Fig. 4

Physiological and artifactual ripple activity in the MTL. a Ripple-locked grand average (blue; N = 18) and the band-bass filtered signal (black; mean ± SEM). b Artifactual ripple events grand average (red) that are characterized by sharp transients that are artificially rendered sinusoidal by band-pass filtering (black). c 1/f-corrected power spectrum (mean ± SEM; irregular resampling) in the MTL during a ripple event exhibits two distinct oscillatory peaks: in the delta and spindle frequency range. Gray shaded areas indicate significant oscillatory activity (cluster test: p = 0.001). d Histogram of ripple occurrence relative to the cortical SO as detected at scalp level. Note ripple occurrence is enhanced during the cortical upstate (i.e., the SO peak, first cluster: p = 0.0010, d = 2.82; second cluster: p = 0.0040, d = 1.42), but significant more ripples occur prior to the downstate (in gray; 400 ms window around the SO peaks, paired t-test: t₁₇ = 5.01, p < 0.0001, d = 1.88). e This relationship was not present for artifactual ripple events, which occurred at random (smallest cluster p = 0.450; no peak differences: t₁₇ = −0.63, p = 0.5381, d = 0.27). f Physiologic ripples were preferentially nested in the trough of the cortical spindle oscillation (V-test against ± pi: v = 5.13, p = 0.0434, mvl = 0.29). g This tight temporal relationship was again not present for artifactual ripple events (v = 1.46, p = 0.3135, mvl = 0.08). h Single subject ripple-locked EEG spectrogram indicates the presence of cortical spindle oscillations during a hippocampal ripple. Note that activity in the spindle range can be observed before and after the ripple (every ~3 s; asterisks indicate elevate spindle power). i Left Spectral analysis of the spindle amplitude (green), its fractal (1/f estimate; black dashed line) as obtained from irregular resampling as well as the wake condition as a control (absence of spindle activity; cluster test: p = 0.0490, d = 0.49). Right: after subtraction of the fractal component from the NREM spectrum, we observed a distinct oscillatory peak indicating a rhythmic comodulation at ~0.37 ± 0.03 Hz (median ± SEM; sign test against zero: p < 0.0001; d = 0.70). All error bars indicate the mean ± SEM