francisco aboitiz, andres ide, and ricardo olivares abstract In this chapter we present data of fine callosal structure in postmortem humans revealing regional differences in fiber composition that indicate differences in interhemi-spheric transfer between primary/secondary sensorimotor areas, on the one hand, and higher-order cortical regions, on the other. In addition, we review evidence from studies concerning a negative relation between interhemispheric communication and brain lateralization in the human as indexed by anatomical asymmetry in the Sylvian fissure and on individual differences in this relation. Since the topic of anatomical asymmetry in perisylvian regions is now a debated issue, it is necessary to delve into some details about the morphology of the Sylvian fissure. Therefore, the chapter begins with a review of callosal anatomy in the postmortem human at both the macroscopic and microscopic levels, continues with a critical review of the current status of anatomical asymmetry in the posterior language region and its relationship to callosal structure, and ends with an evolutionary discussion of the original role of the corpus callosum in placental mammals and its possible participation in the generation of lateralization in the human brain.
Among other things, the brain of placental mammals is unique in that it bears a main fiber tract connecting the two cerebral hemispheres: the corpus callosum. In the human this tract consists of possibly more than 200,000,000 fibers (Aboitiz et al., 1992a). Comparing this number with the approximately 1,000,000 fibers in the optic nerve gives an idea of the potential complexity of interactions that may occur during interhemispheric interaction. However, fundamental questions, such as the adaptive value of the corpus callosum or what role may callosal function play in the development and the francisco aboitiz Departamento de Psiquiatría, Pontificia Universidad Católica de Chile, Santiago, Chile. Andrés ide Departamento de Morfología Experimental, Facultad de Medicina, Universidad de Chile, Santiago, Chile. richardo olivares Facultad de Ciencias, Veterinarias, Universidad de Chile, Santiago, Chile.
evolutionary origin of brain lateralization, are still matters of debate.
In this context we will address two related problems regarding the role of the corpus callosum in brain function. The first is the question of what the advantage of having a corpus callosum is to placental mammals. The obvious answer is interhemispheric communication. But why is interhemispheric communication so important to them? There are many large-brained species among reptiles, birds, monotremes (the platypus and the echidna), and marsupials that seem to do well with a much more limited extent of interhemispheric connectivity through the anterior and pallial commissures. From a comparative viewpoint the increased levels of inter-hemispheric interaction that we observe in placental mammals cannot be considered simply a consequence of having a large and complex brain. Rather, there must be specific adaptive reasons for the origin and maintenance of the corpus callosum in this group.
The second problem is that although the corpus cal-losum participates in lateralized function in the human brain, there has been some controversy as to whether (both in development and phylogeny) the callosum plays a role in the generation of lateralization or whether hemispheric specialization may itself affect callosal function. It has been repeatedly proposed that the corpus callosum is important for the appearance of functional lateralization in the mammalian brain, although hemispheric differences in function are well documented for nonmammalian brains without a corpus callosum, perhaps indicating distinct mechanisms involved in the generation of brain asymmetry in different vertebrates (Bradshaw and Rogers, 1993; Bisazza, Rogers, and Val-lortigara, 1998). Two kinds of ideas regarding the role of the callosum in the generation of mammalian later-alization have been proposed: that it contributes to lat-
eralization through asymmetric inhibitory interactions between the hemispheres (Cook and Beech, 1990; Brad-shaw and Rogers, 1993) and that an intact corpus cal-losum is required for the maintenance of brain symmetry; when callosal function is impaired for some reason, asymmetry develops as a consequence of hemispheric isolation (Ringo et al., 1994; Witelson, 1995). The first concept implies that brain asymmetry might further develop with increasing inhibitory interhemispheric connectivity; the second suggests an inverse relationship between callosal connectivity and brain asymmetry. In addition to these two possibilities, hemispheric specialization may induce alterations in interhemispheric communication, and consequently, the corpus callosum should have more restricted functions in a lateralized brain. Some clues to the first question on the adaptive value of the corpus callosum may be found through the study of regional variation in fiber composition in the postmortem human, while we expect to shed light on the second problem of the callosal role in brain laterali-zation by analyzing individual variability of the corpus callosum.
More specifically, in this chapter we will discuss evidence for regional variability in fiber composition in the human corpus callosum, with a functional interpretation concerning the special role in midline fusion of callosal fibers connecting sensory (especially visual) areas. Then recent findings regarding structural asymmetry in the human brain (especially the Sylvian fissure and planum temporale) and its relationship to lateralized linguistic function will be reviewed. Since this is a controversial topic, it will be discussed in some detail. After this, the relationship between callosal fiber connectivity and the degree of anatomical asymmetry will be analyzed, and a developmental mechanism relating hemispheric asymmetry and callosal connectivity will be proposed. The chapter ends with a phylogenetic discussion of callosal function in mammals and its possible role in the origin of brain lateralization.
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