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_What the Nose Knows: The Science of Smell in Everyday Life_ (2008) by Avery Gilbert

The author's description of himself demonstrates that he is definitely someone who gets to play in the overlap between science and media studies:
"...I'm a sensory psychologist, trained in evolutionary theory, animal behavior, and neuroscience. I'm a rational, evidence-based guy working in the most frothy, fashion-driven, marketing-heavy business outside of Hollywood....
The new science of smell is making us rethink everything from wine tasting to Smell-O-Vision. So it's time for a fresh look at odor perception and how it plays out in popular culture."

1. How many smells are there?

Answer: Despite the fact that both pop science journalists and scientists have been tossing out estimates such as 10,000 smells and 30,000 smells, no one really knows for certain, and most of these writers received this number from some article which claimed to be scientific but failed to cite the source. The oldest of these articles may have been in a 1966 issue of Science Digest which claimed "Industrial chemists have identified some 30,000 different smells." As As Gilbert comments, "Unfortunately, the magazine didn't provide a source either. What this proves, I suppose, is that dubious facts thrived in the media long before the Internet."

Gilbert goes on to find the number 10,000 equally suspect:

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One would like to think that smell scientists have a better grasp of the matter, and indeed they prefer to quote a different estimate. When Linda Buck and Richard Axel won the 2004 Nobel Prize for discovering the olfactory receptors, the Nobel Foundation issued a press release. It noted that people recognize and remember "about 10,000 different odours," a figure the Swedish publicists took from the prize winners themselves. Surely that's a number we can take to the bank. But the number 10,000 didn't originate with Buck and Axel: it had been tossed about for years by other scientists.
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Gilbert continues his hunt for the originator of the number, and finally seems to locate a name mentioned in a footnote: Ernest C. Crocker, a chemical engineer and 1914 MIT graduate. Crocker, however, used a rather unreliable methodology to settle on this number.

2. Everyone knows that professional perfumers have a more refined sense of smell than the average person, but how does a perfumer identify each of the up to 250 ingredients which may go into a perfume?
Answer: This is a trick question on the part of Kestrell, as it turns out that perfumers sense of smell is not actually much more capable of distinguishing individual notes in a perfume. Instead, as in art and literature, what the student does is learn to identify the basic elements and learn how to combine these basic elements to create the desired effect.

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The first step in training is to learn the smell of the available ingredients. The leading teaching technique--the Givaudan method, created by the French perfumer Jean Carles--introduces students to the major ingredients using a matrix approach. Imagine a grid of rows and columns. Each row is a fragrance family: citrus, woody, spicy, and so on. Each column is a training session. In the first session, students smell column-wise one material from each family: lemon oil, sandalwood oil, and clove bud oil, for example. In the second session, the students smell new examples: bergamot oil, cedarwood oil, and cinnamon bark oil. This process continues for about nine lessons, by which time the students are familiar with the olfactory differences between families. Now comes the hard part--learning the "contrasts" within a family. Each subsequent session traverses one row of the matrix. In the citrus lesson, for example, students smell lemon, bergamot, tangerine, mandarin orange, blood orange, grapefruit, and lime. The goal, according to master perfumer and teacher René Morgenthaler, is for the student to create a personal impression of each ingredient. These individualized mental hooks are the key to remembering the fine discriminations needed to do perfumery. The graduate of nasal boot camp must recognize more than 100 natural materials and around 150 synthetics. The professional perfumer eventually becomes familiar with every material in his company's library--anywhere from 500 to 2,000 items--and is able to recognize every grade of each.
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However, perfumers don't waste time trying to identify each individual ingredient in a perfume, but instead identifies d identifies certain combinations which are called an accord.

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An accord is a mixture of raw materials (rarely more than fifteen) that go together particularly well. Accords are the building blocks of perfumery. By combining several of them, the perfumer creates an initial sketch of the perfume, sometimes called a skeleton. In a way, creating a perfume is like writing software: a programmer starts with building-block software modules that already contain many lines of code. A computer program is built with many modules, just as a fragrance is assembled from accords. The analogy goes further--software is tested with iterative debugging; perfume is tested with repeated sniffing and tweaking of the formula.
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CHAPTER 1 The Odors in the Mind Making sense of scents
3. Okay, so perfumers don't have supersmell abilities, but blind people do, right?
Answer: Sorry, but again, no. I was amused, however, to find that Gilbert frequently gets to hear about the supersmell abilities of blind people.
CHAPTER 3 Freaks, Geeks, and Prodigies The wide range of olfactory ability
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When, at a party, I own up to being an expert on the sense of smell, I get peppered with questions. (I don't mind this--if I'm not in the mood for Q&A, I tell people I'm "in the chemical business" and the conversation grinds to a halt.) People often ask about smell ability. Who is better: men or women? perfumers or normal people? Curiously, one comparison doesn't come as a question but as an assertion. Wineglass in hand, someone will inform me in earnest tones that "blind people have a heightened sense of smell." Others confidently assure me that "Helen Keller had an incredibly sensitive nose."
....[I]n the last twenty years, six studies have compared smell in the blind and the sighted. Without exception, they find that the blind are no more sensitive than the sighted--both groups detect odors at about the same concentration. Nor do blind and sighted people differ in the ability to discriminate one odor from another. Even the brain waves triggered by odor stimulation are similar in blind and sighted people.
Blind people may have one advantage: in three of the six studies, they were better at naming odors. Even here, their success depended on cognitive factors such as memory rather than hyperacute perception. Based on her own words, and on what has been observed in experiments, Helen Keller's ability to navigate the smellscape was not the result of a supersensitive nose. Rather, it was a triumph of the adaptable human brain making the most out of a perfectly ordinary nose.

