Expectations Trump Reality in Taste Response


by Steven Stocker

May, 2006

“Our conduct is influenced not by our experience but by our expectations,” said George Bernard Shaw, the great English dramatist. Now, more than a half-century after the playwright’s death, brain imaging is showing that the old master was right, at least regarding our reactions to unpleasant stimuli such as a bitter taste or pain.

In a study published in the March issue of Nature Neuroscience, scientists at the University of Wisconsin demonstrated that people’s responses to a bitter taste, in terms of both their behavior and brain activity, can be influenced by a prior visual cue warning them of how bitter the taste is going to be. When the cue accurately informed them that the taste was going to be highly unpleasant, they responded by rating the taste as highly unpleasant. Moreover, the parts of the brain that respond to negative taste stimuli became very active, as seen in a brain imaging technique called functional magnetic resonance imaging.

However, when the cue misleadingly informed them that the taste was going to be only mildly unpleasant, more than three-quarters of the subjects rated the same taste as less aversive than when they were told the truth. The brain regions that respond to aversive taste stimuli also became less active than they were when the cue was accurate.

“This really points to the power of expectancy on people’s experience,” says lead author and clinical psychologist Jack Nitschke. He says that what occurred in this study is very similar to what happens in the placebo effect, in which a person responds to a pill that he thinks is an active drug but is in reality an inert substance.

“Past studies have shown that expectancy mediates the placebo effect, so instead of giving an inert pill, we decided to just manipulate expectancies directly,” Nitschke says.

In a follow-up study, published in the March 2006 issue of Brain, Behavior, and Immunity, Nitschke and colleagues further investigated the brain mechanisms responsible for the effects of expectancy on taste percep-tion. In this study, they used just the subjects who were consistently tricked by the misleading cue.

bw0606_3
In this image of the cerebral cortex, the shaded region near the center denotes the brain region dedicated to taste, located just above the temporal lobe. Researchers have found that peoples expectations before a taste affect their perceptions of the taste itself. © Anatomical Travelogue/Science
Photo Library.

In these subjects, the researchers found that bursts of activity in the frontal lobe, immediately following cue presentation, likely were responsible for the dampened responses in the taste perception areas following the misleading cue. Nitschke thinks these regions in the frontal lobe may be part of a larger network that informs other brain regions about stimuli they are about to receive and how to react to them.

“This really points to the power of expectancy on people’s experience.”

In this case, the frontal lobe “concludes” on the basis of the misleading cue that the taste stimulus is going to be only mildly unpleasant and inform the taste perception regions to react accordingly, even though the taste in fact turns out to be highly unpleasant.

“The frontal lobe generates information about the situation you’re in and the most appropriate action to take,” says Tor Wager, a cognitive neuroscientist at Columbia University. Wager is first author of a paper showing that a placebo can dampen the brain’s responses to painful stimuli and that the frontal lobe is involved in this process.

“This action can be external—you can take action in the world. An action can also be internal—for example, changing responses in brain regions that process sensory information.” In his placebo study and Nitschke’s taste studies, the frontal lobe acted by reducing the brain’s responses to unpleasant stimuli, Wager says.

Nitschke thinks there may be clinical implications in these studies for patients suffering from anxiety and depression. “If we can lead our patients to believe that an upcoming event isn’t going to be as bad as expected, then that experience will be perceived as less aversive and won’t be as much of a problem,” he says.