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ROBERT SIEGEL, host: Animals have a special place in the human heart. And now, researchers have found creatures great and small also have a special place in our heads. NPR's Jon Hamilton has that story.

JON HAMILTON: When a person sees an animal, any animal, the brain responds with astonishing speed. Christof Koch, a researcher at Caltech, says he was reminded just how fast during a recent run along a mountainous trail in Los Angeles.

CHRISTOF KOCH: As I was about to step down, I saw there was a rattlesnake. By the time I realized it, by the time I felt fear, you say, oh, my God, there's a snake, I had already automatically extended my legs, my stride was larger, so I didn't step on the snake but just, you know, a couple of inches beyond it. And then, of course, I ran past.

HAMILTON: Without getting bitten. Koch says that's a good example of why our brains pay special attention to animals. And he says scientists are beginning to understand precisely how the brain does this. He and other researchers have been looking at how individual brain cells respond to lots of things, including animals.

KOCH: To our surprise, in this part of the brain called the amygdala, there are lots of cells that seem to be specialized to detect animals, you know, spiders and pictures of dogs and puppies and little rodents.

HAMILTON: In other words, our brains have specific circuits for recognizing animals. Koch discovered this while studying the brains of people about to have surgery for severe epilepsy. In order to find the source of their seizures, doctors placed electrodes deep in the brain. Koch says it makes sense that they found a response in the amygdala. It's an almond-shaped part of the brain that appeared quite early in evolution.

KOCH: The amygdala seems to be specialized in alerting us to things that are emotionally important to us, either positive or because they're, you know, they're scary.

HAMILTON: And animals are both. Some want to eat us. Others could be our dinner. And some, we just want to cuddle.

KOCH: We found in one patient a cell that I call the Peter Rabbit cell because it responded to three very cute images. One was a rabbit, one was a white snow hare, and the third one was a cute little mouse.

HAMILTON: The researchers studied several parts of the brain in 41 patients. But it was only in the amygdala that they found cells that responded to animals and not to people or objects. The finding confirms earlier work suggesting that the human brain is particularly responsive to animals. Behavioral studies, for instance, have found that people pay more attention to animals and people than to things. Joshua New at Barnard College in New York did this experiment. He and his team showed people a series of pictures in which a single element would change. The viewers were really good at keeping track of people and animals.

JOSHUA NEW BARNARD COLLEGE: Even though you can embed a tiny, little person or a tiny, little animal anywhere in the scene and they'll notice it changing right away, we actually had other images where entire grain silos in a farm scene were both disappearing and reappearing, and they'd actually report that there was no change in the scene.

HAMILTON: New says more recent work suggests people actually pay as much attention to animals as they do to other people. And he says once a person has detected a living creature, the brain keeps monitoring it, probably because, unlike, say, a bridge or a building, a person or animal can suddenly turn from friendly to hostile. Lynne Isbell from the anthropology department at U.C. Davis says it makes sense that the human brain evolved to focus more attention on animals than bridges or buildings.

LYNNE ISBELL: Long before we had those things, we had animals. And so if we didn't pay attention to them, then, you know, that might not have been such a good idea.

HAMILTON: Isbell says that might have been especially true when it came to snakes. Early humans in Africa and Asia would have encountered lots of poisonous snakes like the rattler that Christof Koch barely avoided.

ISBELL: Our ancestors who weren't able to adjust to those sorts of things would have been bitten and probably would have died.

HAMILTON: Leaving the survivors who responded more quickly to pass along their genes. The research appears in the journal Nature Neuroscience. Jon Hamilton, NPR News.

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