Ever
since Pavlov trained dogs to salivate for meat powder at the sound
of a bell, psychologists have used the principles of classical conditioning
to study how animals and humans learn. But only recently have they
been able to peer into the brain and watch that learning take place.
Now
a team of English researchers, using a sophisticated brain scanning
technique called functional M.R.I., has provided a vivid demonstration
of the neural processes at work in a simple Pavlovian conditioning
experiment.
Like
Pavlov's dogs, the subjects in the study were conditioned to associate
a neutral stimulus — in this case, abstract images presented
on a computer screen — with food. One image was paired with
the smell of peanut butter, wafted to the subjects' noses through
a tube. Another image was paired with the smell of vanilla.
The
13 subjects, said Dr. Jay A. Gottfried, a postdoctoral fellow
at the Welcome department of neuroimaging science at University
College London and the lead author of the study, "were all
young, healthy, right-handed volunteers who came in hungry and
professed to liking both peanut butter and vanilla, which isn't
so easy in England."
The
subjects, who thought they were participating in an experiment
about learning computer tasks, were quickly trained to associate
the images with the food smells. The subjects reacted faster to
the images paired with the food odors than to other images that
had no pleasant associations. At the same time, their brains surged
into action, with areas known to be involved in motivation and
emotional processing — the amygdala, deep in the temporal
lobe, the orbitofrontal cortex, and other structures lighting
up on the brain scan.
Then
the researchers took their study, published last week in the journal
Science, a step further. When the subjects were fed either a peanut
butter sandwich or a bowl of vanilla ice cream, Dr. Gottfried
and his colleagues found, the images associated with that food
no longer drew as strong a response, and the subjects' emotional
brain circuits quieted down. But the image associated with whichever
food the subjects did not receive continued to elicit faster reaction
times and a flurry of chemical activity in the amygdala and other
brain areas.
Psychologists
refer to this as "selective satiation." Dr. Gottfried
calls it "the restaurant phenomenon." "You
go out to Lutèce and have an eight-course meal and just
when you think you can't stuff another crumb into your mouth,
they bring the dessert cart by and, miraculously, you have a spot
for that chocolate cake," he said.
Whatever
its label, added Dr. Gottfried, who did the study with two other
scientists, Dr. John O'Doherty and Dr. Raymond J. Dolan, the effect
reflects the fact that learning is a tool designed by evolution
for survival and as such, is infinitely flexible. "If you
think about a rabbit jumping around a carrot patch," he said,
"it may learn that a pile of rocks comes to predict that
carrot patch. But once the patch is depleted, it behooves the
rabbit no longer to follow the promptings of that pile of rocks."
The
study's findings, he added, may eventually help scientists understand
more about why people with eating disorders fail to become satiated
on foods even after they have eaten them.
Patients
with injuries to the brain's frontal and temporal lobes, Dr. Gottfried
noted, areas that encompass the circuit involved in hunger and
satiation, often have eating problems and may eat indiscriminately
or fail to stop eating when they are full.
But
Dr. P. Read Montague, a professor of neuroscience at the Baylor
College of Medicine and an expert on mental function and the brain,
said the study was most noteworthy for offering a glimpse of what
takes place inside the brain when a rat learns to run a maze or
a human to forage for lunch. "This is what psychologists
really couldn't do" before, he said. "They could sit
and observe the behavior, but they couldn't eavesdrop on the black
box as they were doing it."
Gottfried
said specific brain circuits are involved in this process. The
researchers found heavy involvement of the amygdala — the
area of the brain best known for processing emotions — and
the orbitofrontal cortex. “If every time you drove past a
McDonald’s and saw the golden arches, you felt compelled
to go inside and get a Big Mac, this would be destructive after
time,” he said. Something must tell the brain when to respond
and when not to, and this does not necessarily stop at food. “Whether
we are talking about food or sex or even things on the aversive
scale such as dangers and threats and predators, the brain also
needs to know how to update ... and modulate these associations
so you don’t get stuck in a rut.”
~By
Erica Good, New York Times
