Abstract
BackgroundUntil now there has been no way of distinguishing between physiological and epileptic hippocampal ripples in intracranial recordings. In the present study we addressed this by investigating the effect of cognitive stimulation on interictal high frequency oscillations in the ripple range (80–250 Hz) within epileptic (EH) and non-epileptic hippocampus (NH).MethodsWe analyzed depth EEG recordings in 10 patients with intractable epilepsy, in whom hippocampal activity was recorded initially during quiet wakefulness and subsequently during a simple cognitive task. Using automated detection of ripples based on amplitude of the power envelope, we analyzed ripple rate (RR) in the cognitive and resting period, within EH and NH.ResultsCompared to quiet wakefulness we observed a significant reduction of RR during cognitive stimulation in EH, while it remained statistically marginal in NH. Further, we investigated the direct impact of cognitive stimuli on ripples (i.e. immediately post-stimulus), which showed a transient statistically significant suppression of ripples in the first second after stimuli onset in NH only.ConclusionOur results point to a differential reactivity of ripples within EH and NH to cognitive stimulation.
Highlights
Until now there has been no way of distinguishing between physiological and epileptic hippocampal ripples in intracranial recordings
We investigated whether any differences existed in the duration of high-frequency oscillations (HFOs) between EH and non-epileptic hippocampus (NH), during both resting-state and cognitive stimulation
The reduction of mean ripple rate (RR) during the execution of cognitive task was significant in EH (p < 0.001) but only marginal in NH (p = 0.06; indicating a trend)
Summary
Until now there has been no way of distinguishing between physiological and epileptic hippocampal ripples in intracranial recordings. Clinical macroelectrode recordings within mesial temporal structures in epileptic patients inconsistently report an increase of ripples on the side of epileptogenic tissue This contrasts with observations from previous microrecordings in animals [14]. Brázdil et al BMC Neurosci (2015) 16:47 different electrode types might lead to differing results concerning the contribution of hippocampal ripples to the lateralization of an epileptogenic region. Their clinical/diagnostic utility is compromised by the inability to distinguish normal from pathological ripples in invasive electroencephalographic (EEG) recordings. Achieving such a differentiation is important to understand the fundamental mechanisms behind normal cognitive functions, and for the utilization of ripples as a potential clinical biomarker for the identification of an epileptogenic region
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