Fiber composition in the planum temporale sector of the corpus callosum in chimpanzee and human

In humans the planum temporale is usually larger in the left hemisphere and related to Wernicke's language complex. A slighter leftward asymmetry, unrelated to vocal perception, was reported in the chimpanzee. Searching for differences between the human brain and that of the chimpanzee, we analyzed the fiber composition in the sector of the corpus callosum containing fibers from the planum temporale. This sector was identified in chimpanzee and human myelin-stained materials by comparison with anatomical tract-tracing in the macaque monkey. The axon diameters in the planum temporale sector of the corpus callosum were not different in human and chimpanzee, suggesting that this feature of the output of the planum temporale was preserved since the common ancestor of both species and may not be uniquely related to language. However, the larger size of the human brain probably amplified slow and temporally dispersed conduction between the hemispheres. A trend with thicker axons dorsally and thinner axons ventrally in the corpus callosum was evident in human brain, but was much weaker, or absent in the chimpanzee.