For many years, the scientific consensus has been that humans cannot smell quite as well as our distant cousins, such as canines and rodents.
However, a new research paper asks whether we have been doing our noses an injustice.
Smell – or olfaction, to give it the correct term – is a complex sense and, evolutionarily speaking, it is considered to be our oldest.
However, despite its age, the sense of smell has received much less research than some of our other senses, particularly vision.
Perhaps because of this relative lack of study, the common misconception has lingered on.
According to John McGann, a neuroscientist at Rutgers University-New Brunswick in Canada, our sense of smell might not be any worse than others in the animal kingdom, and it may even outperform animals in certain tasks.
His findings were published today in the journal Science.
Why the disrespect?
McGann has been studying olfaction for 14 years.
During that time, he has delved into reams of data and examined historical writings on the matter.
He believes he has uncovered the reasons why humans have an inferiority complex regarding their own sense of smell.
“For so long, people failed to stop and question this claim, even people who study the sense of smell for a living,” said McGann. “The fact is, the sense of smell is just as good in humans as in other mammals, like rodents and dogs.”
So how have we got it wrong for so long?
The current paper points the finger at Paul Broca – a brain surgeon from the 19th century – who was concerned that olfaction was an animal instinct.
He, and many others at the time, believed that our reduced ability to detect aromas was one of the things that separated us from the beasts.
The general belief was that “to be a reasonable or rational person you could not be dominated by a sense of smell,” said McGann. “Smell was linked to earthly animalistic tendencies.”
According to McGann, the size of our olfactory bulbs has also been used as evidence. These are a pair of neural structures that receive information about smells and pass them on to the brain.
The olfactory bulbs are the only part of the central nervous system that makes physical contact with the environment around us.
In 1879, Broca wrote that because our olfactory bulbs are relatively small in comparison to the rest of our brains, it meant that we had free will and were not dependent on aromas for our survival.
However, according to McGann, this is not necessarily the case and should not be taken at face value.
Although it generally holds true that the size of certain brain areas correlates to an ability to carry out tasks associated with that brain region, McGann believes that the olfactory bulbs are an exception to the rule.
Human olfactory bulbs do appear small when compared with the size of our brains, but they actually house a similar number of neurons to those of other species.
Across a diverse group of animals, there is a 28-fold range in the number of neurons in the bulb, compared with a 5,800-fold range in body weight.
In other words, the number of neurons in the olfactory bulb is relatively constant regardless of an animal’s size.
Genetic studies and folk wisdom
Aside from Broca and olfactory bulbs, certain genetic studies appear to demonstrate that human olfaction is less impressive than in other mammals.
These studies found that rats and mice have around 1,000 different receptors that respond to various smells, whereas humans have only around 400.
McGann counters this, saying that we have got “caught up in numbers” and that 400 is still an impressive amount. He says that “folk wisdom and poorly sourced introductory psychology textbooks” claim that humans can detect around 10,000 aromas. McGann believes that figure should be closer to 1 trillion.
“Dogs may be better than humans at discriminating the urines on a fire hydrant and humans may be better than dogs at discriminating the odors of fine wine, but few such comparisons have actual experimental support,” he said.
McGann hopes that his work will renew people’s interest in this most fascinating of senses.
He believes that the importance of olfaction has been sidelined and underestimated, partly due to this ingrained negative bias that scientists have been carrying around.
Our sense of smell plays a significant and often unconscious role in our everyday lives. It alters how we interact with people, it influences our choice of partner, and it helps us decide what to eat.
In the article in Science, McGann writes, “We can detect and discriminate an extraordinary range of odors; we are more sensitive than rodents and dogs for some odors; we are capable of tracking odor trails; and our behavioral and affective states are influenced by our sense of smell.”
Olfaction might also play a role in certain conditions.
“Some research suggests that losing the sense of smell may be the start of memory problems and diseases like Alzheimer's and Parkinson's,” said McGann. “One hope is that the medical world will begin to understand the importance of smell and that losing it is a big deal.”