A clue! A clue!
July 12, 2004 2:17 PM   Subscribe

What does blue smell like? A novel theory contends that humans smell the frequency of a molecule, rather than shape (full study, pdf). In fact, there is no evidence to support either theory, leading to the question, how do humans smell? Inspired by bonehead.
posted by grateful (31 comments total)

What does blue smell like?
posted by kenko at 2:20 PM on July 12, 2004

oh crap, I didn't see the "inspired by bonehead" thing there. Or rather, overlooked it. In any case, the answer I'm sure you're expecting is "terrible".
posted by kenko at 2:22 PM on July 12, 2004

"...how do humans smell?"

posted by mr_crash_davis at 2:22 PM on July 12, 2004

"...how do humans smell?"

posted by Witty at 2:23 PM on July 12, 2004

Man, we're on a roll here. Try the veal, it's delicious.
posted by kenko at 2:26 PM on July 12, 2004

"...how do humans smell?"

With their noses.
posted by esch at 2:26 PM on July 12, 2004

"...how do humans smell?"

posted by Pretty_Generic at 2:26 PM on July 12, 2004

To begin with, there's no one single frequency of a molecule. Even a simple molecule such as water (with only three atoms) has three vibrational modes, each one with a different frequency, and three rotational modes, most likely each with its own frequency as well. More complex molecules have correspondingly more vibrational modes, each with its own frequency. In fairness to the abstract grateful links to, the authors don't claim that molecules do have just one single frequency that determines a molecule's smell; rather, that seems to be a mischaracterization by grateful.

More importantly is the lack of any proposed mechanism by which the frequencies of a molecule could be distinguished. Biomolecules which essentially act upon a certain of molecule (and this is an oversimplification, because it's not just shape that's detected) are present in virtually every living cell, and are much more widespread than simply giving a sense of taste or smell--they're called receptors. Structures which act upon certain frequencies--in humans, in the eyes and ears--are more specialized and much less common. I'm not aware of any olfactory cells which would have any sort of structure suited to detecting frequency. (Which is not, of course, a proof that it can't be the mechanism for smell, but it makes one highly skeptical.)

For more information about the commonly accepted mechanism of smell, google on olfactory receptor.
posted by DevilsAdvocate at 2:36 PM on July 12, 2004

weird, I just posted about this yesterday on my website... it's interesting stuff.
posted by mdn at 2:37 PM on July 12, 2004

"...how do humans smell?"


Sorry, I couldn't resist
posted by Outlawyr at 2:41 PM on July 12, 2004

"...how do humans smell?"

In too many ways, if you ask me! Everything they touch turns to shit!
They're ungrateful, they disrespect their elders, and they have no sense
of architecture whatsoever! Talk about your wasted potential!
posted by Smart Dalek at 2:52 PM on July 12, 2004

Reminds me of Ralph (I think) in an episode of The Simpsons:

My finger smells like purple!
posted by dagny at 3:17 PM on July 12, 2004

More importantly is the lack of any proposed mechanism by which the frequencies of a molecule could be distinguished.

This just isn't true. Turin has, in fact, proposed a detailed mechanism. You can find a full-text link to his 1996 paper here. Briefly, he suggests that odorant molecules fill a tunneling gap in receptor proteins which use redox chemistry to generate a tunneling current. The current flows only when a molecule with a proper vibrational absorption band fills the gap. A series of receptor molecules, each tuned to a different wavelength, can serve as a rudimentary spectroscope. This is not unlike the way the other spectral senses work: receptors in the eyes are tuned to a fixed set of colors (rather than a continuous spectrum), and those in the ear are arranged along the cochlea, which acts to deconvolve sound into component frequencies.

As for the actual presence of proteins capable of performing these functions, I don't think we have any structures for olfactory receptors: membrane proteins are notoriously difficult structural subjects. Turin, however, makes sequence homology arguments that those proteins identified as receptors would be capable of performing the necessary electrochemistry.

