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Human Perceptions Behind Smell do Have Very Sensitive Noses

Humans can distinguish more than 10,000 different smells (odorants), which are detected by specialized olfactory receptor neurons lining the nose.... It is thought that there are hundreds of different olfactory receptors, each encoded by a different gene and each recognizing different odorants.

Each of the hundreds of receptors are encoded by a specific gene. If your DNA is missing a gene or if the gene is damaged, it can cause you to be unable to detect a certain smell. For example, some people have no sense for the smell of camphor.

Sensory cells in our nose, mouth, and throat have a role in helping us interpret smells, as well as taste flavors. Microscopic molecules released by the substances around us (foods, flowers, etc.) stimulate these sensory cells. Once the cells detect the molecules they send messages to our brains, where we identify the smell.

It's a surprise to many people to learn that flavors are recognized mainly through the sense of smell. Along with texture, temperature, and the sensations from the common chemical sense, the perception of flavor comes from a combination of odors and taste. Without the olfactory cells, familiar flavors like coffee or oranges would be harder to distinguish.

Odors are detected through our olfactory sense by the olfactory "chemoreceptors"...dogs have as much as 40, 000,000 per square centimeter. Odorants are volatile chemicals that are detected in the inhaled air by the olfactory epithelium located in the roof of the two nasal cavities just below and between the eyes and above the roof of the mouth. The olfactory epithelium in humans is about 2.5 square centimeters containing, in total, about 50 million primary sensory receptor cells. The chemoreceptors react to various chemicals like aldehydes...benzaldehyde for example is the odor of almonds...and can be used as an artificial flavoring of almonds in baked goods.

Olfactory, or smell nerve cells, are stimulated by the odors around us--the fragrance of a gardenia or the smell of bread baking. These nerve cells are found in a small patch of tissue high inside the nose, and they connect directly to the brain. Our sense of smell is also influenced by something called the common chemical sense. This sense involves nerve endings in our eyes, nose, mouth, and throat, especially those on moist surfaces. Beyond smell and taste, these nerve endings help us sense the feelings stimulated by different substances, such as the eye-watering potency of an onion or the refreshing cool of peppermint.

Lost in the dark, without sight, sound, or clue? Follow your nose.

New olfactory research suggests that when it comes to tracking scent at ground-level on open terrain, the average human's sense of smell is stronger than most people believe.

"There's this general assumption that people have a bad sense of smell," said study lead author Jess Porter, a Ph.D. candidate in biophysics at the University of California at Berkeley. "But we found that people can certainly sniff their way accurately around a spatial context -- although less successfully and slower if they have only one nostril to work with."

The new American-Israeli study, published online in Nature Neuroscience, reports that people can, in fact, be trained to rely exclusively on ground-level smelling to successfully navigate unknown territory. In fact, they instinctively mimic certain animal behaviors, including enlisting each nostril to independently identify distinct smells and "triangulate" a path.

Porter joined Berkeley psychology professor Noam Sobel and a team of colleagues. Together, they conducted five experiments aimed at assessing people's ability to track scents.

Enlisting anywhere from four to 32 male and female participants for the various tests, the researchers worked in an open field.

The subjects were blindfolded and ear-plugged after being asked to follow a 10-meter trail scented with "chocolate essential oil." They followed the trial by moving close to the ground on their hands and knees and wearing thick gloves, with only their noses to guide them.

Two-thirds of the participants were able to do so. However, when their noses were plugged to cut off the ability to smell, none of them could follow the path.

To test if "practice makes perfect," two men and two women were subsequently trained to complete the same task three times a day for three days, stretched over a two-week period.

Porter and her team found that this type of training allowed subjects to track scent trails faster and with greater accuracy. In fact, their speed of execution doubled within a few days. The researchers believe more training might boost efficiency even higher.

Porter's group also found that as the speed of trained scent tracking increased, so did sniffing frequency. In this regard, they noted that dogs sniff much faster than humans, perhaps accounting for their superior tracking abilities.

A third experiment revealed that each of a person's two nostrils inhales distinct smells, pulled from non-overlapping regions in the air. The researchers pointed out that prior studies have shown that scent "plumes" in the open are often confined to such small areas that only one nostril will pick it up, while the other will not.

The fourth test followed up on this finding by asking 14 subjects to complete the field-tracking experiment, but with one nostril taped shut.

Single-nostril tracking was much less accurate (36 percent versus 66 percent) and 26 percent slower compared to using both nostrils.

Finally, the authors conducted a final test using a special mask that allowed both nostrils to inhale, but forced incoming air to join together into a single air stream inside the middle of the nose.

Porter and her associates found that this "unified nostril" method was 24 percent slower and much less accurate than tracking completed normally.

This shows that optimal human tracking requires the individual use of each nostril to better distinguish smells as people home in on an appropriate route.

The team concluded that, just as happens with animals, people's capacity to smell can be harnessed to great effect.

Still, for most people, eyes remain the key window on the world.

"I think it's fair to say that humans are very, very visual, whereas an animal that lives in the dark is primarily olfactory," noted Porter. "And people do have this idea that as our sense of vision and hearing has become so prominent, we've lost some of our olfactory capability."

That might not be the full story, however. "We think it's maybe that we don't place an emphasis on our sense of smell," Porter suggested. "Because the underlying mechanisms are still there -- to a greater extent than we maybe notice them. And if we place demands on them and train ourselves, our ability to follow a spatial path can improve a lot."

George Preti, a member of Monell Chemical Senses Center in Philadelphia, a research institute devoted the science of taste and smell, agreed that humans' olfactory talents can be honed with practice.

"Our sense of smell may not be as good as dogs or rodents, but it's still pretty acute when we use it correctly," he said. "I'm an expert in the chemistry of human odors, and I can tell you that folks that use it for a living, like perfumers, do improve their use of it and make themselves more consciously sensitive to that kind of input."

"Smell is important in our everyday life," Preti added. "And we use it a lot. Not just for evaluating food, but also for how we're perceived by the rest of the world. So, I'd say that humans certainly do have very sensitive noses -- regardless of what people might think."

People who experience smell disorders experience either a loss in their ability to smell or changes in the way they perceive odors. As for loss of the sense of smell, some people have hyposmia, which is when their ability to detect odor is reduced. Other people can't detect odor at all, which is called anosmia. As for changes in the perception of odors, some people notice that familiar odors become distorted. Or, an odor that usually smells pleasant instead smells foul. Still other people may perceive a smell that isn't present at all.

Patients may have difficulty recognizing smell versus taste dysfunction and frequently confuse the concepts of "flavor" and "taste." While the most common causes of smell disturbance are nasal and sinus disease, upper respiratory infection and head trauma, frequent causes of taste disturbance include oral infections, oral appliances ( e.g., dentures), dental procedures and Bell's palsy. Medications can interfere with smell and taste, and should be reviewed in all patients with reported dysfunction. In addition, advancing age has been associated with a natural impairment of smell and taste ability.