Learning more about this mechanism could unlock secrets of the human brain, perhaps even paving the way for new treatments for eating disorders, the researchers say.
"This study really serves to reemphasize the concept that in essence our reality is nothing but electrical signals in our brain," said Charles Zuker of Columbia University’s Mortimer B. Zuckerman Mind Brain Behavior Institute. Zuker is the lead author of the new research, published in Nature on Wednesday. The results show that taste is hardwired in the brain, and can be triggered as easily by electrical signals as it can be by actual flavor chemicals.
The study built on Zuker's previous work, which showed that specialized cells in the tongue — designed to detect either sweet, sour, bitter, salty or umami tastes — corresponded to specialized cells in the brain.
"Imagine these cells as being the keys of a piano," Zuker said. "You press the key and activate a wire that produces a specific sound. But it doesn't matter how you press the key. You could use finger, an elbow, a nose. It doesn’t matter how we tweak the cells; if you tweak the right cells you get the right signal."
Zuker hypothesized that he could activate or silence the brain cells being tweaked by the tongue cells, creating false perceptions of taste — and he was right.
By taking mice and training them to report what they were tasting ("It's not so easy for a mice to tell me this," Zuker quipped — but they were trained to move in particular directions when tasting bitter or sweet chemicals) Zuker and his postdoctoral co-author Yueqing Peng demonstrated that signals in the brain could create the experience of taste without any real stimuli.
They used optogenetics (explained in the video above) to selectively activate and deactivate the neurons associated with sweetness and bitterness.
"The biggest surprise was that they didn't just report sweet or bitter as they'd been trained to do, but much to our surprised they developed the full behavioral response," Zuker said. "We saw a mouse trying to clean its mouth of a bitter taste. This is extraordinary. There's nothing in there but water and the mouse is gagging, trying to get rid of a nonexistent chemical."
There are more than 100 million neurons in a mouse's brain, but triggering just a few thousand of them was enough to create an experience that's supposed to be triggered by something material. Taste is important: It helps animals seek out the nutrients they need and avoid foods that could hurt them. But just like everything that goes on in the brain, it comes down to electrical signals — and those can always be hacked.
If you're willing to put a wire into your skull, anyway.
Zuker is hoping that further study will produce something useful for humans — no wires required.
"We're trying to figure out how all this is choreographed," he said, "how the brain uses this information to guide actions and behaviors."
If we can better understand the logic behind these circuits, he said, we might be able to find less invasive ways of tweaking them when they go wrong, as could be the case with certain eating disorders.
There are "bigger, sexier" ways to project the findings into the future, too, according to Zucker.
"Here we have a chunk of tissue that uses nearly a third of all of your energy and oxygen, yet it only weighs a small fraction of your total body weight, and it completely transforms the human condition," Zuker said. "It’s able to change fear into courage, sadness into happiness. How does it do it? Even though these are more abstract concepts than taste, I know they have to be encoded in circuits. If we could define the circuits involving courage, we could theoretically activate courage in the brain. But first we need to get the basics of how these processes work."