Future Directions

Giorgio Innocenti suggests that the complexity of callosal connections promises that these functional problems will remain fascinating but frustrating for some time. He points out that there is not yet an adequate understanding of the neural parameters that contribute to differences in size and shape of the callosum, including the number and size of axons that make up the callosum, the proportion of those axons that are myelinated, the thickness of the myelin sheath, and the number and size of blood vessels and other supporting elements. Although work in numerous laboratories is focused on answering some of these questions, until they are answered, studies correlating the size of the callosum or any of its sections to behavior are likely to continue to be frustrating.

Many of the fascinating but unproven hypotheses discussed previously could be more clearly addressed or ruled out if we had a better understanding of the neurophysiology of the human corpus callosum. Relatively little work has been done on the human callosum, although there is a large body of correlative data on differences in the morphology of the callosum derived from MRI and various aspects of behavior. However, there is still a very limited understanding of the anatomy of the callosum, where the crossing fibers originate and terminate, and the differences in the proportion of large and small fibers in different areas. Little is known about the connectivity of interneurons and how it affects the nature of the information transferred.

The advent of MRI led to a major increase in the number of studies examining gross correlations between anatomy and behavior, but the lack of a better understanding of the anatomy of callosal fibers and the basic mechanisms of transmission allowed too much freedom of interpretation. Hence, many of these studies reached inconsistent or conflicting conclusions.

A new method of image analysis known as diffusion tensor weighting is allowing researchers to examine the direction of movement in individual fiber tracts during different cognitive tasks. As this method is refined, and if it can be combined with a more explicit knowledge of callosal anatomy, the plethora of theories regarding the function of the callosum and its contribution to cognition may at last be open to more satisfying investigation.

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