Why do some foods smell good for us?
The new study of psychology professor Don Katz and his lab suggest that he has much more to do with what's going on in our mouths than he often thinks. It turns out that we do not inhale the scent of food and we decide that we like its smell. First, we have to eat it.
Kats's research appears in the magazine's January 7 issue Current biology.
What is the smell?
The smell consists of two components. The first is well known. It involves inhaling the fragrance through the nose. Scientists call this orthonasal smell.
But there is another way you smell, called retinasal smell. When you chew or drink, the food or liquid produces smells in your mouth. These smells rise up to the nose where they are treated by receptor cells on the roof of the nasal cavity – the same region that treats smells entering the nostrils.
Most of us never notice retronasal smell, but it is so important to our system of odor, such as orthopedic smell.
Katz and his colleagues exposed the rats to two odors, one delicious and pleasant to the animals, the other neutral. A group of rats receive only smells through the nose.
The other set received them retrospectively. This is achieved by injecting into the mouth the walnut and ordinary odor liquid, bypassing the nose.
Then Katz's team put the rodents in a small room with a nose filled with the same walnut smell. If animals have a preference for smell, they will go into the nose.
The rats that have experienced the smell have done just that. But the orthonasal group was not attracted to the nose. Researchers had to allow the animals to smell the aromatic flavor twice more before it became a rooted scent and they headed for the nose.
In other words, rats have learned to prefer a faster odor when applied orally, retronasally than the nasal orthonasal.
The results show that Katz is a system of odor of retronasal, which dominates the ortho-nasaal in determining which foods smell good for us. First we like the smells of food, and then this orthogonal system likes the smell afterwards.
According to Katz, this makes sense from an evolutionary point of view. It is an advantage to eat foods before you like the smell you inhale through your nose. "Things that smell perfectly fine are actually not good for us," says Katz. "It's better to try them first.
How are the taste and smell connected?
It has traditionally been thought that two separate systems deal with taste and smell in the brain: the taste signals that originate from the taste buds of our tongue go into the bark of taste. Odors, both ortho- and retro-, are directed to the olfactory cortex.
Katz's research confirms this long-standing guess.
As part of his research, he and his colleagues removed the bark of the rat's taste. He expected this to affect only the taste, but also the smell. The exclusion of the taste cortex also destroyed the retinal sentiment.
This led Kat to conclude that when the rats feed, the delicious cortex treats both the taste and the retinasal odors.
Mouth against a nose
Retronasal and taste signals can go to the same place in the brain when the rats eat, but the orthopedic signals are still directed to the olfactory cortex.
This suggests that the brain does not distinguish between taste and smell, but between the signals that come from the mouth and those that are sniffed by the nose.
Eating against failure
Katz and the findings of his laboratory refer to the feeding of rats. He believes that in other situations involving the smell – a sniff of a flower, for example, or a sense of predator presence – the brain behaves differently.
In these cases, the taste and smell are processed by different brain areas. The taste signals pass to the taste cortex, while the two odor systems – retro- and orthonasal- are processed together where expected in the olfactory cortex.
"Somehow the brain adjusts to what we do," Katz said. "When you eat, the system is different from what you are not, it all depends on the context."
The other authors of the article are Meredith L. Blankenship and Maria Grigorova at Yost X. Meyer at the Wake Forest Medical School.
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