Sense of smell is our most rapid warning system

Date:
October 14, 2021
Source:
Karolinska Institutet
Summary:
The ability to detect and react to the smell of a potential threat
is a precondition of our and other mammals' survival. Using a novel
technique, researchers have been able to study what happens in the
brain when the central nervous system judges a smell to represent
danger. The study indicates that negative smells associated with
unpleasantness or unease are processed earlier than positive smells
and trigger a physical avoidance response.



FULL STORY ========================================================================== [Olfactory bulb in brain | Credit: (c) CLIPAREA.com / stock.adobe.com] Olfactory bulb in brain illustration (stock image).

Credit: (c) CLIPAREA.com / stock.adobe.com [Olfactory bulb in brain |
Credit: (c) CLIPAREA.com / stock.adobe.com] Olfactory bulb in brain illustration (stock image).

Credit: (c) CLIPAREA.com / stock.adobe.com Close The ability to detect
and react to the smell of a potential threat is a precondition of our
and other mammals' survival. Using a novel technique, researchers at
Karolinska Institutet in Sweden have been able to study what happens in
the brain when the central nervous system judges a smell to represent
danger. The study, which is published in PNAS, indicates that negative
smells associated with unpleasantness or unease are processed earlier
than positive smells and trigger a physical avoidance response.


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

The olfactory organ takes up about five per cent of the human brain and
enables us to distinguish between many million different smells. A large proportion of these smells are associated with a threat to our health
and survival, such as that of chemicals and rotten food. Odour 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 olfactory sense seems particularly important for detecting and reacting to potentially harmful stimuli.

It has long been a mystery just which neural mechanisms are involved
in the conversion of an unpleasant smell into avoidance behaviour
in humans. One reason for this is the lack of non-invasive methods
of measuring signals from the olfactory bulb, the first part of the rhinencephalon (literally "nose brain") with direct (monosynaptic)
connections to the important central parts of the nervous system that
helps us detect and remember threatening and dangerous situations and substances.

Researchers at Karolinska Institutet have now developed a method that
for the first time has made it possible to measure signals from the human olfactory bulb, which processes smells and in turn can transmits signals
to parts of the brain that control movement and avoidance behaviour.

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

"It was clear that the bulb reacts specifically and rapidly to negative
smells and sends a direct signal to the motor cortex within about 300
ms," says the study's last author Johan Lundstro"m, associate professor
at the Department of Clinical Neuroscience, Karolinska Institutet. "The
signal causes the person to unconsciously lean back and away from the
source of the smell." He continues: "The results suggest that our sense
of smell is important to our ability to detect dangers in our vicinity,
and much of this ability is more unconscious than our response to danger mediated by our senses of vision and hearing." The study was financed
by the Knut and Alice Wallenberg Foundation, the National Institute
on Deafness and Other Communication Disorders and the Swedish Research
Council. There are no reported conflicts of interest.

========================================================================== Story Source: Materials provided by Karolinska_Institutet. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Behzad Iravani, Martin Schaefer, Donald A. Wilson, Artin Arshamian,
Johan
N. Lundstro"m. The human olfactory bulb processes odor valence
representation and cues motor avoidance behavior. Proceedings of
the National Academy of Sciences, 2021; 118 (42): e2101209118 DOI:
10.1073/ pnas.2101209118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211014100139.htm

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