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The secret of smells

Nearly 25 years ago, US physician and writer Lewis Thomas famously said of the sense of smell: “It may not seem a profound enough problem to dominate all the life sciences, but it contains, piece by piece, all the mysteries.” In humans, the olfactory sense can elicit vivid memories as much as it can evoke the imagination. But for most animals, smell is the primal sense that enables them to find food, detect predators and locate mates. From fruit flies to humans, one question has long puzzled researchers: how does the brain know what the nose is smelling?

A few years ago, US researcher Richard Axel and his student Lind Buck resolved this puzzle and won the 2004 Nobel Prize. In less than four years since, a team of Israeli scientists has shown that smells can be mapped and the relative distance between various odours determined.

The work which lays down the basic laws underlying our sense of smell has appeared in a recent issue of the journal Nature Methods. “This looks like an interesting attempt to classify odourants in relation to function. There is a need for being able to make such predictions with respect to odourants,” says Gaiti Hasan, a scientist at the National Centre for Biological Sciences, Bangalore, who works on the science of smell.

Unlike in smell, the physical attributes of vision and sound can be measured. For instance, one can easily know whether a particular musical note is different from another, because the ear can comprehend the difference in their frequencies. But no such physical relationship has been discovered for smells, partly because odour molecules are much more difficult to pin down than sound frequencies.

In order to create the map, researchers from the Weizmann Institute of Science, led by neurobiologist Noam Sobel, began working with 250 odourants. For each of these odourants, the scientists generated a list of around 1,600 chemical characteristics. Plotting these characteristics, they created a multi-dimensional map of smells that revealed the distance between one odour molecule and another.

Persistent research over the years, however, has helped the Israeli scientists tighten the list of traits needed to locate an odour on the map down to around 40. Subsequently, they checked to see whether the brain recognised this map as it recognises musical scales. Working with fruit flies to rats to honey bees, they studied the neural response patterns to smells and found that in all these species the closer any two smells were on the map, the more similar were the neural patterns.

Subsequently, the scientists tested 70 new smells by predicting the neural patterns that they would arouse. They later matched their predictions with experiments carried out at the University of Tokyo and found that their predictions closely matched the results of the experiments.

These findings lent support to the theory that, contrary to the commonly held view that smell is a subjective experience, there are universal laws governing the organisation of smells. These laws determine how our brain perceives them, says Sobel.

If the parameters they use to classify the odours are relatively simple, this is a significant achievement, Hasan told KnowHow. Hasan thinks such a map will help predict what the brain’s response to an unknown odourant would be.

In the past, scientists had tried to develop a method to measure smell. The method was rather crude and was based on the number of carbon atoms present in a particular compound. It failed miserably as scientists found that two compounds that have a similar chemical structure and differ by just one carbon atom elicited very different responses in the olfactory sensory neurons, the workhorse of the nose in detecting smells.

The smell map may be of potential interest to industry. For instance, characterising a smell on the basis of how the brain recognises it can enable it to be digitised and transferred via the computer in future. This could, for example, help the perfume industry develop superior perfumes.

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