The large numbers of studies carried out on the corpus callosum since the pioneering work of Myers and Sperry in the mid-1950s have largely confirmed, in various animal species including humans, the functions of the corpus callosum reported in this chapter. There is little doubt that the corpus callosum is involved in inter-hemispheric transfer of lateralized information, although it might share this function with other secondary commissures. It is also generally accepted that another function of this great cerebral commissure is to ensure the continuity of lateralized stimuli across the midline (the midline fusion hypothesis). The implication of the corpus callosum in the fusion of three-dimensional objects is still a matter of debate, since the electro-physiological results on the preservation of disparity sensitive cells in split-chiasm cats do not always account for the behavioral deficits observed in the discrimination of random-dot stereograms. The various anatomical, electrophysiological, and behavioral results obtained on animals are parallel to those observed in human patients with callosal deficiencies and support the notion that the corpus callosum is involved in a number of sensory and cognitive functions (see reviews by Lassonde and Jeeves, 1994). The question, however, of how the corpus callosum exerts its influence on its target hemisphere remains to be investigated. Several hypotheses originating from electrophysiological and behavioral experiments have suggested that the corpus callosum has excitatory effects (e.g., Lassonde et al., 1986), whereas some have proposed, on the contrary, an inhibitory action (Cook, 1984). Neurochemical studies in recent years have emphasized a more complex chemical architecture that led to a reconciliation of ideas suggesting that the corpus callosum exerts both excitatory and inhibitory actions on neurons in the contralateral hemisphere (see Conti and Manzoni, 1994). Indeed, several studies have furnished evidence that a number of chemicals belonging to several classes (amino acids, amines, peptides, etc.) are directly associated with callosal transmission. It has been shown that excitatory amino acids are used by the corpus callosum, of which glutamate and aspartate are the most documented. For example, hemidecortication in rats induces a decrease in glutamate levels in the contralateral hemisphere (Peinado and Mora, 1986), whereas electrical stimulation of the cat's suprasylvian cortex increases dramatically the glutamate levels in the homotopic contralateral cortex (Hicks et al., 1985). The use of tritiated transmitter compounds injected into one side of the brain and retrogradely transported to the cell body and visualized by autoradiography allowed researchers to show that the injection of d-[3H] aspartate in the cortex of one hemisphere resulted in a number of labeled neurons in the contralateral hemisphere (Barbaresi et al., 1987; Elberger, 1989). With the advent of immunohistochemical techniques, not only were the latter results confirmed, but it became possible to study the morphology, the laminar distribution, and the percentage of callosally projecting axons. In the somatosensory system of cats, for example, positive immunoreactive callosally projecting neurons to anti-aspartate or anti-glutamate sera represent about half of all retrogradely labeled neurons for each antibody (Conti, Fabri, and Manzoni, 1988a, 1988b). That aspartate- and glutamate-positive neurons do project to the corpus callosum is further supported by electron microscopy immunocytochemical studies that demonstrated that many axon terminals forming synapses in the cerebral cortex are indeed glutamate- and aspartate-positive and are distributed in the layers of termination of callosal axons (see Conti and Manzoni, 1994). At the electrophysiological level, intracellular recordings from several cortical neuronal populations have shown that postsynaptic cells develop both excitatory and inhibitory postsynaptic potentials (EPSP and IPSP) in response to electrical stimulation of the callosal input (reviewed by Conti and Manzoni, 1994). It is generally agreed that most of the transcallosal EPSPs are monosynaptic (Toyama et al., 1969), and there is little evidence that this is also the case for IPSPs.
Although the reported evidence seems to point to an excitatory influence of the corpus callosum, there are some suggestions derived from neurochemical and electrophysiological studies (see above) that some callosal-projecting axons could also have inhibitory effects. This seems to be true in the developing organism, since there is an agreement that in perinatal life, the majority of callosal-projecting neurons are GABAergic inhibitory neurons. In the adult animal the results are still con-flictual (see Conti and Manzoni, 1994).
In conclusion, most studies (behavioral, anatomical, physiological, and chemical) support the suggestion that the corpus callosum exerts an excitatory influence on postsynaptic targets. Since the demonstration that a proportion, albeit small, of callosally projecting neurons have axons with inhibitory effects, the debate has been reopened on the nature of the action of the corpus callosum. In fact, many neuropsychologists, on the basis of work done on brain-damaged patients, have recently proposed an inhibitory action of the corpus callosum (e.g., Cook, 1984; Jeeves, 1986).
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