Concurrent Presentation of Memory-Related Odours and Sounds Nullified Sleep Reactivation Benefits

Reactivation of recently acquired memories during sleep supports their longevity. Reactivation can be altered during sleep using odours 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 odours 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 odour 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 simultaneously presented with these odours. Spatial recall was tested after sleep. Sound cues produced a selective recall benefit for weakly encoded memories. However, memory results did not support the prediction that forgetting could be instilled by the concurrent Forget odour. An encoding-strength-dependent benefit was largest when sounds were presented together with the odour that lacked assigned meaning, whereas the other two odours both disrupted sound-induced memory reactivation. These two odours were linked with instructions and with multiple learning episodes in the Directed-Forgetting task. Accordingly, we infer that reactivation evoked by the Remember and Forget odour cues interfered with the reactivation of spatial memories. Odours also induced a prolonged decline in sigma EEG power (12-16 Hz) that continued at least 10 s after odour offset. Overall, these findings highlight the complexity of memory consolidation during sleep when multiple memories and multiple cues are involved.