What are the structural differences in the brain between animals that are self-aware (humans, apes) and other vertebrates?
Emma Schachner, Salt Lake City

Robert O. Duncan, a behavioral scientist at York College, the City University of New York, responds:

Self-awareness distinguishes humans from most other species. In psychology, self-awareness is defined as metacognition, awareness of one’s own ability to think. In humans, metacognition and other advanced cognitive skills, such as social intelligence, planning and reasoning, are all thought to depend on a region of the brain called the prefrontal cortex.

If we assume that the prefrontal cortex permits metacognition, then the answer is simple: species that fail to demonstrate metacognition tend to lack brain areas that resemble the prefron­tal cortex. But because this area serves many cognitive functions and is well connected to the rest of the brain, the region is probably not the sole locus of metacognition. In other words, the prefrontal cortex may be necessary but not sufficient for self-awareness. Some psychologists speculate that self-awareness may arise in animals with greater overall cognitive ability, larger brain size or a higher degree of connectivity among brain areas.

Identifying the precise structural differences that make some creatures self-aware and others not is quite challenging. Most important, it is difficult to pinpoint and compare subtle structural differences across species in the face of more dramatic differences in brain morphology. For example, dolphins and chimpanzees both demonstrate metacognition, but their brains look completely different.

Additionally, simply identifying which species exhibit self-aware behavior has proved tricky because no reliable behavioral tests for the trait exist. In 1970 Gordon G. Gallup, Jr., of the University at Albany, S.U.N.Y., developed the “mirror test” to assess metacognition in chimpanzees. A chimp passes the test if it uses the mirror to inspect a mark that has been painted on its face. Although the majority of chimps pass, some do fail, causing certain scientists to consider the test unreliable.

The difficulties we have assessing self-awareness demonstrate that it is a complex trait and support the idea that no single brain area is dedicated to it. Overall, the prefrontal cortex may be critical for metacognition, but self-awareness most likely emerges when this region is highly interconnected with the rest of the brain.