It is unclear whether the involvement of the medial prefrontal cortex (mPFC) during encoding is restricted to the evaluative processing of to-be-encoded stimuli or if it is instead actively engaged during memory formation. The difficulty of assessing the contribution of the mPFC to encoding based on previous neuroimaging studies partly arises from the use of several types of stimuli, such as emotional or social ones. These different types of stimulus content could differently modulate mPFC activity during memory formation and thus partly explain the variable contribution of this region to encoding. Using emotional/neutral and social/nonsocial pictures, we conducted an event-related functional magnetic resonance imaging study using a subsequent memory paradigm as the main analytical strategy. We observed that the brain activity in the dorsal and orbital mPFC is significantly and specifically predictive of the successful encoding of social compared with nonsocial pictures. In contrast, the activity in the amygdala specifically predicts the successful encoding of emotional compared with neutral pictures. The modulation of the mPFC by social information in a memory encoding context could be associated with the initiation of self-referential processes whose contribution is to enhance memory formation.

Converging evidence highlights the important role of the amygdala in the enhanced memory for emotional material. However, it is unknown whether any regional and/or hemispheric specificity exists regarding its involvement in the different memory stages, such as encoding and retrieval. We directly addressed this issue by conducting an event-related functional magnetic resonance imaging study of memory for faces with different emotional expressions, in which both encoding and recognition were scanned. We found significant amygdala activation associated with both successful encoding and retrieval of fearful faces. Critically, a direct comparison between both stages revealed that whereas the right amygdala was involved in emotional memory formation, the retrieval of those memories relied on the left amygdala. Thus, this study demonstrates a hemispheric dissociation of amygdala involvement in different stages of emotional memory in humans.

Although results from cognitive psychology, neuropsychology, and behavioral neuroscience clearly suggest that item and associative information in memory rely on partly different brain regions, little is known concerning the differences and similarities that exist between these two types of information as a function of memory stage (i.e., encoding and retrieval). We used event-related functional magnetic resonance imaging to assess neural correlates of item and associative encoding and retrieval of simple images in 18 healthy subjects. During encoding, subjects memorized items and pairs. During retrieval, subjects made item recognition judgments (old vs. new) and associative recognition judgments (intact vs. rearranged). Relative to baseline, item and associative trials activated bilateral medial temporal and prefrontal regions during both encoding and retrieval. Direct contrasts were then performed between item and associative trials for each memory stage. During encoding, greater prefrontal, hippocampal, and parietal activation was observed for associations, but no significant activation was observed for items at the selected threshold. During recognition, greater activation was observed for associative trials in the left dorsolateral prefrontal cortex and superior parietal lobules bilaterally, whereas item recognition trials showed greater activation of bilateral frontal regions, bilateral anterior medial temporal areas, and the right temporo-parietal junction. Post hoc analyses suggested that the anterior medial temporal activation observed during item recognition was driven mainly by new items, confirming a role for this structure in novelty detection. These results suggest that although some structures such as the medial temporal and prefrontal cortex play a general role in memory, the pattern of activation in these regions can be modulated by the type of information (items or associations) interacting with memory stages.