An electronic nose to smell?

Weizmann Institute scientists have 'trained' an electronic system to be able to predict the pleasantness of novel odours, just like a human would perceive them - turning the popular notion that smell is completely personal and culture-specific, on its head. In research published in PLoS Computational Biology , the scientists argue that the perception (observing through senses )of an odour's pleasantness is innately hard-wired to its molecular structure, and it is only within specific contexts that personal or cultural differences are made apparent.

These findings have important implications for automated environmental toxicity and malodor monitoring, fast odour screening in the perfume industry, and provide a critical building block for the Holy Grail of sense technology - transmitting scent digitally.

Over the last decade, electronic devices, commonly known as electronic noses or 'eNoses,' have been developed to be able to detect and recognise odours.

The main component of an eNose is an array of chemical sensors. As an odour passes through the eNose, its molecular features stimulate the sensors in such a way as to produce a unique electrical pattern - an 'odour fingerprint' - that characterises that specific odour. Like a sniffer dog, an eNose first needs to be trained with odour samples so as to build a database of reference. Then the instrument can recognise new samples of those odours by comparing the odour's fingerprint to those contained in its database.

But unlike humans, if eNoses are presented with a novel odour whose fingerprint has not already been recorded in their database, they are unable to classify or recognise it.

So, a team of Weizmann scientists, 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 their colleague Abebe Medhanie of the Neurobiology Department, and Dr. Yehudah Roth of the Edith Wolfson Medical Center, Holon, decided to approach this issue from a different perspective. rather than train an eNose to recognise a particular odour, they trained it to estimate the odour along a particular perceptual axis. The axis they chose was odourant pleasantness. In other words, they trained their eNose to predict whether an odour would be perceived as pleasant or unpleasant, or anywhere in between.

To achieve this, the scientists first asked a group of native Israelis to rate the pleasantness of a selection of odours according to a 30-point scale ranging from 'very pleasant' to 'very unpleasant.' From this dataset, they developed an 'odour pleasantness' algorithm, which they then programmed into the eNose. The scientists then got the eNose to predict the pleasantness of a completely new set of odours 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 generalise and rate the pleasantness of novel odours it never smelled before, and these ratings were about 80 per cent similar to those of naive human raters who had not participated in the eNose training phase. Moreover, if the odours were simply categorised as either 'pleasant' or 'unpleasant,' as opposed to being rated on a scale, it achieved an accuracy of 99 percent.

But these findings still don't determine whether olfactory perception is culture-specific or not. With this in mind, the scientists decided to test eNose predictions against a group of recent immigrants to Israel from Ethiopia. The results showed that the eNose's ability to predict the pleasantness of novel odours against the native Ethipoians' ratings was just as good, even though it was 'tuned' to the pleasantness of odours as perceived by native Israelis. In other words, even though different odours have different meanings across cultures, the eNose performed equally well across these populations. This suggests a fundamental cross-cultural similarity in odourant pleasantness.

The scientists' findings that odour perception is hard-wired to molecular structure and their design of an eNose that is able to classify new odours could provide new methods for odour screening and environmental monitoring, and may, in the future, allow for the digital transmission of smell to scent-enable movies, games and music to provide a more immersive and captivating experience.

ScienceDaily

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