Fig. 4.

Hippocampal ripples. (a) (top) Grand average unfiltered iEEG trace across participants (mean ± s.e.m.), aligned to the maximum of the ripple peak (time 0). The first inset highlights the oscillatory nature of the detected events in the unfiltered raw EEG. The second inset shows the result from a spectral analysis of the grand average EEG (calculated from −1 s to +1 s via FieldTrip’s ‘mtmfft’ function for frequencies from 1–10 Hz in steps of .5 Hz), revealing a spectral peak at 3 Hz (delta wave). (middle) Average of ripple-peak-locked TFR (% change from pre-event baseline), highlighting the band-limited nature of ripples. (bottom) Zoom into 5–30 Hz, showing the grand-average TFR before (top) and after (bottom) statistical thresholding (P < .05, corrected) to highlight the ripple-locked increase in spindle power. (b) Data from a sample participant. (top) ripple-locked average raw EEG trace (solid line) with the bandpass filtered trace (13.5–14.5 Hz; dotted line) superimposed to highlight the nesting of ripples in spindle troughs. (bottom). Corresponding ripple-locked TFR, highlighting that participant’s maxima in the ripple- and spindle ranges. (c) Ripple-ripple peri-event time histogram (PETH). Occurrence probabilities of other ripples relative to individual ripples’ center time. A synthetic 14.5 Hz oscillation is superimposed to illustrate the periodicity of ripple occurrences following successive spindle cycles. Red squares project the histogram maxima from −250 ms to +250 ms relative to the ripple center to better visualize the correspondence of occurrences with individual spindle cycles.