A little of Pavlov’s dog is hiding in all of us, scientists said after an experiment in which they taught human volunteers to associate abstract computer images with ice cream.
Russian scientist Ivan Pavlov’s showed how a dog can be made to salivate when hearing a bell rung every time it is given food. As every school pupil knows, in the end the dog’s mouth will water merely at the sound of the bell, even when no food is present.
Scientists at the Institute of Neurology in London repeated the experiment with human volunteers. But instead of making them drool, they prompted changes in brain activity detected with an imaging scanner.
During an initial training period, 13 hungry volunteers were shown abstract computer images while being exposed to the smell of vanilla or peanut butter. After a while the volunteers began automatically to think of ice cream or peanut butter sandwiches when shown certain images. Meanwhile, their brain activity was
being monitored.
Later the volunteers were allowed to eat either vanilla ice cream or peanut butter sandwiches until they did not want any more. When they were again shown the computer images, the brain activity detected earlier was found to have changed.
The images now triggered a weaker association with the food they had just consumed, demonstrated by a decrease in brain activity. However, the brain activity relating to the uneaten food remained the same.
Dr Jay Gottfried, who led the study, said making connections between eating and various environmental stimuli was a fundamental part of learning, probably common in all animals.
”Say a rabbit is hopping around its little grassy patch and he’s learned how to find clover,” he said. ”So, if he sees a particular tree stump, or rocky slope, that predicts clover.”
But the brain had also evolved a braking system that toned down the response once a particular desire had been appeased. It was this that stopped the rabbit spending all its waking hours nosing round a tree stump that reminded it of clover. An inability to disconnect the anticipation of food from various sights, sounds or other stimuli may play a role in compulsive eating, said Dr Gottfried.
A faulty braking system appeared to be part of the problem for patients with Kluver-Bucy syndrome, who often consume huge amounts of food, even putting inedible items in their mouth.
”The patients have damage to brain regions including those highlighted in the scanning experiments. You could conjecture that a similar thing may be going on in certain eating disorders, where the routine brakes on the whole system are tweaked somehow, so they’re no longer responding to normal cues,” said Dr Gottfried.
The findings were reported in the journal Science. They showed that the changes in brain activity chiefly affected the amygdala and orbitofrontal cortex regions. Previous studies have implicated these regions in conditioning. Some activity differences were also seen in other areas, including the ventral striatum which is associated with drug addiction. – Sapa-DPA