Electrophysiological Evidence against the Magnocellular Deficit Theory in Developmental Dyslexia

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

doi:10.4236/jbbs.2013.32025

Melissa Sue Sayeur, Michelle Mckerral + Show 5 more

Open Access

https://doi.org/10.4236/jbbs.2013.32025

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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.

Highlights

A substantial proportion of children (15% - 20%) have a specific reading disability [1,2]
The main objective of this study was to verify the hypothesis of a magnocellular deficit in developmental dyslexia
Using visual evoked potential (VEP) recording, we dissociated the activity of the M and P pathways

Summary

Introduction

A substantial proportion of children (15% - 20%) have a specific reading disability [1,2]. Much of the recent evidence indicates that dyslexia could result from a deficit in phonological processing. Impaired phonological processing would result from a dysfunction of the neuronal circuits that are responsible for establishing spelling-to-sound correspondences in reading acquisition. Genetic linkage studies have found a locus on chromosome 2 for the transmission of deficits in phonological awareness and subsequent reading difficulties [8]. Numerous studies have shown that phonological skills in pre-school children are a good predictor of their later reading proficiency [9,10,11,12,13,14,15]

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