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When your body gets something it wants,
a pleasure center near the front of the brain buzzes with activity.
Now, psychologists have found that this reward region responds more
strongly when the pleasurable stimuli it encounters are
unpredictable. The results, reported in the 15 April issue of the
Journal of Neuroscience, may lead to a better understanding
of addiction.
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Pop goes the nucleus accumbens. The brain's
pleasure center responds to the unexpected.
CREDIT: PHOTODISC |
A key part of this reward circuitry is the nucleus accumbens, a
patch of tissue in the forebrain about the size of an almond. When
it is experimentally removed from the brains of drug-addicted
animals, their cravings cease. In the late 1980s, researchers
studying this pathway in monkeys found that it responded more
strongly to unexpected, rather than predictable, stimuli. Other
brain-imaging studies showed that the nucleus accumbens is active
when humans receive a reward, whether drugs, money, or just plain
sugar--but before the new study by Gregory Berns of Emory University
and E. Read Montague of the Baylor College of Medicine in Houston,
Texas, the role of surprise in activating the region in humans was
unknown.
Berns and Montague examined neural activity in 25 people as they
were fed 28 teaspoon-sized hits of Kool-Aid and water. Sometimes the
slurps of sweet and tasteless liquid were delivered in a simple
alternating pattern. Other times, the Kool-Aid came at random
intervals that were impossible to predict. Most of the subjects
didn't report noticing a difference between the predictable and
unpredictable sequences. But functional magnetic resonance imaging
revealed that their brains loved the randomness: Only the
unpredictable sequences strongly activated the nucleus accumbens and
nearby regions of the brain.
The finding "fits the animal work precisely" says Kent Berridge,
a University of Michigan psychologist specializing in reward.
Berridge says the next step is examining how the accumbens responds
to nonpleasurable cues associated with pleasurable stimuli--like the
bell Pavlov rang before feeding his dogs--a question key to the
understanding of addiction.
--JOSH GEWOLB
Related sites
Berns lab home
page
Montague lab
home page
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