have different theories on how our noses interpret smells.
by Mary Tucker
Smell is the most mysterious of the five senses - scientists are still not exactly sure how the nose decodes odors.
The sense of smell often seems like
the forgotten sense, perhaps because scent cannot be transmitted
as obviously as images or sound. But watch a dog - with a sense
of smell about a million times more sensitive than ours - identify
a person by their smell or sniff out traces of drugs and it is obvious
what a powerful means of communication it can be. For humans, scent
plays a big role in attraction and is strongly tied to memory.
But how is smell written into molecules? And how do our noses interact with scent molecules? Since classical times, scientists have been trying to pin down solid olfactory rules but they still don't know exactly how the nose works.
Decoding the shape of smell
What we do know is the world is made of atoms and those atoms connect to make molecules. Molecules are what we smell, from wherever they are evaporating, and they reach our nose through the air. Though we know almost everything possible about molecules, we don't know how our nose reads them. Chemists make hundreds of new molecules every week but what each molecule is going to smell like is always a mystery.
The prevailing theory, first refined
in 1952 by John Amoore at Oxford University, is the shape or steric
theory of odor. The theory, simply stated, proposes that the shape
of a molecule determines its smell. In other words, a rose molecule
smells like a rose molecule because its shape is coded precisely
for the nose to interpret this way. It does this by a lock and key
method within the olfactory nervous system: the shape of an airborne
molecule (the key) fits into complementary odorant receptor proteins
on the outside of the nasal cell (the lock). Amoore also proposed
that there are seven primary odours (ethereal, camphoraceous, musky,
floral, minty, pungent and putrid).
But the shape theory is not without its pitfalls. "Shapists" are plagued by the indisputable evidence that not all similarly shaped molecules smell alike, while sometimes differently shaped molecules do. Also unexplained is the fact that humans can detect many more smells than there are odorant receptors in the nose. Aware the shape theory doesn't hold up to watertight scientific scrutiny, scientists have long been pursuing other explanations, with limited success.
In 1996, Luca Turin, a biophysicist
at University College London, thought he may have come up with the
answer to how we smell. In his new book The Secret of Scent
(Faber and Faber 2006), he outlines his hotly contested vibrational
theory of smell, and explains how "‚€¶like the origin of life, the
mechanism of general anaesthesia, the extinction of dinosaurs, the
kinship of the Basque language, [smell] is a scientific Sword in
Photo credit: Gray's Anatomy
shows the receptor neurons in the nose that convert odors
to electrical signals that the brain can interpret.
Turin first came across the vibrational
theory in the mid-1980s, noticing that it had first been conceived
of in the 1930s, later revived in the 1960s, but both times discarded.
With the advent of modern technology, he was able to revisit the
theory and apply new testing methods. Vibrational theory states
that molecules in every substance generate a specific vibration
frequency that the nose interprets as a distinct smell. More specifically,
it speculates that the vibration frequency of odor molecules is
converted to smell recognition via a form of electron tunnelling
with the help of receptors in the lining of the nose. In many ways,
according to vibrational theory, the way we smell is similar to
the way we hear. A molecule's vibrations play out like a chord of
music - but instead of music, we get the chemical melody of scent.
In his investigations, Turin noticed that a vibration producing a wave number of 2500 always produced a smell of sulphur. He then found a different molecule - with the same vibration frequency - that also possessed the same smell: the molecule borane. After looking for molecules that were identical in shape but with different vibrations, he theorised that because they had their own unique "chord patterns", they should have different smells.
Despite achieving an apparent scientific
breakthrough, Turin was immediately confronted with criticism from
members of the scientific community, who doggedly refused to support
the publication of his research. The backstabbing world of scientific
peer review is the central preoccupation of Chandler Burr's new
acclaimed biography of Turin, The Emperor of Scent (Random
House, 2003). (Interestingly, Burr, an ardent supporter of Turin's
work, has recently been named by The New York Times as their
first ever perfume critic). Despite much vindication from Burr and
other members of the press, Turin's vibrational theory - like the
shape theory -- has not been immune to inconsistencies. Experiments
done in 2004 by Vosshall and Keller at Rockefeller University found
three of Turin's proposed predictions on the vibrational nature
of smell to be false.
Whether the shape or vibrational theory, a combination of the two - or something completely different - gains further and credible scientific ground remains to be seen. For the foreseeable future, the debate rages on‚€¶
For more information:
The Guardian - An Explorer Following his Nose
The New York Times - Odorama