Monday, June 5, 2017

The Human Nose Knows More Than We Think - NEUROSCIENCE

A look at the body of olfactory science shows people’s reputation for having a poor sense of smell is a myth.

Humans do not use smell the way other mammals do, and that contributes to our reputation for being lousy sniffers compared with dogs and other animals. But it turns out the human sense of smell is better than we think.
In a review paper published in Science last week neuroscientist John McGann of Rutgers University analyzed the state of human olfaction research, comparing recent and older studies to make the argument our smelling abilities are comparable with those of our fellow mammals.

McGann traces the origins of the idea that humans have a poor sense of smell to a single 19th-century scientist, comparative anatomist Paul Broca. Broca, known for discovering Broca’s area—the part of the brain responsible for speech production—noted that humans had larger frontal lobes than those of other animals, and that we possessed language and complex cognitive skills our fellow creatures lacked. Because our brains’ olfactory bulbs were smaller than those of other mammals and we did not display behavior motivated by smell, Broca extrapolated these brain areas shrank over evolutionary time as humans relied more on complex thought than on primal senses for survival. He never conducted sensory studies to confirm his theory, however, but the reputation stuck.

Now that more sensory tests are being done, the results are mixed.
Experiments conducted in previous decades have found humans are just as sensitive as dogs and mice to the aroma of bananas. Furthermore, a 2013 study found humans were more sensitive than mice to two urine odor components whereas mice could better detect four other sulfur-containing urine and fecal-gland odors tested. A 2017 study also revealed humans were more sensitive than mice to the smell of mammal blood.

One biological feature that does appear to be linked to smelling ability is the number of olfactory bulb neurons an animal has. This number is not linked to the size of the brain or bulb, however. Human women, whose sense of smell is more sensitive than men’s, have more olfactory neurons than the general population of mice but fewer than rats, and have much larger olfactory bulbs than both rodents. Men rank just below mice in olfactory neuron count, but all these species (as well as several other mammals) differ by just 10 million olfactory neurons or fewer.

The lack of a standard metric for scent is the main challenge, McGann says, in comparing absolute olfactory abilities across species. “It’s tempting to say humans are way more sensitive than mice at smelling human blood, and that sounds like a good ecological story,” he says. “But then you look at a whole range of other odors and realize that actually it just seems like there’s quite a lot of odors that humans are better at detecting than mice, dogs or rats, and other odors that we’re less good at detecting.” It’s impossible, therefore, to make sweeping generalizations about which species has the winning nose.

Interesting reading via Scientific American:

Gif via It's Okay to Be Smart
Corina Marinescu

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