The Revenge of Baklava: How the Brain Determines Lasting Disgust

One single negative experience can lead to a lifelong aversion to certain foods. Researchers have recently explored how the brain encodes feelings of disgust and why new foods tend to be marked for avoidance more rapidly.

A case in point is the unpleasant combination of sour mayonnaise on fries or an overload of syrupy baklava--often, a bout of nausea after consuming certain dishes leads individuals to avoid them for extended periods. In some instances, simply the smell or sight of a particular food can trigger feelings of disgust. Understanding why the brain retains such strong gustatory memories has become a focus of recent research conducted on mice.

While pain from touching a hot stove is immediate, nausea from spoiled food typically manifests later. However, the brain can create a retrospective link, effectively placing the offending food item on an internal 'do not eat' list.

In a study conducted by a team from Princeton University and published in the journal Nature, mice were given a novel purple grape-flavored drink. After a brief interval, they received an injection that induced symptoms akin to food poisoning. As anticipated, two days later, the mice avoided the drink in favor of water.

The researchers employed a specialized imaging technique known as FOS-Imaging to observe the brain processes involved. This method allows for the visualization of neuronal activity. Results indicated that the amygdala, a brain region critical for assessing potential threats, registered all stimuli involved in this scenario. The amygdala was active not only while the mice drank but also when they later experienced illness and remembered the negative experience days afterward.

According to the findings, new taste experiences activate specific neurons that remain particularly sensitive to signals from the digestive system for hours. Subsequent nausea signals reactivated these neurons in the amygdala, allowing the brain to make connections between taste and feeling unwell. This ability to associate cause and effect is vital for survival, as it helps distinguish between nutritious and potentially toxic foods.

Mice that had previously encountered the grape drink did not permanently encode the negative experience; they continued to prefer the drink over water, suggesting that a single instance of nausea was insufficient to instigate a lasting aversion.

The study raises intriguing questions about how the brain discriminates which flavors to remember as potentially harmful. A significant factor appears to be novelty. According to the researchers, unfamiliar flavors--those not previously encountered--serve as strong signals for learning from food poisoning experiences, possibly leading to their designation as hazardous.

However, human responses are more complex. Foods that have been enjoyed for years can suddenly become repulsive following a negative experience, such as feeling ill after consuming spoiled potato salad.

Beyond novelty, several additional factors can influence these reactions, including suspicions about specific ingredients, unpleasant odors associated with the food, or the context in which the food was eaten--such as in an unfamiliar restaurant. Moreover, the amount consumed can also play a role in forming aversive memories.

The brain synthesizes all this information to pinpoint the most likely cause of food-related illness. If an individual does become sick, the mental representation of the flavor is 'replayed' in the brain, reinforcing the aversion until it becomes long-lasting.

This learning mechanism is crucial for survival, as it enables individuals to discern between beneficial and harmful food sources.