Evidence That at Least One Mammal Can Smell in Stereo

Most mammals, including humans, see in stereo and hear in stereo. But whether they can also smell in stereo is the subject of a long-standing scientific controversy.

Now, a new study shows definitively that the common mole (Scalopus aquaticus) – the same critter that disrupts the lawns and gardens of homeowners throughout the eastern United States, Canada and Mexico – relies on stereo sniffing to locate its prey. The paper that describes this research, “Stereo and Serial Sniffing Guide Navigation to an Odor Source in a Mammals,” was published on Feb. 5 in the journal Nature Communications.

“I came at this as a skeptic. I thought the moles’ nostrils were too close together to effectively detect odor gradients,” said Kenneth Catania, the Stevenson Professor of Biological Sciences at Vanderbilt University, who conducted the research.

What he found turned his assumptions upside down and opened new areas for potential future research. “The fact that moles use stereo odor cues to locate food suggests other mammals that rely heavily on their sense of smell, like dogs and pigs might also have this ability,” Catania said.

Can You Smell Yourself?

You might not be able to pick your fingerprint out of an inky lineup, but your brain knows what you smell like. For the first time, scientists have shown that people recognize their own scent based on their particular combination of major histocompatibility complex (MHC) proteins, molecules similar to those used by animals to choose their mates. The discovery suggests that humans can also exploit the molecules to differentiate between people.

"This is definitely new and exciting," says Frank Zufall, a neurobiologist at Saarland University’s School of Medicine in Homburg, Germany, who was not involved in the work. "This type of experiment had never been done on humans before."

MHC peptides are found on the surface of almost all cells in the human body, helping inform the immune system that the cells are ours. Because a given combination of MHC peptides—called an MHC type—is unique to a person, they can help the body recognize invading pathogens and foreign cells. Over the past 2 decades, scientists have discovered that the molecules also foster communication between animals, including mice and fish. Stickleback fish, for example, choose mates with different MHC types than their own. Then, in 1995, researchers conducted the now famous “sweaty T-shirt study,” which concluded that women prefer the smell of men who have different MHC genes than themselves. But no studies had shown a clear-cut physiological response to MHC proteins.

In the new work, Thomas Boehm, a biologist at the Max Planck Institute of Immunobiology and Epigenetics in Freiburg, Germany, and colleagues first tested whether women can recognize lab-made MHC proteins resembling their own. After showering, 22 women applied two different solutions to their armpits and decided which odor they liked better. The experiment was repeated two to six times for each participant. Women preferred to wear a synthetic scent containing their own MHC proteins, but only if they were nonsmokers and didn’t have a cold. The study did not determine which scents women preferred on other people, but past studies on perfume have shown that individuals prefer different smells on themselves than on others.

The researchers wanted to know whether the preferences were truly rooted in the brain’s response to the proteins. So next, they used functional magnetic resonance imaging to measure changes in the brains of 19 different women when they smelled the various solutions, in aerosol form puffed toward their noses. “Sure enough, there again was a clear difference between the response to self and non-self peptides,” Boehm says. “There was a particular region of the brain that was only activated by peptides resembling a person’s own MHC molecules.” The brain had a similar response to all non-self MHC combinations, suggesting that any preference for how other people smell is a preference for non-self, not for particular MHC types.

(Image: Getty)

Help for when noses no longer smell properly

A psychological test, available for the first time, is intended to make the counselling and treatment of patients with olfactory dysfunction significantly easier. The new method has been developed by the University Department of Neurology at the MedUni Vienna.

The new investigation method provides the first easy-to-use testing process that measures subjective impairments caused by problems with the sense of smell. The test also examines how the impairment impacts on the patient’s quality of life. The aim of the test is to make targeted treatment and counselling to sufferers significantly easier in the future.

