Beta-1 (10-20 Hz) cortical oscillations observed in the human medial temporal lobe

Interpretation of the Intracranial Electroencephalogram of the Human Hippocampus

Understanding and discriminating the normal and abnormal elements of the intracranial electroencephalogram (iEEG) is essential in decision-making for epilepsy surgery. The hippocampus is widely acknowledged as a key structure in decision-making processes for surgical treatment in temporal lobe epilepsy and epilepsies that involve the mesial temporal structures. This review will provide a summary of the current state of our knowledge and understanding regarding normal and abnormal features of the iEEG of the human hippocampus.

Increased hippocampal and cortical beta oscillations in mice deficient for the HNK-1 sulfotransferase

The HNK-1 carbohydrate is detectable in perineuronal nets around inhibitory neurons in the hippocampus and neocortex. To address the functional contribution of HNK-1 to interneuron function in the adult brain, we recorded EEG and auditory-evoked potential in freely moving mice deficient for HNK-1 sulfotransferase (ST-/- mice) and in wild-type littermates. While ST-/- mice displayed normal theta oscillations, both cortical and hippocampal oscillations within the beta range were enhanced, and gamma oscillations showed an opposite trend. ST-/- mice had amplitudes of auditory-evoked potentials similar to control mice, but the latencies of their hippocampal responses were shorter. Morphological analysis revealed a decreased density of parvalbumin-positive interneurons in the hippocampal CA3 subfield of ST-/- mice, which may contribute to the observed changes in networks oscillations. These findings reveal alterations in ST-/- mice that differ from EEG abnormalities of mice deficient in the HNK-1 carrier molecule tenascin-R.

Frequency bands and spatiotemporal dynamics of β burst stimulation induced afterdischarges in hippocampus in vivo

Temporal and spatial characteristics of hippocampal neuronal network activation are modified during epileptiform afterdischarges. We developed a beta burst stimulation protocol to investigate subregional variations and substrates of rhythmic population spike discharges in vivo in urethane anesthetized Wistar rat hippocampus with a 14-electrode recording array and extracellular single electrode recordings. Our 64 pulse beta burst stimulation protocol was constructed from electrical pulses delivered at intervals corresponding to beta (14-25 Hz), Delta (2 Hz), and slow (0.5 Hz) frequencies. In each experiment these interleaved pulses were all repeated four times with unchanged intervals. Stimulation of either perforant path or fimbria fornix induced a prolonged afterdischarge pattern peaking at 200 Hz fast, 20 Hz beta, and 2 Hz Delta frequencies. Analysis of variance confirmed that the response pattern of the discharges remained constant regardless of the stimulation beta frequency. Within the afterdischarge the fast frequencies were restricted to independent hippocampal subfields whereas beta and slow frequencies correlated across the subfields. Current source density (CSD) analysis revealed that the original signal propagation through subfields of the hippocampus was compromised during the beta burst stimulation induced afterdischarge. In addition, the CSD profile of the epileptiform afterdischarge was consistently similar across the different experiments. Time-frequency analysis revealed that the beta frequency afterdischarge was initiated and terminated at higher gamma (30-80 Hz) frequencies. However, the alterations in the CSD profile of the hippocampus coincided with the beta frequency dominated discharges. We propose that hippocampal epileptiform activity at fast, beta and Delta frequencies represents coupled oscillators at respectively increasing spatial scales in the hippocampal neuronal network in vivo.

Theta oscillation in the human anterior cingulate cortex during all-night sleep: an electrocorticographic study

Ten epileptic patients each with subdural electrodes surgically attached to the anterior cingulate cortex (ACC; two cases), the orbitofrontal cortex (OFC; seven cases), or both (one case) were included in this study. We recorded each patient's ACC or OFC electrocorticogram (ECoG) during the time period that the patient was awake and naturally asleep. We performed a Fast Fourier Transformation (FFT) power spectral analysis on each ECoG to examine its frequency component. We found that the ACC showed regular and continuous theta oscillation (5-7Hz) during wakefulness and rapid eye movement (REM) sleep, but not during slow wave sleep. Theta waves observed in REM sleep were not as distinct as those found in wakefulness. We also discovered that the orbitofrontal signals represented spectral peaks in the theta band only during wakefulness. This suggests the coexistence of theta oscillation in the ACC. Considering our previous observations of gamma and beta oscillations in the human hippocampus, we hypothesize that the human limbic system manifests two oscillatory activities. The results obtained in this study suggest that electrophysiological activity in the ACC could be related to particular psychological functions in wakefulness and in REM sleep. These results are useful in elucidating the human brain mechanism.

