Surface area accounts for the relation of gray matter volume to reading-related skills and history of dyslexia

Abstract

It is unknown whether the abnormalities in brain structure and function observed in dyslexic readers are congenital or arise later in development. Analyzing the 2 components of gray matter volume separately may help in differentiating these possibilities. Gray matter volume is the product of cortical surface area, determined during prenatal brain development, and cortical thickness, determined during postnatal development. For this study, 16 adults with a history of phonological dyslexia and 16 age- and gender-matched controls underwent magnetic resonance imaging and an extensive battery of tests of reading-related skills. Cortical surface area and gray matter volume measures of the whole brain, the inferior frontal gyrus, and the fusiform gyrus were similarly related to phonological skills and a history of dyslexia. There was no relationship between cortical thickness and phonological skills or history of dyslexia. Because cortical surface area reflects cortical folding patterns determined prenatally, this suggests that brain differences in dyslexia are rooted in early cortical development and are not due to compensatory changes that occur during postnatal development and would be expected to influence cortical thickness. This study demonstrates the importance of examining the separate components of gray matter volume when studying developmental abnormalities.

Figure 1.

Relations between reading-related tasks and cortical gray matter volume (A–E), surface area (F–J), and thickness (K–O) for whole brain (A, F, K), the inferior frontal (B–C, G–H, L–M), and fusiform (D–E, I–J, N–O) gyri. Relations are depicted for dyslexic and typical readers separately for gray matter volume and surface area to demonstrate the effect of both the group difference and influence of the whole-brain covariate for the inferior frontal and fusiform gyri. Note that also the relations between reading-related skills and regional gray matter volume and surface area are the same across reading groups (expect for the relationship between alternative phonological awareness and inferior frontal gyrus gray matter volume and surface area); the influence of the whole-brain covariate influences these relations. Relations between reading-related tasks and cortical morphology were not different across hemispheres, so each data point is averaged across the 2 hemispheres.

Figure 2.

Average gray matter volume, surface area, and thickness differences across hemisphere, gender, and reading groups for whole brain (A–C) and regional brain areas (D–F).