|Livescience.com, 12-02-2006, door Ker Than
Mind Rewind: Brains Run in Reverse
When faced with a new learning task, our brains replay events in reverse, much
like a video on rewind, a new study suggests.
This type of reverse-replay is also used in artificial intelligence research to
help computers make decisions. The finding could explain why we learn tasks more
easily if we take frequent study breaks: the pauses between sessions give our
brains time to review information.
The finding was detailed in a Feb. 12 online issue of the journal Nature.
The researchers measured brain activity in rats as the animals ran back and
forth on a linear track. Specifically, they monitored a brain region called the
hippocampus, which is known to be important for memory and navigation in both
rats and in humans.
When the rats completed a lap, they were given a food reward. After eating, the
animals would pause briefly before starting another lap. Outwardly, the rats
didn't seem to be doing much during these rest periods. They would fidget, groom
or remain still. The brain recordings told a different story, however. During
times of rest, a rat's hippocampus was a hotbed of activity.
As the rodents ran up and down the track, hippocampal cells fired in certain
patterns. This sequence of firing repeated when the animals rested, but in
reverse order. The reverse-replays were repeated several times; each replay took
only a few hundred milliseconds.
"In that compressed time, the rat is replaying the entire track from where it
currently is all the way back to the very beginning," said study team-member
David Foster from the Massachusetts Institute of Technology. "This result
suggests that the immediate experience is actually recapitulated several times.
The processing going on outside of the original experience may be important for
The finding could help explain how rats solve something called the "temporal
credit assignment problem." And because the hippocampus in rats and humans
perform many of the same functions, the current study suggests that our brains
may work in the same way.
The problem, a classic dilemma in decision-making theory, is this: If an animal
has to perform a sequence of actions before it can get a reward, how does it
know which actions were ultimately important and which weren't? Actions
performed right before the reward was obtained are easy to identify as
important, but what about actions performed at the beginning of the sequence?
Which of those were important?
Richard Sutton, a computer scientist at the University of Alberta, Canada who
was not involved in the study, likens the problem to playing backgammon for the
"How do you evaluate the opening move if you don't know how to play yet?" he
In the fields of computer science and artificial intelligence, the temporal
credit assignment problem is solved by having the machines work backward,
replaying events in reverse and assigning more credit to actions near the end of
a sequence than to those at the beginning.
"You know that the final move was the right thing to do, so you can send that
information back through the set of actions that were taken leading up to the
final state," Foster said in a phone interview.
If reverse replay also takes place in humans, it could explain why cramming
hours before a test doesn't typically work. The new finding suggests that our
brains learn best when there are frequent pauses between study sessions; during
these breaks, our brains unconsciously reviews the new information several times,
making it easier to commit to memory when the time comes.
How reverse replay leads to learning
Scientists have long known that the release of the chemical molecule dopamine is
an important part of the brain's reward system. The release of this
neurotransmitter floods us with feelings of joy and motivates us to perform
When this knowledge is paired with the new suggestion that our brains may replay
new experiences in reverse, a possible mechanism for learning emerges, Foster
The researchers hypothesize the existence of a special "value area" of the brain
where dopamine signals and reverse-replay signals are fed become paired together.
If the dopamine signal is one that decays over time, meaning that it is stronger
at the beginning of transmission than at the end, then the following might
As a reverse replay signal plays out in the brain's value area, it is associated
with the beginning of a strong dopamine signal; as the replay continues, the
dopamine signal becomes weaker. In this scenario, actions taken near the
beginning of a reverse replay event will be more important to an organism than
actions taken later.
Hints in psychology
Sutton said he would not be surprised if reverse replay occurred in animals as
well as machines. If anything, he said, this mechanism had long been suspected
from early psychological experiments, such as Ivan Pavlov's classical
conditioning experiments with dogs.
"Pavlov rang the bell and gave the dog the steak and after a while, just ringing
the bell was rewarding," Sutton told LiveScience. "So somehow it worked backward
from the steak to the bell."
Foster agrees, but added that the current study suggests we make trains of
associations going much further back than previously thought.
"It's taking the animals several seconds to run around, so this replay could be
sending that information back through several stages and rewarding a long
sequence of actions," Foster said. "It's that long sequence that is new."
The current study looked specifically at spatial learning; however, in rats, and
probably in humans too, the hippocampus is involved in other types of learning
"So [reverse replay] could very well be a mechanism to deal with a broad variety
of information, not just spatial," Foster said.
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