similar to the Wisconsin Card Sort Test, the researchers
presented monkeys with two icons on a computer screen each
showing a line and a shape, and the monkey had to touch one
of the icons and avoid touching the other one. However, the
“rule” was then changed so that the icon that was previously
wrong became correct and vice versa.
Following small lesions in the orbitofrontal cortex, the
monkeys did not adapt to the new rule, and persisted in
choosing the formerly rewarded icon, the researchers found.
This tendency struck Robbins and his colleagues as similar
to that of people with obsessive-compulsive disorder. So, in a
study in press in Neuroimage, they compared the richness of the
orbitofrontal cortex connections in the brains of people with
OCD and those with a history of stimulant use, and found that
both groups had reduced connectivity in the ventromedial and
superior areas of the orbitofrontal cortex.
“In both cases, this reduction in connectivity was negatively
correlated with compulsive behavior,” said Robbins. “The more
compulsive they were, the less connectivity they had.”
In another study, in press in Biological Psychiatry, Robbins
and his colleagues showed that activity in the striatum, a brain
region connected to the orbitofrontal cortex, was reduced
during “reversal learning” in stimulant-dependent individuals
and was also related to their cognitive inflexibility. Researchers
ameliorated this loss of activity and cognitive inflexibility by
giving stimulant-dependent participants pramipexole, which
acts like dopamine in the brain and is often used to control the
symptoms of Parkinson’s disease.
People who are more impulsive to begin with are probably
more likely to try drugs, said Robbins. Stimulants, however,
seem to amplify that trait, according to a study he conducted
with colleagues published in Biological Psychiatry (Vol. 68,
No. 8). They found that non-drug abusing siblings of cocaine
addicts tended to score higher than average on the Barratt
Impulsiveness Scale, agreeing with statements such as “I do
things without thinking” and “I change jobs often.” The
cocaine-addicted study participants scored even higher.
Robbins and his colleagues further tested chronic stimulant
users’ and their siblings’ impulsive tendencies by giving them
the stop signal reaction time test — a game where a computer
prompts you to press a particular key, but as soon as you get into
the rhythm of pressing it, a beep sounds, telling you to cancel
your response. Both stimulant drug users and their siblings were
much slower to respond to the beep than the control group.
“Their siblings clearly have some sort of predisposition to
impulsivity parallel to their drug-taking siblings, which means
that the impulsivity can’t simply be a result of drug abuse and is
a personality trait which may you vulnerable to stimulant abuse.
Of course, abusing the drug probably makes things worse,
possibly through toxic effects in the prefrontal cortex circuits
that mediate self-control,” Robbins said.
Taken together, the findings suggest that stimulant drugs
may produce changes in people’s prefrontal cortices that
make them more likely to continue using drugs, said Robbins.
However, new medications may help people at the early stages
of addiction regain control of their lives. n