Researchers have discovered that an asymmetrical brain feature that’s widely shared among people is barely visible in chimpanzees. While they’re not sure about the function of this uniquely human asymmetry, its location suggests that it has to do with communication and social cognition. The findings, published in Proceedings of the National Academy of Sciences this week, may help us understand the evolution of human language.
A large international team led by François Leroy of the INSERM-CEA Cognitive Neuroimaging Unit studied the brain images of 177 people, who ranged in age from infancy to adulthood. The participants were divided into “typical” and “atypical” groups. The latter included people who are left-handed, show left or right hemisphere lateralization for language (that is, one side of the brain is more dominant than the other in certain aspects of language), as well as patients with autism, corpus callosum agenesis, situs inversus, or Turner syndrome. Some of these factors are known to affect brain asymmetry.
They found an asymmetrical feature in the superior temporal sulcus—an area of the brain that’s important for communication. Called the “superior temporal asymmetrical pit” (STAP), this 45-millimeter-long segment is consistently deeper in the right hemisphere than in the left hemisphere in 95 percent of their “typical” human subjects. It’s been found in people of all ages, and the fact that it shows up in infants suggests that it’s genetic. Its presence is irrespective of handedness, language lateralization, and sex (though it appears slightly more frequently in males than females). The groove was also seen in several “atypical” subjects with situs inversus, Turner syndrome, and corpus callosum agenesis.
Pictured here, you can see the location of STAP (yellow) relative to other brain areas including Heschl’s gyrus (blue) and the ventral tip of the central sulcus (green) on both the left and right inner cortical surfaces of an adult brain. The STAP center is marked with a red cross.
Different sides of the brain are involved in different tasks. The deeper groove in the right brain lies in the region that controls voice and face recognition and working out what other people are thinking, New Scientist explains, while the shallower groove on the left is central to regions previously linked with language.
Furthermore, the team also analyzed brain images of 73 chimpanzees and discovered that the groove is a human-specific brain landmark. "Asymmetrical brain landmarks may be key features to understand what is so specific in our species," Leroy tells New Scientist. "We think that [this asymmetry] is related to either speech or social cognition, which are both abilities for which humans outperform other primates."
Images: shutterstock.com (top), F. Leroy et al., PNAS 2015 (middle)