Smells we relate to repulsiveness and discomfort prompt our body to respond with physical avoidance and are processed before smells linked to appeal, a new study has found.

Sensing and responding to smells that hint of possible danger is a precondition in mammals. Researchers at Karolinska Institutet in Sweden used a new method to analyse how the brain responds to smells that it relates to danger.

Behzad Iravani, a researcher at the Department of Clinical Neuroscience, Karolinska Institutet and the study’s first author, explained: “The human avoidance response to unpleasant smells associated with danger has long been seen as a conscious cognitive process, but our study shows for the first time that it’s unconscious and extremely rapid.”

In order to survive, living organisms must avoid danger. The olfactory sense is said to be vital in identifying and responding to possible danger for humans.

Around five percent of our brain consists of the olfactory organ (olfactory bulb), which helps us differentiate and characterise millions of smells. Many of these smells correlate to risks for our survival and wellbeing, for example mouldy food and chemicals. The brain recognises smells after just 100 to 150 milliseconds of breathing them in through the nose.

It is unknown which neural mechanisms in humans are responsible for translating bad odours into physical avoidance. This is largely due to very little non-invasive approaches of analysing signals from the olfactory organ, the first section of our “nose brain” (rhinencephalon) which connects to significant parts of our nervous system that assist in distinguishing, recalling, and avoiding danger.

The new technique, developed by researchers at Karolinska Institutet, has allowed the analysis of these particular signals from the olfactory organ in humans for the first time.

Published in PNAS, the study consisted of three experiments. Participants had to rate their familiarity with and impression of six smells, varying in positivity and negativity, while researchers analysed their olfactory organ’s electrophysiological activity relating to each individual smell.

The study’s last author Johan Lundström, associate professor at the Department of Clinical Neuroscience, Karolinska Institutet, summarises: “It was clear that the bulb reacts specifically and rapidly to negative smells and sends a direct signal to the motor cortex within about 300 ms. The signal causes the person to unconsciously lean back and away from the source of the smell.

“The results suggest that our sense of smell is important to our ability to detect dangers in our vicinity, and much of this ability is more unconscious than our response to danger mediated by our senses of vision and hearing.”


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