Abstract
AimRipple-band epileptic high-frequency oscillations (HFOs) can be recorded by scalp electroencephalography (EEG), and tend to be associated with epileptic spikes. However, there is a concern that the filtration of steep waveforms such as spikes may cause spurious oscillations or “false ripples.” We excluded such possibility from at least some ripples by EEG differentiation, which, in theory, enhances high-frequency signals and does not generate spurious oscillations or ringing.MethodsThe subjects were 50 pediatric patients, and ten consecutive spikes during sleep were selected for each patient. Five hundred spike data segments were initially reviewed by two experienced electroencephalographers using consensus to identify the presence or absence of ripples in the ordinary filtered EEG and an associated spectral blob in time-frequency analysis (Session A). These EEG data were subjected to numerical differentiation (the second derivative was denoted as EEG″). The EEG″ trace of each spike data segment was shown to two other electroencephalographers who judged independently whether there were clear ripple oscillations or uncertain ripple oscillations or an absence of oscillations (Session B).ResultsIn Session A, ripples were identified in 57 spike data segments (Group A-R), but not in the other 443 data segments (Group A-N). In Session B, both reviewers identified clear ripples (strict criterion) in 11 spike data segments, all of which were in Group A-R (p < 0.0001 by Fisher’s exact test). When the extended criterion that included clear and/or uncertain ripples was used in Session B, both reviewers identified 25 spike data segments that fulfilled the criterion: 24 of these were in Group A-R (p < 0.0001).DiscussionWe have demonstrated that real ripples over scalp spikes exist in a certain proportion of patients. Ripples that were visualized consistently using both ordinary filters and the EEG″ method should be true, but failure to clarify ripples using the EEG″ method does not mean that true ripples are absent.ConclusionThe numerical differentiation of EEG data provides convincing evidence that HFOs were detected in terms of the presence of such unusually fast oscillations over the scalp and the importance of this electrophysiological phenomenon.
Highlights
High-frequency oscillations (HFOs) have attracted attention due to their close relationship with epileptogenicity (Jacobs et al, 2010; Akiyama et al, 2011; Frauscher et al, 2017; Thomschewski et al, 2019)
Ripple band epileptic HFOs can be recorded from a scalp electroencephalogram (EEG; Kobayashi et al, 2010; Andrade-Valenca et al, 2011; Zelmann et al, 2014; von Ellenrieder et al, 2014; Shibata et al, 2016; Bernardo et al, 2018), and they are suggested to indicate disease severity in children with developmental and epileptic encephalopathy, such as West syndrome (Kobayashi et al, 2011; Kobayashi et al, 2015; van Klink et al, 2016; Nariai et al, 2020)
Time–frequency analysis (TFA) supplements HFO detection, time–frequency analysis (TFA) is not a perfect solution because the spectra are based on frequency analysis, and occasionally, noisy and high-frequency spectral blobs may be buried in lower-frequency activities
Summary
High-frequency oscillations (HFOs) have attracted attention due to their close relationship with epileptogenicity (Jacobs et al, 2010; Akiyama et al, 2011; Frauscher et al, 2017; Thomschewski et al, 2019). Tend to be associated with epileptic discharges or spikes, and there is a concern that filtration of steep waveforms such as spikes may cause spurious oscillations or “false ripples” (Bénar et al, 2010). We aimed to exclude the possibility of such false ripples from at least some ripples that were observed in association with scalp spikes using numerical differentiation processing of EEG data, which relatively enhances high-frequency signals and does not generate spurious oscillations or ringing, in theory. There are several methods to reduce slow frequency activity (i.e., whitening) to improve HFO detectability, but avoiding the generation of spurious oscillations has not received much attention (Roehri et al, 2016). We intended to show that ripples really exist in association with spikes using a straightforward method
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