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The smell index: a new method makes it possible to accurately measure similarity between smells

The development by Weizmann Institute of Science scientists makes it possible to predict what the smell of any molecular compound will be and may pave the way for the digitization and reproduction of smells

Odors may be measured by their distance from each other. Prof. Noam Sobel's laboratory, Weizmann Institute
Odors may be measured by their distance from each other. Prof. Noam Sobel's laboratory, Weizmann Institute

"If you aspire to found a new science, go out and measure smell," Alexander Graham Bell urged high school graduates in Washington in 1914. In a new study published today in the scientific journal Nature, scientists at the Weizmann Institute of Science respond to the hundred-year-old challenge posed by the inventor of the telephone, and present a method that makes it possible to accurately measure the degree of similarity between different smells based on the perceptual distance between the smells. This research makes it possible to predict what the smell of any scent compound, no matter how rich and complex, will be based on its molecular structure, and it may pave the way for the digitization and reproduction of smells.

In the field of vision it is known that the difference between blue and yellow is roughly double the difference between blue and green. In the field of hearing, the difference between the sounds "ra" and "do" is always the same as the difference between "ra" and "mi". But olfactory researchers have not, until now, had a reliable tool to measure the difference between two smells. The reason for this is, among other things, related to the challenging nature of the sense of smell: in the nose there are millions of olfactory receptors, divided into hundreds of subtypes, each of which is designed to detect certain molecular characteristics. As a result, our brain is able to absorb millions of smells consisting of many odor molecules combined together in different strengths and doses. In the new study that came out of the laboratory of Prof. Noam Sobel from the Department of Neurobiology, led by research student Aharon Rabia and Dr. Kobi Sanitz, the researchers discovered how it is possible to establish order in the mixture of smells. The key to success in this difficult challenge lay all along in Bell's speech who asked the high school graduates if they "can measure the degree of difference between one smell and another and say if a certain smell is twice as strong as another smell". That is, instead of describing and measuring the smells per se, the researchers turned to calculate the relationships between the smells as they are perceived by our sense of smell.

With the help of the French perfumer Christophe Laudemiel, who created perfumes for international brands such as Clinique, Tommy Hilfiger and Ralph Lauren as well as scents for Procter & Gamble fabric softeners, the researchers created 14 scent mixtures, each of which was composed of about 10 molecular components. Then, in a series of experiments, the researchers presented 200 volunteers with pairs of these mixtures and asked them to rate how similar the odors in each pair were to each other. 

These experiments yielded a database containing thousands of ratings of the degree of similarity between the odor mixtures, as perceived by the experimenters. The researchers used this data to calibrate a multidimensional odor map they had previously created based on physical and chemical properties. In the calibrated map they created, each odorant is represented by a vector that combines 21 indices (from polarity to molecular weight), where the angular distance between any two vectors reflects the degree of perceptual similarity between them - the greater the distance, the more different the odors are from each other. 

To put the perceptual smell map to the test, the researchers turned to data collected in a large-scale study in Prof. Leslie Washall's laboratory at the Rockefeller Institute in New York that tested the ability of experimenters to distinguish between smells. The researchers from Prof. Sobel's laboratory discovered that the smell map accurately predicts the results obtained in the study: the smaller the angular distance between the smells, the more difficult it was for the experimenters to distinguish between them - and vice versa. Encouraged that the model accurately predicted findings collected by others, the team proceeded to the next step: putting the model to the test themselves.

To this end, the researchers concocted more than 100 odor mixtures and invited new experimenters to smell them. The model proved itself in an extraordinary way: in fact, the results were parallel to the results accepted in experiments that test a sensory perception that is known to be based on well-defined parameters - color vision. This was particularly surprising given that each person has a unique composition of olfactory receptor subtypes, with up to about 30% person-to-person variation.

Using this method, researchers can now not only predict what each scent compound will smell like, but also design scents: for example, they can take a perfume that contains a known group of ingredients, and create a new perfume without any of the ingredients in the original perfume, but with the exact same smell. In color vision, compositions that produce the same perceived color are called "metamers of color"; In the current study, the researchers actually created "metamers of smell".

These findings are a significant step towards the realization of the vision of the vice president of the Israel National Academy of Sciences, Prof. David Harel from the department of computer science and applied mathematics at the institute, who was a partner in the research: to enable computers to digitize and reproduce smells. Besides the ability to add the scent of flowers or the aroma of the sea to photos from the last vacation, a computer's ability to interpret scents similar to our sense of smell, can bring about revolutions in a number of fields from environmental monitoring to the biomedical and food industries.

"In our world there are many different kinds of scents - from the smell of violets and roses to aspartida (an Indian spice that is a substitute for garlic and onion). But as long as we do not have the possibility to measure the degree of similarity and difference between different smells, we do not and cannot have a science of smell," Bell said in his famous speech, and Prof. Sobel replies to him: "The distance between roses and violets is 0.202 radians, while the distance between violets and Aspartida – 0.5 radians. The distance between Rose and Aspetida is even greater -0.565 radians. Yes, we managed to turn perceptions of smell into numbers, and this should indeed greatly advance the science of smell."

for the scientific article

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