It is a widespread belief that humans pick up flavors with their taste buds which then send signals to our brains, but a team of scientists demonstrated they could change the way mice perceived taste simply by manipulating groups of brain cells.
The researchers aimed to determine how the brain transforms detection of chemical stimuli into perception, and their findings could change everything we know about the phenomenon, Columbia University Medical Center reported.
"Taste, the way you and I think of it, is ultimately in the brain," said study leader Charles Zuker, a professor of biochemistry and molecular biophysics and of neuroscience, a member of the Kavli Institute for Brain Science and the Mortimer B. Zuckerman Mind Brain Behavior Institute, and a Howard Hughes Medical Institute Investigator at Columbia University Medical Center (CUMC). "Dedicated taste receptors in the tongue detect sweet or bitter and so on, but it's the brain that affords meaning to these chemicals."
The researchers found the tongue possesses taste receptors dedicated to different categories of flavor, and each class of receptor sends a different signal to the brain. Each taste is then sensed by different groups of brain cells located in separate locations in the brain's cortex, which create a "map" of taste qualities in the brain. To make these findings the team used optogenetics, allowing them to stimulate different neurons with a laser. They wanted to find out if they could manipulate the neurons to make the mice feel as if they were tasting something that was either sweet or bitter, without them actually tasting either.
"In this study, we wanted to know if specific regions in the brain really represent sweet and bitter. If they do, silencing these regions would prevent the animal from tasting sweet or bitter, no matter how much we gave them," Zuker said. "And if we activate these fields, they should taste bitter or sweet, even though they're only getting plain water."
When the scientists injected a substance into the mice to silence the sweet neurons, they found the rodents could no longer identify food that was sweet, but could still detect bitter flavors; silencing the bitter neurons had the opposite effect.
The team was able to make the mice think they were tasting something that was sweet or bitter, even when the animals were just drinking water. When mice's "sweet neurons" were activated in the mice's brans as they were drinking water they demonstrated behaviors associated with tasting something that is sweet, such as increased licking. On the other hand, stimulating bitter neurons led the mice to exhibit behavior associated with this taste, such as decreased licking and even gagging. The team also performed these tests on animals that had never tasted anything that was sweet or bitter before, and observed similar responses.
"These experiments formally prove that the sense of taste is completely hardwired, independent of learning or experience," Zuker said. "Which is different from the olfactory system. Odors don't carry innate meaning until you associate them with experiences. One smell could be great for you and horrible to me."
In a final experiment, the team trained mice to perform a cue when they were tasting something that was bitter or sweet, and either fed them things that actually possessed one of these qualities or manipulated their neurons to make them believe they were. They found the mice's responses did not differ under both scenarios.
"In other words, taste is all in the brain," Zuker concluded.
The findings were published in a recent edition of the journal Nature.