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Exactly what is the physical difference, though, between a left prefrontal cortex that is predisposed to happiness and one that isn't? It almost certainly has in part to do with neurotransmitters, the chemicals that ferry signals from one neuron to the next. And while the prefrontal cortex is awash in many neurotransmitters, including dopamine, serotonin, glutamate, GABA and more, Davidson believes dopamine may be especially important. Animal studies have shown that dopamine mediates the transfer of signals associated with positive emotions between the left prefrontal area and the emotional centers in the limbic area of the brain, such as the nucleus accumbens, situated within the ventral striatum. In humans, people with a sensitive version of the receptor that accepts dopamine tend to have better moods, and researchers are actively studying the relationship of dopamine levels to feelings of euphoria and depression.
Dopamine pathways may be especially important in aspects of happiness associated with moving toward some sort of goal (monks achieving a meditative state as well as cigarette smokers allowed to light up after 24 hours of deprivation). But other neurochemicals may be more central to other kinds of happiness, including physical pleasure. "People have made progress differentiating the positive feeling you get when you approach a goal, which maps onto dopamine, and the sensory pleasure of enjoying something, which maps onto the opioid system," says Berkeley psychologist Keltner. "We're just beginning to apply a lens to all those parts of the nervous system in which the positive emotions are embodied. This is really neat territory."
Among those exploring that territory is Brian Knutson, an assistant professor of psychology and neuroscience at Stanford, who, like Davidson, uses MRI to monitor the brains of test subjects. The mental mode he studies is anticipation. "When people think of happiness," says Knutson, "they think of feeling good, but a big part of happiness is also looking forward to something." Knutson's research was inspired by the classic work of Ivan Pavlov, who trained dogs to salivate at the sound of a bell, which they associated with mealtime.
Instead of food, Knutson used money: a small cash payoff if subjects won a video game. "When we looked at their brains just before they got the reward," he says, "we saw this spark that clearly had to do with how positive the idea of making money was." The spark showed up not in the left prefrontal cortex but in what Knutson terms an old section, the nucleus accumbens, located in the subcortex, at the bottom of the brain. The bigger the prize, Knutson found, "the more activation." Knutson believes he is looking at the kind of happy feelings we experience as excitement. The primary focus of his work is to understand the neurophysiology of motivation and decision making--how emotion and reason work together as people make choices. But it could also be a key to mapping out the brain's broader happiness circuitry.