The Frontal Cortex

Natalie Angiers profiles dopamine, which isn't just about rewards:

In the communal imagination, dopamine is about rewards, and feeling good, and wanting to feel good again, and if you don't watch out, you'll be hooked, a slave to the pleasure lines cruising through your brain. Hey, why do you think they call it dopamine?

Yet as new research on dopamine-deficient mice and other studies reveal, the image of dopamine as our little Bacchus in the brain is misleading, just as was the previous caricature of serotonin as a neural happy face.

In the emerging view, discussed in part at the Society for Neuroscience meeting last week in Chicago, dopamine is less about pleasure and reward than about drive and motivation, about figuring out what you have to do to survive and then doing it. "When you can't breathe, and you're gasping for air, would you call that pleasurable?" said Nora D. Volkow, a dopamine researcher and director of the National Institute on Drug Abuse. "Or when you're so hungry that you eat something disgusting, is that pleasurable?"

In both responses, Dr. Volkow said, the gasping for oxygen and the wolfing down of something you would ordinarily spurn, the dopamine pathways of the brain are at full throttle. "The whole brain is of one mindset," she said. "The intense drive to get you out of a state of deprivation and keep you alive."

The caricature of dopamine as the chemical of hedonism and pleasure - it's what drives us to enjoy sex, drugs and rock and roll - was always mostly misleading. While dopamine does predict the arrival of rewards, the neurotransmitter is much more important that. Many dopamine researchers, for instance, refer to the chemical as our "neural currency," since it allows us to quickly assign a value to the multitudes of things and ideas in the outside world. (In other words, dopamine is the price tag of sensory information.) When we see something we want - and it doesn't matter if it's a chocolate cupcake or a glass of water - the mere sight of the object triggers a wave of emotional desire, which motivates us to act. (Emotion and motivation share the same Latin root, movere, which means "to move.") The world is full of possibilities, and it is our dopaminergic feelings that help us choose between them.

And it's not just chocolate cupcakes and lines of cocaine that make our dopamine neurons excited. In an oldish profile of Read Montague, a leading dopamine researcher at Baylor College of Medicine, he notes that one of the innovations of the human brain is that dopamine also evaluates abstract ideas:

From the perspective of the brain, an abstraction can be just as rewarding as the tone that predicts the reward. Evolution essentially bootstrapped our penchant for intellectual concepts to the same reward circuits that govern our animal appetites. "The guy who's on hunger strike for some political cause is still relying on his midbrain dopamine neurons, just like a monkey getting a treat," Montague says. "His brain simply values the cause more than it values dinner." According to Montague, the reason abstract thoughts can be so rewarding, is that the brain relies on a common neural currency for evaluating alternatives. "It's clear that you need some way to compare your options, even if your options come from very different categories," he says. By representing everything in terms of neuron firing rates, the human brain is able to choose the abstract thought over the visceral reward, as long as the abstraction excites our cells more than apple juice. That's what makes ideas so powerful: No matter how esoteric or ethereal they get, they are ultimately fed back into the same system that makes us want sex and sugar. As Montague notes, "You don't have to dig very far before it all comes back to your loins."

The purpose of pleasure, then, is to make it easier for the pleasurable sensation - the delicious taste, the elegant idea, the desired object - to enter the crowded theater of consciousness, so that we'll go out and get it. That's why we've got a highway of nerves connecting the parts of the dopamine reward pathway - the nucleus accumbens, ventral striatum, etc - to the prefrontal cortex. (This also means that a well-turned phrase or pretty painting will be more likely to get stuck in working memory, since it's more rewarding. Aesthetics are really about attention.) Drugs like amphetamine and cocaine, which induce more dopaminergic activity, are chemical shortcuts: because those dopamine neurons in the midbrain are so excited - the neurotransmitter is skulking in the synapse - the world is suddenly saturated with intensely pleasurable ideas, which we can't stop thinking about. We want to talk to everyone and touch everything. If attention is like a spotlight, then these drug makes the filament burn brighter. The end result is that we can't look away.