Drug addicts
routinely throw away the chance of a better future for the
more immediate satisfaction delivered by a fix. This kind of
impulsive choice can be studied in animals, and now
researchers working with rats may have found a brain region
that plays a key role in spur-of-the-moment decisions. The
finding could help clarify human conditions as well, including
addiction.
The biological basis for self-control has become clearer in
recent years. Animal studies, as well as imaging studies in
people with impulsivity problems, have hinted at several brain
structures that might help people hold their horses in hopes
of a later, greater reward. The nucleus accumbens is one of
these, and it's also been shown to be critical for
experiencing pleasure.
Intrigued, Rudolf Cardinal of the University of Cambridge,
United Kingdom, and colleagues decided to zero in on the
nucleus accumbens as a potential center of impulsiveness. They
killed the cells of the nucleus accumbens by chemical
injection in one group of rats and compared those rats'
behavior to that of healthy ones. The researchers offered the
rats a choice of two levers. Pressing one delivered a single
sugar pellet immediately, and pressing the other delivered
four sugar pellets after a delay of up to 60 seconds. In a
study published online today by Science,
the team reports that about 50% of normal rats learned to
choose the large reward, but fewer than 25% of rats whose
nucleus accumbens had been destroyed would wait for the
jackpot.
The findings "provide some different angles on this whole
topic of impulsive choice," says neurologist John Evenden of
AstraZeneca in Wilmington, Delaware. But he points out that
impulsivity is expressed in many ways. It will be interesting
to look at these animals in situations that call for sustained
attention, he says, because lack of concentration is a more
common manifestation of impulsivity in humans.
--CAROLINE SEYDEL
Related site
Magnetic
resonance imaging showing the nucleus accumbens, from the
Whole Brain Atlas
