Fig. 4: EEG-based and fMRI-based replay during cued mental simulation.

a The illustration of two analysis methods for detecting replays. TDLM⁴³ is used primarily with MEG^{3,6,30,32,45,46}, and more recently also with EEG²⁷. The other is a regression method, as per Wittkuhn and Schuck²⁸ and primarily used with fMRI²⁹. Please note that this panel is solely for illustrative purposes. For results based on actual data, refer to Supplementary Fig. 5 and Panel c. b EEG-based replay with TDLM, separately for forward (cued “\(1\to\)”, top row) and backward (cued “\(\leftarrow 4\)”, bottom row) mental simulation conditions. There were significant forward (but not reverse) replays during both forward and backward mental simulation. Sequence strength on the peak time lag (30 ms) is shown on the right, separately for forward and backward mental simulation conditions (two-sided paired t-test, forward condition: t₍₃₂₎ = 2.80, P = 0.009; backward condition: t₍₃₂₎ = 3.09, P = 0.004). The grey dash line represents the permutation threshold, defined as the 95^th percentile of the permutated transitions of interest controlling for multiple comparisons. n = 33. c fMRI-based neural sequence with regression method²⁸, separately for forward and backward mental simulation conditions. There was no significant evidence for sequential activation in the correct order (all P_corr. ≥ 0.06, two-sided one-sample t-test against zero, FDR corrected). The bar plot in the upper right corner shows mean slope coefficients for each period (two-sided paired t-test, forward condition: t₍₃₂₎ = 1.14, P = 0.260; backward condition: t₍₃₂₎ = −0.175, P = 0.862). None of these coefficients were significantly different compared to zero. See Supplementary Fig. 5 for assessing fMRI replay using TDLM, as well as results from single subject for illustration purpose. n = 33. d The parametric modulation of EEG-based replay probability in the whole-brain fMRI during mental simulation showed significant activations in hippocampus and mPFC. We use whole-brain FWE correction at the cluster level (P < 0.05) with a cluster-inducing voxel threshold of P_unc. < 0.001. e The psychophysiological interaction (PPI) between hippocampal activity (anatomically defined) and EEG-based replay probability revealed significant functional connectivity change in mPFC, PCC and visual cortex. See Supplementary Fig. 7c-d for mPFC-based PPI results. We use whole-brain FWE correction at the cluster level (P < 0.05) with a cluster-inducing voxel threshold of P_unc. < 0.01. Each dot is one subject. The grey lines connect results from the same subject. Shaded areas in b and c show SEM across subjects. Error bars in b and c show SEM across subjects. * P < 0.05, ** P < 0.01, ns., not significant. Abbreviation: HPC - hippocampus. Source data are provided as a Source Data file.