Interference in Long-Term Memory: Competition between Intentionally and Incidentally Acquired Memory Traces Leads to Forgetting

Forgetting is a fundamental feature of human memory, yet questions remain about how long-term memories become vulnerable to interference. While previous work has focused on exploring interference between intentionally encoded long-term memories, it is less clear to what extent incidentally acquired long-term memories can also disrupt intentionally encoded ones. We address this question across two experiments (total N = 88). Participants encoded object-location associations into long-term memory. Afterwards, they completed a working memory task in which the same objects reappeared in the same or shifted locations (60° or 120°), or were absent, before a final long-term memory test. Results indicated that the interference induced via the working memory task impaired long-term memory retrieval, and that this effect was driven by swap errors. Occasionally, participants accidentally reported the task-irrelevant working memory location instead of the long-term memory one, indicating that they incidentally transferred the working memory task information to long-term memory. Experiment 2 sought to investigate the mechanisms underlying interference. Results showed that both maintenance and testing in working memory disrupted long-term memory, with the combination of maintenance and testing producing the strongest effects. These findings demonstrate that incidentally acquired long-term memories, especially those that underwent maintenance and testing in working memory, can disrupt intentionally encoded long-term memories.