Uncovering the inferior fronto-occipital fascicle and its topological organization in non-human primates: the missing connection for language evolution.

Whether brain networks underlying the multimodal processing of language in humans are present in non-human primates is an unresolved question in primate evolution. Conceptual awareness in humans, which is the backbone of verbal and non-verbal semantic elaboration, involves intracerebral connectivity via the inferior fronto-occipital fascicle (IFOF). While non-human primates can communicate through visual information channels, there has been no formal demonstration that they possess a functional homologue of the human IFOF. Therefore, we undertook a post-mortem diffusion MRI tractography study in conjunction with Klingler micro-dissection to search for IFOF fiber tracts in brain of Old-World (vervet) monkeys. We found clear and concordant evidence from both techniques for the existence of bilateral fiber tracts connecting the frontal and occipital lobes. These tracts closely resembled the human IFOF with respect to trajectory, topological organization, and cortical terminal fields. Moreover, these fibers are clearly distinct from other bundles previously described in this region of monkey brain, i.e., the inferior longitudinal and uncinate fascicles, and the external and extreme capsules. This demonstration of an IFOF in brain of a species that diverged from the human lineage some 22 millions years ago enhances our comprehension about the evolution of language and social behavior.