Fig. 7

Ripple-mediated information transfer. a Illustration of the information analysis strategy. A single EEG trace featuring SOs and spindles (in blue) and the corresponding MTL trace (in red) are depicted relative to a ripple event (see close-up; in black). Undirectional information was calculated by a moving time-window approach (center left), while directional information was calculated by keeping one window centered on the MTL ripple time point (center right; here fixed MTL window to moving EEG window; EEG to MTL information was calculated accordingly). b Left: time-resolved Mutual Information (MI) relative to the hippocampal ripple event (mean ± SEM). We detected enhanced MI between the hippocampus and prefrontal cortex ~1 s after the ripple event (1.3–2.0 s; cluster test: p = 0.0040, d = 1.06), but also observed two non-significant clusters (positive cluster from 0.25 to 0.35 s, p = 0.0849; negative cluster from −0.15 to −0.10 s, p = 0.1349). Right: spectrally resolved undirected mutual information. Significant (two-tailed p < 0.05) deviations from the average MI. Prior to ripple onset, mutual information was enhanced in the spindle and SO/delta range, which after 1 s dropped. c Left: time-resolved MI relative to the ripple event. We disentangled the information flow from prefrontal EEG to the MTL (purple) and vice versa (cyan). A significant cluster was observed after the ripple (cluster test: negative cluster from 0.15 to 0.50 s; p = 0.0130, d = 1.31), indicating increased information flow from the prefrontal EEG to the MTL. We found a second significant cluster where MI increased after ~1 s, which mainly reflected MTL to prefrontal EEG information flow (positive cluster from 1.1 to 1.85 s; p = 0.001, d = 1.08). Center/right: frequency- and directionality-resolved information flow. Outlined areas (black) reflect significant clusters (p < 0.05), where information flow was enhanced relative to the mean. Center: frequency-specific information flow from the MTL to neocortex was primarily increased in the SO (<2 Hz) and spindle-bands (~16 Hz). Right: information flow from the prefrontal EEG to MTL was not frequency-specific. d Upper panel: phase transfer entropy between the prefrontal EEG and the MTL (>1) is stronger than in the opposite direction (<1), in particular in the spindle-band (from −1–1.4 s; cluster test: p < 0.0010, d = 2.02). Lower left: excerpt during the ripple (t = 0) of the normalized PTE spectrum highlights the peak at ~16 Hz. Lower right: single subject observations in the spindle-band during the ripple. e Normalized spindle occurrence relative to the information peak. Spindle occurrence was significantly enhanced prior to the information peak (−2 to −0.5 s; peak time −0.91 s; cluster test: p = 0.001, d = 3.37). f Left: spindle-locked time-frequency representation of a single subject over 30 s highlights the slow (~0.4 Hz) pattern in spindle power. Superimposed is the rescaled, time-resolved undirected MI between the MTL and scalp EEG (black). Right: average spindle power (red; demeaned) and undirected MI (black; z-scored) indicating an anti-phasic relationship (gray arrows indicate MI peaks that coincide with spindle troughs). Note that the y-axis is truncated at around 0 to highlight the side lobes. Inset: group-level results revealing a statistically significant anti-phasic relationship between spindle power and MI between the MTL and frontal EEG sensors. g MI was enhanced only after a physiologic ripple as compared to an artifactual ripple event. Given that artifactual ripples were noisy and hence, variance was increased, we averaged in the time domain (cluster in panel a) prior to statistical testing. h MI was enhanced only after a physiologic ripple as compared to an artifactual ripple event (paired t-test: t₁₇ = 2.82, p = 0.0117, d = 0.70). Note that artifactual ripples were also not significantly different from the baseline (paired t-test: t₁₇ = −0.04, p = 0.9699, d = 0.01), thus, indicating that no information was transferred. The asterisk indicates that physiologic ripples were different from baseline as tested in panel a. i Topographical depiction of significant MI increases. All electrodes that showed a significant MI increase (z > 1.96, uncorrected p < 0.05) are color-coded according to the z-score, while electrodes without significant modulation are depicted in white. Note, the increase in MI was widespread and not restricted to a circumscribed cortical region. j MI increases per prefrontal sub-region (RM-ANOVA: F_{1.64, 23.01} = 9.13, p = 0.0020, η² = 0.05). The strongest increase was observed in the dlPFC. All error bars indicate the mean ± SEM