Roast turkey. Stuffing. Mashed potatoes and gravy. Pie. Thanksgiving conjures up all sorts of flavors. If you close your eyes you can almost taste them. In fact, one day you may be able to—without food.
Scientists from Columbia University have figured out how to turn tastes on and off in the brain using optogenetics—a technique that uses penetrating light and genetic manipulation to turn brain cells on and off. They reported their findings in an article published last week in Nature . By manipulating brain cells in mice this way, the scientists were able to evoke different tastes without the food chemicals actually being present on the mice’s tongues.
The experiments “truly reconceptualize what we consider the sensory experience,” said Charles Zuker, head of the Zuker lab at Columbia and co-author on the paper. The results further demonstrate “that the sense of taste is hardwired in our brains,” Zuker said, unlike our sense of smell, which is strongly linked to taste but almost entirely dependent on experience.
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Typically when we eat, the raised bumps, or papillae, that cover our tongues, pick up chemicals in foods and transmit tastes to the brain. There are five main types of papillae corresponding to each of the five basic tastes—sweet, sour, salty, bitter and umami. Contrary to popular belief, these aren’t clustered in particular places on the tongue, with bitter in the back and sweet at the front, but are spaced about evenly on the tongue.
A taste map may in fact exist, but it appears to be in the brain rather than on the tongue. First the researchers singled out the mice’s sweet and bitter taste centers in the brain, which are separated by approximately two millimeters in the insula. They concentrated on only sweet and bitter because the two are the most distinct from each other and also the most salient for humans, mice and other animals due their evolutionary importance to survival. Sweet usually indicates the presence of nutrients, whereas bitter signals potential danger of poison.
Zuker and his team then optogenetically stimulated the areas with light and in a series of behavioral tests, were able to have the mice taste sweet or bitter with only plain water. When the researchers activated the sweet neurons, they observed behavior consistent what with happens when mice normally encounter sweet foods: their licking increased significantly, even when the animals’ thirst was satiated. When the scientists stimulated neurons associated with bitter flavors, the mice stopped licking, seemed to scrub at their tongues and even gagged, depending on the level of optogenetic stimulation.
The researchers then performed the tests on animals that had never tasted sweet or bitter in their lives and found the same results. In the last set of experiments the researchers applied to the tongue of the mice chemicals that tasted sweet and bitter and compared their reactions to what happened when they simply stimulated the corresponding neurons optogenetically. There was no difference in the way the animals responded, “proving taste is hardwired in the brain,” Zuker said.
This doesn’t mean that there is no such thing as an “acquired taste,” Zuker clarified. For example, hákarl, fermented shark meat and national dish of Iceland, once called “the single worst, most disgusting and terrible tasting thing,” by famously acerbic food critic Anthony Bourdain is relished by many on the Nordic island nation. Humans are more complicated than mice. Taste can also be shaped by experience and culture. But the basics of this sensation are present from the beginning.
“Every baby smiles to sweet and frowns for bitter,” Zuker explained. “Taste mostly retains that hardwired response unless there is something that supersedes it. There are some things we consume [that] are innately aversive. But we take the gain with the bad if they have a positively reinforcing result.” Coffee or alcohol, for instance, are distinctly bitter, but many people learn to enjoy them over time due to the feelings of stimulation and inebriation they bring, respectively.
Gary Beauchamp, president of the Monell Chemical Senses Center in Pennsylvania, calls the research “a very clear and elegant approach,” confirming the long-standing hypothesis that taste is indeed evolutionarily hardwired. But Beauchamp also notes that sweet and bitter compounds can influence each other in the mouth to affect taste before they reach the brain. “In the real world, where foods are mixtures of things, it’s much more complex than what this study would suggest. Nevertheless, this is excellent work showing that these pathways are innately organized,” he said.
Zuker is aware that sweet and bitter are at the extremes of the taste spectrum and may not be representative of all tastes. But he expects similar results testing other tastes, which are also evolutionarily based. Salt, for example, signals electrolytes. “The next question is how activity in these cortical fields integrates with rest of brain,” to form experience and lasting taste memories – such as those we make at Thanksgiving.