Abstract
Analysis of regional corpus callosum fiber composition reveals that callosal regions connecting primary and secondary sensory areas tend to have higher proportions of coarse-diameter, highly myelinated fibers than callosal regions connecting so-called higher-order areas. This suggests that in primary/secondary sensory areas there are strong timing constraints for interhemispheric communication, which may be related to the process of midline fusion of the two sensory hemifields across the hemispheres. We postulate that the evolutionary origin of the corpus callosum in placental mammals is related to the mechanism of midline fusion in the sensory cortices, which only in mammals receive a topographically organized representation of the sensory surfaces. The early corpus callosum may have also served as a substrate for growth of fibers connecting higher-order areas, which possibly participated in the propagation of neuronal ensembles of synchronized activity between the hemispheres. However, as brains became much larger, the increasingly longer interhemispheric distance may have worked as a constraint for efficient callosal transmission. Callosal fiber composition tends to be quite uniform across species with different brain sizes, suggesting that the delay in callosal transmission is longer in bigger brains. There is only a small subset of large-diameter callosal fibers whose size increases with increasing interhemispheric distance. These limitations in interhemispheric connectivity may have favored the development of brain lateralization in some species like humans.
Highlights
The corpus callosum is a unique feature of the brain of placental mammals, so much so that we may state that it can be a diagnostic character of placentals just like the very placenta is
This structure was first described in 1836 by Owen, who noticed that the corpus callosum was absent in the brain of the wombat, which instead had a well-developed dorsal or hippocampal commissure located ventral to the hippocampi
This observation would agree with the concept of a gradual origin of the corpus callosum, since fibers that originally traveled through the dorsal commissure might have found a different route across the hemispheres
Summary
The corpus callosum is a unique feature of the brain of placental mammals, so much so that we may state that it can be a diagnostic character of placentals just like the very placenta is. The absence of any kind of callosal primordia in nonplacental mammals implied a sudden evolutionary origin of the corpus callosum, with no structures ancestral to it. In 1865, some thirty years later than Owen, Flower [2] and later Smith [3] argued that both the corpus callosum of placentals and the dorsal commissure of marsupials contained fibers connecting the anterior part of the mesial hemisphere and were partly homologues. Flower may have been right in that some neocortical fibers might cross through the hippocampal commissure in marsupials, which would explain its large size Aside from these historical considerations, the corpus callosum remains an evolutionary puzzle. Which circumstances were involved in the origin of the corpus callosum, and why was it maintained in the history of placental mammals? is the corpus callosum related to the emergence of brain lateralization in humans and to conscious experience? [4] In this paper, we will discuss our studies on the comparative fiber composition of the corpus callosum, which in our view may provide important insights into the problems raised above
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