Memories help animals survive by guiding them on what to look for and what to avoid, such as remembering the smell of food or the warning signs of danger. But in a constantly changing world, those memories must also remain flexible. If a reward or threat no longer has the same meaning, the brain needs ways to update what it has learned without completely forgetting the past.

In a new study using fruit flies, neuroscientists in the Felsenberg lab discovered that simply re-experiencing a reward can weaken memories associated with it. The researchers trained flies to associate certain smells with a sugar reward. Later, when the flies encountered sugar again — without any smell attached — they were less likely to approach the odor they had previously learned to seek out. However, not all traces of the memory itself were erased, the researchers found. Brain activity showed that the original memory trace was still present, yet its influence on behavior was abolished.

Whether this memory weakening occurred depended on context: When flies encountered sugar in a familiar environment, their learned responses faded. But when sugar was presented in less familiar settings, or if the animals learned something new at the same time as they encountered sugar again, the memory remained intact. This suggests that the brain uses environmental cues to decide when memories should be updated.

Natural sugar re-exposure weakened memories without relying on dopamine, a brain chemical linked to reward and learning, indicating that the brain has multiple ways to control access to learned information. Another key finding was that re-experiencing sugar weakened all memories linked to that specific reward. This is likely a way for the brain to reassess the importance of past experiences when circumstances change.

Although the work was done in fruit flies, similar principles have been observed in mammals and people, especially in studies of fear and addiction, says FMI group leader Johannes Felsenberg. “By showing that memories can be suppressed rather than erased — and that context determines when this happens — our work offers new insight into how brains balance stability with flexibility,” he says.

In the future, understanding these mechanisms could help to develop strategies to reduce the impact of harmful aspects of memories without destroying them entirely.