SHORT COMMUNICATION Fiber composition in the planum temporale sector of the corpus callosum in chimpanzee and human Giorgio M. Innocenti • Roberto Caminiti • Patrick R. Hof Received: 2 March 2010 / Accepted: 31 May 2010 / Published online: 24 August 2010 Ó Springer-Verlag 2010 Abstract In humans the planum temporale is usually larger in the left hemisphere and related to Wernicke’s language complex. A slighter leftward asymmetry, unre- lated 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. Keywords Cerebral cortex Á Evolution Á Hemispheric interactions Á Lateralization Á Axons Á Primates Introduction In a previous study (Caminiti et al. 2009) we have been searching for differences in the callosal connections of the human brain compared to the chimpanzee. The search was justified by the fact that the corpus callosum (CC) contains fibers interconnecting the cortical areas of the two hemi- spheres which are different both anatomically and func- tionally in humans, but not in non-human primates. However, no differences could be found. Therefore, in the present study we specifically analyze callosal axons inter- connecting the planum temporale, which in the human brain is very asymmetrical, with a leftward dominance, and con- tains cortical areas included within Wernicke’s language complex (Geschwind and Levitsky 1968; Shapleske et al. 1999) although its precise role in language is debated (Hickok and Poeppel 2007). In the chimpanzee, there is also a leftward asymmetry (Gannon et al. 1998; Hopkins et al. 2008), but with smaller differences in surface area (Hopkins and Nir 2010). Cytoarchitectonic differences related to a wider spacing of minicolumns in the left planum temporale are found in humans and lack in non-human primates (Buxhoeveden et al. 2001). Finally, unlike in humans, in chimpanzee vocal perception seems to involve mainly the right temporal region (Tagliatela et al. 2009). Electronic supplementary material The online version of this article (doi:10.1007/s00429-010-0274-9) contains supplementary material, which is available to authorized users. G. M. Innocenti (&) Department of Neuroscience, Karolinska Institutet, Retzius va ¨g 8, 17177 Stockholm, Sweden e-mail: Giorgio.Innocenti@ki.se R. Caminiti Department of Physiology and Pharmacology SAPIENZA, University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy e-mail: caminiti@uniroma1.it P. R. Hof Department of Neuroscience, Mount Sinai School of Medicine, One Gustave L. Levy Place, New York, NY 10029, USA e-mail: patrick.hof@mssm.edu 123 Brain Struct Funct (2010) 215:123–128 DOI 10.1007/s00429-010-0274-9