New Worlds: Smelling the color white

You can see the color white; you can also hear “white noise.” Now, Weizmann Institute of Sciences researchers show that you can also smell a white odor. Their research findings appeared recently in the Proceedings of the National Academy of Sciences (PNAS).

The white we see is actually a mixture of light waves of different wavelengths. In a similar manner, the hum we call white noise is made of a combination of assorted sound frequencies.

In either case, to be perceived as white, a stimulus must meet two conditions: The mix that produces them must span the range of our perception, and each component must be present at the exact same intensity. Could both of these conditions be met with odors, so as to produce a white smell? That question has remained unanswered, until now, in part due to such technical difficulties as getting the intensities of all the scents to be identical.

A Weizmann neurobiology department research team led by student Tali Weiss and Dr. Kobi Snitz – both in the group of Prof. Noam Sobel – decided to take up the challenge. They began with 86 different pure scents (each made of a single type of odor molecule) spanning the entire “smell map,” diluted them to obtain similar intensities and then created blends. Each blend contained a different mixture of odors from various parts of the smell map. These blends were then presented in pairs to volunteers, who were asked to compare the two scent-blends.

The team discovered that the more odors that were blended together in the paired mixtures, the more the subjects tended to rate them as similar – even though the two shared no common components. Blends that each contained 30 different odors or more were thought to be almost identical. The researchers then created a number of such odor blends, giving them a nonsense name – “Laurax.”

Once the subjects were exposed to one of the Laurax mixes and became accustomed to the smell, they were exposed to new blends – mixtures they had not previously smelled. They also called some of these new blends “Laurax,” but only if they contained 30 or more odors and encompassed the range of possible smells.

In contrast, mixtures made of 20 scents or fewer were not referred to as “Laurax.” In other words, Laurax was a “white smell”. In a followup experiment, volunteers described it as being neutral – not pleasant, but not unpleasant.

“On the one hand,” says Sobel, “The findings expand the concept of ‘white’ beyond the familiar sight and sound. On the other, they touch on the most basic principles underlying our sense of smell, and these raise some issues with the conventional wisdom on the subject.”

The most widely accepted view, for instance, describes the sense of smell as a sort of machine that detects odor molecules. But the Weizmann study implies that our smell systems perceive whole scents, rather than the individual odors they comprise.

ANIMALS 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 found that rats 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 and psychology professor Leslie Kay of 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 worked with Daniel Rojas-Líbano of the University of Chile in Santiago, who received his doctorate and the University of Chicago in 2011. Their results were recently published in the Journal of Neuroscience.

Scholars have hypothesized that animals may be able to focus their sniffing, just as humans focus their vision to detect a target – like the face of a friend in a crowd. Humans are also known to be able to adjust their ability to detect specific odors with practice, when cooking or sampling wine, for instance.

The authors wanted to test whether animals can focus their sniffs. In one set of findings, researchers had shown that the nose can act like a gas chromatograph, which separates chemicals in complex blends like flower scents, absorbing substances for different amounts of time depending on how readily they interact with the water-based mucus on the sensory receptors in the nose. Odorants that have high “sorption values” are easily absorbed into the mucus, while odors that do not absorb easily into water have lower sorption values.

The other finding crucial to the current work was the discovery that changes in the airflow rates of scents entering the nose can change which odors the nose readily detects. Different parts of the nose have different airflows, and classes of receptors suited to detecting specific odors. Researchers had speculated that animals might be able to change airflow to target specific odors in a blend of chemicals, like focusing on smelling a particular scent in a perfume.

Rojas-Líbano trained rats to detect a specific odor by rewarding them with a sugar pellet when they had detected a target odor and responded correctly. Electrodes attached to the rats’ diaphragm muscles measured the rate at which they were taking in air. He then tested the animals with many mixtures of two chemicals to see if they could pick out those containing the target scent. The rats were successful in making the distinctions, regardless of which type of odor they were seeking. But the rats learned to look for a highly absorbent odor much more quickly than the rats learning to detect a less absorbent odor. The rats also inhaled differently, depending on which type of odor they were detecting. The animals inhaled for a longer time when they were learning to detect the low-absorbing odor, and then reduced flow rates once they had learned to detect the odor, researchers determined.

“What was happening was that the air was moving through the nose at a slower rate and targeting those parts of the nasal epithelium that are further along in the pathway—those more likely to pick up the low-absorbent odors,” Kay said. For highly absorbent odors, the animals inhaled more quickly because the parts of the nasal cavity that are sensitive to those smells are closer to the beginning of the nose’s air pathway. The researchers thus found differences in the difficulty the rats had detecting different targets from the same set of mixtures, concluded Rojas-Líbano. “This shows that there is more to olfaction than just receptor types and combinations,” he said. “The physical properties of the odors matter a lot, and so does the type of sniff that an individual uses to smell the odors.”