Originally Posted by Redneck Perfumisto
Very interesting stuff, FP. Do you know if there is any support for a combination of the two theories? I get the impression that the cross-terms available by allowing shape (especially chiral ones) to interact with an electronic detection mechanism (or mechanisms) within each receptor could allow all of the facts to be explained. Of course, if vibration is not needed, lop it off with Occam's razor. But some of the facts (and I'm especially thinking the metallocenes) just scream for a non-dead key. You know - like the ones in Fords, that have a chip in the handle. They merge a shaped key and an electronic swipecard to give a gazillion combinations and a non-starting car (or in my case, truck
) should a thief try to mess with it.
I really don't see these so much as competing theories - more like different explanations of different parts of a single complex story, both having made the (in retrospect) regrettable choice of saying they could explain everything.
Also, I see the use of non-trained subjects to differentiate fragrances as being of dubious benefit - if not like using fetal chihuahuas to see if dogs can really track foxes. I suppose if chihuahuas are giving out the grant money it makes sense, but really - I'd rather have bloodhounds. To me it smacks of phony objectivity. Would there be any reason to use non-trained subjects, provided that it was a double-blind experiment? Yes, I can see it if the point of what you are doing has anything to do with finding out what non-trained subjects can do. But if the question you are asking is any function of "can human receptors do this - yes or no?", in order to answer the most fundamental questions of mechanism, then by George, I'd think you'd want the most sensitive and linguistically wired receptors you could lay your hands on.
Or maybe I'm too much of an old dog and a paper chemist to do lab science anymore. Damn whippersnappers. Where'd they put my oxygen now?
I don't possess the expertise to comment on the issue with any kind of authority--I was simply marshaling the information available to me for purplebird7's benefit. So take whatever I say with a huge grain of salt. But it seems to me that both Turin and the establishment agree that shape is part of the picture--the question is whether shape alone suffices (there are some observed phenomena it doesn't seem to explain), whether vibration is the correct theory to explain those discrepancies (there could conceivably be other explanations), and whether inelastic electron tunneling is the mechanism by which it happens. The UCL study suggests that inelastic electron tunneling is possible *in theory*, but it hasn't demonstrated that that's what's going on in our noses.
Re trained versus untrained experimental subjects, I'm not sure I agree that it makes that much of a difference. I would be far more concerned about sample size. The business of attaching verbal descriptors to odors is notoriously tricky, even for trained subjects. Just look at the wild disparities in the "notes" that people write up for fragrances on this site, to say nothing of the vast differences in the visceral like/dislike reactions that people have (and it's not as if trained perfumers and chemists all like the same odors, either). Olfaction is not just a matter of what's going on in the odor receptors, there's also a great deal of cognitive neural processing involved, and clearly at some point or points, whether it be in the odor receptors themselves or in the processing of the signals they send, there are significant individual differences related not just to training but to people's respective genotypes and gene expression. We know that some people are anosmic to certain aromachemicals--who's to say that we all even perceive the ones we *can* smell the same? Already we're at the threshold of epistemology.
One potential problem with trained subjects is that they've been *taught* to attach certain verbal descriptors to certain familiar aromachemicals, which could inadvertently, and paradoxically, be biasing their perceptions. All of which renders these small studies, both for and against vibrational theory, kind of suspect in my book. Ideally we would want an experimental verification of olfactory theory that bypasses subjective verbal descriptors entirely, possibly through things like brain imaging studies and material evidence of inelastic electron tunneling in live G-protein-coupled receptors.
It's such an exciting area of research that I'm surprised more people aren't working on it; in fact, sometimes it seems like it's dead in the water. One thing's for certain, though: the only way the vibration theory is going to make any headway is if Turin himself pushes for more and larger experimental studies, or designs them himself. For whatever reason, he hasn't been doing much of that the last few years.