Theta oscillation in the anterior cingulate and beta-1 oscillation in the medial temporal cortices: a human case report

In previous studies we observed gamma (30-150 Hz) and beta-1 (10-20 Hz) oscillations in the medial temporal lobe (MTL) using subdural electrodes. The beta-1 was present during wake and REM sleep while gamma was present in all states. Recently we studied a patient (35 years M) with electrodes attached to the anterior cingulate cortex (ACC). This structure showed regular theta (5-6 Hz) oscillations. In the first recording, electrodes were attached to the MTL, that demonstrated the beta-1 and gamma oscillations. Two months later, electrodes were placed on orbitofrontal and ACC and an all night sleep recording was carried out. The ACC exhibited a highly regular and continuous theta oscillation during wakefulness and REM sleep, but not during NREM sleep. Since this same subject showed beta-1 oscillations in the MTL, it is probable that the theta in the ACC is independent of beta-1 in the MTL. This single case suggests the existence of two different frequency oscillators in the human limbic system. Elucidating their functional roles will be an interesting challenge for future studies.

A simple measure of correlation across time, frequency and space between continuous brain signals

This paper introduces a simple but systematic method to estimate correlations between the spectral energy of two continuous electrophysiological signals in such a way that it can detect relationships between different frequencies and different latencies. From two series of signals (e.g. electroencephalogram, magnetoencephalogram or local field potentials) recorded from two sites in response to repeated sensory stimulations, the method computes the time-frequency energy of each signal. Then, it computes the Spearman rank order correlation coefficient across all the trials between the energy of the first signal series in one time-frequency region and the energy of the second signal series in a second time-frequency region. The method was designed to analyze interactions between frequency bands, in an effort to describe how the main brain rhythms interact with each other across time and space. It was applied to two simulations and to intracranial electro-encephalogram (EEG) recordings obtained from an epileptic patient performing two verbal discrimination tests (a phonological and a semantic task). It led to the identification of different correlations patterns in the gamma band depending on the level of semantic analysis performed by the patient.

Cortical oscillations in human medial temporal lobe during wakefulness and all-night sleep

We have recorded human medial temporal lobe electrocorticogram during wakefulness and natural sleep in epileptic patients with subdural electrodes. From these recordings, we have found gamma (30-150 Hz) [Neuroscience 90 (1999) 1149] and beta-1 (10-20 Hz) [NeuroReport 10 (1999) 3055] activities during wakefulness in human medial temporal lobe. In this paper, we will report changes of these frequencies across wake and natural sleep. Electrocorticograms during wake, slow wave sleep and rapid eye movement (REM) sleep were subjected to fast Fourier transformation analysis. During wake two spectral enhancements, beta-1 and gamma, were consistently observed across subjects. In the raw signal, beta-1 was observed as a regular rhythmic oscillation. In slow wave sleep, the beta-1 peak disappeared but gamma remained, although slightly reduced in power. During REM sleep, beta-1 appeared again, but the peak frequency was significantly lower than during wake (mean frequency: wake=16.6, REM=12.8 Hz). The gamma peak was also present in REM sleep. It has been known that the rhythmic slow activity (RSA) or theta is observed in some animals. However, it is unclear whether the human hippocampus displays similar activity. Since human beta-1 appears during wake and REM sleep when RSA is observed in other species, and since beta-1 is also a regular rhythmic oscillation, we propose that beta-1 may be the functional equivalent of hippocampal RSA (theta) observed in some animals. Functional significances of the gamma activity should be further investigated.