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Abstract
We investigated the relation between cognitive processing speed and structural properties
of white matter pathways via convergent imaging studies in healthy and brain-injured
groups. Voxel-based morphometry (VBM) was applied to diffusion tensor imaging data
from thirty-nine young healthy subjects in order to investigate the relation between
processing speed, as assessed with the Digit-Symbol subtest from WAIS-III, and fractional
anisotropy, an index of microstructural organization of white matter. Digit-Symbol
performance was positively correlated with fractional anisotropy of white matter in
the parietal and temporal lobes bilaterally and in the left middle frontal gyrus.
Fiber tractography indicated that these regions are consistent with the trajectories
of the superior and inferior longitudinal fasciculi. In a second investigation, we
assessed the effect of white matter damage on processing speed using voxel-based lesion-symptom
mapping (VLSM) analysis of data from seventy-two patients with left-hemisphere strokes.
Lesions in left parietal white matter, together with cortical lesions in supramarginal
and angular gyri were associated with impaired performance. These findings suggest
that cognitive processing speed, as assessed by the Digit-Symbol test, is closely
related to the structural integrity of white matter tracts associated with parietal
and temporal cortices and left middle frontal gyrus. Further, fiber tractography applied
to VBM results and the patient findings suggest that the superior longitudinal fasciculus,
a major tract subserving fronto-parietal integration, makes a prominent contribution
to processing speed.