Visual differences are a consequence, not a cause, of dyslexia

In an article to appear 10 July 2013 in Neuron, Olumide Olulade, Eileen Napoliello, and Guinevere Eden present a series of studies that greatly help educators, psychologists, neurologists, and others understand the relationship between visual deficits and dyslexia. Although most people interested in reading have understood that problems with phonological processes undergird dyslexia, personal accounts of those with dyslexia and some anomalous evidence about the visual cortex and the performance of individuals with dyslexia on certain visual tasks kept the possibility of a visual component open to debate. Professor Eden’s group devised studies and collected the data that shed light on these issues.

In a nutshell, in their first study, Eden’s team found the same results that others had found: When their participants with dyslexia were compared to similar aged children, they showed certain deficits in visual processing associated with a particular part of the brain shown by fMRI. However, when their participants were compared with younger children of like reading ability, there are no deficits in the visual performance; so, these children must not have had the visual problems all along. In their third study, the researchers provided even stronger evidence: The provided powerful remedial reading instruction to their participants and they observed not only improved reading outcomes, but they also found that the students had improved performance on the visual tasks as reflected in fMRI. (Click the accompanying image for a movie of Professors Eden and Olulade explaining the experiments.)

Here is the abstract:

Developmental dyslexia is a reading disorder, yet deficits also manifest in the magnocellular-domi- nated dorsal visual system. Uncertainty about whether visual deficits are causal or consequential to reading disability encumbers accurate identifica- tion and appropriate treatment of this common learning disability. Using fMRI, we demonstrate in typical readers a relationship between reading ability and activity in area V5/MT during visual motion pro- cessing and, as expected, also found lower V5/MT activity for dyslexic children compared to age- matched controls. However, when dyslexics were matched to younger controls on reading ability, no differences emerged, suggesting that weakness in V5/MT may not be causal to dyslexia. To further test for causality, dyslexics underwent a phonolog- ical-based reading intervention. Surprisingly, V5/MT activity increased along with intervention-driven reading gains, demonstrating that activity here is mobilized through reading. Our results provide strong evidence that visual magnocellular dysfunc- tion is not causal to dyslexia but may instead be consequential to impoverished reading.

Olulade, O. A., Napoliello, E. M., & Eden, G. F. (2013). Abnormal visual motion processing is not a cause of dyslexia. Neuron, 79, 1-11. doi:10.1016/j.neuron.2013.05.002

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