new research offers clues about how the two are linked in the brain

In an episode of the popular Black Mirror television series titled Crocodile, an investigator asks a witness to smell a bottle of beer. The goal is to refresh his memory of a crime scene (the crime took place near a brewery).

It might not exactly be a common practice, but our sense of smell, or olfaction, is known for its ability to elicit memories. We all know the feeling. The smell of a particular scent can bring you back to your grandmother’s kitchen, the night of your first dance or to the seaside.

And think of “scent marketing”, where brand designers infuse “signature scents”, for example in fashion stores and hotel lobbies, to improve brand recognition around the world.

Neuroscientists who study olfaction have long questioned the link between our sense of smell and memory. Does this relationship between memory and smell result from the way the brain is wired? A study recently published in the journal Nature opened up an important avenue for answering this long-standing question.

Before we look at the study, some information on how the brain facilitates our sense of smell. Olfactory molecules are initially detected by receptor neurons in the nose. Neurons first send information about these encounters to the olfactory bulb, a brain structure the size of your finger located above the nasal cavity.

The olfactory bulb then sends signals to another brain structure called the piriform cortex. It is believed that odor recognition occurs there, i.e. we identify its potential source, such as an apple, banana or fresh cut grass.



Read more: Six curious facts about smell


What the researchers did

To study how the brain combines olfactory and spatial information, Cindy Poo and her colleagues at the Champalimaud Center for the Unknown in Portugal asked six rats to perform a navigation task.

The rats had to repeatedly navigate a cross-shaped map with four lanes, as shown in the video below from about two and a half minutes. At the start of each trial, a light directed the rat to one of the hallways, where it would be randomly exposed to one of four distinct smells (citrus, grass, banana, or vinegar). The location of a water reward depended on the scent to which the rat was exposed.

For example, the smell of citrus meant that the water reward was at the end of the south hall. If the rat was exposed to the citrus scent in the eastern corridor, it would have to travel south for the reward. If he got the scent in the south corridor in the first place, he could stay put and receive the reward. The idea was that with practice, a given scent would alert the rat to the location of the reward.

https://www.youtube.com/watch?v=BuneEBiU3MO0

The surroundings of the labyrinth were decorated with visual cues so that the rats could orient themselves according to these cues as well. However, the starting point of the rats was different in each trial. If that had been fixed, they could theoretically have just memorized a sequence of turns to find the correct lane, and not using any spatial memory at all. This meant that the success of the task depended on a combination of spatial navigation and olfaction.

After about three weeks of training, the rats did quite well; they were able to locate the water reward in about 70% of the trials. This indicates that the rats may have combined their internal map of the environment with scent locations to locate the reward.

A look at the activity of neurons

Neurons in the hippocampus, a part of the brain involved in memory and navigation, are known to function as “place cells”. These are cells that become active in a specific place in an environment, which allows us to find our way. Similar cells are also found in another part of the brain called the entorhinal cortex.

The most striking finding of the new study is that such selectively localized cells are not only found in the hippocampus and entorhinal cortex, but also in an area of ​​the brain related primarily to olfactory function, namely the piriform cortex. , the place considered to be the main responsible for the recognition of odors.

Researchers in the study monitored the electrical activity of neurons in this area. Surprisingly, they found that only about 30% of the neurons in this region of the rat brain responded to specific smells. Another 30% of neurons fired in response to both a particular smell and location.



Read more: The strange science of scent memory


The remaining 40% of active neurons did not respond to specific smells at all, but rather to places where rats had previously smelled the smells. These selectively localized neurons would even start firing when the rats just walked into the hallway, before encountering a scent.

The researchers then wanted to understand if the hippocampus and the piriformis cortex were “talking” to each other while the rats solved the riddle. They found that cells in both regions tended to fire synchronously as the rats roamed the maze.

So what does this tell us?

These results show that the olfactory system can play a role in spatial navigation, and that spatial memory and olfactory information converge in the piriform cortex. But why has the brain evolved to represent location and smell in the same area?

The answer could be that smells are very useful clues for orientation. For example, a pine forest smells different from a meadow, while a fox burrow smells different from a rat’s nest. The rule is valid even in artificial environments: a metro system smells different from a supermarket, an office different from a restaurant.

So our brains could be hardwired to associate smells with places, as this has been helpful in our evolutionary past.

This study was conducted in rats, which rely more on their sense of smell for navigation than humans because our perception is dominated by vision. But these findings provide new insight into how olfaction and spatial memory are likely connected in the human brain.

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