Declarative memory consolidation is hypothesized to require a twostage, reciprocal cortical-hippocampal dialogue. According to this model, higher frequency signals convey information from the cortex to hippocampus during wakefulness, but in the reverse direction during slow-wave sleep (SWS). Conversely, lower-frequency activity propagates from the information "receiver" to the "sender" to coordinate the timing of information transfer. Reversal of sender/ receiver roles across wake and SWS implies that higher-and lower-frequency signaling should reverse direction between the cortex and hippocampus. However, direct evidence of such a reversal has been lacking in humans. Here, we use human resting-state fMRI and electrocorticography to demonstrate that δ-band activity and infraslow activity propagate in opposite directions between the hippocampus and cerebral cortex. Moreover, both δ activity and infraslow activity reverse propagation directions between the hippocampus and cerebral cortex across wake and SWS. These findings provide direct evidence for state-dependent reversals in human cortical-hippocampal communication.D eclarative memories are initially hippocampus-dependent and gradually become hippocampus-independent over time, that is, consolidated (1, 2). It is theorized that a two-stage reciprocal dialogue between the hippocampus and the cerebral cortex underlies memory consolidation (3-5). According to this model, active behavior generates experiential codes in the cortex that are transmitted to the hippocampus, which houses a labile information store. Later, during slow-wave sleep (SWS), recently acquired hippocampal information is reactivated and transmitted to the cerebral cortex, where it is integrated into a more permanent memory store (5, 6). Thus, the hippocampus and cerebral cortex are proposed to exchange roles in sending and receiving information across wake and SWS (5, 6). Importantly, this model does not imply that all signals travel from the "sender" to the "receiver." Instead, the theory proposes that high-frequency activity carries information from the sender to receiver, that is, from the cortex to hippocampus or the hippocampus to cortex, depending on the stage of memory consolidation (wake or SWS, respectively) (4). Conversely, low-frequency activity propagates from the receiver back to the sender to coordinate the transfer of high-frequency information through modulation of the sender's excitability (4, 7-9). Hence, the two-stage reciprocal dialogue model predicts that lower and higher frequency activity between the hippocampus and cortex should propagate in opposite directions across wake and SWS, as illustrated in the schematic in Fig. 1. However, such reversal has not been directly observed in humans.We have recently analyzed temporal lags (delays) in neural signals to study the net propagation of spontaneous activity. In particular, we investigated resting-state fMRI (rs-fMRI) blood oxygen level-dependent (BOLD) signals and demonstrated directed propagation of infraslow activity (<0.1...