The magnocellular theory of dyslexia.pdf

Developmental Dyslexia

Early Precursors, Neurobehavioral Markers, and Biological Substrates

Edited by

April A. Benasich, Ph.D.
Center for Molecular and Behavioral Neuroscience
Rutgers, The State University of New Jersey

and
R. Holly Fitch, Ph.D.
University of Connecticut

Contents

I Brain Development, Genes, and Behavior Phenotypes

Introduction

R. Holly Fitch

  1. Overview of Early Brain Development: Linking Genetics to Brain Structure
    Richard S. Nowakowski and Nancy L. Hayes
  2. Loss of the Dyslexia Susceptibility Gene DCDC2 Increases Synaptic Connectivity in the Mouse Neocortex
    Joseph LoTurco, Aarti Tarkar, and Alicia Yue Che
  3. The Magnocellular Theory of Dyslexia
    John Stein
  4. Investigation of Candidate Genes in Families with Dyslexia
    Cecilia Marino, Sara Mascheretti, Andrea Facoetti, and Massimo Molteni
  5. What Educators Should Know About the State of Research on Genetic Influences on Reading and Reading Disability
    Elena L. Grigorenko

II Potential Early Precursors of Specific Language Impairment and Dyslexia

Introduction

April A. Benasich
6. Biological Factors Contributing to Reading Ability: Subcortical Auditory Function
Bharath Chandrasekaran and Nina Kraus
7. Timing, Information Processing, and Efficacy: Early Factors that Impact Childhood Language Trajectories
April A. Benasich and Naseem Choudhury
8. Neurolinguistic and Neurophysiological Precursors of Dyslexia: Selective Studies from the Dutch Dyslexia Programme
Ben A.M. Maassen, Aryan van der Leij, Natasha M. Maurits, and Frans Zwarts
9. Phonology and Literacy: Follow-Up Results of the Utrecht Dyslexia and Specific Language Impairment Project
Elise de Bree, Margaret J. Snowling, Ellen Gerrits, Petra van Alphen, Aryan van der Leij, and Frank Wijnen

III Potential Neurobehavioral Markers and Biological Mechanisms of Specific Language Impairment and Dyslexia

Introduction

R. Holly Fitch
10. Cortical Phenotypes Associated with Developmental Dyslexia: Reverse and Forward Genetic Approaches Using Animal Models
Glenn D. Rosen
11. Using Animal Models to Dissociate Genetic, Neural, and Behavioral Contributors to Language Disability
12. Prediction of Children’s Reading Skills: Understanding the Interplay Among Environment, Brain, and Behavior
Jessica M. Black and Fumiko Hoeft
13. A Multifactorial Approach to Dyslexia
Cyril R. Pernet and Jean-François Démonet

IV Using Developmental Neuroimaging for Identification, Intervention, and Remediation

Introduction

April A. Benasich
14. Evolution of Pediatric Neuroimaging and Application of Cutting-Edge Techniques
15. Anatomical Risk Factors for Reading Comprehension
16. Windows into Receptive Processing
17. Neural Correlates of Reading-Related Processes Examined with Functional Magnetic Resonance Imaging Before Reading Onset and After Language/Reading Remediation
Nora Maria Raschle, Michelle YH Chang, Patrice L. Stering, Jennifer Zuk, and Nadine Gaab
18. Transcending Gaps Among Disciplines in Neurodevelopmental Disorders: From Brain Volumetrics to Collaborative Multisystem Assessment
Martha R. Herbert
19. Integration of Left-Lateralized Neural Systems Supporting Skilled Reading
Bruce D. McCandliss and Yuliya N. Yoncheva

Conclusion/Next Steps: Critical Research Directions and Priorities

Peggy McCardle and Brett Miller

About the Editors

April A. Benasich, Ph.D.
Professor of Neuroscience, Director of the Infancy Studies Laboratory, Center for Molecular and Behavioral Neuroscience, Rutgers, The State University of New Jersey, 197 University Avenue, Newark, NJ 07102

Research Background

Her research focuses on the study of early neural processes necessary for typical and disordered language development. Specifically, she studies the development of temporally bounded sensory information processing that predicts language impairment and dyslexia.

R. Holly Fitch, Ph.D.
Associate Professor, Behavioral Neuroscience Division, Department of Psychology, University of Connecticut, 406 Babbidge Road, Unit 1020, Storrs, CT 06269

Research Background

Her research centers on understanding how disruption of early brain development underlies subsequent cognitive disabilities, especially risk factors for language-relevant skills.

CHAPTER 3

The Magnocellular Theory of Dyslexia

VISUAL REQUIREMENTS OF READING

Eighteen percent of students who exit United States (U.S.) schools can only read and write at a level one, a very low level (Fleischman, Hopstock, Pelczar, and Shelley, 2010). This not only consigns such students to risk for later failure, such as very low-paying jobs, unemployment, and criminality, but it is also an appalling waste of talent, because in other respects these students are typically within the normal range of intelligence. They fail because reading is difficult, the most difficult skill that most people ever have to acquire.
Reading is difficult because it requires visual analysis of letters and their order and translation of those letters into sounds. In parallel, it requires learning the phonological structure of a word and recognizing that continuously spoken words can be split down into phonemes.
Although there is current emphasis on learning phonological skills, the very first step in reading relies on visual analysis of the text. A large proportion of the primary information processing required for reading is visual.

What, more precisely, are the visual requirements of reading?

Letters have to be identified correctly; so it is often assumed that the crucial visual process for reading is the system that specializes in object identification. This depends on small neurons (parvocellular [P] neurons) that constitute 90% of retinal ganglion cells. They signal the fine detail and color of visual targets to the ventral or ”what” route that passes from the primary visual cortex toward the visual word form area situated in the anterior part of the fusiform gyrus.

VISUAL-MAGNOCELLULAR NEURONS

It is equally important to be able to sequence letters in the right order. People with dyslexia are less accurate and slower at sequencing letters than identifying each letter individually. Correct letter sequencing depends on properties of the other main visual subsystem, the magnocellular (M) system. The M neurons form only 10% of the ganglion cells in the retina, but they are specialized for timing visual processing and guiding attention.

The M cells project via the magnocellular layers of the lateral geniculate nucleus (LGN) in the thalamus to the primary visual cortex, helping to guide visual attention and eye movements.

MAGNOCELLULAR IMPAIRMENTS IN DYSLEXIA

Research shows that 90% of studies since 2000 that sought evidence for M impairment in dyslexia have found significant results. Visual M cells can only be rigorously defined in the subcortical visual system.

EYE MOVEMENT CONTROL BY THE DORSAL STREAM

Numerous studies have found that not only the direction of visual attention is disturbed in people with dyslexia, but also that their eye control during reading is poor. Abnormal eye control during reading comes from difficulties understanding the text, leading to longer fixations and more reinspections of previous letters.

EVENT-RELATED POTENTIALS

Event-related potential (ERP) studies in dyslexia show weaker responses to moving, low contrast targets than good readers.

AUDITORY TRANSIENT PROCESSING

Identifying and ordering the sequences of sounds that make up speech relies on detecting changes in sound frequency and amplitude. There are auditory equivalents of the visual magnocellular and parvocellular systems that influence reading ability.

CONCLUSIONS

Genetic, developmental, nutritional, and psychophysiological evidence suggest that phonological reading problems may correlate with impaired development of magnocellular systems throughout the brain. Definitive proof will emerge as we better understand genetic influences on development.