The root of humans’ unique intelligence is a perennial question, but one that scientists recently approached from a completely new angle. Past research centered around structural differences between the brains of humans and primates—the hardware—while differences in function and communication—the software—remained a mystery.
Those software differences have come into focus through a study published in the journal Cell . The findings support the idea that an evolutionary trade-off occurred over time: Communication within the brain became more efficient and complex but less reliable. This dynamic may have contributed to human intelligence while also rendering people more vulnerable to mental illness.
“It’s groundbreaking. It’s the first study to go beyond structural differences between humans and animals to look at how the brain computes information,” says Ziv Williams, a neurosurgeon at Harvard Medical School, who was not involved with the research. “It opens an important window into why we’re different from other animals.”
These differences were not easy to observe: They required implanting electrodes to monitor specific neurons in the brain, an invasive procedure that scientists cannot perform on healthy human subjects. To address this challenge, neurobiologist Rony Paz and his colleagues at the Weizmann Institute of Science in Israel collaborated with neurosurgeon Itzhak Fried at the University of California Los Angeles. Fried works with patients with treatment-resistant epilepsy by implanting electrodes in the brain to identify where the seizures originate. Fried obtained data from seven people during the course of their procedures. Paz’s team collected similar data from five macaque monkeys.
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The scientists recorded the electrical activity of 747 single neurons as patients rested, talked, or watched videos and as monkeys rested or heard auditory cues. The variety of tasks allowed the researchers to assess characteristics of brain communication generally rather than brain communication tethered to a specific task. The neurons were located in two brain regions. One was the amygdala, an ancient area devoted to emotion, threats, and survival. The second was the cingulate cortex, a more modern piece of neural real estate that governs thinking, learning, and decision-making.
The team measured two key aspects of communication between neurons: robustness and efficiency. Robustness is the degree to which different neurons express the same signal at the same time, which helps create a reliable message. Efficiency refers to neurons delivering more varied signaling patterns while using less energy, which helps cultivate rich, complex communication within the brain. The two characteristics are similar to conversing at varying levels of language, Paz explains. If you posed a question to a group of people who knew just a few words of a language, their answer would be simple but reliable: Everyone would say the same thing at the same time. If you asked the same question to a group of fluent language speakers, their larger vocabularies would lead to detailed and complex answers, but those answers would be less consistent.
The electrical activity observed in the two groups revealed that the monkeys had more reliable but less efficient signaling than humans. For both animals, the amygdala was more reliable and less complex than the cortex, which falls in line with the different responsibilities of each region.
“The amygdala is where you want all the neurons shouting together, ‘there’s a tiger!’” says Paz, the senior author on the study. “You don’t want some neurons to say, ‘this is a brown tiger’ and some to say, ‘this is a furry tiger.’ That’s informative and interesting, but it’s not for the amygdala. The amygdala wants to say, ‘run!’”
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Yet the four brain regions studied—two in each species—portray a gradient of evolving complexity. At the bottom, with strong reliability but less efficiency, is the monkey amygdala. Next comes the monkey cortex, then the human amygdala, and, finally, the human cortex. The human brain may have sacrificed reliability to develop the advanced cognitive skills that we possess today.
That shift, however, may have also made people more vulnerable to mental illness, Paz says. In the case of anxiety and PTSD, for instance, diminished reliability may lead to a less accurate threat response, Paz says. The brain may respond to a threat that poses no real danger; perhaps the situation was loosely related to a threat experienced in the past.
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“Greater efficiency has created higher cognition, but the price we pay is that we may be more prone to error. This could lead to psychiatric disorders, specifically post trauma and anxiety,” Paz says. The theory may also be relevant for autism, schizophrenia, and other psychiatric conditions, since most disorders involve disrupted communication between the amygdala and cortex, he says.
“The idea is that the less consistent you are, the more you leave yourself vulnerable to failure,” Williams says. “I can see this making sense, but there’s no way to tell unless you test it out. This study gives you the framework to start testing these questions.”
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