It Just Smells

If you play sounds of many different frequencies at the same time, they combine to produce neutral “white noise.” Neuroscientists say they have created an analogous generic scent by blending odors. Such “olfactory white” might rarely, if ever, be found in nature, but it could prove useful in research, other scientists say.

Using just a few hundred types of biochemical receptors, each of which respond to just a few odorants, the human nose can distinguish thousands of different odors. Yet humans can’t easily identify the individual components of a mixture, even when they can identify the odors alone, says Noam Sobel, a neuroscientist at the Weizmann Institute of Science in Rehovot, Israel. Now, he and his colleagues suggest, various blends made up of a large number of odors all begin to smell the same—even when the blends share no common components.

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Although many scents—such as coffee, wine, roses, and dirty socks—are complex blends containing hundreds of components, they are very distinctive. At least two factors are responsible, Sobel says: The individual odorants are often chemically related, and often one or more of them is vastly more intense than the rest.

The team’s findings are “a clever piece of work that shows the olfactory system works exactly as we would predict from our current understanding of it,” says Tim Jacob, a neuroscientist at Cardiff University in the United Kingdom. “That is, if you stimulate every olfactory ‘channel’ to the same extent, the brain cannot characterize or identify a particular smell,” he notes.

“Olfactory white is a neat idea, and it draws interesting parallels to white light and white noise,” says Jay Gottfried, an olfactory neuroscientist at Northwestern University’s Feinberg School of Medicine in Chicago, Illinois. The new study “definitely adds new information about how the brain interprets odors,” he notes.

Even though olfactory white is not likely to be encountered in nature, the concept could be useful, Gottfried says. “Researchers have found that white noise is a useful stimulus in experiments to probe auditory responses,” he notes, and scientists probing the human sense of smell might find similar uses for olfactory white.

How Cells in the Nose Detect Odors

Now a team of scientists, led by neurobiologists at the University of California, Riverside, has an explanation. Focusing on the olfactory receptor for detecting carbon dioxide in Drosophila (fruit fly), the researchers identified a large multi-protein complex in olfactory neurons, called MMB/dREAM, that plays a major role in selecting the carbon dioxide receptors to be expressed in appropriate neurons.

Study results appear in the Nov. 15 issue of Genes & Development.  The research is featured on the cover of the issue.

According to the researchers, a molecular mechanism first blocks the expression of most olfactory receptor genes (~60) in the fly’s antennae. This mechanism, which acts like a brake, relies on repressive histones —proteins that tightly wrap DNA around them. All insects and mammals are equipped with this mechanism, which keeps the large families of olfactory receptor genes repressed.

“How, then, do you release this brake so that only the carbon dioxide receptor is expressed in the carbon dioxide neuron while the remaining receptors are repressed?” said Anandasankar Ray, an assistant professor of entomology, whose lab conducted the research. “Our lab, in collaboration with a lab at Stanford University, has found that the MMB/dREAM multi-protein complex can act on the genes of the carbon dioxide receptors and de-repress the braking mechanism — akin to taking the foot off the brake pedal. This allows these neurons to express the receptors and respond to carbon dioxide.”

Ray explained that one way to understand the mechanism in operation is to consider a typewriter. When none of the keys are pressed, a spring mechanism or “brake” can be imagined to hold the type bars away from the paper. When a key is pressed, however, the brake on that key is overcome and the appropriate letter is typed onto the paper. And just as typing only one letter in one spot is important for each letter to be recognized, expressing one receptor in one neuron lets different sensor types to be generated in the nose.

The Knowing Nose: Chemosignals Communicate Human Emotions

Many animal species transmit information via chemical signals, but the extent to which these chemosignals play a role in human communication is unclear. In a new study published in Psychological Science, a journal of the Association for Psychological Science, researcher Gün Semin and colleagues from Utrecht University in the Netherlands investigate whether we humans might actually be able to communicate our emotional states to each other through chemical signals.

Existing research suggests that emotional expressions are multi-taskers, serving more than one function. Fear signals, for example, not only help to warn others about environmental danger, they are also associated with behaviors that confer a survival advantage through sensory acquisition. Research has shown that taking on a fearful expression (i.e., opening the eyes) leads us to breathe in more through our noses, enhances our perception, and accelerates our eye movements so that we can spot potentially dangerous targets more quickly. Disgust signals, on the other hand, warn others to avoid potentially noxious chemicals and are associated with sensory rejection, causing us to lower our eyebrows and wrinkle our noses.

Semin and colleagues wanted to build on this research to examine the role of chemosignals in social communication. They hypothesized that chemicals in bodily secretions, such as sweat, would activate similar processes in both the sender and receiver, establishing an emotional synchrony of sorts. Specifically, people who inhaled chemosignals associated with fear would themselves make a fear expression and show signs of sensory acquisition, while people who inhaled chemosignals associated with disgust would make an expression of disgust and show signs of sensory rejection.

Scent Into Action

Ferrero, a neurobiologist from Harvard, was visiting the zoo to gather urine specimens for a study linking odors to instinctual behavior in rodents. Early lab results had hinted that a whiff of a chemical in carnivore pee flashed a sort of billboard message, blinking “DANGER” in neon lights — enough to make animals automatically shrink away in fear.

