You have probably been surprised by the headline of this news and it is that, to function in our day-to-day lives, we depend a lot on sight and hearing. On the contrary, for many animals, perceiving a certain smell, however subtle, can make the difference between remaining calmly in the middle of the field or fleeing because a predator is nearby. Well, two experiments carried out by researchers from the Karolinska Institute in Sweden have shown that the nerves that detect odors begin to process those that are negative the moment we detect them, which causes a faster response from our body . The visual and auditory systems take longer to make the body react to the stimulus since they send the sensory responses to the brain for it to process, delaying the time of action.
Previously, it had been proven in experiments with rats that this set of nerves that detect odors, known as the olfactory bulb, is not a mere olfactory mailbox, but is capable of actively classifying stimuli based on learned and innate responses, for example. This becomes especially important when we are faced with a potential danger such as ingesting something harmful or inhaling a toxic gas, and it is that we hardly waste time in calculating the answer.
The researchers wanted to see if this was true in humans or not. To do this, they carried out two experiments. In the first, 19 non-smokers and supposedly healthy people were recruited to quickly smell six different smells. Among these scents there were some pleasant ones such as the perfume of linalol or ethyl butyrate, which smells of fruit, and other unpleasant ones such as that of diethyl disulfide, which is reminiscent of garlic. After allowing each scent to be sniffed several times, the researchers measured the reactivity of the olfactory bulb by shape using electroencephalography. This not only provided them with a timing of responses, but allowed them to take stock of two different types of ” brain waves ” produced by the nerves. One of them was gamma waves, fast processing waves that we depend on for attention and memory. The second were slightly slower beta waves, an oscillation of brain activity used in deliberate decision-making processes. The presence of both waves near our nasal neurons implies that, the moment a fragrance tickles our olfactory bulb, it is preparing other parts of the brain, such as the motor cortex, to get going.
In the second experiment, 21 volunteers were brought together to see if this kind of brain heating translates into a bodily response. Two pleasant smells – strawberry and carvone, an essential oil found in peppermint and dill – were compared with something unpleasant like fish and the cabbage smell of ethanethiol. After smelling them, the volunteers’ movements were measured to see how long it took for the smell to make them physically react.
Putting the data from both experiments together, it is clear that our olfactory bulb processes pleasant and threatening odors at different rates . Faced with an unpleasant smell, it only took us half a second to move away. If it’s not so bad, we take our time. But in that half second a lot happens. In the 250 milliseconds after a smell hits the nose, the two different brain waves “couple” to coordinate a response. If the odor is considered a threat, the signal is sent earlier, taking about 150 milliseconds to reach the motor cortex. Otherwise, it may take much longer before a message is generated. “It was clear that the bulb reacts specifically and quickly to negative odors and sends a direct signal to the motor cortex in about 300 milliseconds,” says Johan Lundström, a biologist in the Department of Clinical Neuroscience at the Karolinska Institute.
Research on visual and auditory reaction time has measured the entire process, from detection to movement, as faster: approximately 150 milliseconds for a reaction to sound and less than 200 milliseconds for sight.
Although it takes a little longer to recoil from a bad smell, it could be much longer if our olfactory bulb was not already deciphering the signal.
“The results suggest that our sense of smell is important to our ability to detect hazards in our proximity, and that much of this ability is more unconscious than our response to danger mediated by our senses of sight and hearing,” says the researcher who participated in the experiments, Johan N. Lundström.
Source: Behzad Iravani, Martin Schaefer, Donald A. Wilson, Artin Arshamian, Johan N. Lundström, PNAS October 19, 2021 118 (42) e2101209118; https://doi.org/10.1073/pnas.2101209118