A magnetic resonance imaging study of planum temporale asymmetry in men with developmental dyslexia

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.

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.

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.

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.

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.