Multiple memories can be simultaneously reactivated during sleep as effectively as a single memory

Memory consolidation during sleep involves reactivation of memory traces. Targeting specific memories by presenting learning-related cues during sleep selectively enhances memory, but the mechanism behind this benefit is not fully understood. To better characterize the consolidation process in humans, we tested whether multiple memories can be reactivated in parallel using a spatial-memory task. After learning the locations of images belonging to semantically related sets of one, two, or six items, half of the sets were reactivated during a nap. Results showed a selective benefit in location recall for cued versus non-cued items regardless of set size, implying that reactivation may occur in a simultaneous, promiscuous manner. Intriguingly, sleep spindles and delta power modulations were sensitive to set-size and reflected the extent of previous learning. Taken together, our results refute the notion that resource availability strictly reduces the capacity of simultaneous sleep-reactivation and bring forward alternative testable models for sleep-related consolidation.