Does a Rose Smell Sweet? Ask an eNose.
In Brain Sense, I try to explain how the sense of smell is hardwired into the human nose and brain. What's the evidence for that? Well, for one thing, newborn babies all over the world smile when they smell vanilla and cry when they smell rotting meat. We are born with a predilection for odors we judge as pleasant. What we learn as we are growing up has only a modest effect on our basic olfactory perception.
If you're skeptical, check out the electronic system that scientists at the Weizmann Institute of Science in Rehovot, Israel, have trained to predict the pleasantness of odors. Their machine, an "eNose," makes the same judgments a human would, using a very humanlike smell-sensing process.
Most previous eNose devices were limited in the odor assessments they could make. Here's why: As an odor molecule passes through a conventional eNose, its molecular structure stimulates sensors to produce an electrical pattern. That pattern is like a fingerprint. It is unique to each odor. An old-style eNose works only when given odor samples to use in building a reference base. The eNose can't recognize or categorize an odor it hasn't encountered before.
So, the Israeli scientists decided to build a better eNose. They started not with the machine, but with the odor judgments made by a group of Israeli volunteers. The people rated a wide variety of odors on a 30-point scale, running from "very pleasant" to "very unpleasant." From that human dataset, the researchers developed a neural network algorithm that they then programmed into the eNose. In this way, they "taught" their machine an "axis" of odor pleasantness.
Then the researchers tested their algorithm to see if the eNose could predict the pleasantness of a set of odors it had never encountered before. Guess what. It could! On the 30-point scale, the eNoses's ratings achieved approximately 80 percent congruence with human judgments made by a completely different group of Israeli volunteers than the ones used to create the algorithm. When the task was made simpler--merely a pleasant-unpleasant dichotomy--the eNose got it right 99 percent of the time.
The next obvious question is, What about cultural differences? Doesn't our society teach us what odors we should consider pleasant or unpleasant? The Israeli researchers probed that question by testing their eNose's predictions against odor judgments made by group of recent Ethiopian immigrants to Israel. The eNose performed just as well as when tested against Israeli natives. This suggests that we are hardwired for smell no matter what our culture.
"Being able to predict whether a person who we never tested before would like a specific odorant, no matter their cultural background, provides evidence that odor pleasantness is a fundamental biological property," says Noam Sobel of the department of neurobiology at Weizmann.
So how are obvious cultural differences explained? "Culture influences the perception of olfactory pleasantness mostly in particular contexts," says Sobel. "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. However, if the smell was presented out of context in a jar, then the French would probably rate the odor just as unpleasant as anyone else would; that is why the French don't make cheese-smelling perfume."
For more, information see
Brain Sense, Part Two: Smell.
Haddad R, Medhanie A, Roth Y, Harel D, Sobel N (2010) Predicting Odor Pleasantness with an Electronic Nose. PLoS Comput Biol 6(4): e1000740.
The delightful eNose cartoon is courtesy of the American Chemical Society.
