Tuesday, February 10, 2009

Of Mice and Men



Scientific progress in olfaction has been slower than in the other senses. There is general agreement about why this is: it is difficult to deliver precise amounts of odor in an experimental setup, there are large differences in odor perception from person to person, and until relatively recently no one knew the nature of the olfactory receptors in the nose.

We’ve overcome some of these obstacles. The receptor question was solved by Linda Buck and Richard Axel in work that earned them the Nobel Prize in 2004. There are now many designs available for computer-controlled olfactometers. Individual differences in smell ability, however, are just a fact of life.

I have come to believe that other, more subtle, obstacles are holding us back, namely certain habits of thought. I mention a couple of these in What the Nose Knows. Take the notion that smell is primarily an emotional sense: this belief lulls us into underestimating how cognitive olfaction truly is. Memory (where did I smell that before?), comparison (I think that one smells woodier), and interpretation (this smells too feminine for a family hand soap), are all higher order cognitive functions. The brain is very thoughtful about the sensations that the nose supplies.

Another conceptual roadblock is the anti-genetic bias of many psychologists. Their default assumption is that differences in odor perception are the exclusive result of experience and learning, and that genetics and biology have little relevance. Yet genetic variability in olfactory receptors and its effect on perception have barely been explored. I argue that genetics may account for many of the individual differences in perception. We certainly won’t know until we look.

A third obstacle is the lazy mental habit of referring sensory properties to the molecules that stimulate them. Chemists, for example, tend to say things like “phenylethanol has a detection threshold concentration of such-and-so parts per million.” Wrong! Perception is a function of the perceiving organism, not the molecule. What we should say is, “The average person’s detection threshold for phenylethanol is such-and-so parts per million.” Odor molecules, like light of varying wavelengths, exist in the external world. But smells, like rainbows, exist only in our heads.

Is this just semantics? Yes, but semantics matter. If we don’t speak and think clearly about a scientific problem, we can end up on a wild goose chase.

These thoughts came to mind when I read a new paper from the University of Lyon, in France. A research team led by Nathalie Mandairon compared the smell preferences of mice and men. How does one do that? By measuring the time that humans and mice spent sniffing each of nineteen different odors. The more a mouse likes a given smell, the longer it spends sniffing it. Similarly, we tend to sniff longer at pleasant smells and more briefly at unpleasant ones.

Using sniff-time data, Mandairon and her colleagues were able to compare apples to apples even though their experimental subjects were different species. The data showed that both groups respond to odors similarly: men find pleasant what mice find pleasant, and vice versa. This is an interesting, if not unexpected, result. After all, we share a broad mammalian heritage with mice and their diet has co-evolved with ours (they’ve been feeding on our grain stores and leftovers for millennia).

Just as interesting to me was how Mandairon et al. discuss the results. They reason that if mice share our perceptions of odor pleasantness, there must be some basis beyond “culture, life experience and learning” that accounts for the commonality. Hmmmm . . . what could that possibly be? They hint at it here:
This suggests that . . . we are endowed with a partly predetermined
neural basis for these odor hedonic representations . . .

They edge closer to it here:
Perception of the hedonic aspects of odorants is thus a complex
process which involves both innate and learned components.

Innate? Partly predetermined? Are they daring to suggest . . . why, yes!
 . . . predispositions in odor preference may be underlain by
genetically programmed neural circuits . . .
In an entire paper on similarities in odor perception between mice and men, genetics is mentioned exactly once. When it comes to this political hot potato, neuroscience researchers can be as timid as church mice.

Tiptoeing around genetics for political correctness is understandable, if not praiseworthy. What I can’t understand is Mandairon’s endorsement of a mathematical model that predicts odor pleasantness. The idea is that odor pleasantness is “partially dependent on the odorants’ physicohemical properties.” Of course this has to be true at some level: different molecules produce different smells because they have different structures. But Mandairon et al. go beyond tautology; the shared response of mice and men suggests
that olfactory preferences are indeed partially engraved
in the structure of the odorant molecule
and
there is an initial part of the [odor] percept which is innate
and engraved in the odorant structure.
Perceptions engraved on the molecule? This is simply a bizarre way to think. What else is engraved on a molecule of phenylethanol: Visual associations to red roses? The name of my florist? An olfactory memory of my dead grandmother?

Perceptions happen in the central nervous system of an organism. To talk as if odor pleasantness resides in the structural features of a molecule is animistic thinking, pure and simple.

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