Smaller Relative Size of the Corpus Callosum

The corpus callosum size is relatively smaller in humans than in other primates. The corpus callosum is the main interhemispheric commissure linking the cerebral cortex of the left and right cerebral hemispheres in placental mammals. Analysis of whole brain MRI scans from 11 anthropoid primate species showed that as brain size increases across the primate order, the mid-sagittal area of the corpus callosum also increases, however this increase does not keep pace with increases in the surface area of the cerebral cortex that the corpus callosum connects. As a predictable consequence of this trend, the large human brain has the smallest corpus callosum of any primate, relative to either brain size or cortical surface area. This negative allometry may reflect the difficulty of maintaining interhemispheric transfer times in larger brains where interhemispheric connections are necessarily longer, and may represent a shift towards processing information in smaller, local networks within a hemisphere. According to this hypothesis, the two hemispheres are expected to be become more independent of each other (i.e., more lateralized) in larger brains like the human brain.  There is a fairly simple rostro-caudal mapping of callosal pathways to cerebral cortex. For example, the posterior fifth of the corpus callosum, known as the splenium, connects primary and secondary visual cortex in the occipital lobe at the back of the brain, as well as some immediately adjacent regions.  The splenium scales at a higher exponent of allometry than other callosal regions, perhaps reflecting the need to preserve integration of left and right visual hemifields with increasing brain size.