Abstract: Forgetting is a cognitive process that individuals tend to avoid under many circumstances, but it does not always have negative effects. Erasing some outdated information (e.g., phone numbers a long time ago) or negative information (e.g., negative emotions) from memory will not only improve one's efficiency of memorization, but also improve one's mental health. In psychology, this kind of forgetting is named ‘intentional forgetting' or ‘directed forgetting'. In a typical experimental paradigm to study directed forgetting, the participants are asked to remember or forget certain items following corresponding cues in the learning stage, and then to recall or recognize these items in the subsequent testing stage. Results of previous studies have consistently shown that the score of memorization on the to-be-forgotten items is significantly lower than on the to-be-remembered items. Regarding whether directed forgetting requires cognitive effort, however, there have been controversial views including two opposing theories, namely passive decay theory and active inhibition theory. In the view of passive decay theory, directed forgetting is not necessarily a mechanism of active inhibition of memory; instead, it is a passive process that does not require cognitive effort - the to-be-forgotten items just decay passively with time for lacking an effective rehearsal as there is a selective rehearsal of the to-be-remembered items. On the contrary, active inhibition theory maintains that directed forgetting is a process of active inhibition of memory induced by the forgetting cues and hence it requires cognitive effort - this theory has not only been supported by behavioral studies but also been evidenced by the ERPs in the frontal lobe as shown in neurocognitive studies. Although the two theories attribute directed forgetting to opposing mechanisms, they are not entirely conflicting as they have consensus on the encoding mechanism of the to-be-remembered items. In addition to these two mainstream theories, there is also a viewpoint that the encoding stage of directed forgetting may involve both active inhibition and passive decay which are two processes independent in time and space, but how the two processes interact and integrate is yet to be explored. Notably, some other studies reported that the performance of forgetting on the to-be-forgotten items was even worse than on the items without any cues（these words don't need to be remembered or forgotten）, suggesting that the to-be-forgotten items have partly entered the memory, which seems to challenge both active inhibition and passive decay theories. Despite a wealth of experimental evidence, the controversy in the theory remains. To further clarify the cognitive mechanism of the encoding stage of directed forgetting, future study needs to take non-cognitive factors such as motivation and emotion into investigation, and to inspect various populations.

Key words: directed forgetting, encoding mechanism, active inhibition, passive decay