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The smell of chlorine wafts through the air. Suddenly, you recall childhood summers spent in a swimming pool. Or maybe it's a whiff of apple pie, or the scent of the same perfume your mom used to wear. Our noses have a way of sniffing out nostalgia.

“I stepped into an elevator and a bunch of people piled in behind me. I was behind a woman with her back to me, her hair was in my nose, and I could smell the perfume, Shalimar, and I hadn’t smelled it in [years]. It seemed like I was transported back to high school,” says Howard Eichenbaum, director of the Laboratory of Cognitive Neurobiology at Boston University.

While all the senses are connected with memories, smell in particular sparks a flurry of emotional memories. Why?   

After a smell enters the nose, it travels through the cranial nerve through the olfactory bulb, which helps the brain process smells. The olfactory bulb is part of the limbic system, the emotional center of the brain. As a member of the limbic system, the olfactory bulb can easily access the amygdala, which plays a role in emotional memories (it’s also where the "fight or flight" reflex comes from).   

“Olfactory has a strong input into the amygdala, which process emotions. The kind of memories that it evokes are good and they are more powerful,” explains Eichenbaum.

This close relationship between the olfactory and the amygdala is one of the reason odors cause a spark of nostalgia. 

“We don’t use emotional memory that much,” says Dr. Ron DeVere, director of the Taste and Smell Disorders Clinic and the Alzheimer’s Disease and Memory Disorders Center, in Austin, Texas, and member of the American Academy of Neurology (AAN). He adds that when people consciously attempt to remember something they focus on the details, not feelings.

“You have an odor, you may not identify the odor, but you are associating that with some memories. The first time you smelled apple pie you may have been at your grandmother’s house,” DeVere says. 

Also at play is a relationship between the olfactory system and the hippocampus, which is critical to developing memories. Even though the olfactory system interacts with the emotion and memory centers in the brain, it does not connect with more developed regions.

“Smells do bring back memories,” says Dr. Ken Heilman, James E. Rooks Jr. Distinguished Professor Neurology and Health Psychology at the University of Florida and a member of AAN. “Smell goes into the emotional parts of the brain and the memory parts, whereas words go into thinking parts of the brain.”

This could explain why memories sparked by smell feel nostalgic and emotional, rather than concrete and detailed. Also, Eichenbaum notes that primates evolved to rely mostly on vision, not smell, so these memories are less reliable. (If you were a rat in his lab, your smell memories would be more detailed).

“When you smell things you remember your emotions … it’s very, very true,” says Heilman. 

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• 'Old person smell' is a real thing
• Life with no sense of smell
• Your cilantro love -- or hate -- may be genetic

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To most readers, this text looks black and white. But to a few, each letter possesses a different color, and reading becomes more than what’s in black and white.   

Those who read in color live with grapheme-color synesthesia, where the brain assigns colors to letter and numbers. Some synesthetes say words possess colors, too (someone might say truth looks gold, for example). Overall, 4 percent of the population experiences a form of synesthesia with 1 percent living with grapheme-color synesthesia.

Synesthesia gives many people a richer experience and it’s believed to be mostly harmless and fixed—people either have it or they don’t.

Until now.

Researchers at the University of Amsterdam found that people, without a history of grapheme-color synesthesia, who read books with some colored letters, associated those letters with the correlating hues. This is the first time anyone has taught synesthesia by reading books. 

“Whenever we give a talk or lecture, people ask if they can learn synesthesia,” says Olympia Colizoli, a doctoral student in the brain and cognition department at the University of Amsterdam.

“Most people would never want to give up their synesthesia and can’t imagine not having these experiences.”

To test whether people could learn grapheme-color synesthesia, Colizoli asked 15 subjects to read books that had four frequently occurring letters paired with four commonly seen colors. Each participant selected a book from Project Gutenberg and Colizoli applied color to the book (prior to the experiment she colored every letter in a book but it made it very difficult to read). Colizoli’s interest isn’t simply professional; she has "time form" synesthesia, which means she sees periods of times, such as days, weeks, or centuries, as shapes.

