Motion-onset VEPs in dyslexia. Evidence for visual perceptual deficit.

The magnocellular deficit theory is one of the prominent theories in dyslexia. However, recent studies have produced conflicting results. In order to assess the validity of this theory, 8 dyslexic children and 14 controls were examined with a motion-onset visual evoked potential (VEP) paradigm at three different velocities (2, 8, and 16 deg/s). VEP elicited by stationary gratings served as a control condition. Amplitudes of motion-onset VEP components (P100, P200) but not of the stationary VEP are significantly attenuated in dyslexic children. Further, there is an interaction of group and velocity for the P200 in the way that group differences are more pronounced for higher velocities than for lower velocities. These results support the hypothesis of an impairment of a specific magnocellular function in dyslexia.