The corpus callosum and the commissural connections of the cerebral cortex

All cortical areas both send fibres to and receive fibres from the opposite hemisphere, although the connections are not necessarily throughout the whole area. In most of the cortex, commissural fibres pass to the contralateral side in the corpus callosum. The anterior commissure carries fibres that interconnect the anterior third or so of the temporal lobe with its partner, as well as fibres that interconnect the olfactory bulbs on each side. Some fibres in the fornix cross the midline, the so-called commissure of the fornix, to interconnect the two hippocampal formations. Commissural fibres are of two types, homotopic and heterotopic. Homotopic fibres pass from one area of cortex to the same area on the other side. Heterotopic fibres pass from one area to a different, although often functionally related, area on the other side. As a generalization, the functions of the commissures can be subdivided into two categories. First, they serve to interconnect representations of the contralateral sensory surround across the midline, for example the representations of the two halves of the body, the two visual hemifields, and so on. In areas containing a lateralized sensory representation, of either the body or the visual field, the callosal fibres are confined, in both origin and termination, to the parts of the area containing a representation of midline and adjacent regions. Thus the representation of the trunk in somatic sensory areas sends and receives commissural fibres, whereas the hand and foot representations are not connected across the midline. Similarly, the vertical meridians in visuotopic representations are interconnected by callosal fibres, whereas the periphery is not. In contrast, the second function of the commissures is to connect areas in one hemisphere with areas on the other side, where the functions of each are represented on only one side, i.e. they are lateralized. Of course, this is most apparent for language areas; for example, objects held in the non-dominant hand cannot be named following callosal section because the sensory cortex of the non-dominant hemisphere cannot communicate across the midline with the language and speech areas in the dominant hemisphere.

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