“Even though [synesthesia] seems to run in families and the evidences suggests it is genetic, language is learned and it comes from the environment … no one is born with the letter a in their brain,” she says. Yet, there seems to be little understanding of the role of environment and synesthesia. 

Prior to reading the colored book, Colizoli asked the participants to take a modified Stroop test, which detects grapheme-color synesthesia, to assure none of the subjects had it. In a modified Stroop test, people look at the words printed in different colored ink. Grapheme-color synethetes have delayed responses when identifying the letters’ colors.

After completing the book, the subjects re-took the Stroop test and showed behavioral signs of synesthesia. Colizoli does not believe these effects are permanent, noting more research needs to be conducted. She and her colleagues also replicated the results with participants who read in Dutch.

“We are bombarded by colored letters all the time,” Colizoli says. “It is interesting to see how adaptive [synesthesia] may be.”

Colizoli also asked the subjects if they noticed any differences since the experiment and they gave a variety of subjective responses (much like synesthetes would). One person claimed to dislike orange until reading in color, while two subjects say they now read faster. Another woman, a musician, enjoyed reading in color so much she asked if Colizoli could print all her sheet music in color for her. (This is not uncommon; artists frequently claim to be synesthetes. Vladimir Nabokov saw the alphabet in rainbow colors with each letter appearing the same shade each time he saw it.)   

“She could remember the music better and fell in love with it. Some people were really sensitive to it.”  

The paper appears in the online journal PLoS ONE. If you want to try reading like a grapheme-color synsethete, check out this link.

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We’ve all heard about the amazing ability of some blind people to hear, smell, or touch with such a high degree of acuity that they become almost savant-like. Daniel Kish, for example, has become famous for his ability to use echolocation, like bats or dolphins, to navigate the world.

But Kish was blinded at the tender age of 13 months, and many other blind people with hyper-ability in other senses were born that way. What about the rest of us? Do we have innate abilities we almost never use?

At this week’s Acoustics 2012 scientific meeting in Hong Kong, a team of Canadian researchers from the auditory neuroscience lab of François Champoux at the University of Montreal presented a study they conducted with sighted people. According to Simon Landry, a graduate student, the researchers exposed subjects to a harmonic tone. Such tones sound like a single note, but they actually have layers of “harmonicity.” So the team slightly altered one layer until the subjects could notice it.

All sighted subjects were about the same in their ability to distinguish an altered layer. But in a second round of testing, those who spent just 90 minutes blindfolded performed significantly better than non-blindfolded participants, and better than they themselves did the first time around.

How the brain can do this hasn’t yet been fully established, but, Landry explained, “the idea is that the brain doesn’t actually change, but vision no longer suppresses the processing of other modalities, which have existing pathways, in the visual cortex.”

By “modalities,” Landry means types of sensory input. He’s using the language of Alvaro Pascual-Leone, a Harvard behavioral neurologist who has spent years studying how the brain processes information.

Pascual-Leone refers to the brain as “metamodal.” He sees the results of the Montreal researchers as “amazingly remarkable…. This is one more illustration” of the metamodal hypothesis “and the implication to me is that clearly there is a cross-talk betweens the senses that goes well beyond what we thought. We have been thinking of these systems as silos, independent of each other, and that is definitely not the case.”

In Pascual-Leone’s hypothesis, parts of the brain aren’t firmly predestined to translate vision or touch or sound, they are simply biased toward one or another by the way they develop. Then, when we open our eyes as newborns, the visual information tends to be translated by the occipital cortex because it’s best suited for the job, not because it’s the only region that can do it, nor because that’s all it can do. All that visual information streaming in just overwhelms information from our ears or our fingers. In his view, the sensory pats of our brains aren’t silos, but a web.

“Why do you close your eyes when you go to a concert?” he asks. “You are suppressing the visual input. That disinhibits the connection between the visual and auditory cortices” so information can flow between them. “It makes more of the brain able to process sound.”       

Take, for example, the case of a Spanish woman who was blind since birth. She worked as a Braille proof reader for the Spanish Organization for the Blind. At age 62, she suffered a stroke that resulted in a coma from which she recovered. But she was no longer able to read Braille. Her stroke had injured her occipital cortex, the center of vision. Her sense of touch was unimpaired. Though she had ever been able to see, her occipital cortex was instrumental in making sense of what she touched and turning it into language.

