Early dysfunction of perceptual processes in developmental dyslexia

Objective: Single-letter reading is an early predictor of later reading success and identifies adult dyslexics [1]. This work investigates the differences between healthy and dyslexic children in brain activity during single-letter reading. Methods: Event-related potentials (ERPs) were recorded from 45 healthy and 45 dyslexic children (age range 8–10 years) during self-paced letter recognition (SPLR), i.e. active reading aloud of letters presented at a self-determined pace. Standardized Low Resolution Electromagnetic Tomography (sLORETA) [2] was applied to individual ERPs and unpaired voxel-by-voxel t-test analysis (P < 0.05) was performed to compare the sLORETA maps between groups of subjects corresponding with the latency of the main grand average peaks. Results: At middle latencies, dyslexics were characterized by a greater activation in the right middle-inferior frontal, insular and superior temporal regions and by impaired engagement of the left middle parietal lobe as compared to the healthy controls. At middle-long latencies, a significantly reduced activation in the left occipital gyrus was observed in dyslexic children compared to controls. Conclusions: Distributed source imaging applied to ERPs describes the spatiotemporal dynamics of cerebral reading functions and allows inference regarding at which stage of the reading process the activation of the involved brain regions is disrupted in impaired readers. The existence of specific compensatory mechanisms may explain why dyslexic children who are characterized by impaired activation of left parietal and occipital regions show a significantly greater involvement of right temporal–parietal regions and premature engagement of right middle–inferior frontal regions as compared to controls