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Most people think that scents are judged subjectively as being pleasant or unpleasant according to people’s personal preference and culture. But a new “electronic nose” built by Weizmann Institute of Science researchers has shown that the perception of an odor’s pleasantness is “innately hardwired” to its molecular structure; and that only in specific circumstances are personal or cultural differences shown to influence opinions on odors.
These findings, published on Friday in the online journal PLoS (US Public Library of Science) Computational Biology, could be applied to the automated testing of toxins in the environment and the monitoring of bad smells, as well as to quick screening of scents for the perfume industry. In the future, this discovery may allow for the digital transmission of smell to scent-enable movies, games and music, providing a more immersive and captivating experience.
The team – comprising scientists at the Rehovot institute led by Dr. Rafi Haddad (then a graduate student of Prof. Noam Sobel of the neurobiology department and co-supervisor Prof. David Harel of the computer science and applied mathematics department), together with Abebe Medhanie of the neurobiology department and Dr. Yehudah Roth of Holon’s Wolfson Medical Center – managed to train their “eNose” to predict whether an odor would be perceived as pleasant or unpleasant, or anywhere in between.
They taught an electronic system to predict the pleasantness of novel odors, just like a human would perceive them. Over the last decade, electronic devices commonly known as electronic noses, or “eNoses,” have been developed to be able to detect and recognize odors. The main component of an eNose is an array of chemical sensors. As an odor passes through the eNose, its molecular features stimulate the sensors in such a way as to produce a unique electrical pattern – an “odor fingerprint” – that characterizes that specific odor.
Like a sniffer dog, an eNose first needs to be trained with odor samples so as to build a database of reference. The instrument can then recognize new samples of those odors by comparing the odor’s fingerprint to those contained in its database. But unlike humans, if conventional eNoses are presented with a novel odor whose fingerprint has not already been recorded in their database, they are unable to classify or recognize it.
So the team decided to approach the problem from a different perspective. Instead of training an eNose to recognize a particular odor, they taught it to estimate the odor along a particular perceptual axis of “odorant pleasantness” and predict how humans would find it.
To achieve this, they first asked a group of native Israelis to rate the pleasantness of a selection of odors according to a 30-point scale ranging from “very pleasant” to “very unpleasant.” From this dataset, they developed an “odor pleasantness” algorithm, which they then programmed into their eNose. They then got the eNose to predict the pleasantness of a completely new set of odors not contained in their database against the ratings provided by a completely different group of native Israelis.
The scientists found that the eNose was able to generalize and rate the pleasantness of novel odors it had never smelled before – and these ratings were about 80 percent similar to those of individual raters who had not participated in the eNose training phase. In addition, if the odors were simply classified as either “pleasant” or “unpleasant,” rather than being rated on a scale, the eNose achieved an accuracy of 99%.
To find out whether smell perception is culture-specific or not, they tested eNose predictions against a group of recent Ethiopian Jewish immigrants to Israel. The results showed that the eNose’s ability to predict the pleasantness of novel odors against the native Ethiopians’ ratings was just as good, even though it was “tuned” to the pleasantness of odors as perceived by native Israelis.
In other words, even though different odors have different meanings across cultures, the eNose performed equally well across these populations. This suggests a fundamental cross-cultural similarity in the assessment of whether odors are pleasant.
“Being able to predict whether a person whom we never tested before
would like a specific odorant, no matter their cultural background,
provides evidence that odor pleasantness is a fundamental biological
property, and that certain aspects of molecular structure are what
determine whether an odor is pleasant or not,” said Sobel.
“We believe that culture influences the perception of olfactory
pleasantness mostly in particular contexts. To stress this point, many
may wonder how the French can like the smell of their cheese, when most
find the smell quite repulsive. We believe that it is not that the
French think the smell is pleasant per se; they merely think it is a
sign of good cheese,” Sobel continued.
“However, if the smell was presented out of context in a jar, the
French would probably rate the odor just as unpleasant as anyone else