Smell is our fastest warning system – ScienceDaily


The ability to detect and respond to the scent of a potential threat is a prerequisite for our survival and that of other mammals. Using a new technique, researchers at Karolinska Institutet in Sweden have been able to study what happens in the brain when the central nervous system judges that a smell is dangerous. The study, published in PNAS, indicates that negative odors associated with annoyance or discomfort are dealt with earlier than positive odors and trigger a physical avoidance response.

“The human avoidance response to unpleasant odors associated with danger has long been considered a conscious cognitive process, but our study shows for the first time that it is unconscious and extremely rapid,” says the study’s first author, Behzad Iravani, researcher in the Department of Clinical Neurosciences, Karolinska Institute.

The olfactory organ occupies about five percent of the human brain and allows us to distinguish several million different smells. Much of these smells are associated with a threat to our health and survival, such as chemicals and rotten food. Odor signals reach the brain within 100 to 150 milliseconds after being inhaled through the nose.

The survival of all living organisms depends on their ability to avoid danger and seek rewards. In humans, the sense of smell seems particularly important for detecting and responding to potentially harmful stimuli.

It has long been a mystery what neural mechanisms are involved in converting an unpleasant odor into avoidance behavior in humans. One of the reasons for this is the lack of non-invasive methods to measure signals from the olfactory bulb, the first part of the rhinencephalon (literally “brain of the nose”) with direct (monosynaptic) connections to important central parts of the nervous system. that help us detect and memorize threatening and dangerous situations and substances.

Researchers at Karolinska Institutet have now developed a method that, for the first time, has measured signals from the human olfactory bulb, which processes odors and can in turn transmit signals to parts of the brain that control movement and avoidance behavior.

Their results are based on three experiments in which participants were asked to rate their experience of six different smells, some positive, others negative, while the electrophysiological activity of the olfactory bulb during the response to each of the odors was measured.

“It was clear that the bulb reacts specifically and quickly to negative odors and sends a direct signal to the motor cortex in about 300 ms,” says the latest study author, Johan Lundström, associate professor at the Department of Clinical Neurosciences, Karolinska Institutet. “The signal causes the person to subconsciously lean back and away from the source of the smell.”

He keeps on:

“The results suggest that our sense of smell is important to our ability to sense dangers in our neighborhood, and much of that ability is more subconscious than our response to danger mediated by our senses of sight and hearing.”

The study was funded by the Knut and Alice Wallenberg Foundation, the National Institute for Deafness and Other Communication Disorders, and the Swedish Research Council. No conflict of interest was reported.

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Material provided by Karolinska Institute. Note: Content can be changed for style and length.


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