Scientists in London have shown that the brain can be trained to hunger for foods on seeing an abstract image.
The brain can be made to want food from looking at a random image
The experiment could hold the key to understanding eating disorders.
The research at the Wellcome Department of Imaging and Neuroscience echoes a famous experiment performed by the Russian scientist Ivan Pavlov, in which dogs were trained to hunger for meat at the sound of a bell.
Using advanced brain scanners - a technique known as magnetic resonance imaging (MRI) - researchers demonstrated that humans could be trained to hunger for foods such as vanilla ice cream or peanut butter at the sight of an abstract computer image.
"The initial aim was to condition the subjects and make sure that they learned the associations between neutral stimuli - abstract pictures or fractal images - and the food odours," said Jay Gottfried of Wellcome Department of Imaging and Neuroscience.
"The pictures and odours were repeatedly paired until the subjects learned the association."
Researchers call the images "predictors".
The work provides an insight into both hunger and how the brain changes once we are full.
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This could, in turn, increase understanding of eating disorders such as bulimia nervosa, where a person "binges" and then makes himself sick, and anorexia, where a person deliberately denies himself certain types of food.
"One big question is: what is the ultimate trigger for putting the break on the system?," Mr Gottfried told BBC World Service's Science In Action programme.
"There are going to be a variety of signals coming from the gut, from the bloodstream, chemical and metabolic factors that all need to converge in order to provide the right signals.
"I think trying to understand a more integrated perspective here is going to be the key to trying to unlock some of the problems of more conventional eating disorders."
The technique of training the brain to hunger for certain types of food based only on a random image is known as "conditioning."
"This was all going on subconsciously - we never told the subjects specifically about what associations were there for them to learn about," Mr Gottfried pointed out.
"It wasn't essential that the subjects were consciously aware of these relationships."
The whole conditioning process takes only about eight minutes.
Once trained, the subjects are placed in an MRI scanning machine and shown the abstract images.
Put off our food
By using MRI, scientists can see what is happening in the brain. The parts affected are the medial temporal lobes in the bottom third of the brain - known for a long time to be involved in conditioning and learning associations - and the orbiter frontal cortex, just above the eyeballs.
"What these bits of the brain are doing is forming predictions - they're telling the subject something nice is going to happen," the scientists explained.
Research has given a better understanding of why we feel full
After the scan, the subjects were given a big lunch involving the food they had been looking forward to - for example vanilla ice cream.
The researchers noted that after the meal, the response in the brain between the image and the food decreased.
In other words, when we are full, something in the brain changes - and we can be put off wanting to eat more of that substance simply by seeing the abstract image.
"What we've done here, which is above and beyond that, is that we're showing that the response is to a predictor," researchers explained.
"If you learn that one individual stimulus predicts the vanilla ice cream, then the response to that predictor decreases - as well as the response to the vanilla ice cream.
"Therefore the brain learns to update its predictions about what is nice in its environment."