Whether or not Turin is right, he certainly has come up with an ingenious and well-considered putative mechanism.
posted by mr_roboto at 3:33 PM on July 12, 2004

I highly recommend Chandler Burr's great, underappreciated book on Luca Turin and the larger topic, The Emperor of Scent.
posted by twsf at 3:43 PM on July 12, 2004

I also highly recommend the paper by Keller & Vosshall (2004) in Nature Neuroscience which finds no evidence whatsoever for Turin's theory, and also the accompanying editorial in that issue. Here's a press release from Rockefeller University about their findings. Following are some excerpts from the Nat Neuro editorial (I would love to post all of it but it's probably illegal):

"The paper by Keller and Vosshall on page 337 of this issue is unusual; it describes a refutation of a theory that, while provocative, has almost no credence in scientific circles. The only reason for the authors to do the study, or for Nature Neuroscience to publish it, is the extraordinary—and inappropriate—degree of publicity that the theory has received from uncritical journalists."

"...The magician James Randi, debunker of paranormal claims, once said that if you claim to have a goat in your back yard, people will probably believe you, but if you say you have a unicorn, you must expect closer scrutiny. The editors at Nature used to classify manuscripts on a 'zoological scale' that ranged from goats to unicorns, and Turin's paper was toward the far end of that scale. Despite the forcefulness of his assertions, most scientists in the field were unconvinced by his proposal. Thus his paper was rejected by Nature, and it was eventually published (without review, according to Turin's own account) by Chemical Senses in 1996.

Turin's theory would probably have vanished into obscurity but for two coincidences. First, one of his former students had become a producer for the BBC, and she decided to make a TV documentary about him. Second, he had a chance encounter with writer Chandler Burr, who was so taken with the theory that he wrote a popular book about it. The Emperor of Scent, which appeared in 2002, is effectively a mouthpiece for Turin's views, and it is intensely hostile to the scientific establishment. It has attracted wide attention, and with the exception of a scathing review in this journal from Avery Gilbert2, the reviews have been almost uniformly favorable. The book is seductively written, and it was recently reissued in paperback, complete with a readers' guide to promote book club discussions."

"...In some sense it does not matter whether the public believes in the vibrational theory of olfaction; the truth will eventually come out. But of course this is not just about olfaction. It is about the public credibility of the scientific process and the biases that affect science reporting in the popular press. It is disturbing that Emperor of Scent received so much favorable publicity from reviewers who were ill qualified to judge its scientific content. The New York Times and The Washington Post, for instance, assigned it to their movie critic and fashion critic, respectively.

The media loves controversy, and ever since David and Goliath, the story of a lone hero taking on the establishment has had enduring appeal. Of course, radical ideas from outside the mainstream do occasionally turn out to be right. Of course scientists are sometimes excessively attached to conventional ideas. But in science at least, the mainstream view is usually based on the accumulation of evidence over many years. Journalists are trained to report both sides of any argument, but this can be misleading when both sides are not equally credible.

A mature body of scientific theory is like a large building, and the impulse to demolish it is often little more than a form of intellectual vandalism, an expression of frustration by those who did not succeed as architects. Some buildings outlive their usefulness, of course, but the threshold for knocking them down should be high. We hope that the paper from Keller and Vosshall will serve as a reminder of why that's so."

posted by adrianhon at 4:08 PM on July 12, 2004

My finger smells like purple!

Perhaps your thinking of...
'Crayons taste like purple' - Ralph from Simpsons

Now how about ...
'What smells like blue?' - Fry from Futurama

That show was always ahead of the curve.
posted by uftheory at 4:14 PM on July 12, 2004

This just isn't true. Turin has, in fact, proposed a detailed mechanism.

Fair enough; I had written that without more carefully looking at the linked article.

The most tantalizing claim, in support of the vibrational theory, is that acetophenone-d8 smells different than acetophenone--the two compounds have nearly identical shapes but very different vibrational frequencies. However, the Keller and Vosshall paper in Nature Neuroscience found no distinguishable difference in smell in a double-blind test. The Turin paper doesn't describe the details under which he found that acetophenone and acetophenone-d8 were perceived as different; if it was not done under double-blind conditions, it throws that conclusion into serious doubt.

