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Sense of smell is our most rapid warning system

According to the study, negative smells associated with unpleasantness or unease are processed more quickly than positive smells and elicit a physical avoidance response.

"While it has long been assumed that the human avoidance response to unpleasant smells associated with danger is a conscious cognitive process, our study demonstrates for the first time that it is unconscious and extremely rapid," says study first author Behzad Iravani, a researcher at Karolinska Institutet's Department of Clinical Neuroscience.

The olfactory organ occupies approximately 5% of the human brain and enables us to discriminate between millions of different smells. Many of these odors are associated with dangers to our health and survival, such as chemicals and rotten food. Odour signals reach the brain within 100–150 milliseconds of nasal inhalation.

All living organisms rely on their ability to avoid danger and seek rewards to survive. The olfactory sense appears to be especially important in humans for detecting and reacting to potentially harmful stimuli.

It has long been unknown which neural mechanisms contribute to the conversion of an unpleasant odor into avoidance behavior in humans.

One reason for this is a dearth of non-invasive methods for measuring signals from the olfactory bulb, the first part of the rhinencephalon (literally "nose brain") with direct (monosynaptic) connections to the critical central nervous system regions that aid us in detecting and remembering potentially dangerous situations and substances.

Researchers at Karolinska Institutet have developed a method that enables the first time that signals from the human olfactory bulb can be measured. The olfactory bulb processes smells and transmits signals to areas of the brain that control movement and avoidance behavior.

Their findings are based on three experiments in which participants rated their experiences with six different smells, some positive and some negative, while the electrophysiological activity of the olfactory bulb was measured in response to each smell.

"It was clear that the bulb responds specifically and rapidly to negative smells, transmitting a direct signal to the motor cortex within about 300 milliseconds," says study co-author Johan Lundström, associate professor at Karolinska Institutet's Department of Clinical Neuroscience. "The signal induces an unconscious leaning backward and away from the source of the odor."

He goes on:

"The findings suggest that our sense of smell is critical to our ability to detect dangers in our immediate environment, and that a large portion of this ability is more unconscious than our response to danger mediated by our vision and hearing."

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