Spatial “replay” in neurons may help rats learn how to navigate toward goals : Study

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“replay” in neurons may help rats learn how to navigate toward goals : Study

the Brain’s Map : Coordinated Brain Activation Supports Spatial Learning and

October 30,
2019 • Specialized brain activation “replays” the possible routes that rats can
take as they navigate a space, helping them keep track of the paths they’ve
already taken and choose among the routes that they can take next, according to
a National Institutes of Health-funded study published in the journal Neuron.

reveal an internal ‘replay’ process in the brain

findings reveal an internal ‘replay’ process in the brain that allows animals
to learn from past experiences to form memories of paths leading toward goals,
and subsequently to recall these paths for planning future decisions,” said Shantanu
Ph.D., assistant professor at Brandeis University, Waltham,
Massachusetts, and senior author of the study. “These results help us better
understand how coordinated activation at the level of neurons can contribute to
the complex processes involved in learning and decision-making.”

The hippocampus,
a structure located in the middle of the brain, is critical to learning and
memory and contains specialized “place” cells that relay information about
location and orientation in space. These place cells show specific patterns of
activity during navigation that can be “replayed” later in forward or reverse
order, almost as if the brain were fast-forwarding or rewinding through routes
the rats have taken.

In previous
research, Jadhav and colleagues had discovered these replay events, marked by
bursts of neural activity called sharp-wave ripples, lead to coordinated
activity in the hippocampus and the prefrontal cortex, an area of the brain
just behind the forehead that is involved in decision-making.

But how
these forward and reverse replay events influence actual learning and
decision-making over time remained unclear. To find out, Jadhav and co-first
authors Justin D. Shin and Wenbo Tang continuously recorded the rats’ brain
activity as the rats learned how to navigate a special W-shaped maze over the
course of one day. This allowed the investigators to see how neural
representations changed as the rats were learning.

researchers trained the rats over eight sessions to follow paths according to
two rules – a simple rule and a complex rule – giving the rats a reward
whenever they reached the correct destination. The simple rule required
remembering the start and end locations of the maze paths. The complex rule
depended on working memory, requiring that the rats remember the previous path
in order to choose the next destination.

scientists focused their analyses on moments of transition when the rats had
paused in between completing one path and choosing the next one.

As the
researchers expected, replay events in the hippocampus showed reactivation of
past paths in a reverse order, as if on rewind, and showed reactivation of the
possible future paths in a forward order, as if on fast forward.

The forward
and reverse replay patterns were so robust that the researchers could use the
recordings to predict where the rats had paused in the W-shaped maze.
Continuous recordings of brain activity throughout the entire task revealed
shifts in activation patterns as the rats learned the simple rule. At different
stages of learning, the researchers could use reverse replay and forward replay
patterns to predict the path the animals had just taken and where they were
about to go next, respectively. These shifts indicated that reverse replay was
important for learning from the previous path, especially in the early stages
of learning, while forward replay was important for planning for the next
route, especially in the later stages of learning.

patterns related to learning the complex working-memory rule were more
consistent over time: Reverse replay events reactivated all possible past
choices and forward replay events reactivated all possible future options
throughout the learning process.

However, when
the researchers looked at coordination between replay events in the hippocampus
and the prefrontal cortex, they found that the coordinated reactivation in the
two brain areas was correlated with the rats’ actual choices – that is,
reactivation was stronger for replay of paths that the rats took than for the
paths they didn’t take.

the findings suggest that coordinated replay across the hippocampus and
prefrontal cortex serves an important function in spatial learning and
memory-guided decision-making. Specifically, the results suggest that reverse
replay is likely to support the ability to reflect on and evaluate paths that
have led to goals in the past, whereas forward replay seems to support the
ability to think ahead and plan choices that will lead to goals in the future.

“The involvement of ‘replay’ in memory processes has been observed across many species, including humans, and this study establishes that replay serves as a key neural substrate underlying an internal dialogue across multiple brain regions to support our ability to learn, plan, choose, and deduce,” Jadhav concluded.

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