The most commonly used scheme for partitioning the human corpus callosum is due to Witelson: the anterior third (genu and rostrum), the anterior and posterior body (middle third), the isthmus (the anterior fifth of the posterior third), and the splenium (the remainder of the posterior third). This scheme defines callosal regions as proportions of linear length and is undoubtedly partly arbitrary. But on the basis of clinical partial disconnection syndromes, the visual function of the splenium, the auditory function of the isthmus, and the motor and somatosensory functions of the anterior and posterior midbody, respectively, are fairly well established (Bogen,
1993). The isthmus also seems to be associated with linguistic (lexical semantic) transfer (Clarke and Zaidel,
1994). Phonetic information may be transferred through more anterior callosum channels (Clarke and Zaidel, 1994; Berman, Mandelkern, Phan, and Zaidel, submitted). More abstract codes, such as generalization sets, may also be mediated by anterior callosal channels that interconnect frontal modules (Sidtis et al., 1981).
Aboitiz and colleagues (this section) analyze the morphological correlates of the standard partition. Jancke and Steinmetz, in turn, analyze its psychometric features, especially its relationship to brain size. Let us approximate the midsagittal cross section of the corpus callosum in the cortex by a rectangle in circle, and let us approximate the cortex by a sphere. Then the area of the corpus callosum is proportional to r2, and the volume of the brain is proportional to r3. Thus, (1) CC = c\r2 and (2) V = C2r3, where CC = corpus callosum area, V = cortical volume, and C1 and C2 are constants. Solving for r in (2) and substituting in (1), we get CC = c3V 2=3, where C3 is a constant. As the brain—that is, r—grows, the area of the corpus callosum grows more slowly than the volume of the cortex. Thus, the ratio of CC to V is larger in smaller brains, such as those of females. This confounding factor needs to be avoided in studies of individual differences or development and aging, in which brain size is known to vary.
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