Cerebral asymmetries in monozygotic twins: an fMRI study

Lateralization for language, spatial judgment and face processing was assessed in 42 pairs of identical twins, 21 discordant and 21 concordant pairs for handedness, using fMRI. Individual laterality indices were calculated based on the observed activation patterns. All tasks showed expected asymmetry, favoring the left-hemisphere for language and the right-hemisphere for spatial judgment and face processing. The intra-class correlations on the laterality indices were significant only on the language task and only for the concordant group, but not the discordant group, suggesting a stronger genetic influence for language asymmetry in concordant twins. The expected asymmetry was greater for the concordant group only on the language task. The difference was not significant, but conformed quite well to Annett's genetic model, which assumes a right-shift (RS) gene with one allele (RS+) biasing toward right-handedness and left-cerebral language dominance, and the other (RS-) leaving both asymmetries to chance. The model also assumes that the genetic influence is additive for handedness but dominant-recessive for left-cerebral language dominance, which explains the high concordance for language dominance in twins discordant for handedness. Our data suggest that the same gene has no influence on right-hemisphere dominance for spatial judgment or face processing, and offer little support for mirror-imaging in MZ twins other than that due to chance.