Directly Testing the Magnocellular Hypothesis of Dyslexia with High-Resolution Functional Magnetic Resonance Imaging of the Human Lateral Geniculate Nucleus

Lay Summary

New fMRI Technique May Reveal Brain Basis of the Learning Disorder Dyslexia

Researchers will use a new functional magnetic resonance imaging (fMRI) technique to explore whether people with dyslexia, a learning disability, have a visual processing impairment in a specific brain region that impedes their reading abilities.

Dyslexia is a common reading disability.  Some evidence indicates that dyslexia may result from abnormalities in the functioning of the brain’s “lateral geniculate nucleus” (LGN).  The LGN is a six-layered structure deep in the brain that serves as the first way-station in the neural pathway leading from the retina to higher brain regions.  A prior autopsy study suggested that the so-called “magnocellular” layers of the LGN are smaller than normal in people with dyslexia, and the Rochester researchers hypothesize that dyslexia is associated with a defect in the processing of visual information by these magnocellular LGN layers.  The layers of the LGN are so thin, however, that standard fMRI techniques do not have sufficient resolution to enable researchers to compare LGN function in people with dyslexia and healthy volunteers.

To address this problem, the investigators will develop a new fMRI data processing technique called “super-resolution” and refine their fMRI data collection methods accordingly.  Through this technique, the investigators anticipate, they will be able to visualize the activity of each layer of the LGN individually.  They will use this fMRI imaging technique to compare the responses of the six LGN layers to visual stimulation in 15 young adults aged 16-22 with dyslexia and 15 age-matched healthy study participants.  They will determine whether the responses to visual stimulation by the magnocellular LGN layers differ in those with dyslexia compared to healthy participants.

Abstract

Directly Testing the Magnocellular Hypothesis of Dyslexia with High-Resolution Functional Magnetic Resonance Imaging of the Human Lateral Geniculate Nucleus

The goal of this study is to develop new high-resolution functional magnetic resonance imaging (fMRI) acquisition and data analysis techniques to measure whether the magnocellular layers of the lateral geniculate nucleus (LGN) are structurally or functionally abnormal in people with dyslexia.

Dyslexia is a common reading-specific learning disorder whose neurological basis may predominantly involve the magnocellular stream, according to the prominent but controversial magnocellular theory of dyslexia. A previous study in post-mortem brains found significant anomalies in the magnocellular layers of the LGN in a group of subjects with dyslexia, but this study has not been replicated, behavioral data on these subjects were unavailable, and the functional implications of the deficits remain unclear. Subsequent studies have only indirectly tested the magnocellular hypothesis using behavioral tests or fMRI in the cortex; however the abilities of these tests to isolate magnocellular function are limited.

The only way to directly test the magnocellular hypothesis is to imaging the magnocellular layers of the human LGN, which is the only location in the visual pathway where the magnocellular stream is segregated. Currently the ubiquity of the magnocellular explanation is uncertain, and a direct test of the magnocellular hypothesis would have important ramifications in the understanding, treatment and diagnosis of dyslexia.