Handedness and the Corpus Callosum: A Review and Further Analyses of Discordant Twins

Handedness effects with respect to regional corpus callosum (CC) anatomy remain open to question. Midsagittal CC tracings were obtained from structural MRIs in 21 female monozygotic twin pairs with discordant handedness (MZHd). The CC was divided into 99 percentile widths which were grouped into seven regions based on Denenberg's (1989, 1991b) factor analysis. Results showed that left handed (LH) twins had significantly larger regional widths in CC region W22-39 compared to right handed (RH) twins, an effect present in 19/21 MZHd pairs. Cross-study analyses revealed CC W22-39 to have the highest variation across female singletons (Cowell et al., 1992, 1993). In the adjacent genu region (W3-18), CC size did not differ between RH and LH twins. However, when twins were reclassified according to handedness direction and consistency to form consistent RH, non-consistent RH, consistent LH and non-consistent LH groups, patterns of CC size in W3-18 closely matched those of singleton women (Cowell et al., 1993). Namely, CC W3-18 was larger in consistent compared to non-consistent RHs. Results support a claim that CC region W22-39, interconnecting premotor cortex in females, provides for environmentally influenced components of handedness, given the difference within MZHd twin pairs. By contrast, CC W3-18, connecting prefrontal cortex, was sensitive to direction and consistency of handedness, both in twins and singletons, a result consistent with combined genetic and environmental effects. Findings highlight the significance of MZHd twin studies in elucidating the developmental mechanisms underpinning structure-function asymmetry, cortical interconnectivity and neurodevelopmental bases of left hand preference.