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2013

Afraid of Snakes? Your Pulvinar May Be to Blame
New York Times, October 31, 2013
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Multiple Walter Whites will walk the streets on Thursday in search of candy. But some frights endure the fashion cycle and never go out of style.

This week in Proceedings of the National Academy of Sciences, a team of scientists examines one particularly long-lasting source of fear: snakes. The researchers found that certain neurons in the brain only respond to these legless reptiles. These snake-dedicated neurons, they argue, are a legacy of our distant primate past, when the animals posed one of the greatest threats to our survival.

The new study builds on years of experiments by psychologists. They found that the widespread fear of snakes stems from a perceptual bias: people recognize snakes faster than other objects.

This bias toward snakes isn’t simply the result of learning to fear them. Children recognize snakes just as quickly as adults. In a study published earlier this year in Developmental Science, psychologists found no difference in the response to snakes when they compared children who grew up in cities with children who grew up in rural areas where they regularly encountered snakes.

Scientists find a similar bias among our primate cousins. In a 2009 study, for example, Japanese researchers showed macaque monkeys sets of pictures of snakes and flowers. In some trials, they saw one snake amid eight flowers; in others, one flower amid eight snakes. The monkeys got a reward — a squirt of juice — when they pointed to the oddball object. They consistently pointed to a snake faster than to a flower. Their swift response was particularly striking because they had never seen a live snake.

The fact that we share this bias toward snakes with monkeys suggests that it evolved in our common ancestors. When primates evolved some 60 million years ago, they adapted to living in trees, searching for food at night and sleeping in the canopy during the day. Snakes creeping through those trees were among their deadliest enemies.

In her 2007 book “The Fruit, the Tree, and the Serpent: Why We See So Well,” Lynne A. Isbell, an anthropologist at the University of California at Davis, argued that a lot of the features that make primates unique evolved as defenses against snakes. Most importantly, the vision of primates became much more powerful so that they could detect snakes lurking in the foliage.

Recently, neuroscientists at the University of Toyama in Japan and the University of Brasilia in Brazil teamed up with Dr. Isbell to put her “Snake Detection Theory” to the test by examining a brain region known as the pulvinar. The neurons in the pulvinar receive signals from the eyes, and previous studies have suggested that they help to direct our attention quickly to particular objects in our view. People with damaged pulvinars can’t help but get distracted.

Other mammals have pulvinars, but the primate pulvinar is special. It contains extra clusters of neurons not found in other species. Dr. Isbell and her colleagues hypothesized that those extra neurons evolved specifically for recognizing snakes.

“I thought that region would be a likely place for finding neurons like this,” said Dr. Isbell.

The researchers threaded electrodes into the pulvinars of two macaque monkeys and then trained them to press a button each time they saw a picture. As the monkeys gazed at the pictures, the electrodes detected signals from the pulvinar. The researchers used the electrode recordings to decipher the individual behavior of 91 different neurons.

Some of the pictures they showed were of snakes. Others were of monkey faces, monkey hands and geometric shapes. The scientists found that when they showed snakes to the monkeys, most of the 91 neurons fired strongly. At the sight of other pictures, they stayed quiet. The result was particularly striking given the fact that the monkeys had been raised at a primate facility where they probably had never before seen a snake.

“The research is very compelling,” said Vanessa LoBue, a Rutgers psychologist who studies fear of snakes in children.

But other experts weren’t yet ready to accept that the “snake neurons” were indeed exclusively responding to snakes. They didn’t think the experiment could rule out the possibility that the neurons fire in response to any enemy of the monkeys, like a leopard or an eagle.

“I would be much more convinced if they also tested photos of other known primate predators,” said Jason Kamilar of Arizona State University. Dr. Isbell and her colleagues are now running those tests.

Since humans have the same extra clusters of neurons in the pulvinar, Dr. Isbell and her colleagues predict that they are also snake-sensitive. But even if we have neurons dedicated to detecting snakes, that doesn’t mean that we have to be terrified of them. Once these neurons become active, they may trigger different responses, depending on our experiences with the serpentine world.

“People who like snakes are experiencing the same visual thing as people who hate snakes,” said Dr. Isbell.

Copyright 2013 The New York Times Company. Reproduced with permission.
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