New research in mice is carried by a variety of neuronal sets over time, suggesting memories of places that “drift” throughout the brain.
Historically, neuroscientists believed that memories of our direct environment locations and features were encoded by specific “location cells.” Cells at these locations in key memory centers known as the hippocampus will light up when the mammal enters a corresponding specific environment. For example, doors to houses on hiking trails and waterfalls. Activation of these location cells was thought to act as a kind of map in the brain by encoding the persistent memory of a particular location and allowing navigation.
“I went back to the 1960s and 1970s and basically thought of that. [spatial] “We’ve seen a lot of experience in the world,” said Daniel Dombeck, professor and chief investigator of neurobiology at Northwestern University.
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In 2013, a paper in the journal Nature Neuroscience sparked some controversy and “blew everyone’s minds off,” Dombeck said. This study employs a new technique to probe mouse hippocampal cells and reveals that representations of brain locations were not as consistent as once thought. Some cells were consistently reactivated when mice were returned to the maze over and over again, but overall, groups of active neurons fluctuated. These spatial representations changed over a few weeks of experiments rather than a static “mental map.”
This phenomenon has come to be known as “expressive drift in the hippocampus,” but the idea has come across several pushbacks. Some scientists have thought that changes in brain activity may actually be related to changes in the mouse environment. Perhaps the smell and sounds of the maze differed between rounds of experiments, or perhaps rodents moved the maze more slowly and more quickly.
In a new study published Wednesday (July 23) in the journal Nature, Dombeck and his team tried to control for these unruly variables.
Related: Can your brain run out of memory?
For each round of the experiment, mice were placed on a treadmill surrounded by screens. A treadmill, similar to a video game controller, served as a conduit for mice to explore the virtual maze. This was exactly the same every time. Teams can directly compare trials in which mice run the same speed, eliminating the variation.
Additionally, cones were placed over each roddent’s nose, pumped with the same scent on all round times, and white noise was played in the background to normalize the auditory landscape.
As mice navigate the virtual maze, researchers monitored the activity of their hippocampal cells in real time. They did this by opening a physical window into the brain and introducing substances that shine when brain cells are activated. This glow can then be monitored under a microscope. The setup does not limit the lifespan of lab mice, so Dombeck said that it allows experiments to be performed over and over again during the course of the research.
By having very strong control over the environment, “we were sure we were going to reduce this expression’s drift,” he told Live Science. “We were sure that memories would look more stable in a few days. That’s not something we found.”
The team observed that only a small subset of approximately 5% to 10% of the recorded cells behaved like traditional place cells and illuminated consistently in each round. These stable cells were also the most excitable overall. This means that there was a high chance that it would fire in response to the stimulus. In fact, teams can predict which cells are least likely to drift based on their level of excitement. On the other hand, less excitatory cells were much more likely to drift.
So why does this drift occur? “This may be a mechanism the brain uses to separate very similar experiences into individual memories. So, while you can go back to repeat locations, such as work, school, or your favorite park, but you can still distinguish between different visits of the mind.
In other words, drifting could be a way for the brain to track the passage of time, he said.
Dombeck suspects that this type of drift generally affects episode memories of certain personal experiences that took place at a particular place and era. Other types of memories, such as motor memories related to learned motor skills, may be expressed differently in the brain.
This study had several limitations. For one, the brain recording approach used in this study captured only a small fraction of the cells in the mouse hippocampus. It is probably 1% of hundreds of thousands of neurons. However, based on previous research, the team suspects that a similar process occurs throughout the hippocampus.
Furthermore, mouse studies are not guaranteed to be applied to humans. However, Dombeck said he expects the processes observed in this mouse study to be “very similar” to those unfolding in the human hippocampus. Because hippocampal cells become less excitatory with age, memory may deteriorate with age, as these few stable cells at the heart of memory lose their excitability, Dombeck suggested.
“If we can somehow fine-tune the excitability of a neuron, or maintain that excitability over time, we can probably maintain the memory,” Dombeck speculated. However, that idea should be backed up with further research.
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