A controversial line of research has suggested the existence of individual differences in callosal gross anatomy, mainly in regard to handedness and sex. Differences in the cross-sagittal size of the corpus callosum may have functional significance, as a larger total or partial callosal size reflects an increased number of small-diameter fibers as seen in light microscopy (Aboitiz et al., 1992a). The sparse large-caliber fibers of more than 3 mm are so variable in density that they fail to show a relationship with callosal size. An additional factor contributing to callosal size is myelin deposition, which also has functional correlates, since, as we discussed, increased mye-lination is associated with higher conduction velocity and hence shorter interhemispheric transmission time.
Some studies indicate a more bulbous splenium in females than in males, while other studies report a sex difference in the size of the isthmus in favor of females. Other findings suggest no sex differences in callosal morphology or fiber composition (for a review, see Aboitiz and Ide, 1998; see also Bishop and Wahlsten, 1997; Oka et al., 1999). Taking into account all the conflicting reports on callosal sex differences, probably the safest conclusion at this point is that if they exist, they are not robust (but see Bishop and Wahlsten, 1997). This perhaps contrasts with the striking difference in brain size between the sexes (Aboitiz, Scheibel, and Zaidel, 1992c). Peters (1988) has argued that sex differences in brain weight relate to cortical areas connected with sensor-imotor surfaces but not to higher-order areas involved in cognition, which make the bulk of callosal fibers (see above). However, as yet there is no strong evidence in his support. A diferent explanation may come from the work of Haug (1987) and Witelson, Glezer, and Kigar (1995), who reported that cortical cell density is higher in females than in males, which may result in similar numbers of cortical cells in the two sexes (Haug, 1987). If this is the case, and if the proportion of cells projecting to the corpus callosum does not differ across the sexes, similar callosal sizes and fiber numbers might be expected in males and females. However, the above findings must be taken with caution. Pakkenberg and Gundersen (1995) estimate that males have a larger total number of cortical neurons than females (the difference being of the same magnitude as the brain size difference), which does not agree with the above hypothesis.
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