By Alex Fradera
In the early 1950s, while investigating rabbits’ sense of smell by recording the activity of their brain cells, the scientist Lord Adrian noticed something curious. As his team mixed up odours of increasing strength, to see at what point the rabbits’ neurons fired in response, they found the critical threshold appeared around the same point that they were able to smell the odour themselves: in other words, this suggested that the smell had become noticeable to animal and man at the same time.
On publication of the research, Lord Adrian mentioned his observation, but it didn’t provoke a serious response, presumably because informed scientists knew that the human sense of smell is generally pathetic. Everyone knew… but they knew wrong. In a new review in Science, John McGann, who runs the Rutgers Laboratory on the Neurobiology of Sensory Cognition, takes us through the historical misunderstandings to reach the truth about what the human nose knows.
The history of the idea that humans can’t smell very well begins with the French neuroanatomist Paul Broca (immortalised through Broca’s brain area), who, facing hostility from the Catholic Church, was defiantly set on making the case for a unique human intelligence that wasn’t due to an invisible soul, but to physical brain features, specifically our large frontal lobes.
To tighten his story, Broca suggested that, in humans, the notably small brain volume taken up by the olfactory bulb, which processes smell (biologists call the sense of smell “olfaction”), reflected a redirection of resources from smell, which guides lower animals, into the higher functions. His argument gained intuitive strength from the fact that in humans the small bulb sits directly against the expanded frontal area. This supported the idea that a poor human sense of smell testifies to human exceptionalism, to our separation from the lowly beast. You can also see this theme in Freud’s work: he claimed smell was connected to instinctual sexual behaviour, and that although it was “usually atrophied” in humans, it connected us “to early animal forms of life.”
But Broca’s original neuroanatomical case doesn’t hold much water. Yes, the human olfactory bulb is small compared to the rest of our (relatively large) brain, but the same is true in other animals. Although across the animal kingdom the brain scales up in relation to body size (larger animals require larger somatosensory systems to represent their greater skin surface, which is why elephants and whales have enormous brains), the olfactory bulb is an important exception to this rule. This might be because however big or small you are, smell always begins at the nostrils.
A different indicator of smell specialisation could be the density of neurons in the olfactory bulb, but this density doesn’t vary greatly from species to species either, and what difference there is doesn’t give us a clear picture: although the bulb of a mouse is neurally denser than that of a human male, it’s less dense than that of a human female.
Admittedly, there are differences in the human olfactory system compared with other animals: for instance, a lack of adult neurogenesis in the olfactory bulb (in humans the growth of new neurons occurs in other regions including the hippocampus and striatum). This could be taken as evidence of less “plasticity” in the human smell system. However, humans appear well-able to adjust and learn to pick up even previously imperceptible smells, for example through aversive conditioning, possibly through reuse of the existing neural resources. So the neuroanatomical evidence that we have feeble odour-detecting isn’t compelling.
Yet the proof is in the smelling. But before you picture the trusted tracker dog, or mouse questing for cheese, ask yourself whether they, like some humans, would be able to distinguish own-brand from top-end potato crisps, or tell a Merlot from a Pinot Noir. Because the truth is, Lord Adrian’s observation reflects a wider truth: while in some instances our sense of smell falls well behind other mammals, in others, it proves itself a match, and in others still, comes out the winner.
As McGann points out, “humans with intact olfactory systems can detect virtually all volatile chemicals larger than an atom or two, to the point that it has been a matter of scientific interest to document the few odorants that some people cannot smell.” Recent research suggests we may be able to theoretically distinguish between, not thousands of different odorants, but up to a trillion! The exact smell matter is critical. For example, on two chemically related compounds, humans outperformed mice on one, but lagged behind them on another.
On reflection, this tallies well with the human experience. It’s clear that, like Proust’s experience with his madeleine cake, smell has a great power to evoke memory; that it guides matters of physical attraction; that it can tip us off to someone’s emotional state. The texts that once categorised us as an “anosmatic” species – without smell, like dolphins and whales – got it way wrong, brought off course by ideology and a reluctance to actually look at the facts. Luckily, many scientists since have gone on to do the necessary work, by following their noses.
Image: 1955: The maitre d’hotel of Londonderry House, London, checks bottles of fine wines chosen for the ‘Golden Banquet’ taking place (Photo by Harry Kerr/BIPs/Getty Images).