Recollections drift throughout neurons over time

The mind’s inner GPS adjustments every time we navigate a well-recognized, static atmosphere, Northwestern University neurobiologists present in a examine published in Nature.

This implies that if somebody walks the identical path each day — and the trail and surrounding circumstances stay equivalent — every stroll nonetheless prompts totally different “map-making” mind cells, or neurons. 

Not solely does this discovery make clear the basic thriller of how the mind processes and shops spatial recollections, nevertheless it additionally might have profound implications for scientists’ understanding of reminiscence, studying and even growing older.

“Our study confirms that spatial memories in the brain aren’t stable and fixed,” mentioned Northwestern’s Daniel Dombeck, the examine’s senior writer. “You can’t point to one group of neurons in the brain and say: ‘That memory is stored right there.’ Instead, we’re finding that memories are passed among neurons. The exact same experience will involve different neurons every time. It’s not a sudden change, but it slowly evolves.”

Dombeck is a professor of neurobiology and the Wender-Lewis Teaching and Research Professor at Northwestern’s Weinberg College of Arts and Sciences. The examine was a collaboration amongst Dombeck and three members of his laboratory: Jason Climer, Heydar Davoudi and Jun Young Oh. Climer, who is without doubt one of the examine’s co-first authors, is now an assistant professor of molecular and built-in physiology on the University of Illinois, Urbana-Champaign.

A reminiscence thriller

Located deep inside the mind’s temporal lobe, the hippocampus shops recollections associated to spatial navigation. For many years, neurobiologists thought the identical hippocampal neurons encoded recollections of the identical locations. In different phrases, the trail somebody may take from their bed room to their kitchen ought to activate the very same sequence of neurons throughout every midnight stroll for a glass of water. 

About 10 years in the past, nonetheless, scientists imaged mice’s brains as they ran via a maze. Even because the mice ran via the identical maze day after day, totally different neurons fired throughout every run. Scientists questioned if the outcomes had been a fluke.

“People in the field started to wonder if the mice were truly having the same experience during each run through the maze,” Dombeck mentioned. “Maybe they run faster on some days. Maybe the smells change from day to day. Maybe there are subtle, unavoidable environmental or behavioral differences that change the overall experience.”

‘We controlled for everything we possibly could’

To probe these questions, Dombeck and his staff designed an experiment that gave them unprecedented management over the mice’s sensory enter. First, the staff employed a cutting-edge multisensory digital actuality system — previously developed in Dombeck’s laboratory — to ensure the animals’ skilled equivalent visible cues. Then, the mice ran via the digital maze on treadmills, guaranteeing exact measurement of velocity. Finally, the scientists put cones on the mice’s noses to offer equivalent smells for each session.

After working the experiment a number of occasions, the outcomes had been clear. Even in a extremely reproducible digital world, the encoded neurons nonetheless drifted. The discovering confirmed that the mind’s spatial maps are inherently dynamic, continually updating no matter how static an area is likely to be.

“We controlled for everything we possibly could,” Dombeck mentioned. “I was convinced we were going to get the opposite result and show that memories really are identical for the same space. But it turns out, they are not. A slightly different group of neurons activated each time.”

“This evidence suggests that memories are fluid. This could be related to deeper questions of why the brain can do things modern AI struggles with, things like learning new things continuously,” Climer mentioned. “It also may play a role in natural forgetting — an active process, often overlooked, but essential for healthy memory function.” 

Implications for growing older

Although few patterns arose all through the course of the experiment, Dombeck and his staff did discover one constant issue. The most excitable neurons, which had been extra simply activated, maintained extra steady spatial recollections all through a number of runs via the digital maze. Because neuron excitability decreases with age, the discovering might assist scientists perceive the position of growing older because it pertains to the mind’s skill to encode new recollections.

“Some neurons do seem to be better at holding onto the original memory than others,” Dombeck mentioned. “Really excitable neurons seem to store memories the best. The ones that fire more weakly are the ones that end up changing. So there does seem to be some small component of the original memory that’s still there in this small fraction of neurons.”

Dombeck and his staff are nonetheless pondering why the activated neurons change although the area stays precisely the identical. Although he’s nonetheless uncertain, Dombeck mentioned the rationale is likely to be associated to time.

“Even if you have the exact same experience, it has to be occurring at a different time,” Dombeck mentioned. “If I hike the same path twice, and it’s identical both times, I probably still want to remember that I did the same hike twice. It’s possible that the brain forces us to take very similar experiences that occur at different times and remember them in slightly different ways. That gives us access to memories of those individual experiences.”

Notes

The examine was supported by the National Institutes of Health.