TITLE:
Electrophysiological Evidence against the Magnocellular Deficit Theory in Developmental Dyslexia
AUTHORS:
Melissa Sue Sayeur, Renée Béland, Dave Ellemberg, Caroline Perchet, Michelle McKerral, Maryse Lassonde, Karyne Lavoie
KEYWORDS:
Dyslexia; Magnocellular and Parvocellular Pathways; Visual Evoked Potentials; Contrast Sensitivity; Spatial Frequencies
JOURNAL NAME:
Journal of Behavioral and Brain Science,
Vol.3 No.2,
May
24,
2013
ABSTRACT: 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.