The distribution of interhemispheric fibers between the corpus callosum and the anterior commissure is even less clear than the topographical order within the corpus callosum. Fiber dissection studies have shown that the human anterior commissure conveys fibers from parts of the orbitofrontal and polar temporal structures, including the amygdala (Klinger and Gloor, 1960). In the macaque monkey, tracing studies have demonstrated that the anterior commissure receives fibers from all the above structures but also from virtually the entire temporal cortex (Demeter, Rosene, and Van Hoesen, 1990).
We have studied the contribution of different cortical regions to the anterior commissure in the human by using the Nauta method for anterogradely degenerating axons in cases with circumscribed hemispheric lesions (Di Virgilio et al., 1999). A large number of degenerating axons were found in the anterior commissure in a case with a lesion spreading over the middle third of the inferior temporal, the fusiform, and the parahippocam-pal gyri as well as the hippocampal formation but sparing the amygdala, the anterior perforated substance, the olfactory tubercle, the diagonal band of Broca, and the prepiriform cortex. Degenerating axons were present in the anterior commissure, although in smaller numbers, in four other cases with lesions in the inferior part of the occipitotemporal cortex and in the middle frontal gyrus (two cases), the superolateral occipital convexity (one case), and the fundus of central fissure (one case). In cases with lesions involving the inferior occipital cortex, the splenium was also analyzed, and degenerating axons were found in its inferior part (Clarke and Miklossy, 1990). No degenerating axons were found in two cases without lesions and in a case with a lacuna inferomedial to the left pallidum (not encroaching on the posterior limb of the anterior commissure). Thus, the human anterior commissure conveys axons from a much larger territory than previously assumed. In particular, there is a heavy contribution from the inferoposterior part of the temporal cortex. The suggestion that cortical regions on the frontoparietooccipital convexity also contribute to the anterior commissure, even if weakly, is puzzling and worth further investigation. Furthermore, the inferior part of the occipitotemporal cortex appears to project through both the corpus callosum and the anterior commissure.
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