PhD, professor of psychology
and neuroscience at the
University of Michigan.
“One important target for
may be the development of
effective strategies to cope
with the insidious effects of
Stuck on signals
Years ago, when Rutgers
University psychologist Arthur
Tomie, PhD, was studying
classical conditioning in his
lab rats, he noticed something
strange. Just as in Ivan
Pavlov’s famous conditioning
experiments, Tomie’s model
involved a first stimulus that
signaled the occurrence of
a second stimulus — in this
case, a metal lever dropped
“One hypothesis is
that, for whatever
reason, these animals
have different brains.
you have this
In the goal-trackers,
or TV commercial) often
have the power to tempt
people even when they
Studies have shown
that obese people are more
attracted to food signals
than people of healthy
weights, Robinson notes.
And drug addicts are more
likely than non-addicts to
be drawn to signals they
associate with their habit.
“A lot of behavior, be it
human or animal, is really
controlled by cues in the
environment that predict
rewards,” he says.
Yet it’s not clear why
some people might be
more focused on cues
than others, nor how that
difference might contribute
into the rats’ cages, signaling
that a food treat was about to
something else is going
to addictions or overeating.
That’s where the rodents
appear. The animals quickly
can help, Robinson believes.
learned to connect the signal
with the reward, and when the
Over the last several years,
he has used a rat model
metal lever appeared, many of
similar to Tomie’s to explore
the rats darted to the corner of
the cage where food would be
University of Michigan
why some animals are so
easily tempted by signals in
Some of the rats, however,
were drawn to the lever itself.
Rather than run to the food
drop, the rats licked and gnawed the inedible metal lever as
though it were rat kibble (Clinical Psychology Reviews, 1995).
This action wasn’t totally unfamiliar, says Tomie. Researchers
had noted similar behavior in pigeons back in the 1960s
(Journal of the Experimental Analysis of Behavior, 1968). But
watching the rats, Tomie was reminded of drug addiction in
people. “What intrigued me is that the animals didn’t appear
to be able to control themselves,” Tomie says. He even tried
withholding the food reward from those rats that contacted
the lever first. Still, a handful of rats couldn’t resist the lever’s
lure. “The animals are unable to stop themselves, to their
detriment,” he says.
Addiction therapists frequently encounter patients who
Robinson has identified
two distinct behaviors among the rodents. For some rats (the
“goal-trackers”) the final reward is most important. They run
for the food cup as soon as they spot the signaling lever. For
others (the “sign-trackers”) the cue itself holds the most sway.
These are the animals that Tomie saw licking and gnawing on
The sign trackers seem to be particularly prone to addiction
or other maladaptive behaviors, Robinson says. Rodents that
keep their eyes on the lever are much more likely to seek drugs
or swallow alcohol from a sipper.
A compulsion for cues seems to extend across many types
of behaviors. Sign-tracking rats have trouble resisting cues in
general, whether they’re associated with food or with drugs,
desperately want to quit drug or alcohol consumption but
experience a triggered relapse, Tomie says. For an alcoholic,
for example, the trigger may be the sight of a cocktail glass
or the flickering neon sign in front of a favored watering
hole. Similarly, food-related signals (such as a fast-food sign
such as cocaine. In fact, by identifying sign-trackers using the
food-pellet model, Robinson can take rats that have never been
exposed to drugs and correctly predict which animals are likely
to exhibit drug-seeking behaviors when introduced to cocaine
(Biological Psychiatry, 2010).