Figure 3.

Hippocampal ripples during encoding and recall predict successful associative memory. Ripple events were detected using a bipolar montage from the electrodes located in or closest to the hippocampus, using a previously published method (80–120 Hz, 20–200 ms duration).¹⁰ To reduce the detections of pathological high frequency oscillations (HFOs), detections were restricted to regions outside of SOZ, and with trials which did not contain an IED. (A) Sample of a raw EEG tracing (blue) with detected ripple event (red arrow), with bandpass filtered (80–120 Hz) tracing (black). Scale bar = 125 ms (B) Characteristics of all detected hippocampal ripples. An average of 1955 ripple events were detected per patient across all conditions. Top: Grand averaged ripple response (left) and spectrogram (right, 10–200 Hz) demonstrates a peak frequency between 80 Hz and 100 Hz. Scale bars = 125 ms. Bottom: Histogram showing detected ripple duration, which follows skewed log distribution. Mean ripple duration 39.8 ms, SD 18.3 ms. (C) Average ripple rate between Successful and Failed Associative Memory Trials (mean ± standard error of the mean). Top: Successful encoding is characterized by a higher hippocampal ripple rate (blue) compared to failed encoding (red) between 750 ms and1375 ms after stimulus presentation (grey box, n = 14, P < 0.05, cluster-corrected). Middle: Successful cued recall is characterized by a higher ripple rate compared to failed cued recall between +1250 ms and +1625 ms after stimulus presentation (grey box, n = 14, P < 0.05, cluster-corrected). Bottom: Successful cued recall is characterized by a higher ripple rate from −750 ms to 0 ms aligned voice response (grey box, n = 9, P < 0.05, cluster-corrected).