they fail to exhibit the "normal" enhanced brain activity in a specific left occipitotemporal brain region in response to the incorrect spellings. These abnormal brain responses were found despite the demonstrated ability of the dyslexic readers to distinguish the correct from the incorrect spellings. The present proposal will substantiate and extend this preliminary evidence for a left occipitotemporal brain dysfunction in several directions: (1) With a second enlarged sample of 30 dyslexics and 30 controls we attempt to replicate the original ERP-fMRI finding. The sample size will avoid that functional abnormalities go undetected when tests of group differences are corrected for multiple comparisons as is the case in ERP- and fMRI-studies. Participants will be recruited from two large longitudinal samples and will also participat in an eye-movement study, so that brain activity data can be related to a rich behavioral data set. To avoid interpretational problems of the ERP-fMRI data we will equalize reading time for a subset of the stimuli. (2) The nature of dyslexic underactivation of the critical left occipitotemporal region in response to visual words will be examined by contrasting the brain response to visual words with the brain response to corresponding environmental sounds in a name retrieval task. This allows to distinguish between a specific impairment of neuronal connections from visual-orthographic to phonological brain regions and a general impairment in accessing phonological brain regions. (3) Structural brain abnormalities in grey matter density of dyslexic readers will be explored by collecting T1 weighted high resolution MR scans from all participants. Analysis will use the recently developed technique of optimized voxel based morphometry. The grant will allow to continue our successful neurocognitive dyslexic research and will allow two young enthusiastic researchers to use their already acquired expertise with ERP and fMRI methodology in a domain of practical educational relevance.

The main goal of the project `Brain Dysfunctions Underlying Developmental Dyslexia` was identification of regions with abnormal brain activity during reading in dyslexic readers. We started with studies examining reading-related brain activity in nonimpaired readers and identified a network of regions of the left hemisphere involved in the fast automatic processing of words during reading. The network was identified by functional magnetic resonance imaging (fMRI) and consisted of two main components: A left posterior region was involved in the rapid identification of the letter strings of words (Visual Word Form Area) and a left inferior frontal region was involved in accessing phonology and meaning. In an fMRI study to be published in Cortex we found underactivation of these two central reading-related brain regions in an adult sample of dyslexic readers. The reduced activation in the VWFA and a left anterior language region stood in contrast to overactivation in brain regions involved in visual and in articulatory processes. In a study published in Human Brain Mapping, we further found reduced gray matter density in the VWFA of dyslexic readers, but also in regions of the cerebellum which may be engaged by automatization of cognitive functions such as reading. An additional fMRI study was done to specify in more detail the functions of the brain regions with reduced activity in dyslexic readers and the functional connectivity between these regions. A further fMRI study with children was done to gain information on the developmental course of the brain dysfunctions underlying dyslexia. Data analysis for these studies is still in progress.