Differential expansion of neural projection systems in primate brain evolution.

Whole brain MRI scans from 11 primate species (43 individuals) spanning more than a 50-fold range in brain volume were used to determine whether the corpus callosum keeps pace with the growth of the forebrain among living anthropoid primates. Interhemispheric connectivity via the corpus callosum and anterior commissure was reduced in larger primate brains, whereas intrahemispheric connectivity was augmented. We also show that the splenium constitutes an increasing proportion of callosal area with increasing brain size. This may function to maintain rapid integration of the left and right visual space as brain size increases. These results indicate that the evolution of larger brain size in primates results in increasingly independent hemispheres.