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Sense of smell - the fastest alarm system for humans -

Sense of smell – the fastest alarm system for humans –

The ability to smell and respond to potential threats is a prerequisite for the survival of humans and other mammals. A study by Karolinska Institutet published in PNAS suggests that negative odors associated with discomfort are processed earlier than positive odors and lead to a rapid and unconscious bodily reaction to avoidance.

The human response to unpleasant odors associated with threats has long been considered a cognitive process controlled by the will. Our study shows that it is a very rapid and unconscious reaction, says Behzad Iravani, a researcher in the Department of Clinical Neuroscience, Karolinska Institutet, and first author of the study.

In humans, the olfactory organ occupies about one twentieth of the brain and we can distinguish several million different odors. A large percentage of these are related to threats to our health and survival, such as the smell of chemicals and spoiled foods. Smell signals reach the brain within 100 to 150 milliseconds after being inhaled through the nose.

Measures the brain’s olfactory response

All living things survive thanks to their ability to avoid dangers and obtain rewards. In humans, the sense of smell appears to be particularly important for detecting and responding to potentially harmful stimuli.

The neural mechanisms that cause foul odor translate into avoidance behavior in humans have long been unknown.

This is partly because there are no non-invasive ways to measure signals from the olfactory bulb, which is the first part of the olfactory brain with a direct (monosynaptic) connection to important central parts of the nervous system that help us detect and remember threats. or dangers.

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Now, researchers at Karolinska Institutet have developed a method that 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 introverted behaviors.

The fastest alarm system

The results are based on three experiments, in which participants initially had to rate their experience with six different scents, some positive and some negative. At a later stage, the electrophysiological activity of the olfactory brain was analyzed in response to negative and positive odors.

– It became clear that the odor bulb reacts specifically and early to negative odors and sends a direct signal to the motor cortex already about 300 milliseconds after the odor system first interacted with the odor. This signal results in the person, without being aware of it themselves, leaning back and away from the source of the smell, says the study’s latest author Johann Lundstrom, associate professor in the Department of Clinical Neuroscience, Karolinska Institutet.


The findings suggest that the sense of smell is uniquely important in our ability to detect dangers in the environment, with a large portion of that ability occurring on a subconscious level to a greater degree than the dangers conveyed by visual and auditory impressions.

Scientific material:

The human olfactory bulb treats the representation of equivalence of smell and is indicative of motor avoidance behaviour (Behzad Iravani, Martin Schaefer, Donald A. Wilson, Artin Arshamian, Johann N. Lundstrom) PNAS Proceedings of the National Academy of Sciences.

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