Nature Neuroscience also published an editorial which very harshly criticized Turin in the same issue in which the Keller/Vosshall paper appeared. (April 2004, although you probably can't get to the full text of the paper and editorial unless you're at an institution which has a subscription.)

[On preview, a bunch of excerpts from the editorial deleted upon finding that adrianhon had done the same thing.]
posted by DevilsAdvocate at 4:14 PM on July 12, 2004

"...how do humans smell?"

posted by undecided at 4:15 PM on July 12, 2004

Hey Man.... Smell My Finger
posted by Eekacat at 4:18 PM on July 12, 2004

I left a fair bit out from the editorial, but if possible I would strongly encourage people to find and read it in its entirety, or at the least read the release from Rockefeller.

While IAAN (I am a neuroscientist) I do not specialise in olfaction. However, based on what I know of sensory perception and signal transduction in humans and other animals, Turin's theory and proposed mechanism sound distinctly improbable, and I am convinced by Keller and Vosshall's study that he is basically wrong.
posted by adrianhon at 4:19 PM on July 12, 2004

"How terrible do humans smell?"

Wait, I told it wrong.
posted by chicobangs at 4:24 PM on July 12, 2004

Turin's theory and proposed mechanism sound distinctly improbable....

Well, I'd certainly agree it sounds improbable; that's why I called it "ingenious" :)
posted by mr_roboto at 4:39 PM on July 12, 2004

This is not unlike the way the other spectral senses work: receptors in the eyes are tuned to a fixed set of colors (rather than a continuous spectrum)...

Umm, no.
posted by euphorb at 4:50 PM on July 12, 2004

Oh, boy. Color vision. Almost no lay person has a correct conceptual understanding of color vision. But it's possible (probable?) that Mr. Roboto roughly knew what he was talking about, he only phrased his assertion badly.
posted by Ethereal Bligh at 4:56 PM on July 12, 2004

Umm, no.

Huh? I see four classes of photopigments: one tuned to a peak transmission wavelength of 410 nm, one to 498 nm, one to 534 nm, and one to 564 nm. That's what I meant by "a fixed set of colors": rather than having receptors which are capable of performing a spectral analysis on incident light (like a spectrophotometer), our minds perceive color by combining four filtered data channels (like some high-end CCD cameras). Sorry for any lack of clarity.

Is your objection that there's some overlap in the pass bands of the filters?
posted by mr_roboto at 5:07 PM on July 12, 2004

And, or course, in defending myself, I confused transmission and absorbance, so that's all pretty much garbled... I mean, it still makes sense, but you know, it's an absorbance band, not a pass band, and it's not exactly filtering (well, it is in a camera, but not in an eye), and I'm gonna stop digging the hole now.
posted by mr_roboto at 5:14 PM on July 12, 2004

Well, the 420nm receptor, the "rod", is not combined with the others for the perception of color. But, otherwise, yeah, you knew what you meant, it just sounded like you may have been saying something else.

By the way, the Voyager CD included color photographs, which is really weird (and possibly stupid) if you think about it. However, it also included a way to decode them. Furthermore what this says about human color vision implies a great deal about Earth's environment! Which is cool.

Perhaps you know: are there any cameras that are photospectrometers at every point of resolution? I guess you'd also need the spectra of the illuminating light, although it'd probably be derivable from the data.
posted by Ethereal Bligh at 5:19 PM on July 12, 2004

I think you'd be interested in imaging spectrographs. The most sophisticated examples of these are custom-built devices used in astronomy.
posted by mr_roboto at 5:33 PM on July 12, 2004

Thanks for that link. I'm guessing the resolution can't be very good for imaging? I'm not a graphic artist or photographer, but I do know a bit about color vision and related problems; and the idea of a camera that's a spectrograph at every point of resolution, with a flat-broad spectrum illuminating light, seems really cool to me.
posted by Ethereal Bligh at 6:05 PM on July 12, 2004

would this be related to the air smelling like it's going to snow (right before it does)?
posted by amberglow at 7:07 PM on July 12, 2004

ok, i guess not.

"...how do humans smell?"


: >
posted by amberglow at 8:12 PM on July 12, 2004

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