As was mentioned above, many callosal connections tend to be homotopical, that is, they connect equivalent regions in both hemispheres (Innocenti, 1986, 1995a, 1995b). However, in the newborn, callosal connections are overdeveloped, many of them connecting areas that are devoid of callosal projections in the adult. In the perinatal period there is a massive decrease in the number of callosal fibers (LaMantia and Rakic, 1990), producing the adult pattern of restricted callosal connections (Innocenti, 1986, 1995a, 1995b). It has been postulated that if two homotopic cortical areas are asymmetric, there will be a higher than normal process of callosal fiber retraction during this period owing to topographic and functional incongruities between the two regions (Aboitiz et al., 1992b, 1992c). Alternatively, an increased retraction of callosal terminals may induce the generation of hemispheric asymmetry and laterali-zation, especially in males (Witelson and Nowakowski, 1991; Witelson, 1995), presumably owing to the emphasis on intrahemispheric processing and progressive isolation of the two hemispheres. Witelson and Now-akowski found that very premature infants show a high proportion of left-handedness, which they interpret as showing that the normal course of callosal axon loss has been interfered with, resulting in loss of laterality. Supporting this view, Lassonde, Bryden, and Demers (1990) report that subjects with callosal agenesis tend to be more lateralized than control subjects, as seen in di-chotic listening tests (note that this diverges from the findings in acallosal mice mentioned above). However, in this proposed mechanism there is no explanation of why language should be localized to the left instead of the right. Furthermore, evidence indicates that anatomical asymmetry (see above) seems to appear earlier in development than the period of callosal axon retraction, which apparently takes place between the thirty-fifth gestational week and the first postnatal month (Clarke et al., 1989). A likely possibility is that the two factors reinforce each other; that is, after early-generated anatomical asymmetries have induced an increased retraction of terminals in the corpus callosum, the reduced callosum itself plays a role enhancing the incipient functional lateralization by constraining the communication between the two hemispheres. In this way, the development of language dominance would depend on the interplay between anatomically asymmetric structures and their respective degree of inter-hemispheric communication.
One interesting and recurrent finding is that the relationship between callosal connectivity and hemispheric asymmetry tends to be stronger in males. This might imply that in females the process of perinatal reaccommodation of connectivity is not as intense as in males. For example, if asymmetry determines an increased retraction of callosal terminals in males, for some reason in females there might be less competition between axons, resulting in a less intense regression of projections than in males, which in turn would imply that, at least for some processes, females might be able to tolerate a higher level of interhemispheric interaction with increasing hemispheric specialization than males. Perhaps sex hormones have an effect on the process of terminal retraction in late neural development, thus causing the proposed developmental differences between sexes. Sex hormones have been reported to modulate callosal morphology (Moffat et al., 1997; Fitch and Denenberg, 1998). Furthermore, different fiber types can have different sensitivities to sex hormones, yielding the observed finding of a negative correlation of the number of relatively thick fibers with asymmetry only in males. It has been reported (Juraska and Kopcic, 1988) that male rats that are raised in an enriched environment tend to develop larger myelinated callosal axons than those raised in an isolated environment. Female rats that are raised in an enriched environment tend to show increased numbers of callosal fibers rather than increased fiber size. Recall that among humans, relatively large fi bers (between 1 and 3 mm in diameter) showed a negative correlation with asymmetries only in males, which might suggest that males but not females tend to respond to different developmental conditions (be they increasing hemispheric asymmetry or an enriched environment) by changing the proportions of relatively large-diameter fibers (Aboitiz et al., 1992b). Nevertheless, our more recent findings indicate that age-related increases in the numbers of large-diameter callosal fibers tend to be female-specific (Aboitiz et al., 1996; see above), indicating that sex differences in the process of myelination and fiber growth may follow rather complex rules.
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