It’s pretty easy for most people to recognize a strong smell — a new cedar dressing table — and associate it with a memory — the smell of cedar in a pet rabbit’s cage, for example. But what brain processes bring on this phenomenon? How can someone smell a warm breeze, and its scent automatically brings to mind all of the events that unfolded during a trip to the Bahamas, where that person first smelled that scent? New research by Norwegian scientists sheds light on the brain activity that leads to this phenomenon.
“We all know that smell is connected to memories,” Kei Igarashi, lead author of the study from the Kavli Institute for Systems Neuroscience, in a press release. “We know that neurons in difference brain regions need to oscillate in synchrony for these regions to speak effectively to each other. Still, the relationship between interregional coupling and formation of memory traces has remained poorly understood.”
So, the researchers set out to determine how the two neuronal pathways — for smell and memory — converged to create these unique memories. They fitted a group of mice with electrodes on their hippocampus and different parts of the entorhinal cortex, both of which are involved in memory consolidation, with the latter also being responsible for receiving input from the senses. They then had the mice run through a maze containing holes big enough for their noses to peak through. When they smelled what was on the other side, they were given two different smells, of which one told them the direction that food was in. After three weeks, 85 percent of the rats were able to determine, based on smell and memory, where to go for a reward.
They found that the more the rats became familiar with the maze’s purpose, their brain waves began to synchronize. “Immediately after the rat was exposed to the smell there is a burst in activity of 20 Hz waves in a specific connection between an area of the entorhinal cortex, lateral entorhinal cortex, and an area in the hippocampus,” called distal CA1, Igarashi said in the statement. The same activity was not seen at a comparable strength anywhere else in the brain. The researchers found that this activity ranged between 20 and 40 Hz.
While this isn’t the first time the scientists heard of such a coordination among brain wave activity, it’s the first time that they saw specific bands of activity, or oscillations, develop with memory in the hippocampus. Besides that, it gives us an interesting look into the inner workings of our brains, so that the next time you smell fresh apple pie — or whatever else — and it brings you back to the first time grandma made it for you, you’ll know what processes brought you to that memory.
Source: Igarashi K, Lu L, Colgin L, et al. Coordination of entorhinal–hippocampal ensemble activity during associative learning. Nature. 2014.