Our Sense of Smell Gives Us a Startlingly Fast Warning System For Danger

A new study has shown that our sense of smell is exceptionally sensitive and alert to danger.
Two experiments by the Karolinska Institute in Sweden have shown that the smell-detecting nerves deep within our nose begin processing negative odors as soon as they are detected. This triggers our body to respond more quickly.

This contrasts with our visual and auditory system, which send sensory responses to specialized units in the brain for processing. It delays the time it takes the body to get up to move.

Given how dependent we are on sound and sight to navigate the world, this might surprise some. For many animals, the ability to quickly pick up subtle scents can be the difference between feeding or fleeing.

Research in rodents has shown how brain-like this bundle of scent-detecting neurons, also known as the Olfactory Bulb, can be. This collection of neurons is not a post-box for scents. It can actively sort stimuli using learned and innate responses.

This is especially true in potentially dangerous situations. You don't want precious seconds wasted calculating your response when you are just a few steps away from eating a poisonous morsel or worse, inhaling toxic fumes.

But what's true in mice and rats might not be the same for us. Even though millions of years of evolution have separated us from our closest relatives and our smelly bulbs have not withered, that doesn't mean we aren't evolving.

The team recruited 19 healthy, non-smokers to test the odors.

Some, such as the fruity-smelling ethylbutyrate or the scent of linalool, can be considered pleasant. Others, such as the garlicky tang from diethyl diulfide, were more appealing.

Presenting the volunteers with each odor a number of times, the researchers measured the reactivity of the olfactory bulb noninvasively using electroencephalography. This gave them an indication of the timing of the responses and allowed them to assess the two types of brain waves that the nerves were producing.

The first was gamma wave fast-processing waves that we depend on for memory and attention. Beta waves, which is an oscillation of brain activity used in deliberate decision-making processes, was the second.

These waves are present near our nasal neurons and signify that when a scent touches our olfactory bulbs, other parts of our brain like our motor cortex get ready to go.

The team also tested the warm-up on 21 volunteers to see if it translated into a body response.

The pleasant odors of strawberry and carvone, an essential oil found in mint or dill, were compared to something fishy and the cabbage-stink smell of ethanethiol. To get an idea of how long it took the smell to make the volunteers react, we measured their movements.

When you combine the data, it becomes clear that our olfactory bulbs process pleasant and threatening smells at different rates. We can quickly get rid of something unpleasant smelling in less than a half second. We'll wait until it's not too bad before we take our time.

There's much more going on than that. Within 250 milliseconds after an odor is detected in the nose, two brain waves 'couple' to coordinate a response.

A signal will be sent faster if the smell is considered a threat. It takes around 150 milliseconds for the signal to reach the motor cortex. It could take longer if the signal is not sent immediately.

Johan Lundstrm from the Karolinska Institute's Department of Clinical Neuroscience, says that it was evident that the bulb responds quickly to negative smells. It sends a signal directly to the motor cortex in about 300 milliseconds.

The entire process of detection to movement, from sound to sight, is measured in terms of visual and auditory response time. It takes around 150 milliseconds to detect sound and less than 200 milliseconds to see it.

Although it may take us longer to recognize a bad odor, it can be much longer if the signal wasn't already being teasingly received by our olfactory bulb.

Lundstrm says that the results show that our senses, including smell, are important in our ability to detect dangers around us. This ability is much more unconscious than our responses to dangers mediated by vision and hearing.

We can all let out a huge sigh of relief, knowing that our noses know best.

This research was published by PNAS.