An article from Norwegian SciTech News at NTNU

Degrees of synchronization in oscillatory activities (coherence) between two brain regions of well-trained rats are color-coded in these figures. Top: Synchronization between lateral entorhinal cortex (LEC) and distal part of hippocampal area CA1 (dCA1). Middle: Synchronization between medial entorhinal cortex (MEC) and dCA1. Bottom: Synchronization between LEC and proximal part of hippocampal area CA1 (pCA1). 20-40 Hz oscillations were synchronized only between LEC and dCA1 (arrow). (Photo: NTNU)
Degrees of synchronization in oscillatory activities (coherence) between two brain regions of well-trained rats are color-coded in these figures. Top: Synchronization between lateral entorhinal cortex (LEC) and distal part of hippocampal area CA1 (dCA1). Middle: Synchronization between medial entorhinal cortex (MEC) and dCA1. Bottom: Synchronization between LEC and proximal part of hippocampal area CA1 (pCA1). 20-40 Hz oscillations were synchronized only between LEC and dCA1 (arrow). (Photo: NTNU)

Why certain smells trigger memories

The signals from your nose connect to memories in an orchestrated symphony of signals in your brain.

Published

Gemini, NTNU Trondheim - Norwegian University of Science and Technology

NTNU is the second largest of the eight universities in Norway, and has the main national responsibility for higher education in engineering and technology.

VIDEO:The researchers designed a maze for rats, where a rat would see a hole to poke its nose into. When poking into the hole, the rat was presented with one of two alternative smells. One smell told the rat that food would be found in the left food cup behind the rat. The other smell told it that there was food in the right cup. The rat would soon learn which smell would lead to a reward where. After three weeks of training, the rats chose correctly on more than 85 percent of the trials.

When I was a child, I used to sit in my grandfather’s workshop, playing with wood shavings. Freshly shaven wood has a distinct smell of childhood happiness, and whenever I get a whiff of that scent my brain immediately conjures up images of my grandfather at his working bench, the heat from the fireplace and the dog next to it.

Researchers at the Kavli Institute for Systems Neuroscience in Norway have recently discovered more about the process behind this phenomenon. 

Synchronized brain waves

The brain, it turns out, connects smells to memories through an associative process where neural networks are linked through synchronized brain waves. The brain waves make smells stick to our memories and inner maps.

“We know that smell is connected to memories,” says Kei Igarashi, lead author of the article.

“We also know that neurons in different 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," he says.

Igarashi and his colleagues have investigated how odor and place representation evolved in the entorhinal and hippocampal region, to figure out whether learning depends on coupling of oscillatory networks.

The results are published in the latest edition of Nature.

Smell guides the way in maze

The researchers designed a maze for rats, where a rat would see a hole to poke its nose into.

When poking into the hole, the rat was presented with one of two alternative smells:

One smell told the rat that food would be found in the left food cup behind the rat. The other smell told it that there was food in the right cup. The rat would soon learn which smell would lead to a reward where.

16 electrode pairs

After three weeks of training, the rats chose correctly on more than 85 percent of the trials.

In order to see what happened inside the brain during acquisition, 16 electrode pairs were inserted in the hippocampus and in different areas of the entorhinal cortex.

After the associations between smell and place were well established, the researchers could see a pattern of brain wave activity (the electrical signal from a large number of neurons) during retrieval.

Specific connection

Immediately after the rat is exposed to the smell, there is a burst in activity of 20Hz–40Hz waves in a specific connection between an area in the entorhinal cortex, lateral entorhinal cortex (LEC), and an area in the hippocampus, distal CA1 (dCA1).

"While a similar strong response was not observed in other connections,” Igarashi says.

This coherence of 20 Hz activity in the LEC and dCA1 evolved in parallel with learning, with little coherence between these areas before training started.

By the time the learning period was over, cells were phase locked to the oscillation and a large portion of the cells responded specifically to one or the other of the smell-odor pairs.

Long distance communication

“This is not the first time we observe that the brain uses synchronised wave activity to establish network connections, Edvard Moser, director of the Kavli Institute for Systems Neuroscience comments.

Both during encoding and retrieval of declarative memories there is an interaction between these areas mediated through gamma and theta oscillations. However, this is the first study to relate the development of a specific band of oscillations to memory performance in the hippocampus,

"Together, the evidence is now piling up and pointing in the direction of cortical oscillations as a general mechanism for mediating interactions among functionally specialised neurons in distributed brain circuits," says the director.

So, there you have it – the signals from your nose connect to memories in an orchestrated symphony of signals in your brain. Each of these memories connects to a location, pinpointed on your inner map. 

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Read the Norwegian version of this article at forskning.no

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