The inferior frontal sulcus in the chimpanzee brain: morphological examination and association with oro-facial communication

There has been considerable interest in the neuroanatomical homologue of Broca's region in nonhuman primates, particularly in our phylogenetically closest relative the chimpanzee (Pan troglodytes), to further our understanding of the evolution of language in hominins. The inferior frontal sulcus (ifs) is a critical landmark below which lie language production areas in the language-dominant hemisphere of the human brain, and its homologue is found in the chimpanzee brain. The present study examined the variability in the morphology of the ifs in 73 chimpanzee brains (ie 146 hemispheres). Examination was carried out on surface reconstructions derived from in vivo 3 T magnetic resonance imaging scans, which permitted detailed intra-sulcal assessment. Based on the connections formed by the ifs and adjacent sulci, two frequent and three less frequent morphological types were identified. The average location and spatial variability of the ifs were quantified in the form of spatial surface probability maps. Relationships were also established between specific morphological features of the ifs in the left hemisphere and oro-facial communication abilities, as measured by attention-getting sound production. These findings refine our knowledge of prefrontal sulcal organization in hominids and offer valuable insights into the structural blueprints that supported the emergence of speech in humans.