Does your mouth water when you hear the sound of the ice cream man coming down your street? If so, then you have been conditioned to salivate at the sound. In the early 1900s, Ivan Pavlov studied this conditioned response when he trained dogs to salivate at the sound of a bell. Pavlov knew that dogs naturally salivated when meat was fed to them. To condition this salivation, Pavlov rang a bell each time he presented the food to the dogs. After the ringing bell was paired many times with the presentation of the meat, the dogs salivated when they heard the bell, but BEFORE they saw the food. In other words, the dogs had become conditioned because they expected that the meat reward would follow the sound of the bell.

Humans can become conditioned in much the same way as Pavlov's dogs. In a two-part experiment, scientists, using brain imaging methods, have learned which parts of the brain are active when people associate visual images with food smells. This research also suggests that if Pavlov's dogs had been allowed to eat all the meat they wanted, then the conditioned response would have disappeared.

Conditioning Experiment, Part 1

During the experiment, the scientists used functional magnetic resonance imaging (fMRI) to scan the subjects' brains. After eight minutes of training, the abstract pictures alone -- without the accompanying smells -- caused increased activity in two parts of the brain:

1. the amygdala, a part of the limbic system well known for its role in fear conditioning
2. the orbitofrontal cortex (OFC), a part of the prefrontal cortex.

Previous brain imaging studies have shown that both the amygdala and OFC are involved in conditioning and reward effects, so it wasn't a surprise to see increased activity in these areas when the pictures were shown. The researchers considered this activation to be a hunger response: the activation of the amygdala and the OFC showed that the subjects expected to experience peanut butter or vanilla when they saw the pictures.

Conditioning Experiment, Part 2

Changes in other areas of the brain were seen during different parts of the training. For example, areas in the ventral striatum, insula, and cingulate cortex showed increased activity during the initial training phase and decreased when an abstract picture of the food eaten was paired with that image. Activity in these brain areas, however, increased for the pictures associated with the opposite food: subjects who ate vanilla ice cream showed more activity for peanut butter after they ate vanilla ice cream than before they ate anything at all. If this brain activity can be viewed as food desire, then changes in activity would indicate that the subjects showed more desire for vanilla after eating peanut butter than they showed when they had had no food--and vice versa for the subjects who ate vanilla ice cream.

Experiments with people who have been conditioned to food rewards may provide clues to the causes of compulsive eating and addictions. Some researchers think that visual signals encourage certain behaviors. For example, in this latest study, the visual signals were the abstract pictures. However, many things can become visual cues in daily life. It is possible that some people are conditioned to want fattening or sweet foods when they go to a particular restaurant; perhaps other people eat bags of potato chips when they watch TV or drink too much alcohol when they are with certain friends. People who want to change certain eating behaviors may benefit from changing their environment. A change in a person's environment may help to avoid behaviors that they hope to change. In severe overeating or addiction cases, it may be that abnormal activity in the amygdala or OFC impairs the "I've had enough" response that most of us get when we are full.

Our tendency to get sick of a food after moderate consumption makes sense from an evolutionary point of view. It is important for us to want to consume many different types of food, so that we can get many different vitamins and minerals. Our desire for different tastes (even when we're no longer hungry!) hints at why, after we eat appetizers, bread, a salad, and an entree, a piece of chocolate cake still looks so good.

Reference:

Gottfried, J.A., O'Doherty, J., Dolan, R.J. Encoding predictive reward value in human amygdala and orbitofrontal cortex, Science, August 22, 2003: 1104-1107.