Pascual-Leone pointed out that the Canadian experiment underscores the difference between those blind from birth and those who become blind later in life. Those who have experienced vision “have a brain that’s been calibrated with vision.” They have, metaphorically speaking, acquired a library of references that have physically reshaped their brains. A congenitally blind person knows what, for example, a cube and a sphere feel like. But if they gain sight later in life, and see a cube and sphere together for the first time, they can’t tell which is which “unless they can match the visual with the tactile,” he said. After that integration, they do fine.

“What we find with sighted, blindfolded people,” he said, “may be showing us something about what it is to see, rather than to be blind.”  

Brian Alexander (www.BrianRAlexander.com) is co-author, with Larry Young PhD., of "The Chemistry Between Us: Love Sex and the Science of Attraction," (www.TheChemistryBetweenUs.com)  to be published Sept. 13.

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To a very vocal online contingent, cilantro is the very worst.

On "I Hate Cilantro" websites and Facebook pages they gripe that the herb tastes like soap, mold, or dirt. Cilantro haters not only despise its flavor, they also detest its smell. Stories in publications as serious as the New York Times, the Wall Street Journal and, yes, even msnbc.com have even covered the sharp divide in taste preferences when it comes to this particular herb.  And when a study of identical twins found an aversion to cilantro stems from a genetic glitch, the herb's bashers finally had a good reason why they found the leaves of the Coriander plant so offensive.

But who are these people in the anti-cilantro community? No one had a clue -- until now.

There has been no attempt to quantify which people hate the herb until two nutrition experts from the University of Toronto took a stab at it. They recently published their findings in the journal Flavour. In the study, they surveyed nearly 1,400 young adults ages 20 to 29 in Canada. 

Volunteers completed a 63-item preference checklist in which they rated each food on a 9-point scale from 1 (dislike extremely) to 9 (like extremely). They could also select "never tried" or "would not try."

Researchers found an aversion to cilantro ranged from a low of 3 percent to a high of 21 percent among six different ethnic groups.

Young Canadians with East Asian roots, which included those of Chinese, Japanese, Korean, Thai and Vietnamese descent, had the highest prevalence of people who disliked the herb at 21 percent. Caucasians were second at 17 percent, and people of African descent were third at 14 percent. 

Among the herb's fans, the group with the fewest number of people who disliked cilantro were those of Middle Eastern background at 3 percent, followed by those of Hispanic and South Asian ancestry at 4 percent and 7 percent respectively.

Exposure to the herb at an earlier age and with greater frequency in Mexican, Asian, and Indian cooking likely helps shape a positive flavor preference. Another possibility is that genetic differences among the cultural groups might influence someone's taste perception of the herb.  

Although researchers have yet to evaluate all 63 items on the food-preference checklist, study author Ahmed El-Sohemy, PhD, is sure of one thing: "Cilantro is perhaps the most polarizing with large numbers either loving it or hating it." The paper calls this the "unusual divisive nature of cilantro."

"People who dislike cilantro extremely describe it very, very differently from those who love it," explains El-Sohemy, an associate professor of nutrition at the University of Toronto. The reason? "These individuals live in very different sensory worlds and are not perceiving the same thing," he says.

As for El-Sohemy's opinion of cilantro, count him among the lovers. "I remember loving the taste as a child," he says. "I distinctly remember my mother's Egyptian cooking, which used cilantro frequently."

The study is a first step in determining how widespread a dislike for cilantro is, at least in a sample of young Canadians. It's unclear whether older Canadians feel similarly or how much the herb is despised by people in other countries.

Eventually, the Toronto scientists hope to pinpoint the genetic basis for why cilantro is an herb some people love to hate.

Related:

• Your cilantro love -- or hate -- may be genetic
• Phantom smells may be a sign of trouble
• The nose doesn't know: Life with no sense of smell
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From Starbucks to spoiled milk to freshly baked snickerdoodles, most of us take our sense of smell -- and the world's good, bad and ugly odors -- for granted. For people born with isolated congenital anosmia (ICA), though, the world smells as bland as a soggy soda cracker.

