The caudal infrasylvian surface in dyslexia: novel magnetic resonance imaging-based findings.
OBJECTIVE: To detect anatomic abnormalities of auditory association cortex in dyslexia by measuring the area of the perisylvian region known as the caudal infrasylvian surface(s) (cIS) in dyslexic and control subjects. BACKGROUND: Several quantitative morphometric investigations of cortical areas in dyslexia have focused on the cIS, which encompasses the supratemporal plane and the inferior bank of the posterior ascending ramus of the sylvian fissure. Inconsistencies in the results of these studies may be attributable in part to the use of measurement methods that do not account fully for surface undulations of the cIS. METHODS: The authors used an MRI-based surface reconstruction technique that models the curvature of the cerebral cortex in three dimensions to obtain whole-hemisphere and regional surface area estimates. Measurements were obtained in both hemispheres of eight right-handed male dyslexic subjects and eight right-handed male control subjects. RESULTS: The cIS area of dyslexic subjects was significantly larger than that of control subjects, and this result was not attributable to a difference in whole-hemisphere surface area. Neither the dyslexic nor control subjects showed a left or right asymmetry in this region, although there was a trend toward less variance of the asymmetry scores in dyslexic subjects. CONCLUSIONS: The gross anatomic organization of this region is different in dyslexic subjects, and elucidation of the precise nature of these differences may be aided by surface modeling techniques.