Portraying the unique contribution of the default mode network to internally driven mnemonic processes

Abstract

Numerous neuroimaging studies have implicated default mode network (DMN) involvement in both internally driven processes and memory. Nevertheless, it is unclear whether memory operations reflect a particular case of internally driven processing or alternatively involve the DMN in a distinct manner, possibly depending on memory type. This question is critical for refining neurocognitive memory theorem in the context of other endogenic processes and elucidating the functional significance of this key network. We used functional MRI to examine DMN activity and connectivity patterns while participants overtly generated words according to nonmnemonic (phonemic) or mnemonic (semantic or episodic) cues. Overall, mnemonic word fluency was found to elicit greater DMN activity and stronger within-network functional connectivity compared with nonmnemonic fluency. Furthermore, two levels of functional organization of memory retrieval were shown. First, across both mnemonic tasks, activity was greater mainly in the posterior cingulate cortex, implying selective contribution to generic aspects of memory beyond its general involvement in endogenous processes. Second, parts of the DMN showed distinct selectivity for each of the mnemonic conditions; greater recruitment of the anterior prefrontal cortex, retroesplenial cortex, and hippocampi and elevated connectivity between anterior and posterior medial DMN nodes characterized the semantic condition, whereas increased recruitment of posterior DMN components and elevated connectivity between them characterized the episodic condition. This finding emphasizes the involvement of DMN elements in discrete aspects of memory retrieval. Altogether, our results show a specific contribution of the DMN to memory processes, corresponding to the specific type of memory retrieval.

Fig. 1.

Recruitment of the DMN during the word fluency tasks. Whole-brain group activation maps of regions showing deactivation during the phonemic (blue), semantic (yellow), and episodic (pink) word fluency tasks compared with baseline fixation. Overlaps between the phonemic and semantic conditions are presented in green, overlaps between phonemic and episodic conditions are presented in purple, and overlaps between the three conditions are presented in gray (with black outline). Random, n = 16, P < 0.05 FDR-corrected, cluster size > 5 × 3³ voxels. Cluster size (in thousands of voxels) is presented in six key regions. The amPFC and left hippocampus were less deactivated during semantic word fluency, whereas posterior DMN nodes (right IPL and precuneus) were less deactivated during episodic word fluency. *Significant differences in cluster size between the word fluency conditions (Tukey posthoc comparisons, P < 0.05).

Fig. 2.

Selectivity to the word fluency tasks. (A) Whole-brain group activation maps of regions showing increased activity during the phonemic (blue), semantic (yellow), and episodic (pink) word fluency tasks. Each condition was compared with the other two conditions. The green outline marks the regions that were deactivated during any of the tasks (that is, the DMN). Selectivity to the episodic and semantic word fluency tasks includes DMN components, whereas selectivity to the phonemic word fluency condition is seen in classic language areas. Random, n = 16, P < 0.05 FDR-corrected, cluster size > 5 × 3³ voxels. (B) Whole-brain group activation maps of regions showing greater response during the mnemonic conditions (semantic and episodic word fluency) compared with the nonmnemonic condition (phonemic word fluency). Greater recruitment of the posterior cingulate nodes of the DMN was seen during mnemonic word fluency. The RSC is lateralized and not shown in the midsagital section. Random, n = 16, P < 0.05 FDR-corrected, cluster size > 5 × 3³ voxels. The bar plots present the averaged β-values in mnemonic-selective regions (marked in orange).

Fig. 3.

Functional connectivity within the DMN. Significant differences between the degree of correlation of each pair of DMN nodes in the three word fluency conditions (P < 0.05, Bonferroni-corrected). Posterior parts of the DMN are more interconnected during episodic memory compared with other word fluency conditions, whereas the anterior aspect of the DMN is more connected to posterior aspects of the DMN in the semantic condition compared with other word fluency conditions.