The smell is able to transport us through time and space. It could be the sweet scent of jasmine or the scent of fresh grass. Once you smell a familiar scent, you can suddenly return to the home of your childhood or under the scorching sun on the distant seashore. This connection between smells and places seems to be a deeply ingrained aspect of human consciousness. But how are they connected in the brain? A study published in the journal Nature provides a possible explanation.
The neural connection between smell and space
“Smell molecules, by their very nature, do not carry spatial information. However, animals in the wild use smells for spatial navigation and memory, which allows them to find valuable resources such as food, ”said Cindy Poo, the study's first author. "We wanted to understand the neural basis of this behavior, and so we decided to study how the brain combines olfactory and spatial information."
The researchers focused on the primary olfactory cortex. "The olfactory system is unique among the senses," say the scientists. "Only the sense of smell has direct reciprocal connections with the hippocampal system, which is involved in memory and navigation."
The neurons in the hippocampus are known to function as "site cells". This is because each cell becomes active at a specific location in the environment. Together, these neurons encode the entire region, effectively creating a neural map of space.
The hippocampal site cells, whose discovery in rats led to the Nobel Prize in Physiology or Medicine in 2014, are so reliable that scientists can determine where an animal is simply by observing their activity.
“We know that the hippocampal system sends signals to the primary olfactory cortex,” said Cindy Poo. "So, we suspected that this area of the brain might do more than just recognize different smells."
Olfactory neuron testing
To test this idea, the researchers developed a custom puzzle for rats, which have a well-developed sense of smell. Rats caught odors at the four ends of the maze. Then, depending on the scent, they had to figure out where the reward was hidden. “In this task, the rats had to remember the exact associations of smells and places,” explained Cindy Poo.
While the animals were solving the puzzle, the researchers observed the activity of neurons in a part of the primary olfactory cortex called the posterior piriform cortex.
“Neurons communicate with each other by emitting electrical impulses. By recording electrical signals emitted by hundreds of individual neurons in this region of the brain, we were able to decipher what specific neurons were doing. For example, did they become active when the animal smelled a certain smell, or when it was in a certain place in the maze, ”the researchers say.
“Our results have exceeded our expectations. We predicted that some neurons might take care of location to some extent. However, by carefully studying the activity of neurons in the olfactory cortex as the animal moved through the maze, we found that these neurons studied the entire map of the environment. "
Olfactory neurons encode spatial maps
The researchers found a large population of neurons that, like hippocampal cells, became active at a specific location in the maze.
Interestingly, the map did not cover the entire environment in the same way. Instead, it was mostly limited to behaviorally significant points in the maze: where animals smelled and received rewards.
“It seems that important places have been learned firsthand and encoded on the map. It is noteworthy that these cells of the olfactory system began to react in a certain place, when there was no smell, even when the rat just walked without participating in this task, "added Cindy Poo.
The scent of space
Isn't this how we form memories that associate certain smells with certain places?
“We found that some neurons here reacted to smell, others to location, and still others to both types of information to varying degrees. All of these different neurons are mixed together and are likely connected. Therefore, it can be assumed that the activation of associations of smell and space can occur through activity in this network, ”the scientists suggested.
“Our research also opens a new window into understanding how senses are used for navigation and memory. People rely more on visual cues than smells, but it is likely that the principles of how we remember where we have been and how we get where we are going are very similar. "
The study was published in the journal Nature.
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