The ability to identify and react to the odor of a potential threat is a necessary requirement for our existence and that of other mammals. Researchers at Sweden’s Karolinska Institutet have been able to analyze what happens in the brain when the central nervous system interprets a smell as dangerous. According to the study, unfavorable odors linked with unpleasantness or unease are processed more quickly than good aromas and elicit a bodily avoidance reaction.
“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 around 5% of the human brain and enables us to discriminate between millions of different odors. Many of these odors are related with dangers to human health and survival, such as chemicals and rotten food. Odour impulses reach the brain within 100–150 milliseconds of nasal inhalation.
All living organisms rely on their abilities to avoid danger and seek rewards to survive. The olfactory sense appears to be especially crucial in humans for identifying and reacting to potentially hazardous stimuli.
It has long been unknown which brain 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 discovered a technology that enables the first time that signals from the human olfactory bulb can be measured. The olfactory bulb analyzes scents and provides signals to areas of the brain that govern movement and avoidance behavior.
Their findings are based on three tests in which people rated their experiences with six different smells, some favorable and others negative, while the electrical activity of the olfactory bulb was monitored in response to each fragrance.
“It was apparent that the bulb responds specifically and fast to unfavorable odours, transmitting a direct signal to the motor cortex within around 300 milliseconds,” explains 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 imply that our sense of smell is critical to our ability to recognize threats in our immediate environment, and that a large portion of this ability is more unconscious than our response to danger mediated by our eyesight and hearing.”
The Knut and Alice Wallenberg Foundation, the National Institute on Deafness and Other Communication Disorders, and the Swedish Research Council all contributed to the study’s funding. There are no conflicts of interest that have been disclosed.