Attenuated hemispheric lateralization in dyslexia: evidence of a visual processing deficit.

There is controversial evidence that deficits in the processing of low contrast and low spatial frequency stimuli are of importance in the pathogenesis of dyslexia. Fifteen adult dyslexics and 19 controls were examined using visual evoked potentials (VEP) at varying spatial frequencies (2 and 11.33 cpd) and contrasts (0.2, 0.4, 0.6, 0.8). Our results show that the amplitude of VEPs following different spatial frequencies and contrasts did not differentiate between dyslexics and controls. Further, we found significantly higher amplitudes of the P1 and P2 over the right occipital cortex. For the P2, this hemispheric asymmetry was not found in the dyslexic group suggesting a specific low level visual processing deficit in the right occipital region in dyslexia.