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Memories of locations “drift” throughout the mind as they’re carried by completely different units of neurons over time, a brand new examine in mice suggests.
Historically, neuroscientists thought that reminiscences of places and options of our fast setting had been encoded by particular “place cells.” These place cells, positioned in a key reminiscence heart referred to as the hippocampus, gentle up when a mammal enters the precise setting they correspond to — say, the door to a house or a waterfall on a mountaineering path. It was thought that the activation of those place cells acted as a sort of map within the mind by encoding lasting reminiscences of particular locations in addition to enabling navigation.
“Going back to the 1960s and 1970s, we basically thought that [spatial] memories were encoded by specific neurons in the brain,” mentioned senior examine creator Daniel Dombeck, a professor and principal investigator of neurobiology at Northwestern University. “That was the thought for probably 30, 40 years — until about 10 years ago.”
In 2013, a paper within the journal Nature Neuroscience stirred some controversy and “blew everyone’s mind,” Dombeck mentioned. The examine employed newer methods to probe cells within the mouse hippocampus, revealing that the mind’s illustration of locations wasn’t practically as constant as as soon as thought. Some cells persistently reactivated when mice had been returned to a maze time and again, however total, the group of lively neurons fluctuated. Rather than being a static “mental map,” these spatial representations modified over the weeks-long experiment.
This phenomenon got here to be often known as “hippocampal representational drift,” however the thought met some pushback. Some scientists questioned whether or not the shifts in mind exercise had been really associated to modifications within the mice’s environments — maybe the smells or sounds within the maze differed between rounds of the experiment, or the rodents moved by means of the maze extra slowly or shortly in a given spherical.
In their new examine, printed Wednesday (July 23) within the journal Nature, Dombeck and his crew got down to management these unruly variables, they usually did so using virtual reality and a tiny treadmill.
Related: Can your mind run out of reminiscence?
In every spherical of the experiment, the mice had been positioned on a treadmill surrounded by screens. Akin to a online game controller, the treadmill acted as a conduit for the mice to discover a digital maze, which was precisely the identical each time. The crew might then immediately evaluate the trials the place the mice ran the identical pace, thus eliminating that variability.
Additionally, a cone was positioned over every rodent’s nostril to pump in the identical scent throughout each spherical, and white noise was performed within the background to normalize the auditory panorama.
As the mice navigated the digital maze, the researchers monitored the exercise of their hippocampal cells in actual time. They did this by opening a bodily window into the mind and introducing a substance that glowed when mind cells had been activated. They might then monitor this glow below a microscope. This setup does not restrict the longevity of the lab mice, so they may run the experiment time and again over the course of the examine, Dombeck famous.
By controlling the setting so tightly, “I was sure we were going to reduce this representational drift,” he informed Live Science. “I was sure that the memory was going to look more stable over days — and that’s not what we found.”
The crew noticed that solely a small subset of cells — round 5% to 10% of these recorded — behaved like standard place cells, lighting up persistently in every spherical. These steady cells had been additionally essentially the most excitable total, that means they had been extra prone to hearth in response to a stimulus. In truth, the crew might predict which cells had been least prone to drift based mostly on their degree of excitability. Meanwhile, the less-excitable cells had been far more susceptible to drift.
So why does this drift happen? “It might be a mechanism that the brain uses to separate highly similar experiences into discrete individual memories so that we could access them separately later,” Dombeck prompt. So though you may return to a spot repeatedly — work, faculty or a favourite park — you possibly can nonetheless distinguish the completely different visits in your thoughts.
In different phrases, the drift could also be a manner for the mind to trace the passage of time, he mentioned.
Dombeck suspects this sort of drift impacts episodic memories, generally, that are about particular private experiences that befell at specific places and occasions. Other forms of reminiscence — resembling motor memories, about realized motion abilities — could also be represented in a different way within the mind.
The examine had just a few limitations. For one, the brain-recording strategy used within the examine captured solely a fraction of the cells within the mouse hippocampus — possibly 1% of its tons of of 1000’s of neurons. But based mostly on previous research, the crew suspects related processes are occurring throughout the hippocampus.
Additionally, analysis in mice is not assured to use to people. But Dombeck mentioned he would count on the processes noticed on this mouse examine to be “fairly similar” to these unfolding within the human hippocampus. Because cells of the hippocampus change into much less excitable with age, it may very well be that reminiscence worsens with age partly as a result of these few steady cells on the core of our reminiscences lose excitability, Dombeck prompt.
“If we could somehow tweak the excitability of our neurons or maintain that excitability over time, we could probably maintain memory,” Dombeck speculated. But that concept will must be backed up with additional analysis.
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