What do one trillion different scents smell like?

I've been really happy with the comments on my upcoming detective thriller CHIMERAS. The book will be released in two weeks, but I've already heard back from some early readers (and yes, I'm still offering free ARC's, see details here), and many have praised Track's sensitivity to smells. Apparently, it's a trait many relate to and yet you don't find so often in fiction.

Most of our memories are stored as images. So, even when we write, we tend to over-emphasize visual descriptions and forget all about our nose. From a scientific point of view, though, how does olfaction compare to other senses? Can we "see" and "hear" more than we can "smell"?

"Humans can discriminate several million different colors and almost half a million different tones, but the number of discriminable olfactory stimuli remains unknown. The lay and scientific literature typically claims that humans can discriminate 10,000 odors, but this number has never been empirically validated [1]."

In a study published last week in Science, Bushdid et al. calculated that, contrary to previous estimates, humans can discriminate at least one trillion different olfactory stimuli -- far more than colors and tones.

How did they make such an estimate?

Colors are created from light: changes in wavelength create different hues and saturations. Similarly, sounds are created from air waves and changes in frequencies create different tones. Because we can physically measure both the frequency and wavelength of waves, it is relatively easy to determine the ranges within which human eyes and ears can detect these stimuli:

"Humans can detect light with a wavelength between 390 and 700 nm and tones in the frequency range between 20 and 20,000 Hz [1]."

But while colors and tones are created by waves, olfactory stimuli are created by mixtures of numerous distinct odor molecules. Even the "simple" scent of a rose contains 275 components. The odor molecules bind to the olfactory receptor cells in the nasal cavity, sending a signal up to the olfactory nerve.

One trivia that I discovered while writing CHIMERAS is that olfactory receptors are not restricted to the nasal cavity. They are also found in sperm cells [2] where they are possibly involved in the control of sperm migration and fertilization.

To measure the resolution of the human visual or auditory system, scientists measure how close two signals need to be in frequency in order to become undistinguishable. In other words, if the signals are like hair and our ability to pick them up is a comb, how fine are the comb teeth? How far apart do two light wavelengths need to be in order for our eyes to discern them as distinct colors?

Bushdid et al. used a similar criterion to measure how good we are discerning scents. They used 128 odor molecules to make different scent mixtures of 10, 20, or 30 components. Each mixture yields a different smell, and the more components the mixtures shares the harder they are to distinguish from one another. So, similarly to what's typically done for the visual and auditory system, in order to measure the resolution of the human nose, Bushdid et al. measured how much two mixtures need to overlap in order to become indistinguishable to the human nose. Of the 26 subjects in the study:

"At least half of the tested subjects could discriminate mixture pairs that overlapped by less than 75% of their components. Some could also discriminate mixture pairs that overlapped by 75 and 90%, but none could discriminate mixture pairs with more than 90% overlap [1]."

Bushdid et al. then used mathematical extrapolations to predict that the majority of individuals can distinguish mixtures that overlap less than 51%, which amounts to over one trillion mixtures made with 30 components. This is only a lower limit since for their experiment the researchers used 128 different components while in nature you can find many more, and in mixtures of often more than just 30 components.

I wonder what they would've concluded had Track been part of the study. :-)

[1] Bushdid C, Magnasco MO, Vosshall LB, & Keller A (2014). Humans can discriminate more than 1 trillion olfactory stimuli. Science (New York, N.Y.), 343 (6177), 1370-2 PMID: 24653035

[2] Vanderhaeghen P, Schurmans S, Vassart G, & Parmentier M (1993). Olfactory receptors are displayed on dog mature sperm cells. The Journal of cell biology, 123 (6 Pt 1), 1441-52 PMID: 8253843