Abstract
Until now there has been no way of distinguishing between physiological and epileptic ripples in intracranial recordings. In the present study we addressed this by investigating the effect of cognitive stimulation on interictal high frequency oscillations in ripple range (80–250 Hz) within epileptic and non-epileptic hippocampus. We analyzed depth EEG recordings in 10 patients with intractable epilepsy, in whom hippocampal activity was recorded during quiet wakefulness and subsequently during a simple cognitive task. Using automated detection of ripples based on amplitude of power envelope, we analyzed ripple rate (RR) in the cognitive and resting period, within epileptic (EH) and non-epileptic hippocampi (NH). Compared to quiet wakefulness we observed a significant reduction of RR during cognitive stimulation in EH, but not 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 the stimuli onset in NH only. Our results thus point to differential reactivity of ripples within EH and NH to cognitive stimulation.
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