Corpus Callosum Morphology, MRI, in Dyslexic Men as Measured with Judith M. Rumsey, Manuel Casanova, Glenn B. Mannheim, Nicholas Patronas, Nathan DeVaughn, Susan D. Hamburger, and Tracy Aquino To test the hypothesis of anomalous anatomy in posterior brain regions associated with language and reading, the corpus callosum was imaged in the midsagittal plane with magnetic resonance. The areas of the anterior, middle, and posterior segments were measured in 21 dyslexic men (mean age 27 yrs, SD 6) and in 19 matched controls. As predicted, the area of the posterior third of the corpus callosum, roughly equivalent to the isthmus and splenium, was larger in dyslexic men than in controls. No differences were seen in the anterior or middle corpus callosum. The increased area of the posterior corpus callosum may reflect anatomical variation associated with deficient lateralization of function in posterior language regions of the cortex and their right-sided homologues, hypothesized to differ in patients with dyslexia. Key Words: MRI, dyslexia, corpus callosum BIOL PSYCHIATRY 1996;39:769--775 Introduction Decades of behavioral research support a contemporary view of dyslexia as a form of language disorder involving phonological deficits, making the left temporoparietal cortex a site of particular interest (Rumsey 1992). Emerg- ing anatomical and physiological data support this view, and neuroimaging techniques now offer an opportunity for further exploration and clarification of the biological basis of dyslexia. Autopsy and structural and functional neuroimaging studies support the involvement of posterior brain regions From the Child Psychiatry Branch, National Institute of Mental Health, Bethesda, MD (JMR, GBM, SDH, TA), the Department of Psychiatry and Health Behavior, Medical College of Georgia, Augusta, Georgia (MC, NDV), and the Clinical Center, National Institutes of Health, Bethesda, MD (NP). Address reprint requests to Dr. Judith M. Rumsey, Child Psychiatry Branch, National Institute of Mental Health, Building 10, Room 6N240, Bethesda, MD 20892. Received August 26, 1994; revised April l 1, 1995. subserving language function in developmental dyslexia (Duara et al 1991; Galaburda et al 1985; Hagman et al 1992; Humphreys et al 1990; Hynd et al 1990; Kushch et al 1993; Larsen et al 1990; Leonard et al 1993; Rumsey et al 1992, 1994a). Such studies have reported an absence or reduced frequency of the usual left-greater-than-right asymmetry of the planum temporale. Galaburda et al (1985) hypothesized a reduction in cell death during fetal development as a causative mechanism, resulting in an excess of neurons in the fight planum. Should these neurons form connections by sending their axons through the corpus callosum, an increase in its size should result. Because neuronal axons from the temporal lobes cross in the isthmus (Witelson 1989), such an increase would likely be localized to the posterior portion of the corpus callosum. Thus, the corpus callosum, which subserves interhemi- spheric communication and integration, is of interest in dyslexia. It is clearly imaged and easily measured on © 1996 Society of Biological Psychiatry 0006-3223/96/$15.00 SSDI 0006-3223(95)00225-6