Researchers at MIT's Picower Institute for Learning and
Memory have found that rats use a mental instant replay of their actions to
help them decide what to do next, shedding new light on how animals and humans
learn and remember.
The work will appear in the Aug. 27 issue of the journal
"By understanding how thoughts and memories are
structured, we can gain insight into how they might be disrupted in diseases
and disorders of memory and thought such as Alzheimer's and
schizophrenia," said study author Matthew A. Wilson, the Sherman Fairchild
Professor of Neuroscience at the Picower Institute. "This understanding
may lead to new methods of diagnosis and treatment."
Wilson's laboratory explores how rats form and recall
memories by recording - with an
unprecedented level of accuracy - the activity of single neurons in the
hippocampus while the animal is performing tasks, pausing between actions and
sleeping. The hippocampus is the seahorse-shaped brain region researchers
believe to be critical for learning and memory.
Wilson's previous work has shown that after the animals run a maze, their brains "replay" during sleep the sequence of events
they experienced while awake. Researchers believe this process is key to
sleep-reinforced memory consolidation in both animals and humans.
The latest study shows that these sequences also occur
when the animals are awake and may help them decide what to do next.
When a rat moves through a maze, certain neurons called
"place cells," which respond to the animal's physical environment,
fire in patterns and sequences unique to different locations. By looking at the
patterns of firing cells, researchers can tell which part of the maze the
animal is running.
While the rat is awake but standing still in the maze,
its neurons fire in the same pattern of activity that occurred while it was
running. The mental replay of sequences of the animals' experience occurs in
both forward and reverse time order.
"This may be the rat equivalent of 'thinking,'"
Wilson said. "This thinking process looks very much like the reactivation
of memory that we see during non-REM dream states, consisting of bursts of
time-compressed memory sequences lasting a fraction of a second.
"So, thinking and dreaming may share the same memory
reactivation mechanisms," he said.
Memory's building blocks
"This study brings together concepts related to
thought, memory and dreams that all potentially arise from a unified mechanism
rooted in the hippocampus," said co-author Fabian Kloosterman, senior
The team's results show that long experiences, which in
reality could have taken tens of seconds or minutes, are replayed in only a
fraction of a second. To do this, the brain links together smaller pieces to
construct the memory of the long experience.
The researchers speculated that this strategy could help
different areas of the brain share information - and deal with multiple
memories that may share content - in a flexible and efficient way. "These
results suggest that extended replay is composed of chains of shorter
subsequences, which may reflect a strategy for the storage and flexible
expression of memories of prolonged experience," Wilson said.
Moreover, by comparing the content of the replay with the
rat's physical location on the track and his actual behavior immediately before
and after the replay event the researchers could tell the rat was not just
thinking about his most recent experience but also about other options, such
as: "What if I turned around and went back the way I came?" or
"How would I get here if my starting point is at a distant location?"
This suggests that the same brain mechanisms come into
play to remember the past and consider future actions, reinforcing recent work
by neuroscientists outside of MIT who determined that in humans, cognitive
processes related to episodic recall and evaluation of future events overlap to
a high degree.
Memory formation and future planning are among the
cognitive functions ravaged by diseases such as Alzheimer's disease,
schizophrenia and psychosis.
"A better understanding of how we use memories, not
only to learn from past experiences but also to explore our future options, can
give us insights into how the system fails under these disease
conditions," Kloosterman said.
The MIT researchers plan to further explore the link
between awake replay and cognition in animals engaged in more cognitively
demanding tasks such as those involving multiple choices, where the rat has to
make a decision ("do I go left or right?") based on a prior learned
In addition to Wilson, the study was led jointly by
Kloosterman and MIT brain and cognitive sciences graduate student Thomas J.
This research was supported by National Institutes of