Science News - February 26, 2011

“These neurons were not registering whether something aversive was surprising,” Salzman says, suggesting they play a specialized role in rewarding surprises.

Other amygdala neurons did the opposite — registering surprise when it was aversive but not rewarding. By carrying two different sets of neurons for surprise, the amygdala may be better able to predict what’s coming in order to prepare appropriate actions, Salzman says.

“If you get something that’s highly rewarding by surprise, you want to be able to take actions in advance that would help you acquire that rewarding stimulus again,” he says. “By contrast, if you receive a bad surprise you normally would want to take some defensive actions or avoidance actions to avert a similar outcome.”

Ensuring a good hand

But the same stimulus, depending on the circumstances, can be rewarding or not, Salzman explains. He uses an example from the card game blackjack.

The object of the game is to hold cards with a total value as close as possible to 21, without going over. Players are initially dealt two cards and then can request additional cards.

Say a player is dealt two cards with a total value of 11 and then gets a 10 of hearts, worth 10 points, to reach a perfect 21. Now suppose that in the next hand, after the cards are shuffled, the same player is dealt two cards that total 15.

Cell-by-cell responses different nerve cells in the amygdala respond differently depending on the stimulus. a study in monkeys found that while one set of cells (a) responded more robustly to a reward, a sip of water, another (b) responded more to an annoying puff of air.

0123 4

Neuron set A response to stimuli Neuron set b response to stimuli

Reward trials Air-puff trials

Neural response

Listening in on nerve cell chatter,
on the fly. “If you’re an animal out there
and you’ve just learned something about
either a harmful stimulus or a predator,
you may not get a second chance.”
Salzman says, evidence that the amyg-
dala’s emotional response to the same
sensory stimulus can be flexible.

Murray sees such flexibility among monkeys making food choices in the lab.

Murray’s group is now using a simi-
lar method to see how values assigned
to various rewards can be linked with
actions. In a recent experiment, Sarah
Rhodes, also of the National Institute
of Mental Health, trained monkeys to
Given a choice between two
foods, monkeys (like most
people) will invariably reach
for their favorite. But after
gorging on a favored food
for a while, the reward value
of that treat diminishes, the
way the value of the 10 drops
when a blackjack player
already has 15 points in hand.

“I’m kind of
glad that the
brain isn’t as
simple as we
sometimes
make it out

perform two actions, tap and hold, to receive a reward. Six rapid taps to a touch-sensi-tive screen were rewarded with one kind of food, while a continuous touch for two seconds was rewarded with another.

to be.”

PEtEr rudEbECk

At that point the monkeys shift their choices and select a new food.

Monkeys without a working amygdala, however, show little change in behavior and continue to stuff themselves with the first food choice, Murray’s studies have found. These findings suggest that the amygdala is needed to revise the assessment of a reward’s value.

Neural response

Rhodes, Murray and their colleagues wanted to see if the animals, once filled with their favorite food, would shift their actions along with their choices. Normal monkeys did, choosing a new action once they got their fill of the first food.

Animals without an amygdala, however, didn’t make the switch.

“What we think is happening is that the amygdala is helping update the food value, and sending that information somewhere else — probably the orbitofrontal cortex,” Murray says. This same circuitry may also help animals, including humans, automatically calculate and update the value of goals or outcomes

Reward trials Air-puff trials

Those findings, reported in November in San Diego at the annual meeting of the Society for Neuroscience, suggest that the intact control animals engaged in goal-directed behavior, and that the amygdala played a role, Murray says.

“It means the animals literally know and somehow represent a goal. And that if the value of the goal changes, they can therefore change their behavior,” she says. “We think this is what humans do, so it’s really helpful to understand the neural circuitry.”

Reward trials Air-puff trials

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Another recent study shows that people, like monkeys, do indeed use the amygdala to judge the value of a particular item. Rick Jenison, who studies neuroeconomics at the University of Wisconsin–Madison, and his colleagues recruited three volunteers who were already undergoing a procedure that required electrodes to be implanted in the brain. Using the electrodes, the researchers eavesdropped on 51 amygdala nerve cells as the volunteers put price tags on a series of junk food items.