Complexity of Human Interhemispheric Connections

stephanie clarke

Francisco Aboitiz, Andres Ide, and Ricardo Olivares review very interesting evidence concerning regional differences in axonal diameters within the corpus callosum, an inverse relation between the degree of hemispheric asymmetry and callosal size, and models of the development of this relationship in ontogeny and in phylogeny. They argue that diameters of callosal axons reflect their cortical origin, large axons originating in sensory and motor cortices and small axons in association cortices. On the basis of reports that the changes in callosal size associated with different degrees of hemispheric asymmetry affect some parts of the corpus cal-losum but not others, the authors argue that the variable regions within the corpus callosum are those that convey fibers from asymmetric regions. Finally, they propose that in both ontogeny and phylogeny the density of cal-losal connections arising from a cortical region may be influenced by the degree of asymmetry between this region and the homotopic one in the opposite hemisphere. All three arguments rely on the assumptions that there is a rather precise topographic arrangement of axons within the corpus callosum according to their origin and that callosal connections are essentially homotopical. One of my criticisms is that these fundamental assumptions may be false, since human tracing studies indicate that both aspects of interhemispheric connectivity may be more complex than presumed.

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