"I don't know what it is to smell," says Martin Angel, a 48-year-old urban planner from Santa Ana, Calif. "People try to describe it to me and I'm like, 'That's like trying to describe the sun to a blind man.'"

While Angel says he doesn't miss his sense of smell because "when you've never had something, there's nothing to miss," a new study published in the Public Library of Science's journal PLoS ONE has found that ICA can impact people in a number of ways, causing everything from enhanced social insecurity to an increased risk of household accidents.

"The sense of smell is one of the oldest sensory systems and provides a lot of important information influencing human behavior," says lead researcher Ilona Croy of the Smell and Taste Clinic at the University of Dresden Medical School in Germany. "Olfactory cues can transport emotional information, are critical for detecting edible food and help to prevent microbial threats. We were curious how people who are not able to smell lead their lives."

To find out, Croy and her colleagues interviewed 32 people with ICA along with 36 age-matched "controls" (people who could smell just fine). Questions ranged from how often they scorched food to how often they showered to the number of sexual partners they'd had in their lifetime. After tallying the results, researchers found people with ICA had a slight increase in social insecurity (i.e., they worried more about social situations and their own body odor), an increased risk for depressive symptoms and an increased risk for household accidents, such as eating spoiled food or burning clothes while ironing.

Julie Solo, a 45-year-old international health worker from Durham, N.C., who was born without a sense of smell, says she read about the study's findings but doesn't believe she suffers from either depressive symptoms or an increase in social anxiety.

"The only down side is worrying about not smelling a fire," she says. "Although I did once have a boyfriend who told me my feet smelled. He was just joking around, though."

On the other hand, ICA sufferer Carol Tedesco, a 53-year-old historic shipwreck professional from Key West, Florida, says some of the findings definitely ring true for her.

"I live in the tropics and I'm very active and I usually shower three times a day," she says. "I'm always concerned I might not be fresh. Plus I'm more dedicated to emptying the cat box and keeping it clean. You don't want someone to walk into your house and have it not smell good and not know it."

While approximately one-fifth of the population have an impaired sense of smell,  ICA affects only about 1 in 5,000 to 10,000 people, says Croy.

"In most ICA patients, there is no olfactory bulb, meaning they are missing the most significant part of the olfactory system in the brain," she says.

While people with ICA can still taste, Croy says they won't "perceive flavors as they are perceived via the sense of smell."

The inability to smell may also affect their sex life. The new study found people with ICA had about half the number of sexual partners as those with no smell impairment.

"This might be related to the enhanced social insecurity," says Croy. "However, it could also be that they are less responsive to odorous sexual stimuli."

While Croy's research points to the downfalls of ICA, those without a sense of smell quickly point to the upside of their condition.

"I can go into places that most people won't go into, like Porta-Potties," says Angel. "People will come out gagging, but I'm okay."

Solo, who does a lot of work in developing countries, says her inability to smell is actually a boon.

"A friend came to visit me once and we went to a slum in Nairobi where they were doing garbage cleanup," she says. "I guess the smell of a large garbage dump in a slum in Nairobi was pretty awful. My friend was dying, but I was just fine."

Tedesco, too, has found that her ICA has certain benefits.

"I can drive by where a skunk has been run over and not even notice," she says. "And my partner says I'm great to live with because I can't smell when he breaks wind. To me, it's just a sound."

Related: 

Sniff test: Living without a sense of smell

Phantom smells may be a sign of trouble

Your cilantro love -- or hate -- may be genetic

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One of the less-talked-about side effects of being blind: fielding many (many!) questions from us sighted folks -- enough questions, apparently, to program a YouTube channel. Tommy Edison, who is blind, hosts the popular YouTube series "The Tommy Edison Experience," where he answers viewer questions: How do blind people use an ATM? How do blind people use paper money?

In one of his latest videos, he posts his answer to a fascinating question: How do blind people dream? 

Edison explains that he's been blind since birth, so, no, he doesn't "see" in his dreams. "I think because I’ve never seen in real life, that my sub-conscious doesn’t know what it’d be like to see, either, so, no. I don’t see in my dreams,” Edison explains in the video.

