Abstract
Ripple oscillations (80–200 Hz) in the normal hippocampus are involved in memory consolidation during rest and sleep. In the epileptic brain, increased ripple and fast ripple (200–600 Hz) rates serve as a biomarker of epileptogenic brain. We report that both ripples and fast ripples exhibit a preferred phase angle of coupling with the trough-peak (or On-Off) state transition of the sleep slow wave in the hippocampal seizure onset zone (SOZ). Ripples on slow waves in the hippocampal SOZ also had a lower power, greater spectral frequency, and shorter duration than those in the non-SOZ. Slow waves in the mesial temporal lobe modulated the baseline firing rate of excitatory neurons, but did not significantly influence the increased firing rate associated with ripples. In summary, pathological ripples and fast ripples occur preferentially during the On-Off state transition of the slow wave in the epileptogenic hippocampus, and ripples do not require the increased recruitment of excitatory neurons.
Highlights
In the epileptic brain, ripple oscillations (80–200 Hz) exhibit increased rates in epileptogenic mesial-temporal regions [1, 2]
To examine if the phase of the slow wave correlates with the generation of fast ripples and ripples (Figures 1A,B) first we compared the probability density function (PDF) of the phase angles of coupling between fast ripples and slow waves (Figure 1C1), and the PDF of ripples and slow waves in the seizure onset zone (SOZ, Figure 1C1)
We show in the hippocampal SOZ and NSOZ, fast ripples occur preferentially during the trough-peak or On-Off state transition of the slow wave
Summary
Ripple oscillations (80–200 Hz) exhibit increased rates in epileptogenic mesial-temporal regions [1, 2]. Ripples are important in memory consolidation during rest and sleep [3]. Neocortical ripples during the trough-peak (or On-Off) state transition of the non-rapid eye movement (NREM) sleep slow wave are found at a higher density in epileptogenic tissue and are considered pathological [4,5,6] (Figures 1A,B). Ripples are associated with a 5- to 6-fold increase in stratum pyramidale principal cell firing and 2- to 3-fold increase in stratum pyramidale and oriens non-principal firing. Normal ripples are involved with memory consolidation and generated preferentially during the Off-On transition of the neocortical slow wave [13]
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