The hippocampus is coupled with the default network during memory retrieval but not during memory encoding - PubMed (original) (raw)

The hippocampus is coupled with the default network during memory retrieval but not during memory encoding

Willem Huijbers et al. PLoS One. 2011.

Abstract

The brain's default mode network (DMN) is activated during internally-oriented tasks and shows strong coherence in spontaneous rest activity. Despite a surge of recent interest, the functional role of the DMN remains poorly understood. Interestingly, the DMN activates during retrieval of past events but deactivates during encoding of novel events into memory. One hypothesis is that these opposing effects reflect a difference between attentional orienting towards internal events, such as retrieved memories, vs. external events, such as to-be-encoded stimuli. Another hypothesis is that hippocampal regions are coupled with the DMN during retrieval but decoupled from the DMN during encoding. The present fMRI study investigated these two hypotheses by combining a resting-state coherence analysis with a task that measured the encoding and retrieval of both internally-generated and externally-presented events. Results revealed that the main DMN regions were activated during retrieval but deactivated during encoding. Counter to the internal orienting hypothesis, this pattern was not modulated by whether memory events were internal or external. Consistent with the hippocampal coupling hypothesis, the hippocampus behaved like other DMN regions during retrieval but not during encoding. Taken together, our findings clarify the relationship between the DMN and the neural correlates of memory retrieval and encoding.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Experimental Task.

Encoding trials consisted of three periods: a 1-second cue period, a 3-second encoding period, and a 1.5-second rating period. During the cue period, participants were introduced with a cue word together with an icon that indicated the trial condition. During the encoding phase, dependent on the icon, they either imagined an image or sound associated with the word (internally oriented conditions: Int-Enc) or they either perceived a sound or image associated with the word (externally oriented conditions: Ext-Enc). During the rating period, participants rated the imagery quality or perceptual richness of their experience. Retrieval trials, presented on the subsequent day, consisted of two periods, a 4-sec retrieval period, and a 1.5-sec confidence rating period. During the retrieval period, participants viewed the cue words from the previous day and retrieved the correct encoding source (1 = imagined sound, 2 = heard sound, 3 = imagined image, 4 = observed image). During the confidence rating period, they rated their confidence about their retrieval decision (“unsure”/“sure”).

Figure 2. fMRI Results.

Brain regions identified by the resting state analysis are depicted in blue (p<0.005, FDR corrected, cluster size >25 voxels): (A) posterior cingulate cortex, (B) ventral parietal cortex, (C) medial and (D) superior prefrontal cortex and (E) hippocampal formation. Bar graphs indicate mean cluster activity based on the memory-task (averaged over left and right sides in the case of bilateral activations). Left bar graphs show activity during encoding for internal (Int) and external (Ext) orienting conditions combined over hits and misses. Right bar graphs reflect the difference in activity between hits and misses for encoding (Int-Enc / Ext-Enc) and retrieval (Int-Ret / Ext-Ret). Vertical lines indicate SEMs.

Figure 3. fMRI Results.

Direct comparison of two ventral parietal regions: the angular gyrus (ANG – blue) and supramarginal gyrus/temporoparietal junction (TPJ - yellow). Bar graphs indicate mean cluster activity based on the memory-task (averaged over left and right sides. Left bar graphs show activity during encoding for internal (Int) and external (Ext) orienting conditions combined over hits and misses. Right bar graphs reflect the difference in activity between hits and misses for encoding (Int-Enc / Ext-Enc) and retrieval (Int-Ret / Ext-Ret). Vertical lines indicate SEMs.

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The hippocampus is coupled with the default network during memory retrieval but not during memory encoding - PubMed (original) (raw)