4. Sometimes I find myself distracted by a scent, either pleasant or unpleasant, but my spouse/housemates insist that either it smells fine to them or they can't smell it at all. Am I having olfactory hallucinations?
Answer: You are not imagining it: individuals can possess very different combinations of smell receptors, so they also experience smells differently.

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The person-to-person variability in odor perception is enormous. To get an idea of the scale, compare it to color vision. Imagine that instead of three kinds of color blindness there were dozens, and that each type affected up to 75 percent of the population instead of only 6 percent. Smell scientists struggle to explain this variability; it remains one of the biggest mysteries about the sense of smell. Why are some people able to smell a particular molecule and others not? Why do some people find it pleasant and others do not?
Cultural factors--the favorite explanation of academic researchers--certainly play a role in odor preferences. But cultural explanations don't go too far in explaining the extensive differences between people within the same culture. Biological factors, which receive surprisingly little attention, may account for much of this variation. For example, certain specific anosmias--the inability of a person with otherwise normal smell to detect a specific type of molecule--have a biological basis, namely the lack of a receptor for the molecule in question. There are a couple of dozen specific anosmias, but they account for merely a fraction of the total variation in odor perception.
The key to the mystery may reside more broadly in the human genome. A tantalizing possibility is that your olfactory receptor genes determine how you smell the world, and why you smell it differently than other people. Everyone has roughly 350 olfactory receptors, but not necessarily the same 350 as the next person. In addition, the gene for a given receptor can show subtle variation in DNA sequence from person to person.
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5. Compared to animals, the sense of smell in humans is pretty poor, and geniuses such as Freud and Darwin claimed that, as humans evolved, the sense of smell was reduced because it isn't very important. Is this true?
Answer: No, and no. Not only does the human sense of smell compare pretty favorably to that of animals such as dogs and rodents, but there is no evidence that the human sense of smell has been reduced due to evolution.

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New evidence suggests that humans and animals may be more similar in odor perception than we thought. In 1991, Linda Buck and Richard Axel discovered a large family of mammalian olfactory receptor genes, work for which they eventually received the Nobel Prize. Each gene produces a different receptor. In general, more receptors means more detectable odors, and therefore greater smell ability. Rats have about 1,500 functional receptors, followed by dogs with about 1,000, mice with about 900, and chimpanzees with about 350. Humans have somewhere between 340 and 380. Dolphins have zero.
Does this mean rats are five times better smellers than we are? Not really. We can use DNA sequence similarity to arrange odor receptors into families and subfamilies. In theory, similar receptors detect similar odor molecules, so a receptor subfamily detects a class of related odors. When we compare odor receptor subfamilies, the human-animal gap doesn't look too large. Humans and dogs have about 300 subfamilies, rats have 282, and the mouse 241. The overlap between species is substantial. About 87 percent of human receptor subfamilies have counterparts in the mouse genome, while 65 percent of mouse subfamilies are shared by humans. This suggests to Linda Buck and her colleagues that "the majority of odorant features [i.e., smells] detectable by one species may also be recognized by the other." Perhaps a mouse can smell more of our world than we can smell of his. (Unlike us, he may have a whole subfamily of receptors devoted to cat urine.) For man and mouse the differences are not as big as the similarities. For man and chimp this is even more the case--there is a corresponding human gene for 85 percent of chimp odor receptor genes. Chimp, dog, man, or mouse, we perceive the general features of the smellscape in much the same way.
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Lastly, because there can never be too many Richard Feynman anecdotes: "The physicist Richard Feynman had a great party trick in which he would identify by smell objects briefly handled by other guests when he wasn't looking. He said it was easy to do because peoples' hands have surprisingly different scents. (A 1977 study confirmed that hand odor is individually distinctive and discriminable.)"

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