A magnetic resonance imaging study of planum temporale asymmetry in men with developmental dyslexia
J. M. Rumsey, B. C. Donohue, D. R. Brady, K. Nace, J. N. Giedd and P. Andreason
Child Psychiatry Branch, National Institute of Mental Health, Bethesda, Md., USA.
BACKGROUND: Imaging studies have suggested anomalous anatomical asymmetries
in language-related regions of the temporal and parietal lobes in
individuals with developmental dyslexia. Autopsy studies have reported
unusual symmetry of the planum temporale (PT) in patients with dyslexia.
Methodological limitations characterize much of this literature, however.
OBJECTIVE: To examine the size and asymmetry of the PT and its extension
into the parietal lobe (planum parietale [PP]) in men with
well-characterized, persistent dyslexia by using magnetic resonance imaging
and 3-dimensional surface rendering techniques. METHODS: The brains of 16
right-handed dyslexic men aged 18 to 40 years and 14 matched control
subjects were studied with magnetic resonance imaging. Most of these
subjects were previously studied with positron emission tomography, which
demonstrated functional abnormalities in temporal and parietal brain
regions in the dyslexic group. The area of the PT was determined with the
aid of 3-dimensional surface-rendering techniques. The size of the PP was
estimated by measuring the length of the posterior ascending ramus on 3
parasagittal slices. RESULTS: Approximately 70% to 80% of both groups
showed equivalent leftward (left > right) asymmetries of the PT;
approximately 50% to 60% showed equivalent rightward (right > left)
asymmetries of the PP. These asymmetries showed equivalent moderate inverse
correlations with each other in both groups. CONCLUSIONS: These results
challenge the notion that anomalous asymmetry of the PT is strongly
associated with developmental dyslexia. Given the heterogeneity of the
dyslexic population, some subgroup of dyslexic individuals (i.e., those
with developmental language disorders) may show unusual symmetry or
reversed asymmetry in this region. However, anomalous asymmetry of the
planum did not contribute to functional abnormalities demonstrated in these
patients by positron emission tomography.
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