The primate brain comprises functional networks: different cerebral regions communicate among themselves by way of privileged pathways to carry out specific functions (analyzing information, controlling movement, etc.). Humans however stand out as the only primate to have certain cognitive capacities, such as a strong aptitude for abstract thought. Among the brain's major networks, the default mode network (DMN) is particularly enigmatic. First identified in humans, the DMN is active when the subject is not focused on outwardly tasks. It is associated instead with introspection, self-referential thought and future planning.

Does this DMN exist in other primates as well? 

Partnering with colleagues from France¹, Canada and the United States, researchers from MIRCen's Neurodegenerative Diseases Laboratory (CNRS/CEA/Paris-Saclay University) employed functional magnetic resonance imaging (fMRI) to characterize the cerebral networks of four primate species: humans, macaques, marmosets, and mouse lemurs, the latter being the world's smallest primate. Their study was brought about by the earlier discovery of mouse lemur cerebral networks by another MIRCen team. For that earlier work, the researchers developed a methodology for the complex analysis of fMRI images. For the work presented here, the international team adapted that methodology so that it was able to explore the brains of the four primates.

Published in Cell Reports, the results of the study showed that the DMN of humans did not exist identically in the non-human primates under study. In awake-resting humans, the DMN shows connections between specific brain regions. Included therein is the medial prefrontal cortex, responsible for information manipulation, and the posterior cingulate cortex, involved in the regulation of interactions between cerebral regions.

In the study's non-human primates, a number of the connections characteristic of the human DMN were not present, and importantly, their medial prefrontal cortex was only poorly connected with the posterior cingulate cortex.

The results suggest that the connective association of these two brain regions occurred only recently in primate evolution and led to a remodeling of the DMN in humans. That remodeling may have facilitated abstract thought capacities and thus contributed to a cognition increase in humans.

Indeed, the medial prefrontal cortex has an important role in cognitive processes, such as the flexible manipulation of information, the elaboration of non-impulsive decision-making strategies, emotional regulation, motivation and sociability. The posterior cingulate cortex is important for internally-directed cognition and the regulation of attention toward a task.

1 - Marc Jeannerod Cognitive Neuroscience Centre (CNRS/Claude Bernard Lyon 1 University)