With the advent of modern tract-tracing techniques, it has been possible to determine precisely the sites of origin and termination of transcallosally projecting neurons (see Rosenquist, 1985; Innocenti, 1986) as well as the trajectory of their axons through the corpus callosum (Lomber, Payne, and Rosenquist, 1994). Briefly, each hemisphere projects callosal axons not only to homologous (homotopic) areas in the contralateral hemisphere but also to heterologous (heterotopic) areas. The callosal tract consists of myelinated and unmyelinated axons with a wide range of diameters (0.08—5 mm) and conduction velocities (1.4-30 m/s) (reviewed by Innocenti, 1986). The estimate of callosal axons has radically changed since the advent of electron microscopy, which can detect small unmyelinated fibers, undetected by light microscopy studies. In the cat, for example, electron microscopy studies have reported an estimate of 23 million callosal axons, compared to the 2 million previously reported with light microscopy analysis (In-nocenti, 1986). In a previous study (Tremblay et al., 1987), we examined the pattern of callosal projecting neurons in all the visual cortical areas of the cat's brain by sectioning the posterior half of the corpus callosum and exposing the cut-end axons to the retrograde tracer HRP. The results, depicted in Figure 6.1, indicate that the labeled cellular bodies were absent in area 17 and were mainly focused in the 17/18 border corresponding to the representation of the vertical meridian. The extrastriate areas exhibit a high density of callosal projections. This overall topography of the distribution of callosal cells is similar to what has been reported by Seagraves and Rosenquist (1982). More recently, Lom-ber and colleagues (1994), using HRP exposition to discrete severed portions of the corpus callosum, have furnished the first description of the complete cortical field that gives rise to purely callosal axons. Moreover, these authors were able to link the functional divisions of the cerebral cortex to fiber trajectory through the corpus callosum. The motor cortex sends fibers through the rostrum and genu of the corpus callosum. The adjacent somatosensory cortex projects fibers through the anterior half of the body, whereas axons arising from auditory regions course in the posterior two-thirds of the body and the dorsal splenium, where they match the distribution of axons originating in the limbic cortex. Finally, axons from visual cortices, which occupy the greatest single fraction of the cortical mantle, pass through the largest portion of the corpus callosum; the fibers are present throughout the splenium and extend well into the body and the anterior portion. These results also indicate the presence of unimodal and multimodal regions within the rostrocaudal extent of the corpus callosum.
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