The sense of smell is our fastest warning system

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Published 17 October 2021 at 11.43

Domestic. Man's avoidant response to unpleasant odors associated with threats has long been considered a will-driven cognitive process. But a new study from KI shows for the first time that it is an unconscious and very fast reaction.

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In humans, the olfactory organ occupies about one twentieth of the brain and we can distinguish many millions of different odorants. A significant proportion of these are associated with threats to our health and survival, such as the smell of chemicals and rotten food. Odor signals reach the brain within 100 to 150 milliseconds after inhalation through the nose.

All living things survive thanks to their ability to avoid dangers and seek rewards. In humans, the sense of smell seems especially important for detecting and responding to potentially harmful stimuli.

It has long been unknown which neural mechanisms cause an unpleasant odor to translate into avoidant behavior in humans. This is partly due to the lack of non-invasive methods for measuring signals from the olfactory bulb, the first part of the olfactory brain with a direct (monosynaptic) connection to the important central parts of the nervous system that help us to detect and remember threats or dangers.

Now researchers at Karolinska Institutet have developed a method that has made it possible for the first time to measure signals from the human olfactory bulb that processes odors and then sends signals to parts of the brain that control movement and avoidant behaviors.

The results are based on three experiments, where participants initially had to estimate their experience of six different smells, some positive and others negative. At a later stage, the electrophysiological activity of the olfactory brain was analyzed in response to the negative and positive odors. the olfactory system first reacted to the odor. That signal then results in the person, without being aware of it, leaning backwards and away from the source of the smell, says the study's last author Johan Lundström, associate professor at the Department of Clinical Neuroscience, Karolinska Institutet, in a statement.

He continues:

– The results indicate that the sense of smell has a unique significance for our ability to detect dangers in the environment, where a significant part of that ability occurs on an unconscious level to a greater degree than dangers mediated by visual and auditory impressions.