Abstract
Variations in levels of processing affect memory encoding and subsequent retrieval performance, but it is unknown how processing depth affects communication patterns within the network of interconnected brain regions involved in episodic memory encoding. In 113 healthy adults scanned with functional MRI, we used graph theory to calculate centrality indices representing the brain regions’ relative importance in the memory network. We tested how communication patterns in 42 brain regions involved in episodic memory encoding changed as a function of processing depth, and how these changes were related to episodic memory ability. Centrality changes in right middle frontal gyrus, right inferior parietal lobule and left superior frontal gyrus were positively related to semantic elaboration during encoding. In the same regions, centrality during successful episodic memory encoding was related to performance on the episodic memory task, indicating that these centrality changes reflect processes that support memory encoding through deep elaborative processing. Similar analyses were performed for congruent trials, i.e. events that fit into existing knowledge structures, but no relationship between centrality changes and congruity were found. The results demonstrate that while elaboration and congruity have similar beneficial effects on source memory performance, the cortical signatures of these processes are probably not identical.
Highlights
Episodic memory formation is dependent on the hippocampus and medial temporal lobe (MTL) structures[7,8], while memory storage depends on representations that are distributed across cortical networks, in a process of modality-specific cross-cortical storage[9]
We regarded less than ten trials in any of the categories used in the fMRI analyses to be insufficient for analysis, and ten participants who met these exclusion criteria were excluded from further analyses
(1) We first defined regions that were involved in episodic memory encoding by contrasting events that were subsequently remembered with full source information with those that were not. 40 nodes, including both hippocampi brought to the graph analyses. (2) centrality measures were calculated for all nodes, (3) and we identified nodes that increased centrality during deep processing
Summary
Episodic memory formation is dependent on the hippocampus and medial temporal lobe (MTL) structures[7,8], while memory storage depends on representations that are distributed across cortical networks, in a process of modality-specific cross-cortical storage[9]. The beneficial effect processing depth has on episodic memory may be reflected through tighter integration and interaction between neocortical brain regions, and changes in the organization of the network of encoding-relevant regions during the initial binding of cortical memory representations. While elaboration traditionally refers to how strongly a mental representation has been enriched during encoding by semantic evaluations[25], congruity is a dimension of the LOP framework that refers to how well target words or items fit into existing knowledge structures or schemas[26] Both elaboration and congruity are related to subsequent memory performance and can be operationalized as attributes of the encoding situation that can have different levels[2,5,25,27,28,29]
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