Dyslexia: EEG-Analysis of Deficient Reading Mechanism

Research project P 14494 Dyslexia: EEG-Analysis of Deficient Reading Mechanisms Heinz WIMMER 26.6.2000 The present project uses electrophysiological methods within an experimental cognitive-neuropsychological framework to explore the physiological basis of deficient reading processes. For this research we can rely on the participation of dyslexic adolescents and control subjects with unimpaired reading performance, who already had participated in two large longitudinal studies on reading development (beginning at school entry with altogether about 900 children) done in our laboratory. For these children an extensive data base on reading development and on linguistic and cognitive measures exists, which will be related to the EEG findings. The new idea in the design of the EEG studies is that the different experimental tasks are directly focussed on a well-defined set of processing modules involved in word reading (visual letter string processing, indirect phonological lexical access based on grapheme-phoneme coding, direct orthographically mediated lexical, access to word semantics). The plan is realize two EEG studies starting and a follow-up study. The first study is based on the dual-route conception of word reading (direct orthographic and indirect phonological lexical access) and contrasts reading of high frequency number words with reading of corresponding pseudowords (constructed parallel to the number words). Control tasks are digit reading (same number words as in the direct orthographic access task) and visual processing of unfamiliar letter strings (Greek letters). The second study is modelled after the recent work of Bentin, Mouchetant- Rostaing, Giard, Echallier & Pernier (1999) and will use the oddball-task paradigm to examine, in a set of experiments, deficits in visual, phonological, lexical, orthographic and semantic processes in word reading. Based on the dyslexic deficits observed in these two experiments follow-up studies will be designed to replicate and to further analyse these deficits. For all experimental studies, the EEG will be analysed not only by calculating event-related potentials (ERPs), but also by applying new methods of analysing event-related band power changes. In particular, we will use the measurement of induced band power (IBP) which was developed by the psychophysiology research group in Salzburg and has been shown to reflect different cognitive processes. These event-related measures in the frequency domain have - to our knowledge - not yet been applied to analyse the EEG of dyslexic children and may provide new insights in the physiological basis of dyslexic deficits.

The main goal of the project was to contribute to the scientific understanding of the neurocognitive dysfunctions underlying dyslexia. Altogether 13 peer-reviewed publications resulted from the project, with some publications still in preparation. Besides the EEG-studies we published studies on neuropsychological deficits (e.g., eye movement recordings) and lately we published magnetic resonance imaging (fMRI) findings on functional neuroanatomical dysfunctions. Important findings: The EEG-studies revealed dyslexic abnormalities in the fast electro-physiological brain response to visual words and in the power of certain frequency bands during reading. These EEG abnormalities suggest a sluggish brain response to visual words. The fMRI-data showed a dyslexic underactivation of a specific regions in the left occipito-temporal cortex which is assumed to connect visual brain regions with phonological and semantic brain regions. The occipitotemporal underactivation was accompanied by overactivation in the left frontal and in subcortical regions in correspondence with the high effort experienced by dyslexic readers. In correspondence with this interpretation were eye movement data showing short forward movements and long fixation durations (but few regressive saccades) as hallmark of dyslexic reading. Consistent with these eye movement findings were data showing a deficit in fast simultaneous visual processing of multiple visual forms.