It is often assumed that human callosal connections exist predominantly between homotopic regions of the cortex. At the same time what is considered a homotopic connection is rarely specified. Homotopy could be considered at the level of lobes, areas (Brodmann's areas, functional areas such as V1 and V2), or topographic representations within a given area (e.g., the same parts of the visual field). Recent evidence from human tracing studies, using the Nauta method for anterogradely degenerating axons in cases with circumscribed lesions, suggests that heterotopic interhemispheric connections are numerous and widespread.
Heterotopic connections between lobes were demonstrated in two instances. The right calcarine region was shown to send interhemispheric connections not only to the whole occipital cortex, but also to the posterior part of the temporal and parietal cortices, including regions proposed to be involved in specifically human functions such as the angular gyrus (Clarke et al., 1995, and unpublished results). The middle portion of the inferior temporal, the fusiform, and the parahippocampal gyri and of the hippocampal formation on the right side was shown to send relatively dense heterotopic connections to the posterior part of the superior temporal gyrus, the planum temporale, and the supramarginal and the angular gyri (corresponding to Wernicke's area) as well as weak connections to the posterior part of the inferior frontal gyrus (corresponding to Broca's area; see Di Virgilio and Clarke, 1997).
Heterotopic connections were demonstrated within the occipital lobe (Clarke and Miklossy, 1990). Hetero-topic connections were observed from the medial to the lateral part and from the superolateral to the super-omedial part of the occipital cortex.
Heterotopic connections between visual areas were demonstrated in a case with a small lesion in the upper part of Brodmann's area 19. As derived from the pattern of interhemispheric and intrahemispheric connections (Clarke and Miklossy, 1990; Clarke, 1994), the lesion was most likely in the putative human equivalent of macaque area PO/V6. Most of the interhemispheric connections originating in the damaged cortex terminated in the symmetrical region of the opposite hemisphere but some also in other parts. Among the latter were the boundaries between visual areas V1 and V2
and between areas V3 and V3A, as well as areas V4 and V5.
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