Do visual neurophysiological tests reflect magnocellular deficit in dyslexic children?

To address the question of a possible magnocellular visual deficit in children with reading problems (dyslexia), we examined pattern ERG and VEP responses to stimulation with checks of 24', 49' and 180' in size and of 5%, 42% and 100% contrast level. Neurophysiological difference between children with reading problems and those without them was found confined to VEP which showed a significant prolongation of P100 wave in dyslexic children at highest contrast (100%) and smallest checks (24'). Pattern ERG was normal. These results support the assumption of a visual deficit in dyslexic children. However, they are not consistent with an isolated deficit of the magnocellular function, which, theoretically, would cause VEP changes to lower contrast and largest check stimuli.