Forgetting in Drosophila consists of an increase in uncertainty rather than a stochastic loss of memory

While forgetting has been studied extensively in various organisms, its precise nature has often been unclear. Here, we used behavioral experiments in Drosophila to determine that a significant aspect of forgetting consists of a decrease in the ability of a memory to induce an appropriate behavior. We tested flies for memory retention at various times after training and then separately retested both flies that chose correctly and those that chose incorrectly. Although the ability to choose correctly decreased over time, we could not measure any differences in memory between flies that initially chose correctly and those that chose incorrectly upon retest. This suggests that forgetting is unlikely to consist of a spontaneous loss of a memory but instead consists of a decrease in the probability of flies that remember choosing the correct behavioral response. Thus, although flies maintain memory over time, there is an increase in uncertainty associated with this memory. We find that forgetting of long-term memories and accelerated forgetting in old flies occur in a similar manner.