Kent Berridge, a psychologist at the University of Michigan, has spent more than thirty years trying to understand the biology of rewards. How and where do pleasure and motivation—the impulse to eat a piece of chocolate cake, say, or to resist it—arise in the brain? Berridge divides the components of reward into “liking,” “wanting,” and “learning.” When human newborns, young orangutans, chimpanzees, monkeys, or even rats and mice taste something sweet, for instance, they all stick out their tongues just a little; this is liking. (When they taste something bitter, they all open their mouths in a remarkably similar gape.) They will try to get more of what they like; this is wanting. And, over the course of their lives, they will build up huge reservoirs of associations around whatever they like; this is learning.
In the science of reward, Berridge’s framework is both influential and controversial. “Our fundamental starting point,” he has written, “is that the temptation and pleasure of sweet, fatty, or salty foods arise actively within the brain, not just passively from physical properties of foods themselves. ‘Wanting’ and ‘liking’ reactions are actively generated by neural systems that paint the desire or pleasure onto the sensation—as a sort of gloss painted on the sight, smell or taste.”
In other words, the experience of reward is generated from within as well as from without. Presumably, there are dedicated places in the brain—Berridge calls them “hedonic hot spots”—where the gloss of pleasure is painted on, where a certain sheen and glimmer is cast over life’s rewards. But when scientists go hunting for these hot spots, using neural recordings and neuroimaging studies, they find them all over the place. So many brain areas light up in response to a sweet taste, a hit of intravenous cocaine, a jackpot win, or a subliminal glimpse of a smiley face that researchers can’t make much sense of what’s going on. They get lost in whole South Sea archipelagoes of hot spots. In one recent review of the literature, Berridge notes reward-related activity in the orbitofrontal cortex, the anterior cingulate, and the insula, as well as in deeper, subcortical structures, such as the nucleus accumbens, the ventral pallidum, the ventral tegmentum, the amygdala, and some of the dopamine pathways between them. Dopamine, a hormone secreted by certain neurons, is known to play a major role in pleasure and reward. But, amid all the neural activity generated by such broad categories of behavior as liking, wanting, and learning, it’s hard to figure out where it all starts in the mammalian brain.
Read the full article "The Thirsty Mind" at The New Yorker.