Brain asymmetries and sex differences in developmental dyslexia

thirty years ago, geschwind and galaburda (1985a, b, c) hypothesized complex links among genetic factors, prenatal environment, sex, brain asymmetries, and the susceptibility to developmental disorders, in par-ticular, developmental dyslexia. this chapter reviews studies our team conducted on the neuroanatomy of developmental dyslexia. among other results, we replicated galaburda’s original finding of an altered asymme-try of the planum temporale (PT) in dyslexia by using magnetic resonance imaging (MRI) in a large group of children. this difference in asymmetry, however, was only found in boys. we observed that the cortical region ded-icated to visual word recognition (located in the left hemisphere) is thicker in readers who do not have dyslexia than in readers who have dyslexia, but only among girls. we also found that two white matter tracts connect-ing posterior and anterior regions of the brain and participating in reading show different hemispheric asymmetry patterns in children with dyslexia and control children. finally, we discovered a difference in the asymmetry of the depth of the central sulcus (a major fold of the brain) between chil-dren with dyslexia and control children, with a different pattern in boys and in girls. overall, we found that when individuals with dyslexia and control individuals differ in brain anatomy, the differences depend on the cerebral hemisphere, and they are not the same in males and females. In other words, brain asymmetry seems key to understanding the neuroana-tomical basis of dyslexia, and this neuroanatomical basis seems to partly differ between the sexes. Several possible explanations are discussed that are consistent with geschwind and galaburda’s original ideas.