Another possibility is that the involvement of the DMN in encoding and retrieval is not related to whether attention is oriented to internal or external events but to whether the HF is coupled or uncoupled with other components of the DMN. Although resting state coherence studies have associated the HF with DMN [4], [6], [8], in memory studies HF and DMN may show similar or different activation patterns depending on the memory phase, encoding or retrieval. During successful retrieval, HF activity tends to increase just like other components of the DMN [15], [32]. During successful encoding, however, HF activity tends to increase [15], [33] whereas DMN activity tends to decrease [19], [21], [22], [23], [24]. Thus, one possible explanation of the memory functions of the DMN is that the DMN is beneficial to memory when it is coupled with the HF, as in the case of retrieval, but it is not beneficial to memory when it is uncoupled with the HF, as in the case of encoding. Although this hippocampal coupling hypothesis is consistent with available evidence, it has never been directly tested within the same participants and the same experiment.
