One hundred million years of interhemispheric communication: the history of the corpus callosum

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.

Estimation of the Mean Axon Diameter and Intra-axonal Space Volume Fraction of the Human Corpus Callosum: Diffusion q-space Imaging with Low q-values

“…Fiber composition along the corpus callosum is a rough representation of the topography of the different cortical areas, in which the prefrontal areas are connected through the anterior part (genu), the primary and secondary sensorimotor areas are connected through the midbody, and the temporal, parietal, and occipital areas are connected through the posterior part of the corpus callosum (splenium). 23,37,38 In our in vivo experiment in healthy volunteers, we observed the estimated mean axon diameter was higher in the midbody of the corpus callosum (A3 to A6) and smaller in the genu (A2) and splenium (A7). This anterior-to-posterior trend of estimated mean axon diameter is generally consistent with findings reported by Aboitiz and colleagues 23 from a study of the post-mortem human corpus callosum using light and electron microscopy.…”

In Vivo Mapping of Fiber Pathways in the Rhesus Monkey Brain

“…It presents with a topographical representation of the different cortical areas as established in different placental mammals (reviewed in [22]). In macaque the topography reflects the wide diversity of fiber calibers [12,[23][24][25]. DTI confirms that many callosal connections are homotopic and connect equivalent regions between the two hemispheres (for details see [12]).…”

The correlation between white-matter microstructure and the complexity of spontaneous brain activity: A difussion tensor imaging-MEG study

“…Correlations have been found in regions including commissural (Corpus Callosum), projection (Sagittal stratum), and association fibers (cingulum/parahippocampal WM), predominantly in posterior regions of the left hemisphere. The splenium of the CC, where FA is greater (Chepuri et al, 2002), is traversed by three functionally specialized groups of myelinated fibers that bidirectionally connect parietal, temporal and occipital lobes with a roughly ordered arrangement of axons according to their origin (Aboitiz and Montiel, 2003;Huang et al, 2005;Dougherty et al, 2007;Zarei et al, 2006;Hasan et al, 2009). Fibers from the occipital lobe, from the lateral and caudal portions of the parahippocampal gyrus and from the caudal temporal lobe travel through the splenium, where our "target" correlation areas seem to be located.…”