LISBON, Portugal — Scientists have discovered the reason smell has such a strong association with memories and places. A team at the Champalimaud Centre for the Unknown says it all comes down to neurons in the brain.
These brain cells in the olfactory cortex – the area of the brain that controls smell – make the link between specific scents and places. Smell often has the power to transport us through time. It could be the calming fragrance of lavender that brings someone back to their childhood home or the smell of cinnamon that transports your mind back to a Christmas with loved ones.
This clear association between smells and memories is critical piece of how we think and process information. To figure out where this link travels to and from in the brain, researchers conducted a study using a maze for rats.
“Odor 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” says postdoctoral researcher and study first author Cindy Poo in a media release.
“We wanted to understand the neural basis of these behaviors, and so we decided to study how the brain combines olfactory and spatial information.”
‘Place cells’ can reveal your location
The research team focused on the primary olfactory cortex, which sits in the front of the brain.
“The olfactory system is unique among the senses,” explains senior author and principal investigator Zachary Mainen.
Dr. Mainen adds 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.
“Only olfaction has direct reciprocal connections to the hippocampal system, which is involved in memory and navigation,” Mainen says.
Together, these neurons, which scientists call “place cells,” create a map of space. These cells, whose discovery in rats led to scientists winning a Nobel Prize for Physiology or Medicine in 2014, are so reliable that researchers can tell where an animal is simply by observing their activity.
“We know that the hippocampal system sends signals to the primary olfactory cortex,” Dr. Poo continues. “So we suspected that this brain region might do more than just identify different smells.”
Decoding where smells send the brain
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 to find the reward.
“In this task, the rats had to learn and remember exact associations of odors and locations,” Poo explains.
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.
“Neurons communicate with one another by emitting electric impulses,” Mainen says. “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.”
“Our results exceeded our expectations,” adds Poo. “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.”
The brain builds a memory of smells and locations
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 only focused on behaviorally significant spots on the maze, like where the animals received the awards for smelling the odors.
“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,” Mainen adds.
“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,” Poo suggests.
“This study also opens up a new window to understand how the senses are used for navigation and memory,” Mainen concludes. “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 appear in the journal Nature.
South West News Service writer Georgia Lambert contributed to this report.