Ferrero and Harvard neurobiologist Stephen Liberles are among a cadre of researchers trying to understand the basis of instinctual animal behaviors. In the last few years, scientists have made progress by studying smell — unmasking the molecular identities of behavior-triggering odors and charting these odors’ routes to the brain. One early stop, a sensory structure known to spur mice into action when they encounter odors from other mice, can actually rev the rodents up when they run into cats or rats, too.

In fact, studies have shown that odors from different species can spark varying patterns of neural activity in mice. And new evidence from researchers including Ferrero and Liberles suggests behavior-triggering odors don’t always travel to the brain in the way scientists once thought.

Recent research has even revived interest in the once-ridiculed idea that humans also respond instinctually to odors from other humans — though some scientists still think the idea is kooky. No matter who has it right, the new work may hold clues to the brain areas responsible for complex behavior in people.

“We used to think it was beyond the reach of what we could study,” says neurobiologist Lisa Stowers of the Scripps Research Institute in La Jolla, Calif. “There was just too much going on in the brain.”

Human heads are big, complicated and tricky to access, so researchers are zeroing in on rodent brains instead.

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Study suggests that a poor sense of smell may be a marker for psychopathic traits.

People with psychopathic tendencies have an impaired sense of smell, which points to inefficient processing in the front part of the brain [orbitofrontal cortex]. These findings by Mehmet Mahmut and Richard Stevenson, from Macquarie University in Australia, are published online in Springer’s journal Chemosensory Perception.

Psychopathy is a broad term that covers a severe personality disorder characterized by callousness, manipulation, sensation-seeking and antisocial behaviors, traits which may also be found in otherwise healthy and functional people. Studies have shown that people with psychopathic traits have impaired functioning in the front part of the brain – the area largely responsible for functions such as planning, impulse control and acting in accordance with social norms. In addition, a dysfunction in these areas in the front part of the brain is linked to an impaired sense of smell.

Mahmut and Stevenson looked at whether a poor sense of smell was linked to higher levels of psychopathic tendencies, among 79 non-criminal adults living in the community. First they assessed the participants’ olfactory ability as well as the sensitivity of their olfactory system. They also measured subjects’ levels of psychopathy, looking at four measures: manipulation; callousness; erratic lifestyles; and criminal tendencies. They also noted how much or how little they emphasized with other people’s feelings.

The researchers found that those individuals who scored highly on psychopathic traits were more likely to struggle to both identify smells and tell the difference between smells, even though they knew they were smelling something. These results show that brain areas controlling olfactory processes are less efficient in individuals with psychopathic tendencies.

Scientists have restored the sense of smell in mice through gene therapy for the first time — a hopeful sign for people who can’t smell anything from birth or lose it due to disease.

The achievement in curing congenital anosmia — the medical term for lifelong inability to detect odors — may also aid research on other conditions that also stem from problems with the cilia. Those tiny hair-shaped structures on the surfaces of cells throughout the body are involved in many diseases, from the kidneys to the eyes.

The new findings, published online in Nature Medicine, come from a team at the University of Michigan Medical School and their colleagues at several other institutions.

The scent of love: Decomposition and male sex pheromones

A team of researchers, led by Christian von Hoermann from Ulm University, Germany, filled olfactometers with different volatile scents and recorded which scents female hide beetles were attracted to. The scents used were pig cadaver, collected at different stages of decay, male pheromone gland extract, synthetic pheromones, and a control, pentane (an organic solvent which was used to extract the other odours).

The females ignored both the control and synthetic pheromone. In fact they pretty much ignored everything apart from the odour of piglet in the dry remains stage, as long as it was enhanced by male pheromones.

Christian von Hoermann explained, “Although cadaver odour alone is not sufficient to attract two to three week-old virgin female hide beetles, it is enough to attract newly emerged males.” Release of pheromones by these males appears to signal the cadaver as an appropriate site for feeding, mating and egg laying. Evolution seems to have ensured that hide beetle females only respond to a mate (or a food source for their larvae) when the other is also present, so that they can optimise the chances of their offspring’s survival.

Smelling a skunk after a cold: Brain changes after a stuffed nose protect the sense of smell

A new Northwestern Medicine study shows that after the human nose is experimentally blocked for one week, brain activity rapidly changes in olfactory brain regions. This change suggests the brain is compensating for the interruption of this vital sense. The brain activity returns to a normal pattern shortly after free breathing has been restored.

Previous research in animals has suggested that the olfactory system is resistant to perceptual changes following odor deprivation. This new paper focuses on humans to show how that’s possible. The study is published in the journal Nature Neuroscience.

"You need ongoing sensory input in order for your brain to update smell information," said Keng Nei Wu, the lead author of the paper and a graduate student in neuroscience at Northwestern University Feinberg School of Medicine. "When your nostrils are blocked up, your brain tries to adjust to the lack of information so the system doesn’t break down. The brain compensates for the lack of information so when you get your sense of smell back, it will be in good working order."