Snakes in the brain

On a trip to the Greensboro Sciquarium this past summer, my poor sister suffered from her 4-year-old son’s love of creepy crawlies. He insisted on being held up so he could press his face against the tanks containing giant tarantulas, and happily reached out to pet a giant snake (held by a zookeeper) while my sister watched from several yards away. His excitement and her dread were on my mind as I read about the recent discovery of neurons in the brain that detect snakes, and only snakes. This has brought new attention to the “snake detection theory”, the idea that the need to detect snakes (and other threats) drove the evolution of our brains.

Snakes on the brain, by Design_Discord at zazzle.com

Snakes on the brain, by Design_Discord at zazzle.com

The idea that neurons respond to specific stimuli is nothing new. Neuroscientists have been using fine electrodes to record the activity of single neurons (not in humans) for longer than we’ve had MRIs and the potential to get fancy images of the brain. Back in the 1950s, through painstaking work showing all varieties of images to cats and monkeys (apparently including “sexy women” at one point, probably in the midst of an all-nighter), neurobiologists David Hubel and Torsten Wiesel determined that individual neurons in the visual cortex respond to very specific sights. Some neurons only respond to perfectly horizontal lines, others to lines of only 15 degrees. This work won them a Nobel prize. Building on that work, other neuroscientists have identified neurons that only respond to specific objects, like hands, or specific facial expressions, like a grimace.

The surprise to me was the location, the pulvinar, partly because I didn’t recognize the name. This turns out to be another example of the fractal brain at work, because the pulvinar is actually a small region of the thalamus, the central hub of our senses. Information for your eyes passes through the thalamus on the way to the visual cortex for processing.

Detecting snakes in the pulvinar instead of the usual object-recognition pathways of the brain makes sense, in the same way that most reflexes do – this kind of automatic, unconscious processing is faster and probably saves lives from snake bites. That doesn’t necessarily mean that it’s evolutionarily hardwired into our genes, though.

The more compelling evidence was brought to my attention by a New York Times article that highlights the possibility that children don’t seem to learn to recognize snakes. Children from the Indian city of Bangalore – where snakes are understandably rare, and children have probably only seen them in books or movies – are just as fast at picking snakes out of a group of (non-threatening) reptiles as children from rural areas where snakes are common and children are taught what to do if they encounter one. If recognizing snakes is something we learn, then it should depend on experience or instruction – not be faster than detecting other kinds of animals, no matter what kind of snake background you have.

And what does all this mean for my nephew’s love of creep crawlies? So far, the evidence is only for a visual recognition of snakes being hard-wired in somehow; the pulvinar has nothing to with emotion (so far as we know). My nephew’s attraction to snakes could have the same basis in a speedy recognition of these animals as my sister’s aversion to them. Evolution might give us a heads-up on these creatures, but may not have programmed everything we feel about them.

ResearchBlogging.org
Van Le Q, Isbell LA, Matsumoto J, Nguyen M, Hori E, Maior RS, Tomaz C, Tran AH, Ono T, & Nishijo H (2013). Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes. Proceedings of the National Academy of Sciences of the United States of America, 110, 19000-19005 PMID: 24167268
Penkunas MJ, & Coss RG (2013). A comparison of rural and urban Indian children’s visual detection of threatening and nonthreatening animals. Developmental Science, 16, 463-475 PMID: 23587043

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