9. Evidence from diffusion tensor imaging

A "disconnection syndrome" in which functional connectivity of the relevant cortical networks in the left hemisphere is disrupted has been proposed as a potential basis for reading difficulties [82]. Diffusion Tensor Imaging (DTI), a technology similar to fMRI, allows probing the distance and direction of water molecule movement in the brain, producing form and orientation information about the underlying white matter structures [83]. White matter exhibits anisotropic water movement, with water molecules showing various degrees of diffusion in each direction. In typical DTI studies, diffusion images from at least six directions are analyzed using an ellipsoid tensor model—a symmetrical 3 3 matrix. Parallel and perpendicular diffusivities are then calculated and used to estimate properties of underlying tissues [84]. DTI has demonstrated a correlation between the microstructural integrity of the left temporo-parietal white matter and reading ability in dyslexic and control adults [85]. It seems that this technology could be instrumental in measuring not only the degree of connectedness between crucial brain features, but also in determining the amount of pressure needed by these systems to change functioning.

Fractional anisotropy (FA) is a related technology that is used to index structural information regarding a brain area. It measures the anisotropy of the diffusion of water molecules [86] and is sensitive to axonal density, size, myelination, and the coherence of organization of fibers within a voxel, thus providing an index of the structural integrity of white matter. FA is measured from 0 (isotropic diffusion) to 1 (anisotropic diffusion) [83]. Beaulieu et al. propose that FA may be reduced in
