BRAIN (A)SYMMETRY IN MONOZYGOTIC TWINS

Origin and ontogenesis of human brain laterality are unknown. Using in vivo magnetic resonance morphometry we measured cerebral hemispheric asymmetry of the planum temporale, a structural substrate of functiona l laterality, in pairs of monozygotic twins concordant or discordant f or handedness. In both groups, intraclass (i.e., within twin pair) cor relations were low. The right-handers showed leftward asymmetry wherea s the left-handers lacked asymmetry. The discordance for lateralized b rain anatomy can be accounted for by ontogenetic models assuming twinn ing of an asymmetrical germ or differential action of nongenetic facto rs within twin pairs in utero. The findings confirm a coupling of late ralized structure and function of the human brain. At least in monozyg otic twins, early epigenetic factors must play a role in anatomofuncti onal laterality development.