In the human, MRI and postmortem studies have detected an inverse relationship between total callosal size and functional brain lateralization as determined by several behavioral parameters, including handedness (Witelson, 1995). When the sexes are separated, it has been observed that the callosal isthmus (the region between the posterior third and the posterior fifth) correlates negatively with visual behavioral laterality measures in males but not in females (Witelson and Goldsmith, 1991; Clarke and Zaidel, 1994). Supporting these findings, Habib and colleagues (1991) determined that the total size and the size of the anterior half of the corpus callosum were larger in nonconsistent than in consistent right-hander males. However, they also found that the posterior midbody was larger in consistent than in nonconsistent right-handed females, a result that was not found by Clarke and Zaidel (1994).
Aboitiz and colleagues (1992b, 1992c) analyzed the relationships between callosal connectivity and hemispheric specialization, as indexed by the degree of peri-sylvian asymmetry. The Sylvian fissure was measured from Heschl's gyrus to the end of the ascending branch, and the magnitude of the asymmetry (regardless of direction) was plotted against callosal measures. An inverse relationship was found between asymmetry and the size and fiber numbers in the isthmus that was specific for males, supporting the above findings. At the level of fiber composition, in males the significant correlation was observed for the numbers of small, medium-sized, and moderately large fibers of the isthmus (between 0.4 and 3 mm in diameter) but not for the fibers larger than 3 mm. However, in a small segment immediately posterior to the isthmus (the anterior third of the splenium), small and medium-sized fibers (between 0.4 and 1 mm) showed a significant, negative correlation with asymmetries, this time only in females. This may indicate either a differential mapping of asymmetric regions of males and females or that distinct cortical regions are asymmetric in the two sexes. However, we cannot reject the possibility that in both males and females the isthmus and the anterior splenium contain fibers connecting asymmetric areas, the observed sex differences resulting from the high interindividual variability and the relatively small sample used in the study (10 subjects of each sex). Another intriguing finding is that only in males, fibers with diameters between 1 and 3 mm correlated with asymmetry, perhaps indicating that in males but not females, relatively fast interhemi-spheric transfer is affected by the asymmetry of language-related areas. When the correlation between the asymmetry levels of the different components of the Sylvian fissure and callosal connectivity was tested, no significant relationship was found, presumably because these components involve smaller cortical regions whose contribution to the total fibers in the isthmus becomes less important.
The above indicates that in the human, increasing anatomical asymmetry and increasing functional later-alization tend to be associated with a decreased inter-
hemispheric communication, especially in males. Thus, the interplay between callosal connectivity and hemispheric asymmetry may be of great importance during the development of language lateralization. Some findings in rats tend to support this conclusion, as the density of somatomotor callosal terminals decreases as volumetric cortical asymmetries increase (Rosen, Sherman, and Galaburda, 1989). However, later the same group (Rosen et al., 1990) found no relationship between anatomical asymmetry measures and callosal size in different mice strains. Other results in rodents indicate a complex, if not confusing, relationship between asymmetry and callosal connectivity (Bradshaw and Rogers, 1993). Some findings are compatible with models of inhibitory callosal interactions that mask an underlying hemispheric asymmetry (Denenberg, 1981). The controversial nature of some of these findings aside, it is possible that during human development, the relationship between corpus callosum and brain lateralization is different than in other species. Below, we will discuss a possible model for the interaction between anatomical asymmetry and callosal connectivity in the human, followed by a discussion of the role of the corpus callosum in the phylogeny of placental mammals, including its possible role in the origin of brain lateralization.
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