Functional imaging suggests that people with addictions may have difficulty making decisions based on long-term consequences versus impulsive short-term need
Functional magnetic resonance imaging suggests that people
with addictions may have cognitive difficulty making decisions based on long-term
consequences rather than impulsively choosing short-term fulfillment, according
to an article in the December issue of the Journal of Neuroscience.
"It's perhaps not just that people are slaves to pleasure,
but that they have trouble thinking through a decision," said Charlotte Boettiger,
PhD, an assistant professor of psychology at the University of North Carolina
at Chapel Hill, and lead author of the study.
"Our data suggest there may be a cognitive difference
in people with addictions," Boettiger said. "Their brains may not fully process
the long-term consequences of their choices. They may compute information less
efficiently."
The study also found that a variant of the COMT gene,
which controls the level of dopamine in the cortex, was associated with a tendency
to make impulsive decisions and with high activity in certain brain areas during
decision making.
"What's exciting about this study is that it suggests a new approach to therapy.
We might prescribe medications, such as those used to treat Parkinson's or early
Alzheimer's disease, or tailor cognitive therapy to improve executive function,"
said Boettiger, who led the study as scientist at the University of California,
San Francisco's Gallo Clinic and Research Center.
"The genetic findings raise the hopeful possibility that
treatments aimed at raising dopamine levels could be effective treatments for
some individuals with addictive disorders," said Howard Fields, MD, senior author
of the study.
Most addiction imaging studies have focused on brain
response to drug-related stimuli.
Boettiger used functional magnetic resonance imaging
(fMRI) to evaluate what happened when sober alcoholics and people in a non-alcoholic
control group made decisions between immediate and delayed rewards.
Boettiger recruited 24 subjects; 19 provided functional
imaging data (9, abstinent alcoholics; 10, no history of substance abuse). Another
five were included in the genotyping analysis.
At the research facility at the University of California,
Berkeley, the subjects were asked to decide between receiving a small monetary
award immediately or waiting for a larger payoff. The scenarios were hypothetical,
but the tasks measured rational thinking and impulsivity; sober alcoholics chose
the "now" reward almost three times more often than the control group, reflecting
more impulsive behavior.
While decisions were being made, imaging detected activity
in regions associated with decision making - the posterior parietal cortex, the
dorsal prefrontal cortex, the anterior temporal lobe and the orbital frontal cortex.
The dorsal prefrontal cortex and the parietal cortex
often form cooperative circuits, and this study found that high activity in both
was associated with a bias toward choosing immediate rewards.
The frontal and parietal cortex are also involved in
working memory - being able to hold data in mind over a short delay. When asked
to choose between $18 now or $20 in a month, the subjects had to calculate how
much that $18 (or what it could buy now) would be worth in a month and then compare
it to $20 and decide whether it would be worth the wait.
The parietal cortex and the dorsal prefrontal cortex
were much more active in people unwilling to wait. This could mean, Boettiger
said, that the area is working less efficiently in those people.
The COMT gene has two common variants with a single amino
acid difference at position 158; valine (Val) or methionine. The Val form of the
gene is associated with lower dopamine levels, and Boettiger's study showed that
people with two copies of the Val allele (resulting in the lowest dopamine levels)
had significantly higher frontal and parietal activity and chose now over later
significantly more often.
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