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Impulsive, Weak-Willed or Just Too Much Dopamine? Brain Study Highlights
Role of Dopamine in Impulsive Behavior

ScienceDaily (June 29, 2010) � It's a common scenario: you're on a diet,
determined to give up eating cakes, but as you pass the cake counter,
all resolve disappears� Now, scientists at the Wellcome Trust Centre for
Neuroimaging at UCL (University College London) have shed light on the
brain processes that affect our will power and make us act impulsively.

In a study published in the Journal of Neuroscience, funded mainly by
the Wellcome Trust, researchers led by Professor Ray Dolan have shown
that increased levels of dopamine -- a chemical in the brain involved in
mediating reward, motivation, and learning through reinforcement, --
make us more likely to opt for instant gratification, rather than
waiting for a more beneficial reward.

The research may help explain why people affected by conditions such as
Attention Deficit/Hyperactivity Disorder (ADHD), characterised by high
levels of dopamine in the brain, tend to show extremely impulsive
behaviour. Similarly, it highlights why such behaviour can be a
potential negative side-effect of L-dopa, a drug used to help alleviate
the symptoms of Parkinson's disease.

To test the effect of dopamine on decision-making, Professor Ray Dolan
and colleagues carried out a test with 14 healthy volunteers under two
conditions: once when given a small (150mg) dose of L-dopa, once when
given a placebo. Under each condition, the subjects were asked to make a
number of choices consisting of either a 'smaller, sooner' option, for
example receiving �15 in two weeks, or a 'larger, later' option, such as
receiving �57 in six months.

"Every day we are faced with decisions that offer either instant
gratification or longer-term, but more significant reward," explains Dr
Alex Pine, first author of the study. "Do you buy your new iPhone today
or wait six months till the price comes down? Do you diet or eat that
delicious-looking cake? Do you get out your books to study for a future
exam or watch some more TV?"

The researchers found that every subject was more likely to behave more
impulsively -- choosing the 'smaller, sooner' option -- when levels of
dopamine in the brain were boosted. . On the whole, the number of sooner
options chosen increased by almost a third, although each subject varied
on this measure.

Dr Pine believes that this finding may also explain why we tend to
behave more impulsively when influenced by external 'cues'.

"We know that sensory inputs -- sights, sounds smells and anticipation
of rewards, or even of neutral cues which have been associated with
rewards -- momentarily boost dopamine levels in our brains, and our
research shows that higher dopamine levels make us act more
impulsively," he says.

"But this research is important for more than just explaining our day to
day lapses in self-control. It also helps us understand why disorders
which are associated with abnormal dopamine functioning can also lead to
extremely impulsive behaviour."

The researchers also tested the subjects under the influence of small
doses of haloperidol, a dopamine suppressant; however, the results were
inconclusive, showing little difference from the effect of the placebo.
Dr Pine cautions against the idea that dopamine suppressants might be
used to help combat impulsivity and addiction.

"Dopamine plays a wide role in the brain, from movement through to
cognition," he explains. "Lowering dopamine levels may be able to reduce
impulsivity, but we need to be certain that this didn't come at the
expense of other, important functions."

The test was conducted whilst the subjects were in a functional magnetic
resonance imaging (fMRI) scanner, which looks at activity in the brain
by measuring changes in blood flow. A network of brain regions,
including the striatum and prefrontal cortex, tends to be more active
when considering a sooner versus a more delayed reward. The researchers
showed that this differential activity was more magnified after the
subjects were given L-dopa.

They also found that greater individual susceptibility to the influence
of the drug was associated with an increase in activity in the brain
region known as the amygdala when volunteers made choices. The amygdala
is known to play a role in processing emotions, which affects
decision-making, though the mechanism of this influence it is not yet
fully clear.


Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily
staff) from materials provided by Wellcome Trust, via EurekAlert!, a
service of AAAS.


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