We all know that when it comes to enjoying food, taste and smell go hand in hand. But how and where they hold hands in the neural circuits of the brain has been something of a mystery.
Neuroscientists have known for a while that odor receptors in the nose send signals to the the brain’s taste center, also known as the gustatory cortex. But does the converse happen? Do taste receptors in the tongue talk to the smell center, the olfactory cortex?
New research suggests the answer is yes.
The smell center gets and uses information from the tongue even if an animal is not consciously sniffing — or even inhaling.
“We know there is a sense of smell in the taste system. What’s new is that we now know that smell, like taste, can’t really work on its own, either,” said Donald B. Katz, a neuroscientist at Brandeis University who co-authored the study.
“What this means is that the different senses are really interacting with each other at a much earlier level than previously thought,” said Joost X. Maier, the postdoctoral researcher at Brandeis who did the experiments reported in the current issue of the Journal of Neuroscience.
One can construct reasons why this might be the best way to design the brain. But the brain arose by chance, interacting with the world and sculpted by natural selection. For virtually all forms of life, taste and smell were experienced together in the act of finding and consuming food.
“They happen together, and evolution is going to handle them together,” said Katz. “Honestly, it shouldn’t be a surprise that the part of the brain that is supposed to be devoted to the sense of smell is also doing taste.”
Recent research has shown there is cross talk of other senses. For example, the hearing center (the auditory cortex) responds to visual input as well as sound.
When a monkey is shown a video of one of its friends cooing or grunting, a visual stimulus goes to the auditory cortex, where it changes — enhances or suppresses — sound perception. That doesn’t happen when an image of a mechanical disk with a fake mouth accompanies the same sound. In other words, what the monkey sees literally affects what it hears.
The new research also sheds light, albeit indirectly, on the phenomenon of synesthesia. That’s the rare and strange commingling of sensations. People with synesthesia perceive specific shapes (often letters and numbers) as inherently colored, or hear sounds when certain motions occur in their visual fields.
“These people reflect that all sensory systems are connected,” Maier said. “But in them, the mechanisms that form appropriate interactions between sensory signals are broken.”
For his research, Maier inserted tiny wires into the brains of 19 rats. Then he gave them liquids with four different tastes — salty, sweet, sour and bitter — or plain water and recorded the electrical activity of single cells in their olfactory cortexes.
Certain cells responded to specific tastes, either speeding up their firing or slowing it down. A few responded to certain taste pairings: sour and salty, or salty and bitter. The briskest responses were to unpalatable flavors, sour and bitter.
Why that’s the case isn’t known. Possibly those flavors telegraph danger. Many plant poisons are bitter compounds known as alkaloids.
To make sure he was actually stimulating taste receptors with the flavored liquids, Maier anesthetized the tongue with Anbesol. (Yes, it’s the same stuff parents put on the gums of teething infants.) The firing of cells in the olfactory cortex declined because the taste receptors in the tongue were asleep.
Maier then tested the other possibility — that he was inadvertently stimulating smell receptors with his liquids. He washed out the rats’ noses with a chemical that destroyed the smell receptors for three days. When the rats drank the flavored liquids, the cells in the olfactory cortex continued to fire as they had before. The cortex was clearly getting taste signals, not smell signals.
The combined sensory information — taste along with smell, and smell along with taste — determines behavior even when the animal doesn’t know it.
For example, when rats were given a sweet, cherry-scented liquid — a source of easy calories — they grew to like it. When the sweetener was removed but the scent remained, they continued to prefer it to plain water even though it provided no nutritional advantage.
In an experiment by another Brandeis neuroscientist, the taste cortexes of rats were temporarily inactivated. The animals were then presented food with a spicy smell that they grew to like. When the functioning of their taste centers was restored, they no longer liked the food. However, if the taste centers were inactivated a second time, the food once more had appeal.
Ultimately, smell and taste work together to produce a menu of food preferences.
Evolutionarily speaking, in a world where one rotten carcass or a mouthful of the wrong berries can spell death, knowing what food has been nutritious and safe in the past — in other words, knowing what you “like” — can make all the difference.