Reason smells are so evocative has been discovered by scientists

By Georgia Lambert via SWNS

The reason smell has such a strong association with times and places has been discovered by scientists and it is down to neurons in the brain.

Found in the olfactory cortex - the area of the brain that controls smell - they make the link between specific scents and places, according to new research.

Smell has the power to transport us through time - it could be the calming fragrance of lavender that brings you back to your childhood home, or the spice of cinnamon that transports your mind back to a Christmas spent with loved ones.

This clear association between smells and places is deeply embedded in how we think and process information.

To figure out where this link travels to and from in the brain, researchers from the Champalimaud Centre for the Unknown conducted a study using a maze designed for rats.

Cindy Poo, a postdoctoral researcher at the Centre, was the study's first author.

She said: "Odour molecules do not inherently carry spatial information.

"However, animals in the wild use odors for spatial navigation and memory, which allow them to locate valuable resources such as food.

“We wanted to understand the neural basis of these behaviors, and so we decided to study how the brain combines olfactory and spatial information."

The research team focused on the primary olfactory cortex, which is centered at the front of the brain.

As Zachary Mainen, the senior author and principal investigator at the Centre in Portugal explained: "The olfactory system is unique among the senses."

Dr. Mainen went on to explain that the neurons in the hippocampus - which are cells in the area of the brain that controls memory - become active at a specific location within an environment.

He said: "Only olfaction has direct reciprocal connections to the hippocampal system, which is involved in memory and navigation."

Together, these neurons, which are famously known as "place cells" create a map of space.

These cells, whose discovery in rats led to scientists bagging a Nobel Prize for Physiology or Medicine in 2014, are so reliable that researchers can tell when an animal is simply by observing its activity.

Dr. Poo said: "We know that the hippocampal system sends signals to the primary olfactory cortex.

"So we suspected that this brain region might do more than just identify different smells."

To put these olfactory neurons to the test, the team developed a custom-made puzzle filled with different odors for rats to make their way through.

The rats sampled odors at four ends of the plus-shaped maze.

Then, depending on the scent, they had to figure out where the reward was hidden.

In light of this task Dr. Poo said: "In this task, the rats had to learn and remember exact associations of odors and locations."

While the animals were solving the puzzle, the researchers monitored the activity of their neurons in a part of the primary olfactory cortex called the posterior piriform cortex.

Dr. Mainen went on to explain how neurons work, saying: "Neurons communicate with one another by emitting electric impulses.

"By recording the electric signals emitted by hundreds of individual neurons in this brain area, we were able to decode what specific neurons cared about.

"For instance, whether they became active when the animal was smelling a specific odor, or when it was at a certain location in the maze."

Dr. Poo added: "Our results exceeded our expectations.

"We had predicted that some neurons here might care about location to a certain degree.

"However, by carefully studying the activity of olfactory cortex neurons while the animal was navigating in the maze, we found that these neurons had learned an entire map of the environment."

During the study the researchers discovered a busy population of neurons that, similarly to the place cells, became active at a specific location in the maze.

Interestingly, the map didn't equally cover all of the maze.

Instead, it was largely restricted to behaviourally significant spots on the maze, like where the animals received the awards for smelling the odors.

Dr. Mainen added: "It appears that important locations were learned through experience and encoded into a map.

"It was remarkable that these cells in the olfactory system started responding in a given location when no odors were present, even when the rat was just walking around not engaged in the task."

Dr. Poo went on to suggest an explanation behind how we form memories: "We found that some neurons here responded to smell, others to location, and yet others to both types of information to varying degrees.

"All these different neurons are mixed together and are probably interconnected.

"Therefore, one can speculate that activating smell-space associations can happen through activity within this network."

Dr. Mainen explained the future impact of this study. He said: "This study also opens up a new window to understand how the senses are used for navigation and memory.

"Humans rely on visual landmarks more than odors, but it's likely that the principles of how we remember where we've been and get to where we're going are very similar."

The study's findings were published in the Nature journal.