ANNEX BOX 3–2 Neural Bases of Dyslexia
Recent studies using brain imaging techniques suggest that dyslexia is in some degree due to specific abnormalities in the way the brain processes visual and verbal language information (see Thompson, 2000). Guenevere Eden and associates at the National Institute of Mental Health used functional magnetic resonance imaging (fMRI) to examine the extent of brain activation in area V5/MT—an area particularly involved in the perception of movement—in response to moving stimuli in dyslexic men and normal control subjects. The control group showed substantial activation in this area, while the dyslexic subjects did not. In contrast, presenting the subjects with stationary patterns resulted in equivalent activations in other visuocortical areas in each group. A key point here is that area V5/MT is a part of the magnocellular visual system, which is critical to normal perception of motion. Perceptual studies suggest that dyslexics are deficient in motion detection.
A study at the National Institute on Aging used positron emission tomography (PET) to study the degree of activation of the angular gyrus, relative to occipital regions, during reading in normal and dyslexic men. In the normal subjects, there was a strong correlation between activation (i.e., increased blood flow) in the angular gyrus and occipital regions. In the dyslexic group, by contrast, there appeared to be a disconnection between the angular gyrus and the occipital regions; there was no correlation between changes in blood flow in the two regions. Additional PET studies of reading tasks (Shaywitz et al., 2000) also found striking differences between dyslexic and nondyslexic subjects in the degree of activation of different brain areas.
Studies conducted by Merzenich, Tallal, and colleagues showed that