Representational Drift of Hippocampal Neurons Alters Spatial Reminiscence in Mice

We usually consider reminiscence as secure—a psychological archive that shops experiences in neat, retrievable recordsdata. But what if these recordsdata quietly shift positions, even when the unique expertise hasn’t modified? A brand new examine titled “Hippocampal representations drift in stable multisensory environments,” revealed in Nature, means that spatial reminiscence is extra dynamic than beforehand thought. Researchers at Northwestern University discovered that hippocampal neurons—particularly, CA1 place cells—replace their exercise patterns day-to-day, even when an animal is uncovered to the very same setting.

“Our study confirms that spatial memories in the brain aren’t stable and fixed,” stated senior creator Daniel Dombeck, PhD, professor of neurobiology at Northwestern University. “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.”

To take a look at whether or not this “representational drift” was intrinsic or formed by delicate adjustments in conduct or sensory enter, the researchers developed a tightly managed experimental setup. Mice have been head-fixed on a cylindrical treadmill and ran by means of a multisensory digital actuality (VR) monitor with visible cues held fixed throughout classes. They additionally obtained a gradual airstream of α-pinene by means of a nostril cone to get rid of olfactory variability—an necessary management, given mice’s excessive sensitivity to scent. White noise was used to masks ambient sound.

Using two-photon imaging, the staff tracked the exercise of round 1,678 CA1 pyramidal cells throughout 5 consecutive days. The setting and conduct have been extremely reproducible: mice ran at constant speeds and exhibited robust anticipatory licking earlier than reaching a digital reward web site. Even so, the spatial tuning of particular person place cells shifted over time. Only 41.9% of cells that fired in a selected location on day one did so once more by day 5.

To discover what is perhaps driving this drift, the researchers analyzed intrinsic properties of the neurons themselves. They discovered that extra excitable neurons have been considerably extra more likely to preserve secure spatial representations throughout days.

These findings problem the concept hippocampal spatial codes are mounted over time, even beneath secure exterior situations. Instead, the outcomes recommend that reminiscence representations could also be inherently fluid, maybe enabling the mind to differentiate between repeated experiences or encode time as a part of reminiscence itself.

The examine additionally raises new questions on how age-related adjustments in neuronal excitability may contribute to reminiscence decline. “Some neurons do seem to be better at holding onto the original memory than others,” Dombeck stated. “Really excitable neurons appear to retailer reminiscences the perfect. The ones that fireplace extra weakly are those that find yourself altering. As excitability decreases with age, so too may the mind’s capacity to retain constant spatial maps—a speculation the staff hopes to discover additional.

“Even if you have the exact same experience, it has to be occurring at a different time,” Dombeck stated. “If I hike the identical path twice, and it’s an identical each instances, I in all probability nonetheless need to do not forget that I did the identical 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,” stated Dombeck. “That gives us access to memories of those individual experiences.”

Ultimately, the examine means that reminiscence isn’t static. Instead, just like the paths we stroll every day, it’s dynamic, reshaped frequently by the neurons tasked with remembering it.