The pleasure we get from eating is less about the mouth and more about the mind.
A slab of double-chocolate cake, for example, triggers a cerebral hotspot, a little clump of neurons about two inches behind the nose, right in the middle of the brain.
This pea-sized area in the forebrain, not the tongue or palate, is the source of pleasure one derives from experiencing a fluffy fork-full of cake, says Kent Berridge, the James Olds Collegiate Professor of Psychology and Neuroscience in LSA.
"The pleasure of that sweetness and creaminess isn’t in the food itself, it’s in the brain," Berridge says.
We’re actually born with an innate sense of yummy and yucky. Scientists have studied how various taste samples are recognized by newborn infants, who smack their lips to sweetness but cringe in disgust at bitterness.
Starting from this basic set of preferences, taste rewards or aversions then change through a lifetime of learning and can be shaped by culture. That is to say, part of why you love chocolate cake is because your brain rewards you with a little tingle of joy every time you encounter it. Your brain wants you to repeat the experience. Who are you to say no?
"Taste is a natural key to pleasure," Berridge says. It triggers a thin circuit of connected hotspots in the forebrain that amplify the sensation by putting out neurochemical signals of pleasure, including opioids, orexins, and endocannabinoids. These are the rewards that become key to learning to like a particular food.
"In a sense, culture does what orexin does," he adds. Cultural cues, like everyone around the table oohing and aaahing over that first sip of 2007 Chateauneuf du Pape, can set the reward circuits to trip. "Reward circuits are very sensitive to learning."
"Neural systems paint the desire or pleasure onto the sensation as a sort of gloss painted on the sight, smell, or taste," Berridge says. (No word yet on sexual sensations, but the Dutch are working on it, he says.)
The Bitter Truth
As his research team catalogues the genetic differences that set humans apart, Jianzhi “George” Zhang, a professor of Ecology and Evolutionary Biology in LSA, has found that humans have shed many of the genes used to make the pheromone-binding receptors that create a sense of smell. We’ve also lost some of the genes that would give us the ability to taste bitterness.
Zhang offers a possible explanation. “The ability to taste bitterness is important for detecting toxins in food, and most of those toxins are in plants. About one to two million years ago, we started eating more meat rather than plants,” he says. We also acquired a taste for throwing our food onto a fire, which can detoxify.
Since this 2006 finding on bitterness receptors, Zhang’s lab has done a series of investigations on genes in other animals that support the idea that dietary preferences have shaped the senses for taste and smell.
Perhaps due to disuse, bats have lost the taste of umami, a fifth flavor added to sweet, sour, bitter, and salty that is found in shrimp, savory fish, cheeses, and tomatoes. Pandas, almost exclusive eaters of bamboo leaves, have lost their sense of umami too. And vampire bats, eating nothing but blood, have shed their sense of sweetness and umami.
Humans, says Zhang, will continue to taste bitterness less and less.
“Humans lost two bitter receptor genes since the separation from chimps,” and there are “at least two more” that are leaving humans at present.
But he thinks the other four taste receptors (sweet, salty, sour, and umami) are still being used “so they will likely stay.”
Luckily, rats inherently respond to sweet and bitter much the same way as the newborn humans Berridge tested, and that has opened up a fruitful line of research. Having pinpointed some of the hedonic hot spots, or "pleasure locks," in the rat brain, Berridge and colleagues are able to painlessly inject minute amounts of various drugs that might enhance the pleasure or desire for a given taste. Doing this, they’ve been able to make a rat like sweetness more, or want something it didn’t even like at first, or want it past the point of liking.
The most surprising thing they’ve found by doing this is that dopamine, the neurotransmitter most often associated with sensations of pleasure (and its evil twin, addiction) isn’t the key player in food pleasure at all. Berridge now distinguishes between liking and wanting because they are rooted in two different neural circuits.
This was starkly illustrated in a 1991 experiment in which a rat’s brain was electrically stimulated to signal wanting, but not necessarily liking. The animals ate twice as much of the treat, while actively disliking it. Such mismatches between the wanting for food and the actual need may be important avenues into understanding the eating extremes of morbid obesity or anorexia, Berridge says.
This set of questions also leads to an exploration of the neurobiology of addiction, work that Berridge is pursuing with LSA colleague Terry Robinson, the Elliot S. Valenstein Collegiate Professor of Behavioral Neuroscience.
Just know that it really is all in your head. When you’re physically hungry and the aroma of that turkey in the oven is just about driving you mad with desire, what’s really going on is that the hunger has set up an added neural reward: a squirt of orexin that goes from the hypothalamus to the ventral pallidum in the brain’s basal ganglia, which "primes the (pleasure center’s) lock to open," Berridge says. That first few bites of the bird sets off a cascade of happy signals in the pleasure circuit.
Your body enhances the brain’s reward system to make sure it gets what it needs. "It’s a system for valuing things out there in the world," Berridge says.