Electrophysiological evidence for the influence of unitization on the processes engaged during episodic retrieval: enhancing familiarity based remembering.

Episodic memory depends upon multiple dissociable retrieval processes. Here we investigated the degree to which the processes engaged during successful retrieval are dependent on the properties of the representations that underlie memory for an event. Specifically we examined whether the individual elements of an event can, under some conditions, be unitized, leading to an enhancement of familiarity based responding. Retrieval processes were examined using event-related potential (ERPs) old/new effects, recorded during an associative recognition memory task. The nature of to-be-remembered information was manipulated by using word-pairs as stimuli. At study, participants were asked to remember word-pairs sharing an association (traffic-jam); association+semantic relationship (lemon-orange); or a semantic relationship only (cereal-bread). A behavioural pre-test revealed that association word-pairs were rated as having the most unitized representation. At test, participants were required to recognize if word-pairs were presented in the same pairing as at study, were rearranged from at study, or were entirely new. Behavioural recognition performance was clearly influenced by the nature of the to-be-remembered stimuli, memory being strongest for pairs related purely by association, and weakest for semantic only pairs. ERP old/new effects recorded at test also showed significant differences in the neural correlates of retrieval, depending on stimulus characteristics. The bilateral frontal old/new effect (typically associated with familiarity) was solely elicited by association only pairs. By contrast, the left parietal old/new effect (associated with recollection) was elicited equally by all three conditions. In addition, the late right frontal old/new effect (typically associated with some form of strategic/executive processing) was modulated. This latter effect was initially largest for association only pairs, and subsequently largest for semantic pairs. These findings suggest that the pattern of engagement of familiarity and recollection during successful episodic retrieval is dependent on the properties of the representations that underlie memory for an event.