Author(s)

Melissa Sue Sayeur, Renée Béland, Dave Ellemberg, Caroline Perchet, Michelle McKerral, Maryse Lassonde, Karyne Lavoie

National Institute of Health and Medical Research, Montreal, France.
Research Centre in Neuropsychology and Cognition, University of Montreal, Montreal, Canada.
Research Centre, Sainte-Justine Hospital, Montreal, Canada.

Over the last two decades, the hypothesis of a magnocellular deficit in dyslexia has raised considerable interest and controversy. Using an electrophysiological procedure (visual evoked potentials, VEP), we compared magnocellular and parvocellular contrast and spatial frequency-response functions between phonological dyslexics (n = 16) and a typical reading group (n = 12) matched for age and socioeconomic background. No significant differences were found between the two groups in the amplitude of the VEP components associated with either magnocellular or parvocellular responses. However, topographic analyses revealed a group difference in the distribution of amplitude in the right frontal and left temporal regions, which appeared to be underactivated in dyslexics. These results suggest a deficit in the higher-level cortical regions involved in phonological and/or linguistic processing, and calls into question the notion of a magnocellular involvement in dyslexia.

Dyslexia; Magnocellular and Parvocellular Pathways; Visual Evoked Potentials; Contrast Sensitivity; Spatial Frequencies

M. Sayeur, R. Béland, D. Ellemberg, C. Perchet, M. McKerral, M. Lassonde and K. Lavoie, "Electrophysiological Evidence against the Magnocellular Deficit Theory in Developmental Dyslexia," Journal of Behavioral and Brain Science, Vol. 3 No. 2, 2013, pp. 239-251. doi: 10.4236/jbbs.2013.32025.