 “I mean, the way it works for me, is just the way my life occurs, right? So it’s all smell, sound, taste and touch," he continues. "That’s all there is. Just like your life works. I mean, you see in your life, so, obviously, you’d see in your dreams."

To someone who's always been able to see, though, that description might be surprising. (Edison dreams in Smell-O-Vision?) Most sighted people remember the images and emotions from a dream -- but smells, sounds, tastes and touches, maybe not so much. 

"You guys, you’re visually driven," Edison said to me in a phone interview. "I don’t know, 'cause I’ve never seen, but I would think if there was something very prevalent -- like if there was a fire in your dream -- I would think you would remember the smell of it. Or take a bite of the hamburger, and it tastes like lobster -- that’s going to be a funny thing you'd remember." 

In the video, he explains a typical dream: “I mean, just like you guys, right? Weird things happen in dreams. I mean, so here I am, it’s the bottom of the ninth. Runners on second and third, two men away -- and all of a sudden, it’s my seventh birthday. Strange!”

And as it turns out, the question "How do blind people dream?" has been tackled a surprising number of times by academics. 

"One of the interesting things about this idea is people have actually studied it, specifically looking at what kind of content (blind) people have in their dreams," says Dr. Rachel Vreeman, assistant professor of pediatrics at Indiana University School of Medicine, co-author of "Don't Cross Your Eyes ... They'll Get Stuck That Way!" and frequent answerer of some of our odder questions. 

"It shows in these studies that people who have been blind since birth or very early in childhood have no images in their dreams," although that seems to vary, Vreeman explains. Some studies report that if a person loses his or her sight before the age of 5 will almost never have images in dreams, but a few exceptions exist. For those who go blind in middle childhood, it seems dreams can go either way -- visual or non-visual. And, as it logically follows, those who become blind later in life continue to experience some images in their dreams -- but that can fade over time, Vreeman explains. 

She says at least one study suggests that transportation is a recurring theme in the dreams of many blind people -- perhaps because that's something that often gives them trouble in real life.

"The key with that is it’s your brain that’s making the dream ... It’s really what your brain has experienced and what your brain continues to experience," Vreeman says. "People who are blind tend to have a lot more smells, hearing, tactile (sensations), which people who have vision tend to not have many of those. I can’t remember a dream that I’ve ever had, and I feel like a lot of sighted people feel the same way, where there were lots of textures, lots of smells." 

Edison's videos are an offshoot of his main online presence as the Blind Film Critic -- he reviews movies from the perspective of a blind person. He says after he reviewed "Inception" -- which he says he found easier to follow than most sighted people seemed to -- his viewers started asking him the "how do blind people dream" question. (A question, incidentally, he parodied in "Sh-- Sighted People Say To Blind People," his take on the sh-- X says to Y meme that was everywhere a few months ago: "Hey, when you're dreaming, can you see?")

In each video, Edison cheerfully, charmingly, gives us sighted folks his take on what it's like to live life the way he does. 

"You see people -- they’re not sure what I can and cannot do. People raise their voice: 'HI, IT'S NICE TO MEET YOU.' People don’t know how to be, and that’s OK," he says. "People are curious. You don’t run into a lot of me in the world. Really, you don’t -- there’s not a lot of us, especially people who have been blind since birth."

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When Bridget Lewis began to lose her sense of smell, her friends and family thought she couldn't be serious.

"Everyone thought I was joking," says Lewis, who's 42 and lives in Arlington, Texas. "My mom even had fun with it, and 'tested' me. She asked me to hold out my tongue and had my little sister grab things from the kitchen, like Tabasco sauce and lemon juice and they squeezed drops of it on my tongue! When they saw I had no reaction, they believed me."

In Lewis's case, a severe bronchial infection stole her sense of smell in 2002; nine years later, it's only partially returned. Most of us have experienced a temporarily diminished sense of smell -- it's part of the super fun side effects of a cold, along with the dripping nose and general malaise. But before it happened to her, Lewis says she didn't even realize a long term loss of smell was even possible -- there is blindness, and deafness, but many people aren't familiar with the word for a lost sense of smell: anosmia.

