STRATEGIC OR RANDOM , HOW THE BRAIN CHOOSES
Many of the choices we
make are informed by experiences we've had in the past. But occasionally we're
better off abandoning those lessons and exploring a new situation unfettered by
past experiences. Scientists at the Howard Hughes Medical Institute's Janelia
Research Campus have shown that the brain can temporarily disconnect
information about past experience from decision-making circuits, thereby
triggering random behavior
In the study, rats
playing a game for a food reward usually acted strategically, but switched to
random behavior when they confronted a particularly unpredictable and
hard-to-beat competitor. The animals sometimes got stuck in a random-behavior
mode, but the researchers, led by Janelia lab head Alla Karpova and
postdoctoral fellow Gowan Tervo, found that they could restore normal behavior
by manipulating activity in a specific region of the brain. Because the
behavior of animals stuck in this random mode bears some resemblance to that of
patients affected by a psychological condition called learned helplessness, the
findingsmay help explain that condition and suggest strategies for treating it.
Karpova, Tervo and their colleagues published their findings in the September
25, 2014, issue of the journal Cell.
The brain excels at
integrating information from past experiences to guide decision-making in new
situations. But in certain circumstances, random behavior may be preferable. An
animal might have the best chance of avoiding a predator if it moves
unpredictably, for example. And in a new environment, unrestricted exploration
might make more sense than relying on an internal model developed elsewhere. So
scientists have long speculated that the brain may have a way to switch off the
influence of past experiences so that behavior can proceed randomly, Karpova
says. But others disagreed. "They argue that it's inefficient, and that it
would be at odds with what some people call one of the most central operating
principles of the brain -- to use our past experience and knowledge to optimize
behavioral choices," she notes.
Karpova and her
colleagues wanted to see if they could create a situation that would force
animals to switch into this random mode of behavior. "We tried to create a
setting that would push the need to create behavioral variability and unpredictability
to its extreme," she says. They did this by placing rats in a competitive
setting in which a computer-simulated competitor determined which of two holes
in a wall would provide a sugary reward. The virtual competitor, whose
sophistication was varied by the experimenters, analyzed the rats' behavior to
predict their future choices.
"We thought if we
came up with very sophisticated competitors, then the animals would eventually
be unable to figure out how to outcompete them, and be forced to either give up
or switch into this [random] mode, if such a mode exists," Karpova says.
And that's exactly what happened: When faced with a weak competitor, the
animals made strategic choices based on the outcomes of previous trials. But
when a sophisticated competitor made strong predictions, the rats ignored past
experience and made random selections in search of a reward.
Now that they had
evidence that the brain could generate both strategic and random behavior,
Karpova and her colleagues wanted to know how it switched between modes. Since
that switch determines whether or not an animal's internal model of the world
influences its behavior, the scientists suspected it might involve a brain
region called the anterior cingulate cortex, where that internal model is
likely encoded.
They found that they
could cause animals to switch between random and strategic behavior by
manipulating the level of a stress hormone called norepinephrine in the
anterior cingulate cortex. Increasing norepinephrine in the region activated
random behavior and suppressed the strategic mode. Inhibiting release of the
hormone had the opposite effect.
Karpova's team
observed that animals in their experiments sometimes continued to behave
randomly, even when such behavior was no longer advantageous. "If all
they've experienced is this really sophisticated competitor for several
sessions that thwarts their attempts at strategic, model-based
counter-prediction, they go into this [random mode], and they can get stuck in
it for quite some time after that competitor is gone," she says. This, she
says, resembles the condition of learned helplessness, in which strategic
decision-making is impaired following an experience in which a person finds
they are unable to control their environment.
The scientists could
release the animals from this "stuck" state by suppressing the
release of norepinephrine in the anterior cingulate cortex. "Just by
manipulating a single neuromodulatory input into one brain area, you can
dramatically enhance the strategic mode. The effect is strong enough to rescue
animals out of the random mode and successfully transform them into strategic
decision makers," Karpova says. "We think this might shed light on
what has gone wrong in conditions such as learned helplessness, and possibly
how we can help alleviate them."
Karpova says that now
that her team has uncovered a mechanism that switches the brain between random
and strategic behavior, she would like to understand how those behaviors are
controlled in more natural settings. "We normally try to use all of our
knowledge to think strategically, but sometimes we still need to explore,"
she says. In most cases, that probably means brief bouts of random behavior
during times when we are uncertain that past experience is relevant, followed
by a return to more strategic behavior -- a more subtle balance that Karpova
intends to investigate at the level of changes in activity in individual neural
circuits.
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