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 ca udal infrasylvian surface(s) (cIS) in dyslexic and control subjects. Backgr ound: Several quantitative morphometric investigations of cortical areas in dyslexia have focused on the cIS, which encompasses the supratemporal plan e and the inferior bank of the posterior ascending ramus of the sylvian fis sure. Inconsistencies in the results of these studies may be attributable i n part to the use of measurement methods that do not account fully for surf ace undulations of the cIS. Methods: The authors used an MRI-based surface reconstruction technique that models the curvature of the cerebral cortex i n three dimensions to obtain whole-hemisphere and regional surface area est imates. Measurements were obtained in both hemispheres of eight right-hande d male dyslexic subjects and eight right-handed male control subjects. Resu lts: The cIS area of dyslexic subjects was significantly larger than that o f control subjects, and this result was not attributable to a difference in whole-hemisphere surface area. Neither the dyslexic nor control subjects s howed a left or right asymmetry in this region, although there was a trend toward less variance of the asymmetry scores in dyslexic subjects. Conclusi ons: The gross anatomic organization of this region is different in dyslexi c subjects, and elucidation of the precise nature of these differences may be aided by surface modeling techniques.