Human neocortical and hippocampal near-DC shifts are interconnected

Advances of Intracranial Electroencephalography in Localizing the Epileptogenic Zone

Intracranial electroencephalography (iEEG) provides the best precision in estimating the location and boundary of an epileptogenic zone. Analysis of iEEG in the routine EEG frequency range (0.5-70 Hz) remains the basis in clinical practice. Low-voltage fast activity is the most commonly reported ictal onset pattern in neocortical epilepsy, and low-frequency high-amplitude repetitive spiking is the most commonly reported ictal onset pattern in mesial temporal lobe epilepsy. Recent studies using wideband EEG recording have demonstrated that examining higher (80-1000 Hz) and lower (0.016-0.5 Hz) EEG frequencies can provide additional diagnostic information and help to improve the surgical outcome. In addition, novel computational techniques of iEEG signal analysis have provided new insights into the epileptic network. Here, we review some of these recent advances. Although these sophisticated and advanced techniques of iEEG analysis show promise in localizing the epileptogenic zone, their utility needs to be further validated in larger studies.

Signal characteristics of intraventricular electrodes recordings in human epilepsy: a case report

The case of a patient with intractable temporal lobe seizures and inadvertent unilateral intraventricular depth electrode placement is presented. The resting electroencephalograph (EEG) showed marked amplitude differences between the intraventricular electrode on the left and the parenchymal electrode on the right. All recorded seizures originated on the left side and in spite of its intraventricular location, frequency power spectra during the early ictal phase showed a marked increase in power for all frequency bands in the left depth electrode, exceeding that on the right. Analysis with Brain Electrical Source Analysis (BESA) software demonstrated marked ictal baseline shifts which were initially limited to the left side but changed to the right during clinical secondary generalization. In the immediate postictal state, all, except for infraslow, frequencies were markedly reduced in power. We conclude that intraventricular depth electrode contacts placed adjacent to the hippocampal structure can record interictal and ictal activity for all frequency bands, albeit at reduced amplitudes. Furthermore, infraslow activity can provide supplementary information about the epileptogenic zone.

Large-scale calcium waves traveling through astrocytic networks in vivo

Macroscopic changes in cerebral blood flow, such as those captured by functional imaging of the brain, require highly organized, large-scale dynamics of astrocytes, glial cells that interact with both neuronal and cerebrovascular networks. However, astrocyte activity has been studied mainly at the level of individual cells, and information regarding their collective behavior is lacking. In this work, we monitored calcium activity simultaneously from hundreds of mouse hippocampal astrocytes in vivo and found that almost all astrocytes participated en masse in regenerative waves that propagated from cell to cell (referred to here as "glissandi"). Glissandi emerged depending on the neuronal activity and accompanied a reduction in infraslow fluctuations of local field potentials and a decrease in the flow of red blood cells. This novel phenomenon was heretofore overlooked, probably because of the high vulnerability of astrocytes to light damage; glissandi occurred only when observed at much lower laser intensities than previously used.

Ripples in the medial temporal lobe are relevant for human memory consolidation

High-frequency oscillations (ripples) have been described in the hippocampus and rhinal cortex of both animals and human subjects and have been linked to replay and consolidation of previously acquired information. More specifically, studies in rodents suggested that ripples are generated in the hippocampus and are then transferred into the rhinal cortex, and that they occur predominantly during negative half waves of neocortical slow oscillations. Recordings in human epilepsy patients used either microelectrodes or foramen ovale electrodes; it is thus unclear whether macroelectrodes, which are routinely used for pre-surgical investigations, allow the recording of ripples as well. Furthermore, no direct link between ripples and behavioural performance has yet been established. Here, we recorded intracranial electroencephalogram with macroelectrodes from the hippocampus and rhinal cortex contralateral to the seizure onset zone in 11 epilepsy patients during a memory consolidation task while they were having an afternoon 'nap', i.e. a sleep period of approximately 1 h duration. We found that ripples could reliably be detected both in the hippocampus and in the rhinal cortex and had a similar frequency composition to events recorded previously with microelectrodes in humans. Results from cross-correlation analysis revealed that hippocampal events were closely locked to rhinal events and were consistent with findings on transmission of ripples from the hippocampus into the rhinal cortex. Furthermore, hippocampal ripples were significantly locked to the phase of hippocampal delta band activity, which might provide a mechanism for the reported phase-locking to neocortical slow oscillations. Ripples occurred with the highest incidence during periods when subjects lay awake during the nap time. Finally, we found that the number of rhinal, but not hippocampal, ripples was correlated with the number of successfully recalled items (post-nap) learned prior to sleep. These data confirm previous recordings in animals and humans, but move beyond them in several respects: they are the first recordings of ripples in humans during a cognitive task and suggest that ripples are indeed related to behavioural performance; furthermore, they propose a mechanism for phase-locking of ripples to neocortical slow waves via phase coupling to hippocampal delta activity; finally, they show that ripples can be recorded reliably with standard macroelectrodes in the human brain.

An epigenetic induction of a right-shift in hippocampal asymmetry: selectivity for short- and long-term potentiation but not post-tetanic potentiation

In humans, it is well established that major psychological functions are asymmetrically represented between the left and right cerebral cortices. The developmental origin of such functional lateralization remains unknown. Using the rat as a model system, we examined whether exposing neonates briefly to a novel environment can differentially affect synaptic plasticity in the left and right hippocampi during adulthood. During the first 3 weeks of life, one half of the pups from a litter spent 3 min daily away from their familiar home environment (Novel) while their littermates remained in that familiar environment (Home). At adulthood (7-months old), post-tetanic potentiation (PTP) of excitatory post-synaptic potentials (EPSPs), a very short-lasting form of plasticity, was greater among the Novel than the Home rats in both left and right hippocampi. In contrast, the novelty-induced increases in short- and long-term potentiation (STP, LTP), two relatively longer-lasting forms of plasticity, were found only in the right hippocampus. These findings demonstrate that a phase-selective asymmetry in hippocampal synaptic plasticity can be induced epigenetically by seemingly small systematic differences in early life environment. The selectivity of this asymmetry for the longer-lasting forms of synaptic plasticity suggests that the observed asymmetry in plasticity may contribute specifically to an asymmetric learning process which, in turn, may contribute to a functional asymmetry in the neocortex.