The corpus callosum Its anatomy and variability

As has been mentioned, the corpus callosum consists of nerve fibers connecting the cerebral cortices of the two cerebral hemispheres. During embryonic development and also in phylogeny, it originates as a component of the hippocampal (pallial) commissure, an ancient fiber tract that connects the medial cortices (hippocampi) of the two hemispheres (Rakic and Yakovlev, 1968). Eventually, the corpus callosum separates from the hippo-campal commissure and locates in the dorsal part of the hippocampal formation, becoming the main fiber tract connecting the two hemispheres. There is a topographic organization of callosal fibers in relation to the cortical regions that they connect; that is, fibers connecting frontal regions travel through the front of the corpus callosum, while fibers connecting occipital cortices tend to travel in the posterior part of this structure. In this way, a map of the different cortical regions can be established along the callosum (Pandya and Seltzer, 1986). However, in smaller-brained animals such as the cat and the rat, the segregation of callosal fibers belonging to distinct cortical areas is less clear-cut (Nakamura and Kanaseki, 1989; Clarke et al., 1995; Kim, Ellman, and Juraska, 1996). A commonly used and straightforward method to subdivide the corpus callosum is the partition proposed by Witelson (1995), in which the corpus cal-losum has been arbitrarily divided into three regions according to maximal straight length (see the top part of Figure 2.1). The anterior third (denominated by the genu) is a rather bulbous region that contains fibers connecting prefrontal cortices. The middle third is the midbody of the corpus callosum, a slender region that contains projections from motor, somatosensory, and auditory cortices. The posterior third is divided into the posterior fifth (the splenium), which in anteroposterior sequence contains temporoparietal and occipital (visual) fibers, and the isthmus, a region between the midbody (middle third) and the splenium (posterior fifth), which is believed to contain fibers connecting superior temporal and parietal regions, including perisylvian areas, related to language processes (Witelson, 1995).

Figure 2.1. Top: Diagram of the corpus callosum, indicating the regions through which fibers connecting different cortical areas pass. F, frontal; M, motor; Ss, somatosensory; A, auditory; P/T, parieto-temporal; V, visual. Bottom: Distribution of fiber diameters in different callosal regions.

Figure 2.2. Top: Callosal cells and their terminations in the contralateral cortex in the primary and secondary visual cortices of the cat are concentrated in the midline of the visual field. Bottom: Callosal cells in higher-order visual cortices are dispersed throughout the respective area. (Modified from Ber-lucchi, 1972.)

Figure 2.2. Top: Callosal cells and their terminations in the contralateral cortex in the primary and secondary visual cortices of the cat are concentrated in the midline of the visual field. Bottom: Callosal cells in higher-order visual cortices are dispersed throughout the respective area. (Modified from Ber-lucchi, 1972.)

Many callosal fibers are homotopical, that is, they connect equivalent regions on the two sides of the brain. Callosal cells and nerve terminals that connect early-processing sensory areas (somatosensory, auditory and especially visual) and motor regions tend to be restricted to the borders between left and right areas rather than being dispersed along the surface (Berlucchi, 1972; In-nocenti, 1986; Clarke and Miklossy, 1990). In these regions there is a sophisticated topographic map of the sensory surface, and callosal cells and terminals are located in the region representing the sensory or motor midline (Figure 2.2). Since each hemisphere contains a representation of the contralateral sensory or motor field, it has been postulated that callosal cells participate in fusing the two hemirepresentations in each hemisphere. In the case of the visual system, midline fusion is important to maintain a continuity of the visual scene, and in some animals, such as humans, it is also useful for depth vision at and around the midline. Interestingly, the auditory areas are slightly different from visual and somatosensory areas in that the representation of the sensory surface (the cochlea) is tonotopic, that is, organized in stripes that represent auditory tones rather than spatial locations. Callosal cells here are also located in a band transverse to these isofrequency contours in the borders of the areas (Innocenti, 1986).

In so-called higher-order or association areas in the temporoparietal and frontal lobes, increasingly abstract and complex aspects of sensory and motor processing take place, and the topography of the sensory/motor surface becomes blurred. In many of these regions, cells respond much better to stimuli characteristics than to the location of the stimulus in the sensory field. Callosal cells and terminals connecting these regions are dispersed along the respective cortical areas instead of being restricted to the borders that represent the midline (Berlucchi, 1972; Innocenti, 1986).

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