Snakes And The Brain: How Our Visual Systems Evolved To Protect Us From Slithering Predators

cobra
Snakes have a curious effect on human and primate brains, which seem to have developed innate fears of the reptiles over millions of years. RussBowling, CC BY 2.0

We humans have a near-perfect set of tools with which to protect ourselves against potential threats. Our acute sense of hearing picks up the faintest rustle in the grass, and our visual systems stay trained ahead, forming depth perception as our eyes work in tandem, with one another and with our other senses. It should come as no surprise that when Lynne Isbell was passing through a Kenyan glade in 1992, she instinctively stopped, right at the doorstep of a ready-to-strike cobra.

An anthropologist from the University of California Davis, Isbell grew curious after the experience. She wondered why her brain knew to halt all motion, even if she hadn’t had time to consciously form the thought in her head: A deadly snake is in front of me, I should stop walking. Her brain’s automatic, evolutionarily honed mechanism for fear response took over. The incident has since spurred decades of research in Isbell’s lab, and it’s gotten medical science ever closer to understanding humans’ and primates’ innate senses of fear.

"At first I thought it was luck," she said in a recent interview with NPR. "But now I'm pretty sure that it's not luck. It's a reflection of 60 million years of evolutionary history working on my visual system."

Isbell’s latest findings shine a spotlight on a region in the brain called the pulvinar. Derived from the Latin pulvinus for “cushion,” the bundle of nuclei sits beneath the thalamus, in the middle of the brain. It’s only found in humans, primates, and monkeys. Scientists believe lesions to this area can trigger attention deficits and neglect syndromes. Isbell’s research adds to its list of duties, as studies of primate pulvinar reveal a potential answer for her unconscious reaction in Kenya in 1992.

She found out, for instance, that primates in parts of the world where snakes are less common tend to have worse visual systems. Monkeys in Madagascar have terrible eyesight, she says, and it’s no accident: Madagascar has no venomous snakes.

To root these claims in empirical evidence, Isbell designed a test involving two monkeys who, in theory, would’ve never seen snakes before. She teamed up with researchers from Japan and put two macaques in front of screens that either displayed fear-producing images of snakes, or benign images of hands, faces, and shapes. Researchers found that out of 100 neurons that fired in the presence of at least one image, 40 percent of the neurons fired when that image featured a snake. The next largest group was faces, at 29 percent.

"There are neurons that are very sensitive to snake images and much more sensitive to them than the faces of primates," Isbell told NPR. Part of the explanation is how humans and primates have their visual systems oriented in the body.

"We have our forward-facing eyes," she says. "We have our excellent depth perception. We have very good visual acuity, the best in the mammalian world. We have color vision. So there has to be some sort of explanation for it."

Isbell’s research sets an important foundation for studying how primates’ brains develop sense of fear. While prior research has shown that Malagasy lemurs do not display outward responses of fear to snakes, perhaps sensing they aren’t venomous, Isbell’s findings echo across the scientific community as having great salience.

"There have been a lot of people suspecting that there must be something like this going on," Sue Mineka, a clinical psychology professor at Northwestern University, explains. Mineka’s research pokes holes in Isbell’s theory of innate fear, though she concedes there is much to be gained from the present study.

"We have known that many species of monkeys either have an innate fear of snakes or pick up a fear of snakes very readily,” she told the Agence France-Presse. “This provides a probable mechanism. That has been a huge question in the literature."

 

Source: Van Le Q, Isbell L, Matsumoto J. Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes. PNAS. 2013.

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