"It's not as dehabilitating [as losing other senses]," says Dr. Beverly Cowart of the Monell Chemical Senses Center. "But I think people find it hard to relate to; I think people don't appreciate how much they use their sense of smell."

An estimated 1 to 2 percent of people in North America say they have a smell disorder, according to government figures. It's more common in men than in women, and it's much more common in older people, occurring in nearly 25 percent of men aged 60 to 69 and 11 percent of women in that age range, according to the National Institutes of Health.

High inside your nose is a grouping of cells called olfactory sensory neurons. These neurons each possess an odor receptor, which picks up on the tiny molecules released by things like a just-peeled orange, a bag of microwaved popcorn or your coworker's tuna fish lunch. The neurons then pass the information to your brain, which interprets the smell.

As Cowart explains, there are a few common ways to lose your sense of smell: a chronic nasal sinus disease, a viral infection, inhaling something toxic or a head injury. Unlike most nerves, the brain's olfactory nerve fibers are continuously replaced, "so occasionally people can gradually regain their sense of smell as new fibers grow in." And some people are born without the ability to smell.

Lacking a sense of smell can be dangerous -- anosmics wouldn't notice a gas leak, or the smell of smoke. Personal hygiene is a big worry for some. "The biggest effect is on food-flavor perception," Cowart says. "It actually creates social difficulties for people. Sometimes, people become more isolated; they don't go out as much."

Smelly stuff reaches the olfactory neurons in one of two ways: either through your nostrils, or through a pathway connecting the roof of your throat to your nose. If that second passageway is blocked, we're unable to pick up on the odors -- and, consequently, the flavors -- emitted by food.

One rainy day four years ago while Lewis was pregnant, she remembers bursting into tears over her anosmia."I was sad and I think depressed because I realized I couldn't smell the rain ... and realized I wouldn't be able to smell the scent of my newborn," Lewis remembers. "Horrible day."

On the upside, potentially smelly tasks like cleaning out the fridge don't bother her at all -- but she adds that she tends to go overboard. "I have always been scared that I could make our daughter sick — if I even suspect something has been in the fridge for too long, I throw it out.  I've also burned many pots and foods in the oven because as they cooked, I couldn't smell them!"

Have you or a family member lost the ability to smell? How has it impacted your life?

You can find The Body Odd on Twitter and Facebook, and follow Melissa Dahl @melissadahl.

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Scientists have sniffed out a novel idea to stop hearing loss in its tracks: A new procedure essentially provides a way for people to hear with their noses. (Can you smell me now?)

Australian scientists say that taking stem cells from the nose and transplanting them to the ear may help preserve hearing for those whose auditory problems begin in infancy or childhood, according to a new study published Thursday in the journal Stem Cells. The research focused on early-onset sensorineural hearing loss, which is caused by a loss of sensory cells or neurons in the cochlea. (That's the part of the inner ear that holds the actual sensory organ of hearing.)

It was a study in mice, but the researchers believe the findings may apply to human ears, too. A team of scientists, led by Dr. Sharon Oleskevich of the University of New South Wales, injected mucosa-derived stem cells (ick) into the cochlea of mice who were showing signs that their hearing was deteriorating. (Mice were used in the study because the way early hearing loss works in the little critters is similar to the way it works in humans.)

“One of the challenges in tackling this condition is that the regenerative ability of the human cochlea is severely limited," Oleskevich said in a statement. “It has been proposed that the transplantation of cells from other parts of the body could treat, prevent or even reverse hearing loss. The transplanted cells have the potential to repair tissue by replacing damaged cells and enhancing the survival of existing cells, preventing the condition from developing further.”

A month later, researchers tested each mouse's hearing threshold, using an auditory brainstem response test, which measures the lowest sound level to which the brain responds -- and the mice with the transplanted nasal stem cells did better when compared to mice without.

This is the craziest story I've smelled all day. What about you? What do you think of the new research?

You can find The Body Odd on Twitter and Facebook, and follow Melissa Dahl @melissadahl.

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