Imagine yourself eating dinner. You're hungry, and the roast chicken tastes delicious. You recognize notes of thyme, rosemary, and sage, from the herbs it was rubbed with. You like the brussels sprouts better than when you were a kid, though their flavor hasn't changed, just your taste for it. Then you're full, and you don't want any more chicken. But you do want dessert, because you love dessert. Who doesn't?
And though you weren't really thinking about any of that while you were eating, your brain was processing all of it. After all, it's your brain's job to translate the material being mushed up in your mouth into pleasure, and to piece together complicated combinations of scent and the five basic tastes — sweet, salty, sour, bitter, and umami — into specific flavors. While telling you you're full, it's also nagging you to eat dessert. How all this happens is a complicated and still often mysterious process.
As you chew, different sweet, salty, sour, bitter, and umami molecules are binding to their corresponding receptors on your tongue. Fitting like keys into locks, they trigger neurons that send messages to your brain. For a long time, scientists thought that just one part of the brain recognized all of these messages. But as it turns out, it's not the message itself that tells you whether something is sweet or bitter, but the part of the brain it goes to. A single, tiny part of the brain tells you when you're eating something sweet, while a separate part nearby tells you when a food is bitter. And as scientists have shown in mice, the brain can be tricked into tasting something it's not actually tasting — or vice versa — by turning these parts on and off. That holds true for all of the five tastes except for sour, which is still something of a mystery. Scientists don't know what part of the brain experiences that taste, or if it's even just one part, and they don't know why it's different. But they do know that, no matter what, all flavor is really just in your head.
How we react to those different tastes is also, to a large extent, hardwired into our brain, and we all react the same, even if to different degrees. We love sweet and salty and umami things, and we're usually not so keen on bitter and sour things. The reasons are biological: In nature, sweetness indicates readily available energy, umami usually indicates the presence of protein, and salt is necessary for certain basic bodily processes. Bitterness, on the other hand, is often a sign of toxins, and things that are sour usually aren't ripe or have gone bad.
Both environmental and genetic factors can affect how strong the brain's reaction will be to any one taste. If your body needs the nutrients that come with a taste, you may like that taste more. For example, scientists have shown that rats who usually hate the taste of a salty solution will happily lap it up when they've been deprived of sodium. You may also just learn to like unpleasant tastes if they bring you pleasure in other ways. You learn to like the bitter tastes of coffee and beer because you learn that they aren't poisonous, but that they give you energy, or make you tipsy.
Genes, meanwhile, might also change the way you actually experience a flavor. When a brain recognizes sweetness, it releases dopamine, connecting flavor with pleasure. Many people get the biggest dopamine surge after their first bites of food, and then the effect lessens as other hormones tell the person they're getting full. But some brains are genetically wired to handle enjoyable tastes like that differently. Some are more receptive to that dopamine release, get more pleasure out of eating, and therefore are inclined to eat more. Others get a bigger dopamine rush from the anticipation of food than from actually eating it, and also eat more to try to match the joy they thought they would get from that first bite.
That makes the brain's response to different tastes about more than recognizing the difference between a piece of cake and a piece of cheese. There's still a lot scientists don't know about taste and about the brain, but studying both could help us learn more about health issues like obesity, about how our tastes change, and about how these things could be connected.