Across three primates, a brain network linked to self-processing differs in its connections

The human brain did not invent from scratch every structure associated with processing information about one’s body and oneself. A comparison of human, chimpanzee and macaque brain scans found similar local characteristics across species but differences in how distant regions are connected. Published in the Journal of Neuroscience, the analysis describes the architecture of a network proposed to support such processing; it does not measure subjective experience or show that one species is more conscious than another.

A team led by Ning Liu of the Chinese Academy of Sciences’ Institute of Biophysics examined scans from 46 humans, 46 chimpanzees and 43 macaques. It started with a set of human brain regions linked by earlier studies to sensing the body’s internal state, external information concerning oneself and reflection on oneself. To find comparable regions in the other brains, the researchers used connectivity patterns across 42 major white-matter tracts—the fibers linking parts of the brain—that are shared among the three species.

At the local scale, an MRI contrast sensitive to myelin, the coating around nerve fibers, showed a highly similar spatial distribution in the human and macaque maps; comparable gene-expression patterns were also partly conserved. Those measures describe tissue properties, not performance on behavioral tasks. Across larger-scale connections among regions, however, the organization varied more between species, with chimpanzees intermediate in the overall pattern examined.

The differences were not uniform. In subdivisions of the network defined from human research, maps associated with signals from inside the body were most alike in macaques and chimpanzees; for external information related to oneself, chimpanzees and humans were closer. The subdivision associated with mental reflection on oneself showed greater separation. The locations of highly connected hubs shifted as well: from the insula in macaques toward the cingulate cortex in humans, with both represented in chimpanzees.

The narrower conclusion is that similarity in some local features need not imply the same large-scale wiring. The map suggests testable hypotheses about the evolution of circuits, but assigning a region to a network mapped in humans does not prove it has the same function in every primate. Linking these anatomical differences to measured tasks and behaviors is the next step toward understanding what they changed in the animals’ lives.