Hippocampal-cortical coupling differentiates long-term memory processes.

Reactivation of long-term memories enables experience-dependent strengthening, weakening, or updating of memory traces. Although coupling of hippocampal and cortical activity patterns facilitates initial memory consolidation, whether and how these patterns are involved in postreactivation memory processes are not known. Here, we monitored the hippocampal-cortical network as rats repetitively learned and retrieved spatial and nonspatial memories. We show that interactions between hippocampal sharp wave-ripples (SPW-R), cortical spindles (SPI), and cortical ripples (CXR) are jointly modulated in the absence of memory demand but independently recruited depending on the stage of memory and task type. Reconsolidation of memory after retrieval is associated with an increased and extended window of coupling between hippocampal SPW-Rs and CXRs compared to the initial consolidation. Hippocampal SPW-R and cortical spindle interactions are preferentially engaged during memory consolidation. These findings suggest that specific, time-limited patterns of oscillatory coupling can support the distinct memory processes required to flexibly manage long-term memories in a dynamic environment.

Interictal epileptiform discharges shape large-scale intercortical communication.

Dynamic interactions between remote but functionally specialized brain regions enable complex information processing. This intercortical communication is disrupted in the neural networks of patients with focal epilepsy, and epileptic activity can exert widespread effects within the brain. Using large-scale human intracranial electroencephalography recordings, we show that interictal epileptiform discharges (IEDs) are significantly coupled with spindles in discrete, individualized brain regions outside of the epileptic network. We found that a substantial proportion of these localized spindles travel across the cortical surface. Brain regions that participate in this IED-driven oscillatory coupling express spindles that have a broader spatial extent and higher tendency to propagate than spindles occurring in uncoupled regions. These altered spatiotemporal oscillatory properties identify areas that are shaped by epileptic activity independent of IED or seizure detection. Our findings suggest that IED-spindle coupling may be an important mechanism of interictal global network dysfunction that could be targeted to prevent disruption of normal neural activity.

Chaotic Analysis of Hippocampal and Cortical Sleep EEG during Various Vigilance States.

In this paper, we investigate the hippocampal and cortical sleep EEG of adult rats at different sleep stages by employing Lyapunov exponent and third-order cumulant measures to quantify and compare the chaotic and nonlinear behavior of EEG obtained during vigilance states of quiet- waking, slow-wave sleep, and rapid eye movement (REM) sleep. Lyapunov exponent was computed to characterize the EEG for chaos and third-order cumulant was used to measure the deviations from Gaussianity of the signal. Our results show positive Lyapunov exponents for all EEG states indicating a Iow- dimensional chaos for both REM and non-REM system. Furthermore, REM sleep EEG exhibits the largest Lyapunov exponent in both hippocampal and cortical EEG amongst other vigilance states. We also identified non-zero third-order cumulant for all the vigilance states which suggests their non- Gaussian behavior.

Developmental differences of dimension complexity of hippocampal EEG during REM sleep.

This paper uses spectral analysis and correlation dimension index to examine the developmental differences between the EEG measured from two hippocampal subfields, CA1 and the dentate gyrus. The study was focused on the hippocampal EEG during the vigilance state of REM sleep in freely moving rats of 15 and 90 days of age. Power spectra, magnitude-squared coherence, and correlation dimension were estimated. The correlation dimension adds a new interpretation from the nonlinear dynamics perspective, and we found that there are no significant developmental differences between the granule cells of the dentate gyrus and the pyramidal cells of area CA1 when the theta activity was present during REM sleep.