Research ArticleNEUROSCIENCE

Selective and coherent activity increases due to stimulation indicate functional distinctions between episodic memory networks

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Science Advances  22 Aug 2018:
Vol. 4, no. 8, eaar2768
DOI: 10.1126/sciadv.aar2768
  • Fig. 1 Experiment design overview.

    (A) We selected subject-specific left parietal stimulation locations on the basis of seed-based resting-state fMRI connectivity with anatomically defined hippocampal locations. Circles indicate these locations for each participant. (B) Before and ~24 hours after five consecutive daily stimulation sessions, participants completed an fMRI memory task. We administered stimulation and sham conditions within subjects in a counterbalanced order. Representative electrical fields (e-fields) for one subject demonstrate stimulation and sham intensities, with red indicating peak intensity (color bar range, 1 to 210 V/m). (C) In separate blocks, participants studied trial-unique objects paired with either scene or location contexts during fMRI scanning. After a delay, we assessed object recognition memory and contextual recollection memory. Responses were used to identify trials during study that were later correct, thereby providing fMRI signals of successful memory formation. We used different stimuli for each fMRI task assessment.

  • Fig. 2 Stimulation coherently increased PM network fMRI activity.

    (A) We selected PM and AT network regions of interest a priori (18, 29). L, left; R, right. (B) Mean fMRI activity evoked by stimuli during memory formation averaged for the PM and AT networks for the Post-Stim and Post-Sham conditions, demonstrating selective increases in PM network activity due to stimulation (Post-Stim versus Post-Sham). (C) Mean fMRI activity changes due to stimulation (Post-Stim minus Post-Sham) averaged for each region within the PM and AT networks demonstrate consistent increases for the PM network but not for the AT network. (D) Coherence of activity changes due to stimulation is shown via a correlation graph, with coloration indicating between-region correlations of activity changes across subjects. We quantified the coherence as the mean correlation for each network and between networks, as indicated via bar graphs, demonstrating network-specific coherence of changes that were greatest for the PM network. Error bars indicate SEM. *P < 0.05, **P < 0.01, and ***P < 0.001.

  • Fig. 3 Effects of stimulation on voxel-level activity and memory performance.

    (A) Whole-brain, voxel-wise analyses show regions of significant activity change due to stimulation (Post-Stim minus Post-Sham) unconstrained by the PM-AT network regions of interest. Coloration indicates voxels meeting the significance threshold (see Materials and Methods), which all showed greater activity Post-Stim versus Post-Sham in the PM network regions (table S3). (B) Mean item recognition did not change because of stimulation. (C) Mean contextual recollection increased because of stimulation, relative to Baseline and relative to sham. (D) Each bar represents a single subject change in contextual recollection accuracy Post-Stim relative to Baseline and Post-Sham relative to Baseline, showing that stimulation consistently improved contextual recollection, whereas increases occurred in only ~50% of subjects due to sham. Error bars indicate SEM. *P < 0.05 and **P < 0.025 (one-tailed).

  • Fig. 4 Prefrontal control stimulation did not affect the PM network.

    (A) Prefrontal locations used for control stimulation are shown following the format of Fig. 1A. (B) Mean fMRI activity evoked by stimuli during memory formation averaged for the PM and AT networks for the Post-Stim and Post-Sham conditions, indicating no significant effects of stimulation. Effects of stimulation on the PM network were significantly greater for subjects receiving parietal stimulation than those receiving prefrontal control stimulation (not shown, see text). (C) Whole-brain, voxel-wise analyses indicated that there were no significant effects of stimulation on activity (Post-Stim versus Post-Sham) for the prefrontal control group in PM-AT network areas, but there were significant decreases in right dorsolateral prefrontal cortex activity (not shown, see text and table S4). Coloration indicates areas where there were significant between-group differences, reflecting greater activity increases due to stimulation (Post-Stim versus Post-Sham) for the parietal stimulation group relative to the prefrontal control stimulation group. Notably, these areas were located within the PM network, confirming that PM network activity increased selectively for parietal stimulation relative to prefrontal control stimulation (table S5).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/8/eaar2768/DC1

    Table S1. fMRI activity in individual regions of the PM network.

    Table S2. fMRI activity in individual regions of the AT network.

    Table S3. Clusters showing increased activity due to parietal stimulation (Post-Stim versus Post-Sham).

    Table S4. Activity changes due to prefrontal active control stimulation.

    Table S5. Clusters showing greater activity increases (Post-Stim versus Post-Sham) due to parietal stimulation versus control prefrontal stimulation.

  • Supplementary Materials

    This PDF file includes:

    • Table S1. fMRI activity in individual regions of the PM network.
    • Table S2. fMRI activity in individual regions of the AT network.
    • Table S3. Clusters showing increased activity due to parietal stimulation (Post-Stim versus Post-Sham).
    • Table S4. Activity changes due to prefrontal active control stimulation.
    • Table S5. Clusters showing greater activity increases (Post-Stim versus Post-Sham) due to parietal stimulation versus control prefrontal stimulation.

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