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Putting Distinct Memories of Similar Events in Their Place

Summary: A new study reports, contrary to popular belief, place cells in the dentate gyrus do not remap. Instead, memory discrimination is controlled by increased co-firing of place cells and the neurons that organize which place cells discharge.

Source: NYU.

Neuroscientists have found new evidence on how distinct memories of similar events are represented in the brain.

Its findings, which appear in the journal Neuron, correct a previous misconception of how such memories are stored in the hippocampus–a part of the brain crucial for memory and understanding space.

“Previous research suggested that brain cells were ‘re-mapped’ in making distinctions between memories of similar and distinct experiences,” says André Fenton, a professor in New York University’s Center for Neural Science and the senior author of the paper. “However, it’s clear from our results that neuron activity is, in fact, synchronous–like a flock of starlings that takes on different formations while still maintaining cohesion as a flock.

“These new findings provide strong evidence in favor of a fresh, dynamic conceptualization of how neurons signal information.”

The study was co-authored with Milenna Tamara van Dijk, a doctoral student at NYU’s Langone Medical Center, working in the Center for Neural Science.

It’s been established that the electrical discharge of hippocampus cells signals places; each “place cell” contributes to a neural map-like representation of space by discharging only when we are in discrete parts of an environment called the cell’s “place field.” As we move from place to place through a space, one set of discharging place cells ceases to fire while another set starts to discharge and then ceases as we move away while the next set discharges, and so on to trace out the real-world path in neural activity such that the same particular sets of place cells are active whenever we return to the same places. Scientists John O’Keefe, May-Britt Moser, and Edvard Moser were awarded the 2014 Nobel Prize in Physiology or Medicine for their discovery of these and related cells.

While place fields have aided neuroscientists’ understanding of how the brain represents memories and information, a specific question remains unanswered: how do brains learn and discriminate between similar and distinct experiences of similar things?

For instance, consider a commonplace experience such as parking your car in a familiar garage on different days of the week. Sometimes you remember parking in the same parking spot and other times you remember parking in different parking spots. How does the hippocampus discriminate between multiple memories, storing some as the same and others as different?

The prevailing view is that distinct memories are signaled by distinctive neural activity in a part of the hippocampus, dentate gyrus. Prior work has shown that a given set of dentate gyrus place cells readily change the relative locations of their firing fields in different environments, so if the fields of two cells overlapped in one environment, they would probably not overlap in the other, and vice versa. This “remapping” is assumed to underlie distinctive memories. However, dentate place cell remapping was never tested during a rigorous memory discrimination task.

In the Neuron study, the researchers explored this dynamic through a series of memory tests using mice.

Their results showed that, in fact, dentate place cells did not remap; their place fields were constant. Moreover, instead of remapping, memory discrimination was controlled by increases in the co-firing of dentate place cells and the neurons that organize which place cells discharge synchronously and asynchronously.

dentate gyrus

In other words, the firing of dentate place cells occurs globally, but are timed in different ways in order to express distinctive memories–instead of changing where cells fire overall. NeuroscienceNews.com image is in the public domain.

In other words, the firing of dentate place cells occurs globally, but are timed in different ways in order to express distinctive memories–instead of changing where cells fire overall.

“Different flock patterns occur when just a small number of starlings change course, with each starling maintaining strongly correlated movements with its nearest neighbors,” explains Fenton, who also holds appointments at the Neuroscience Institute at NYU Langone Medical Center and SUNY Downstate. “Similarly, differences in the timing of the co-firing of neurons can signal differences in memory formation–all within a globally maintained spatial tuning and correlation structure.”

About this neuroscience research article

Funding: The research was funded by grant from the National Institute of Aging, part of the National Institutes of Health (R01AG043688).

Source: James Devitt – NYU
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is in the public domain.
Original Research: Abstract for “On How the Dentate Gyrus Contributes to Memory Discrimination” by Milenna Tamara van Dijk, and André Antonio Fenton in Neuron. Published May 3 2018.
doi:10.1016/j.neuron.2018.04.018

Cite This NeuroscienceNews.com Article
NYU”Putting Distinct Memories of Similar Events in Their Place.” NeuroscienceNews. NeuroscienceNews, 11 May 2018.
<http://neurosciencenews.com/distinct-memory-categorizationi-9035/>.
NYU(2018, May 11). Putting Distinct Memories of Similar Events in Their Place. NeuroscienceNews. Retrieved May 11, 2018 from http://neurosciencenews.com/distinct-memory-categorizationi-9035/
NYU”Putting Distinct Memories of Similar Events in Their Place.” http://neurosciencenews.com/distinct-memory-categorizationi-9035/ (accessed May 11, 2018).

Abstract

On How the Dentate Gyrus Contributes to Memory Discrimination

Highlights
•Dentate gyrus-dependent memory discrimination does not require place cell remapping
•Dentate neural correlates of memory discrimination are transient, lasting seconds
•Sub-second dentate network discharge correlations signal memory discrimination
•Dentate excitatory-inhibitory coupling is increased at memory discrimination sites

Summary
The dentate gyrus (DG) is crucial for behaviorally discriminating similar spatial memories, predicting that DG place cells change (“remap”) their relative spatial tuning (“place fields”) for memory discrimination. This prediction was never tested, although DG place cells remap across similar environments without memory tasks. We confirm this prior finding but find that DG place fields do not remap across spatial tasks that require DG-dependent memory discrimination. Instead of remapping, place-discriminating discharge is observed transiently among DG place cells, particularly when memory discrimination is most necessary. The DG network may signal memory discrimination by expressing distinctive sub-second network patterns of co-firing at memory discrimination sites. This involves increased coupling of discharge from place cells and interneurons, as was observed during successful, but not failed, behavioral expression of memory discrimination. Instead of remapping, these findings indicate that memory discrimination is signaled by sub-second patterns of correlated discharge within the dentate network.

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