Putting the pieces together: the role of dorsolateral prefrontal cortex in relational memory encoding

Abstract

Results from fMRI have strongly supported the idea that the ventrolateral PFC (VLPFC) contributes to successful memory formation, but the role the dorsolateral PFC (DLPFC) in memory encoding is more controversial. Some findings suggest that the DLPFC is recruited when one is processing relationships between items in working memory, and this processing specifically promotes subsequent memory for these relationships. However, previous studies could not rule out the possibility that DLPFC promotes memory during all elaborative encoding conditions and contributes to memory on all subsequent associative memory tests. To address this question directly, we used fMRI to examine activity during two encoding tasks that prompted participants to encode either relational or item-specific information. On relational trials, participants imagined pairs of items interacting, whereas on item-specific trials, participants imagined the items spatially separated and in different sizes. After scanning, we examined memory for relational information and item-specific information. fMRI results showed that DLPFC activity specifically promoted memory for relational information during relational encoding and not memory for item-specific information during item-specific encoding. In contrast, activity in the VLPFC predicted memory for both relational and item-specific information. The present results are consistent with the idea that the DLPFC specifically contributes to successful memory formation through its role in building relationships among items.

Figure 1

Trial structure and timing. On each trial, participants were presented with a noun pair for 3.5s (cue) during which time they were instructed to perform one of two different mental imagery tasks.. During interact trials participants imagined the object referents interacting in a meaningful way (e.g. goat kicking the clock). During separate trials participants imagined the object referents separated as if on different sides of a room and in different relative sizes (e.g. a small clock on the left side of the room and a large goat on the right side of the room). A probe was then presented for 1s which prompted participants to rate the vividness of their mental image that they formed during the cue period. A white fixation cross was presented as the ITI (jittered 2-14 sec.)

Figure 2

DLPFC activity specifically correlates with subsequent memory for relational information. (A) At upper right, a cortical surface rendering shows regions (colored in yellow) for which activity was increased during relational encoding (interact) trials for which the relational information was remembered with high confidence compared to relational trials for which this information was forgotten (see methods for details). Also depicted are regions (colored in blue) for which activity was increased on item-specific encoding (separate) trials for which the location of both items were subsequently remembered with high confidence compared to trials for which the location the items were subsequently forgotten (see methods for details). Regions colored in green exhibited suprathreshold responses in both contrasts. Note that green colored regions fall exclusively within VLPFC in the PFC. (B-C): Activity in two prefrontal regions of interest are separately plotted for interact trials (yellow bars) as a function of subsequent memory for relational information and for separate trials (blue bars) as a function of subsequent memory for item-specific information. (B) Activity in a region of left DLPFC (approximate BA 9/46) was correlated with memory for relational information on interact trials, and was not correlated with memory for item-specific information on separate trials in either map-wise or ROI analyses. (BC) Activity in left aVLPFC (approximate BA 45, 47/12) was correlated with memory relational information and item-specific information. Error bars depict the standard error of the mean.