Humans sense negative smells faster than positive ones, meaning we can react to unpleasant odors literally in the blink of an eye, according to new research.
The discovery has been outlined in a study by scientists mostly from the Karolinska Institutet medical research center in Sweden.
They were aiming to find out what happens in the brain when the central nervous system deems a smell to be dangerous.
The human sense of smell has been a major topic of interest over the past year or so since COVID is known to cause a loss of smell and taste.
The reason humans can distinguish between different smells is thanks to a section of our brain called the olfactory organ, also called the olfactory bulb (OB).
In nature, this provides us with the benefit of being able to avoid something that could be dangerous by associating certain odors with threats—toxic fumes, for instance.
Scientists already thought that the OB played a part in rapid-avoidance behavior when we encounter bad smells, but little is known about how this actually takes place.
The researchers conducted experiments to find out. Until recently, there wasn't a non-intrusive way of measuring neural activity in the OB.
In total, they recruited 19 participants who took part in 540 trials each and were exposed to six different smells. Their brain activity was recorded using electrodes attached to their scalp and above their eyebrows.
They found that bad smells were associated with early avoidance. "It was clear that the OB reacts specifically and rapidly to negative smells and send a direct signal to the motor cortex within about 300 milliseconds," Johan Lundström, study co-author and associate professor at the Karolinska Institutet's Department of Clinical Neuroscience, said in a press release.
Blink of an Eye
That's about the same amount of time it takes people to blink on average, which falls within a range of 100 to 400 milliseconds, according to Harvard University's Bionumbers database.
Lundström continued: "The signal causes the person to unconsciously lean back and away from the source of the smell."
He added that the way our sense of smell helps us react to danger is "more unconscious" than when our other senses, such as vision and hearing, do the same.
According to the study, the OB is able to do this by processing smell information in two time periods. First, there's a brief initial period of brain activity along with a "privileged" amount of activity that deals with unpleasant odors. These are both linked to a physical avoidance response.
Second, there is a follow-up processing period called a beta response that allows us to form a subjective perception of the smell.
The study has been published in the journal Proceedings of the National Academy of Sciences of the United States of America.
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