BACKGROUND: Until 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). METHODS: We 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. RESULTS: Compared 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. CONCLUSION: Our results point to a differential reactivity of ripples within EH and NH to cognitive stimulation.

Behavioural and Social Neuroscience Research Group CEITEC Central European Institute of Technology Masaryk University Brno Czech Republic

Department of Neurology Brno Epilepsy Center St Anne's University Hospital and Medical Faculty of Masaryk University Pekařská 53 Brno 65691 Czech Republic

Department of Neurology Mayo Systems Electrophysiology Laboratory Mayo Clinic Rochester MN USA

Department of Physiology Medical Faculty of Masaryk University Brno Czech Republic

Institute of Scientific Instruments Academy of Sciences of the Czech Republic Brno Czech Republic

International Clinical Research Center St Anne's University Hospital Brno Czech Republic

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