We measured transient and steady-state checkerboard contrast-reversal visual evoked potentials (VEPs) in 10 dyslexics, 5 patient controls, and 11 normals over a range of contrasts and luminances. Latency, amplitude, and phase measurements failed to distinguish the responses of dyslexics from those of normals or patient controls. Decreases in luminance or contrast resulted in an increased latency of the transient VEP in all groups, but these changes also did not distinguish the responses of dyslexics from those of the controls. Response variability was similar in dyslexics and normals, but was increased in subjects with attention deficit-hyperactivity disorder (ADHD). Performance on standardized psychometric testing did differentiate the dyslexics from controls, but did not correlate with VEP responses.
Comment
In this paper, we attempt to replicate the often-cited study of Livingstone et al. (Livingstone, M.S., Rosen, G.D., Drislane, F.W., & Galaburda, A.M. (1991). Physiological and anatomical evidence for a magnocellular defect in developmental dyslexia. Proceedings of the National Academy of Sciences (USA), 88, 7943-7947). We found that VEP responses reported by Livingstone et al. to be selectively absent in dyslexics were also absent in normal subjects.
The reasons for this discrepancy are not entirely clear. We suspect that one important difference is that we used a rigorous signal-detection procedure.
We emphasize that our negative results do not mean that we have ruled out a role for visual sensory abnormalities in dyslexia. However, our findings make it unlikely that a simple loss of magnocellular function readily manifest in the VEP is causally and specifically related to dyslexia.