Electrophysiological evidence for the magnocellular-dorsal pathway deficit in dyslexia.

In adults, the onset of coherent motion compared to random motion in a random dot kinematogram leads to a right hemispheric amplitude advantage of the N2 response. The source of this asymmetry is believed to lie in the motion selective MT+ cortex. Here, we tested whether the right tempo-parietal N2 component shows a similar regularity in children. In particular, we were interested in whether coherent vs. incoherent motion modulates the amplitude of N2 similarly in dyslexic and control children. We found higher N2 amplitude for coherent compared to random motion in the right hemisphere for controls but not for dyslexics. This effect was related to topographical differences of N2 amplitude for random motion between the studied groups and was accompanied by longer reaction times to random motion in dyslexic compared to control children. Furthermore, a negative correlation between the amplitude of N2 for random motion and spelling errors was observed in both groups, which is consistent with previous findings linking the magnocellular-dorsal (MD) pathway with orthographic skills. These data support the hypothesis of subtle deficiencies in the MD pathway in dyslexia.