ver 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
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
The 13 subjects, said Dr. Jay A. Gottfried, a postdoctoral fellow
at the Wellcome 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
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
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."