Now, a new University of Michigan study gives new evidence about what's going on in the brain when we taste something we like, or develop a liking for something we once hated.
And although the study used rats instead of people, it has direct implications for understanding the way we perceive pleasure ?and the reasons why some people develop problems, such as drug abuse, depression or anorexia, that knock their pleasure response off balance.
In a new paper in the November issue of the Journal of Neurophysiology, U-M neuroscientists and psychologists report the findings from direct monitoring of an area of the brain known as the ventral pallidum. Located deep in the brain, it's a kind of traffic center for signals from different areas of the brain that process tastes and pleasurable sensations.
The researchers were able to track the activity of brain cells in that area while the rats received water, salt water and sugar water directly into their mouths. They also recorded how the rats behaved while they tasted those different solutions, including signs that they liked or disliked the tastes. And, they repeated the tests when the rats had been treated with drugs that greatly reduced their bodies' salt levels.
At first, the rats all behaved negatively after tasting a strong salt-water solution, compared to the water or sugar water. Their ventral pallidum brain activity was also much lower in response to the salt water.
But when the researchers put the rats into a salt-deprived state using a combination of diet and hormones that cause the body to get rid of salt, the picture changed. Suddenly, the rats' brain activity rose as high when they received the salt water as it had when they received sugar water. The effect lasted for a while af ter the rats' bodies returned to normal salt levels, but soon enough it wore off.
"We converted something that wasn't pleasing to something that suddenly became pleasurable, and when we did that the neurons we were studying switched their response," says senior author J. Wayne Aldridge, Ph.D., a research associate professor in the Department of Neurology at the U-M Medical School. "Pleasure has traditionally been one of the hardest problems for neuroscience to measure, but these results shed light on how it is represented in brain activity."
The study was designed so that the signals from the ventral pallidum were only related to the "liking" ?or disliking ?of the taste, and not to a salt-seeking drive or movement. The ventral pallidum is part of the limbic system of the brain, which is involved in motor-muscle control as well as pleasure and reward.
In an accompanying editorial, University of North Carolina researchers Robert Wheeler and Regina Carelli call the study "elegant" and the results a "profound step" toward understanding the nature of pleasure itself, rather than the behaviors and actions triggered by it.
Aldridge collaborated on the study with former U-M Psychology graduate student Amy Tindell, Ph.D., and with Kent Berridge, Ph.D., a professor in the Department of Psychology in the College of Literature, Science, and the Arts.
"This finding reveals a type of brain Morse code for pleasure," says Berridge. "The faster these neurons fire, the more pleasant the taste seems to become. The hardest test for a pleasure code is whether the brain signal can track the change from nasty to nice. The amazing fact is that these neurons pass that test."
Aldridge notes that an analogous effect occurs in everyday human life, when a formerly favorite food becomes less attractive after we have over-indulged in that food.
"Moment by moment, this low-level information processing in the brain helps us react t o what we like or don't like," he says. "These neurons respond to a taste as pleasurable, or as not pleasurable."
But research on how these neurons fire to signal pleasure is important for more than just curiosity's sake, he adds. The ventral pallidum is an important brain region for both pleasure and craving. If firing patterns go wrong in ventral pallidum, it could possibly contribute to eating disorders, anorexia and drug addiction.
Eventually, activation in the ventral pallidum in response to pleasurable tastes could also be useful in brain-mapping techniques in humans. All in all, Aldridge says, the new paper is "a really good example of how animal experiments help us understand the human brain. If we can understand how the brain generates normal pleasures, we may have a new focus for effective treatments in people who don't experience normal pleasure."