PFC and Episodic Encoding

In the course of a typical day, humans experience countless events, yet, at the end of the day, only some experiences are memorable, with many of the day’s events having been forgotten. Central to understanding memory is determining why some experiences can be later remembered, whereas others are forgotten. One factor that may influence memory is the nature and extent of the cognitive control operations engaged during episodic encoding—that is, the process of transforming an experience into a durable memory trace such that it can be subsequently remembered. We have been testing the hypothesis that PFC control mechanisms guide episodic encoding by bringing task-relevant representations on-line and elaborating upon them, rendering the accessed and elaborated information available for input into the MTL memory system.

Our efforts to specify the underpinnings of episodic encoding have yielded novel insights into the role of cognitive control in modulating and enhancing episodic learning. Our fMRI data suggest that distinct regions in PFC impact memory formation depending on the content of the to-be-learned experience, with the extent of engagement of PFC processes during an experience being predictive of whether that experience will be later remembered or forgotten. For example, our results implicate left VLPFC and left MTL structures in the encoding of stimuli that can be elaborated upon at the semantic and phonological levels, and right VLPFC and bilateral MTL in the encoding of visuo-spatial events [Wagner, Poldrack et al., 1998; Wagner, Schacter et al., 1998; Kirchhoff, Wagner et al., 2000; Davachi, Maril, & Wagner, 2001 ; Clark & Wagner, 2003; for a review see Wagner, Koutstaal, & Schacter, 1999; Paller & Wagner, 2002]. We have argued that this content-sensitive pattern of PFC activation reflects the recruitment of distinct frontal processes that allocate attention to semantic, phonological, and visuo-spatial representations, guiding their input into declarative memory [Wagner, 1999; 2002]. Consistent with this hypothesis, we have observed that these content-sensitive encoding patterns in PFC are accompanied by correlated patterns in posterior neocortical association areas that putatively represent the stimulus features being encoded [Kichhoff et al., 2000; see also, Dobbins & Wagner, in press].

An important question surrounding these fMRI-identified frontal correlates of effective encoding is whether they index neural computations that play a causal role in learning or are mere correlates of learning. To begin to test the necessity of PFC mechanisms for effective encoding, we recently used fMRI-guided single-pulse TMS to disrupt PFC function during verbal encoding. Our results revealed that transient disruption of left VLPFC function during verbal encoding impairs memory performance 20 min later, whereas disruption of right VLPFC enhanced later memory performance perhaps by inducing a functional shift to reliance on left-lateralized PFC control processes that are more effective for verbal encoding [Kahn et al., 2005]. We are actively pursuing open questions surrounding the role of PFC control processes in memory formation, including efforts to more precisely specify the impact of PFC processes in modulating posterior neocortical and MTL mechanisms.