Concurrent presentation of memory-related odors and sounds nullified sleep reactivation benefits.

Reactivation of recently acquired memories during sleep supports their longevity. Reactivation can be biased during sleep using odors or sounds through a technique termed targeted memory reactivation (TMR). Here, we attempted to selectively weaken memories by reactivating them together with forgetting instructions. We delivered sounds to reactivate spatial memories and concurrent odors to reactivate instructions. Participants learned about the instructions in a Directed-Forgetting task performed with a list of to-be-remembered and to-be-forgotten words. One odor was linked with instructions to forget, one with instructions to remember, and a third was not assigned any meaning. During a nap, sounds previously linked with object-location learning were presented with these odors. Spatial recall was tested after sleep. Sounds produced a selective recall benefit for weakly encoded memories. However, results did not support the prediction that forgetting could be instilled by the concurrent forget odor. The encoding-strength-dependent benefit was greatest when sounds were presented together with the odor not assigned meaning, whereas the other two odors both disrupted sound-induced memory reactivation. We infer that remember and forget odors, due to the linked instructions and/or multiple learning episodes in the Directed-Forgetting task, evoked sleep-based retrieval that interfered with reactivation of spatial memories. Odors also induced a prolonged decline in sigma EEG power (12-16 Hz) that continued at least 10 s after odor offset. Overall, these findings highlight the complexity of memory consolidation during sleep when multiple memories and multiple cues are involved.