Our recognition of taste may stem from detection with taste receptors in the tongue, but it is our brains that truly recognize flavors for what they are. It is not the tongue “telling” the brain what we’ve tasted, a common idea that has just been flipped on its head by scientists in a new study. The researchers demonstrated that manipulating cells in the brains of mice allowed them to change the way something tasted.

“Taste, the way you and I think of it, is ultimately in the brain,” said study leader Dr. Charles S. Zuker, professor of biochemistry and molecular biophysics and neuroscience at Columbia University Medical Center (CUMC), in a press release. “Dedicated taste receptors in the tongue detect sweet or bitter and so on, but it’s the brain that affords meaning to these chemicals.”

Zuker and his team hoped to reveal how the brain transforms detection of chemical stimuli into perception. In the past, Zuker and his colleagues had proven the existence of dedicated receptors for each taste on the tongue, and that each type of receptor sends a specific signal to the brain. Recently, they demonstrated that a unique set of brain cells is responsible for sensing each taste, which are located in separate places in the brain’s cortex — essentially generating a map of taste qualities in the brain.

The scientists utilized optogenetics for the study, which allowed them to directly activate specific neurons with a laser. Yueqing Peng, a postdoctoral associate in Zuker’s lab, wanted to know if manipulating these specific neurons could evoke the perception of sweet or bitter in a mouse — without the mouse actually tasting either. Sweet and bitter were chosen for the experiment because they are the most recognizable tastes for humans, and most critical evolutionarily (sweetness allows for identification of energy-rich nutrients, while bitter warns against the intake of potential poisons and noxious chemicals).

“In the study, we wanted to know if specific regions in the brain really represent sweet and bitter,” Peng said. “If they do, silencing these regions would prevent the animal from tasting sweet or bitter, no matter how much we gave them. And if we activate these fields, they should taste bitter or sweet, even though they’re only getting plain water.”

The researchers were correct in their hypotheses. When they injected the mice with a substance to silence the sweet neurons, the animals could not identify sweet, but they could still detect bitter. Conversely, silencing the bitter neurons prevented mice from recognizing bitter but not sweet.

The researchers could also trick the animals into thinking they were tasting bitter or sweet even though they were drinking plain water. When the team activated the sweet neurons while the mouse was drinking, they observed behaviors associated with sweet, like increased licking. Again, the same was true of bitter neurons — when activated, mice showed bitter-associated behaviors, like gagging.

In another set of experiments, the researchers performed the optogenetic tests on animals that had never been exposed to sweet or bitter chemicals. Still, the activation of the corresponding neurons produced the appropriate behavioral response.

“These experiments formally prove that the sense of taste is completely hardwired, independent of learning or experience,” Zuker said. “Which is different from the olafactory system. One smell could be great for you and horrible for me.”

Of course, as humans, we can eventually learn to enjoy bitters and dislike sugars.

As far as the mice experiments went, there was no difference between mice who actually tasted sweet or bitter and mice who had the respective neurons stimulated. Their behavior did not differ between the two, demonstrating that the light mimicked the perception of bitter and sweet.

“In other words,” Zuker said, “taste is all in the brain.”

Source: Peng Y, Gillis-Smith S, Jin H, Trankner D, Ryba N, Zuker C. Sweet and bitter taste in the brain of awake behaving animals. Nature. 2015.