A Morphological Comparison of the Caudal Rami of the Superior Temporal Sulcus in Humans, Chimpanzees, and Other Great Apes

For centuries, anatomists have charted the folding patterns of the sulci of the cerebral cortex in primates. Improvements in neuroimaging technologies over the past decades have led to advancements in the understanding of the sulcal organization of the human cerebral cortex, yet comparisons to chimpanzees, one of humans' closest extant phylogenetic relatives, remain to be performed in many regions, such as the caudal temporoparietal region (cTPR). For example, while several posterior branches, or rami, of the superior temporal sulcus (STS) have been identified within the cTPR in great apes since the late 1800s, no study has yet comprehensively identified and quantitatively compared these rami across species. To fill this gap in knowledge, in the present study, we defined the three caudal branches of the STS (cSTS) in 72 human and 29 chimpanzee brains (202 total hemispheres) and then extracted and compared the morphological (depth and surface area) properties of these sulci. We report three main findings. First, modern methods replicate classic findings that three rami of the posterior STS are unique to the hominid lineage (i.e., humans and great apes). Second, normalizing for brain size, the cSTS rami were relatively deeper in chimpanzees compared to humans. Third, the cSTS branches were relatively larger in surface area in humans compared to chimpanzees. Finally, we share probabilistic predictions of the cSTS to guide the identification of these sulci in future studies. Altogether, these findings bridge the gap between historic qualitative observations and modern quantitative measurements in a part of the brain that has expanded substantially throughout evolution and that is involved in human-specific aspects of cognition.