The corpus callosum is the largest connecting pathway between the CEREBRAL HEMISPHERES. Estimates of the number of nerve fibres it contains vary from 500 to 800 million. It contains two types of fibres: the larger-diameter mediate sensori-motor coordination, the more numerous small-diameter fibres connect association areas.
For the first half of the twentieth century, the corpus callosum seemed to have no function at all. This stood in sharp contrast to the eighteenth century when some authors believed that this ‘callous body’ or ‘hard part’ of the brain was the ‘seat of the soul’. By 1941 McCulloch & Garrell concluded that thus far attempts have ‘failed to produce any characteristic disorders except, possibly, impairment of co-ordination of the hemispheres in complicated symbolic activity’ attributable to lesions of, or even surgical sections of, the corpus callosum. This view was radically revised starting from the classical pioneering studies of Myers & Sperry in the late 1950s, on the INTERHEMISPHERIC TRANSFER of information through the corpus callosum. Fibres originating unilaterally in different regions of the cerebral cortex consistently cross the midline in specific locations. Most of these fibres are found in the corpus callosum. This is true for the interhemispheric fibres of primary motor and somatic sensory cortices, as well as the OCCIPITAL LOBE. At the same time, fibres from certain cortical areas intermingle with fibres coming from other regions. This overlap of callosal trajectories tends to occur between those cortical areas that have contralateral interconnections. A natural corollary of the anatomical studies is that different topographical regions within the FOREBRAIN commissures contain fibres relating to different functional specializations. Somatic sensory motor functions are localized to the central portion of the body of the corpus callosum; the SPLENIUM deals mainly with visual-related functions. Such a view finds support from clinical and experimental evidence indicating that discrete lesions of the forebrain commissures differentially disrupt cortical functions.
The physiological functions of the corpus callosum have been most intensively studied in relation to the VISUAL SYSTEM. Visually responsive cortex in the OCCIPITAL LOBE, TEMPORAL LOBE and PARIETAL LOBE of cat and monkey has been subdivided into several areas, each of which contains a more or less complete and continuous representation of visual space. Whilst the various visual cortical areas receive information chiefly from the contralateral half of the visual field (see VISUAL FIELD) they may in addition, owing to the corpus callosum, be reached by visual information from the ipsilateral hemifield. It has been shown that callosal fibres running between areas 17 and 18 (see BRODMANN’S AREAS) of the two sides are endowed with visual fields on the vertical meridian, and distribute themselves to cortical neurons that are also concerned with the vertical midline of the visual field. Such a pattern of organization appears to apply not only to areas 17 and 18 but to the other visual cortical areas as well.
Sperry argued that the visual information carried by the callosal connections and that supplied by the direct input to the cortex are both supplementary, because they make an addition to one another, and complementary, because this mutual addition results in a homogenous whole. The relation of the callosal connections to sensory maps differs for the visual and somesthetic systems. Many physiological studies of the information transmitted between these connections in cats have shown that in the visual modality, of 100 visual receptive fields of the callosal fibres studied, only one lay more than 4° from the vertical meridian. In the somesthetic modalities almost 20% of the receptive fields had a distal location on a forepaw. The evidence also supports the hypothesis that the callosal connections of the primary VISUAL CORTEX may intervene in some binocular processes such as binocular STEREOPSIS along the mid sagittal plane, which requires either some overlap along the vertical meridian of the projections from the nasal and temporal hemi-retinae, or a set of interhemispheric connections limited to the vertical meridian representation, or both. Evidence for callosal dependent inhibitory surrounds of excitatory receptive fields lying on the other side of the vertical meridian supports the evidence that callosal influences are compatible only with an uninterrupted visuo-topic representation of the visual space on both sides of the vertical meridian.
Results of extensive studies on patients indicates that in the intact cortex, the corpus callosum exerts a facilitatory, or modulating influence on the neural activity in both hemispheres. Observations of callosotomized patients suggest that callosal section may not only abolish interhemispheric propagation of seizure discharges but may also reduce, or even arrest, abnormal activity in an initial focus. Neurophysiologically almost all callosal fibres are excitatory whilst their functional effects may be either excitatory or inhibitory depend-ing on the nature of the interneurons. It is likely that the normal process of interhemispheric regulation requires a balance between both inhibitory and excitatory influences.
References
Pandya D.N. & Seltzer B. (1986) The topography of commissural fibres. In Two Hemispheres—One Brain. Functions of the Corpus Callosum, ed. F. Lepore, M. Ptito & H.H. Jasper, Alan R. Liss: New York.
Sperry R.W. (1964) The great cerebral commissure. Scientific American 210:42–52.
MALCOLM A.JEEVES
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