For centuries, poets and novelists have pondered the nature of emotions, and in recent decades scientists got into the emotion business as well. If you search the National Library of Medicine for papers under the heading of “emotion,” you’ll find 142,443 papers since 1959. In those papers, scientists have classified emotions like taxonomists classifying beetles. They’ve studied how experiences and genes influence our emotional life. They’ve looked at how emotions differ from culture to culture. There are entire journals dedicated to this research. In the latest issue of the aptly named journal Emotion, for example, you can find papers on envy, on how anxiety can sharpen attention, on the recall of unpleasant memories.

But what exactly do all these studies have in common? What do scientists mean when they talk about emotion? “There is little consensus about what emotion is,” Joseph LeDoux writes in a provocative new paper  in the journal Neuron. Given the bargeloads of paper that have been dedicated to scientific studies of emotions, such a statement comes as a surprise. And it comes as a particular surprise from someone like LeDoux. He’s a leading neuroscientist at New York University and the author of books such as The Emotional Brain. In other words, he’s made a career of studying this thing we call emotion. But if there’s little consensus on what the word means, how do scientists like LeDoux manage to do their work? “The short answer,” LeDoux writes, “is that we fake it.”

LeDoux doesn’t mean that he and his colleagues fabricate data. He means that they use an assortment of short-hand definitions of emotion that work well enough for them to make some sense of the experiments they conduct. Most people would agree, for example, that some of our mental states have a certain feeling associated with them. If I tell you how I nearly stepped into an open elevator shaft the other day, you can probably guess how that made me felt. A lot of research is based on this idea that the words, feelings and emotions are essentially interchangeable.

But there are a lot of limits that this equivalence puts on scientists. It makes it hard for scientists to figure out where emotions ultimately came from, for example. Many studies indicate that emotions are not unique to humans. Other animals behave in ways that are reminiscent of how we behave when we’re scared or playful and so on. But our sense of human feelings comes from our own introspection, from questions we ask each other, from William Shakespeare and William James. There is no baboon version of William James to tell us what it feels like to narrowly escape being eaten by a leopard. Maybe they feel the same terror. Or maybe they don’t feel anything at all. Emotions-as-feelings get in the way of tracing the continuity from our own emotional life to its origins.

To get beyond this impasse, LeDoux suggests we find ways to talk about emotion without actually using the word itself. This is actually easier than it sounds. Trying to impose an anthropomorphic vision on the animal kingdom gets things precisely backwards. Our own emotional life has emerged through our evolution from animal ancestors. So we should begin with what we can actually study in those animals, and then use that knowledge to make sense of our late-arriving emotions.

LeDoux argues that the most interesting thing about animals in this regard is how they’re wired to survive. Even as LeDoux avoids the term emotion, he introduces a new one: “survival circuits.”

Survival circuits have inputs and outputs. Their inputs may be signals from the eyes and other senses, for example, and their outputs are changes in an animal’s body that boost its chances of survival. Survival circuits are partly hard-wired in an animal’s genes, but they can also change in response to experiences, allowing animals to learn to respond to certain situations in adaptive ways.

Among the best studied survival circuits are the ones that allow an animal to defend itself. Certain odors–of predators or of dominant rivals–automatically switch on neurons in a region of the rat brain called the amygdala. Those neurons are linked to other brain regions, such as the hypothalamus. As signals travel through this defense circuit, they trigger innate defenses, such as freezing, running away, or fighting. But, as LeDoux and others have shown, rats can also learn to associate arbitrary sensations with danger. If a rat always sees a red light go off right before it gets an electric shock, the red light will eventually neurons in the amygdala.

Other animals have survival circuits for defense as well. But what’s really important about them is that, from species to species, they share the same basic wiring patterns. All mammals have a lot of the same amygdala-centered circuitry, for example, as do birds and reptiles. Humans have it too, and it appears to play much the same function as it does in rats.

One feature these survival circuits have in common is their selectivity. Only certain things will activate our defense circuit without learning, for example. An angry face will do it, for example, as will a surprisingly loud noise. Our survival circuits key into very simple cues–just the eyes on a face are enough for us to unconsciously switch on the defense circuit. The same underlying survival circuits have adapted to the ways in which different animals live. A rat will not respond to the subtleties of our faces, but a faint whiff of a pheromone or the odor of cat urine will switch on its own circuitry.

Survival circuits for different behaviors do not run in isolation. They tap into each other and influence one another. For example, when a rat hears a tone that signals an electric shock, it loses its appetite. That occurs because the defense circuits and the feeding circuits link together in places like the amygdala. The interplay between the circuits is complicated, however: the feeding circuit can be suppressed by the defense circuit for a while, but eventually its signals will overcome this interference. Animals may lose their appetite when they’re scared, but fear won’t starve them to death.

Darwin recognized that two species are similar either because they descend from a common ancestor, or because they independently evolved a particular trait. The same goes for survival circuits. Insects, for example, have a radically different nervous system from our own, which is the product of some 600 million years of separate evolution. There is no amygdala in a fly. And yet flies also have survival circuits for defending themselves, seeking food, and so on. What’s especially intriguing is the fact that those survival circuits–while different at the surface–actually use a lot of the same neurotransmitters and other chemicals found in our own survival circuits. So there might be an even older common ancestry to survival circuits that we realize.

LeDoux believes that this view of life can make human emotion a lot easier to understand. We have feelings, he argues, because we have evolved a conscious awareness of our survival circuits are doing. When our consciousness detects a survival circuit coming online, we can give that experience a name. If LeDoux is right, then understanding whether animals have emotions like we do comes down to determining whether they’re conscious of their survival circuits, too.

Is this perhaps throwing the question of emotions out of the frying pan and into the fire? After all, consciousness has been one of the most vexing puzzles of neuroscience for many decades. LeDoux’s pretty optimistic. We may not ever be able to fully understand consciousness as a phenomenon, but neuroscientists are starting to work out which parts of the human brain are important for conscious awareness. By studying animal brains, they can see whether other species share some of that consciousness hardware as well. “Then some basis for speculating about animal feeling and their nature would exist,” LeDoux concludes. For those who want to know whether dolphins are conscious or not, LeDoux’s theory may not be very satisfying. But if we want, instead, to know more about the biology that underlies our emotional life, learning to avoid the word emotion may let us learn a lot more about ourselves.

Top image: Ojos Grandes. Courtesy Flickr user Claudio.Núñez

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Carl Zimmer is the author of ten books about science, most recently Science Ink: Tattoos of the Science Obsessed. He has written hundreds of articles for the New York Times and magazines including National Geographic, Time, Scientific American, Science, and Popular Science. Since 2003, he has written the award winning blog The Loom.