Abstracts

Rationale: Interictal HFOs (ripples and fast ripples) have been repeatedly identified in recordings from depth macroelectrodes in epileptics. In contrast to fast ripples, which are believed to reflect the neuronal substrates of epileptogenicity, ripples are considered to be a signature of both normal and epileptic brain processes. The differentiation of physiological and epileptic ripples in intracranial recordings remains unavailable. Methods: We analyzed SEEG recordings in ten patients with intractable partial seizures in whom hippocampal activity was recorded in resting state and subsequently during simple cognitive task with randomly presented frequent and rare visual stimuli. Using automated detection of ripples based on length of power envelope, we analyzed potential differences in ripple rate (RR) in the cognitive versus resting period, within epileptic (EH) and non-epileptic hippocampi (NH). Further direct impact of cognitive stimuli on ripples (immediately after the stimulus) was investigated.Results: Ripples have been detected within hippocampal recordings in all the investigated subjects. Mean RR in resting periods was 16.38 12.18/min within EH, and 19.38 17.4/min within NH. In the cognitive task periods mean RR within EH and NH decreased to 11.22 7.74/min, and 17.16 15.0/min respectively. The reduction of RR during cognitive stimulation was significant in EH (p<0.001), but not in NH (p=0.06). Interestingly we observed a transient suppression of ripples in both EH (slight) and NH (significant) in the first second after the stimuli onset, followed by a significant increase in RR for both EH and NH.Conclusions: Our results point to different reactivity of ripples within EH and NH to the cognitive stimulation.