How can associative memory be directly assessed? To measure neural processes related to episodic association, one should construct a contrast between degrees of association that controls for successful memory. As discussed earlier, several studies using scalp, MEG, and intracranial EEG have used this approach to demonstrate increases in theta power associated with episodic memory formation and retrieval. A common approach is to compare encoding or retrieval events in which subjects remember items with or without retrieving the correct contextual or associative information (as in [26]). Some studies use an explicit metric of association, such as spatial distances (during encoding) between successively recalled items [14] or semantic-temporal distances between remembered words [54]. Another study [23] assessed contextual memory by comparing recognition for items presented in matching or non-matching contexts. And yet another approach is to ask subjects for a subjective report of confidence, and compare correctly retrieved items based on their confidence ratings, under the assumption that high-confidence judgments tend to accompany retrieval of source or contextual information. Crucial to all of these studies is a contrast between groups of successful
