Neuroimaging studies of the corpus callosum are easier to understand if its elaborate internal organization is considered. The corpus callosum connects the cortical surfaces of the two brain hemispheres, and there is a topographically specific organization of callosal fibers in relation to the cortical regions they connect. Tract-tracing studies using anterograde or retrograde labels such as biocytin or rhodamine-labeled latex micro-spheres (Innocenti, 1994) have established the topographic distribution of callosal connections at the cortex in several species. A massive perinatal loss of callosal axons, lasting from the thirty-fifth gestational week to the end of the first postnatal month (Clarke et al., 1989; LaMantia and Rakic, 1990), is thought to lead to a restricted pattern of adult callosal connections (Innocenti, 1994). In the adult callosum the genu (or anterior third) connects prefrontal cortices; the midbody (middle third) connects motor, somatosensory, and auditory cortices; and the splenium (posterior fifth) carries temporal, parietal, and occipital (visual) fibers. Perisylvian fibers from superior temporal and parietal cortex relay information from critical language and association areas and cross mainly in the isthmus ( just anterior to the splenium; see Figure 5.1). To a certain degree, callosal fiber types are also organized topographically. Fast-conducting, large-diameter (>3 mm) sensorimotor fibers are concentrated in the posterior midbody and splenium, while thinner, more lightly myelinated fibers are found at the genu. These fibers at the genu offer a lower conduction velocity, connecting prefrontal regions implicated in longer-
term planning and organization of behavior (Aboitiz et al., 1992a). Nonetheless, the idea of a sharply defined cortical map at the callosum has been mitigated by recent anterograde tracer studies in humans (Di Virgilio and Clarke, 1997). These suggest that heterotopic connections (i.e., between nonequivalent cortical areas in each brain hemisphere) are numerous and widespread, even in the genu and splenium where callosal axons are most highly segregated.
Partitioning Approaches Because there are no gross anatomical landmarks that clearly delimit anatomically or functionally distinct callosal regions, several geometric partitioning schemes have been designed to subdivide the callosum into subregions whose fiber topography is expected to be different (Figure 5.1). These partitions define subregions that might be affected differently in development or disease and whose structural parameters (such as size, shape, or MRI signal intensity) might correlate more or less strongly with cognitive test data that evaluate different channels of interhemispheric communication (Clarke and Zaidel, 1994).
Vertical partitions Most studies of gender and handedness effects on callosal structure have been based on the Witelson (1989) partition (see Figure 5.1c). This scheme defines callosal subdivisions based on fractions of its maximum anterior-posterior length. Nonetheless, the curvature and shape variability of the midsagittal cal-losum can bias the proportions of callosal area represented in each of the resulting segments. This difficulty has led several investigators (e.g., Clarke et al., 1989) to base their partitions on a curvilinear reference line (Figure 5.1d), which takes the global curvature of the callosum into account.
Radial Partitions On the basis of the centroid, or center of mass, of the corpus callosum, angular rays can be defined (Figure 5.1e) that intersect the callosal boundary above and below. These rays can be used to produce an equiangular partition with 100 separate elements (Figure 5.1e) (see Rajapakse et al., 1996). Clarke and colleagues (1989) partition this medial reference line into nodes of equal separation, before defining 30 sectors based on the shortest line through each node connecting outer and inner boundaries (Figure 5.1d). Stievenart and colleagues (1997) partition the callosum by defining rays normal to a series of equidistant nodes on the ventral callosal boundary (Figure 5. 1 g), which provide the basis for thickness and curvature measurements. Allen and colleagues (1991) noted that the tip of the rostrum is occasionally difficult to identify, which may add error in defining the curvilinear partitions while affecting only the rostral sector in the straight-line-based approaches (Figures 5.1a and 5.1c).
To avoid making arbitrary definitions, Denenberg, Kertesz, and Powell (1991) performed a factor analysis to determine a ''natural'' partition of the callosum. Thickness measurements were obtained from a population of 104 normal adults (by connecting 100 equally spaced points on the inner and outer callosal boundaries; see Figure 5.1f), and these measures were used to determine seven regions with consistent variations (seven factors). While the partitioning scheme chosen ultimately depends on the application objectives and the scale of the expected structural effects (Bookstein, 1996), many apparent conflicts among different callosal studies derive from hidden or overt methodological differences, as will be seen in the following sections.
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