According to Gisela Pusswald, the developer of the test who works in the University Department of Neurology, patients often complain that their food no longer tastes like it used to and that they are unable to perceive perfumes or body odour at all, or only to a limited extent. The associated uncertainty of everyday living is often an even greater challenge. Says Pusswald: “Many patients are afraid that they will be unable to smell a gas leak if one occurs. The same goes for smoke, since they are unable to detect its smell.”

Worldwide, one in five people are affected by olfactory disturbances. The head of the test’s development, Johann Lehrner from the University Department of Neurology, explains why these conditions are extremely serious: “The debilitation of people with olfactory disturbances can be quite significant and can even cause persistent depressive states.” According to Lehrner, it is a worldwide phenomenon: “International studies estimate that one in five people worldwide aged between 20 and 90 have a disturbed sense of smell.”

English version of the test currently in development
The test, which has had its première in Vienna, was developed for the entire German-speaking region. Clinicians therefore now, for the first time, have a method that they can use and evaluate easily and one that delivers fast results. It gives experts the ability to very quickly obtain a good estimate of the extent of the problem. The German version of the test is currently being adapted for the English-speaking world.

A direct line through the brain to avoid rotten food

Consuming putrid food can be lethal as it allows bacterial pathogens to enter the digestive system. To detect signs of decay and thus allowing us and other animals to avoid such food poisoning is one of the main tasks of the sense of smell. Behavioural scientists and neurobiologists at the Max Planck Institute for Chemical Ecology in Jena, Germany, have now for the first time decoded the neural mechanisms underlying an escape reflex in fruit flies (Drosophila) activated in order to avoid eating and laying eggs in food infected by toxic microorganisms. A super-sensitive and completely dedicated neural line, from olfactory receptor, via sensory neuron and primary brain neurons, is activated as soon as the tiniest amount of geosmin is in the air. Geosmin is a substance released by bacteria and mold fungi toxic to the fly. This stimulus overrides all other food odour signals, irrespective of how attractive they are on their own. Consequently, geosmin is a full STOP signal that prevents flies from eating and laying eggs in toxic food, similar to when we open the fridge and smell last week’s forgotten dinner.

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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.

Animals learn to fine-tune their sniffs

Animals use their noses to focus their sense of smell, much the same way that humans focus their eyes, new research at the University of Chicago shows.

A research team studying rats found that animals adjust their sense of smell through sniffing techniques that bring scents to receptors in different parts of the nose. The sniffing patterns changed according to what kind of substance the rats were attempting to detect.

The sense of smell is particularly important for many animals, as they need it to detect predators and to search out food. “Dogs, for instance, are quite dependent on their sense of smell,” said study author Leslie Kay, associate professor of psychology and director of the Institute for Mind & Biology at the University of Chicago. “But there are many chemicals in the smells they detect, so detecting the one that might be from a predator or an explosive, for instance, is a complex process.”

Kay was joined in writing the paper by Daniel Rojas-Líbano, a postdoctoral scholar at the University of Chile in Santiago, who received his PhD from UChicago in 2011. Rojas-Líbano, who did the work as a doctoral scholar, was the first author on the publication. Their results are published in an article, “Interplay Between Sniffing and Odorant Properties in the Rat,” in the current issue of the Journal of Neuroscience.

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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Babies Learn the Smell of Mum

Researchers show for the first time that a mammal begins to suckle its mother’s milk through a learned response built on learning her unique combination of smells. When it is born, the newborn is exposed to the smell of its mother’s amniotic fluid and the baby then responds to those smells to feed.

Prevailing thought has been that pheromones –chemicals that trigger an innate behavior – drove the suckling response as an automatic behavior. The new work determines that, in mice, the smells must be learned before the behavior can occur.

Suckling is a critical step for survival in mammals, which are defined by giving birth to offspring that need to feed from their mother’s milk. The newborn must begin to feed soon after birth or it will die. It is a crucial, defining behavior in mammals and offers researchers an opportunity to investigate the biology of instinct.

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.