The studies summarized above stress the complexities and subtleties in the organization of adult callosal connections revealed by new anatomical methods and therefore widen our understanding of the morphological basis of interhemispheric interactions.
Several questions remain. First, the information about the individual callosal axons to "higher-order" visual areas is scanty, and nothing is known about those interconnecting higher association areas. Second, as this volume demonstrates, research on the corpus callosum has developed over several decades on a strong interdisciplinary basis. The challenge facing this particularly advanced field, but common to other areas of the neurosciences, is that of mapping structural, electrophys-iological, and psychological data onto each other. Our analysis of adult material has tentatively identified basic operations that are performed by callosal axons. However, it is unclear how these operations are reflected at the levels of neuronal response properties and of visual perception. The role of callosal connections in "binding by synchrony" the activity of neurons in the two hemi spheres in sensory processing (Engel et al., 1991) offers a promising direction for future investigations. One breakthrough in this area may come from the development of techniques for assaying the functionality of interhemispheric connections. The analysis of inter-hemispheric coherence of EEG signals during sensory stimulation may provide such a tool. Indeed, an increased interhemispheric coherence of EEG signals in the gamma band was observed in both animals and humans with stimuli that activate the callosal connections of the visual areas (Kiper et al., 1999; Knyazeva et al., 1999). With this technique the role of the corpus cal-losum in perceptual binding could be clarified by further investigations using chimeric figures (Levy, Trevarthen, and Sperry, 1972).
The studies of developing callosal connections stress their potential modifiability (plasticity) in early development. Callosal connections could be modified either by the altered growth of the axon, by the maintenance of axonal structures that are normally eliminated, or by the elimination of axonal structures that are normally maintained. Any of these modifications could occur at one or another stage of axonal differentiation detailed above. In addition, axonal morphology could be modified at myelination, not mentioned in the present review, in spite of its potential relevance for the temporal operations the axons perform. The stage of callosal synapto-genesis and the subsequent partial synaptic elimination are particularly important, since both events occur post-natally in humans and might be use-dependent. With the exception of myelination the other stages of axonal differentiation take place largely in utero and might be affected by prenatal events.
It is important to stress that the kind of plasticity exhibited by callosal axons in animal experiments is not necessarily of an adaptive kind. None of the animals with callosally modified connections have been tested in this respect. It should also be noticed that the information on the cellular mechanisms involved in the development of callosal connections is still extremely scanty. In particular, with the exception of preliminary studies on the maturation of some cytoskeletal proteins (reviewed in Innocenti, 1991), the biochemical events involved in the elimination or maintenance of callosal connections remain to be elucidated.
acknowledgments Supported by Swiss National Science Foundation grant 3100-039707.93 and Swedish Medical Research Council grant 12594. We wish to thank Mr. Philippe Gaudard, Mr. Eric Bernardi, and Mrs. Kristina Ingvarsson for their help at various stages in the preparation of this manuscript. This chapter was prepared in 1997 and partially updated in 1999.
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