The sense of smell is the fastest warning system that humans have.

The ability to detect and respond to potentially threatening odors is a prerequisite for our survival, as well as for other mammals. Using a new technique, researchers have been able to study what happens in the brain when the central nervous system judges that an odor represents danger. The study, published in PNAS, shows that negative odors associated with unpleasantness or discomfort are processed earlier than positive odors and trigger a physical avoidance response.

The olfactory organ occupies about 5% of the human brain and allows us to distinguish millions of different smells. A large proportion of these smells are associated with threats to our health and survival, such as the smell of chemicals and rotting food. After the odor signal is inhaled through the nose, it reaches the brain within 100 to 150 milliseconds. In humans, the sense of smell appears to be particularly important for detecting and responding to potentially harmful stimuli. The human avoidance response to unpleasant odors associated with danger has long been considered a conscious cognitive process, but this study shows for the first time that it is unconscious and very fast.

The neural mechanisms involved in converting unpleasant odors into avoidance behavior in humans have long been a mystery. One reason for this has been the lack of non-invasive methods to measure signals from the olfactory bulb, the first part of the rhinencephalon (literally), with direct (monosynaptic) connections to important central parts of the nervous system that help us detect and remember threatening and dangerous situations and substances.

Researchers from Karolinska Institutet have now developed a method that, for the first time, makes it possible to measure signals from the human olfactory bulb, which processes odors and then transmits them to the part of the brain that controls 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 and some negative, while the electrophysiological activity of the olfactory bulb in response to each odor was measured.

“It is clear that the olfactory bulb responds specifically and rapidly to negative odors and sends a direct signal to the motor cortex within about 300 milliseconds,” says Johan Lundström, associate professor at the Department of Clinical Neuroscience at Karolinska Institutet and one of the authors of the study. “This signal causes the person to unconsciously lean back and away from the source of the odor.”

The results suggest that our sense of smell is important for our ability to detect nearby danger and that this ability is largely more unconscious than our responses to danger mediated by vision and hearing.

The research paper, titled "The human olfactory bulb processes odor valence representation and cues motor avoidance behavior," was published in the journal PNAS.

(Source: Qianzhan.com)