Gamma Amplitude Research Articles

In Vitro Recordings of Human Neocortical Oscillations

Electrophysiological oscillations are thought to create temporal windows of communication between brain regions. We show here that human cortical slices maintained in vitro can generate oscillations similar to those observed in vivo. We have characterized these oscillations using local field potential and whole-cell recordings obtained from neocortical slices acquired during epilepsy surgery. We confirmed that such neocortical slices maintain the necessary cellular and circuitry components, and in particular inhibitory mechanisms, to manifest oscillatory activity when exposed to glutamatergic and cholinergic agonists. The generation of oscillations was dependent on intact synaptic activity and muscarinic receptors. Such oscillations differed in electrographic and pharmacological properties from epileptiform activity. Two types of activity, theta oscillations and high gamma activity, uniquely characterized this model-activity not typically observed in animal cortical slices. We observed theta oscillations to be synchronous across cortical laminae suggesting a novel role of theta as a substrate for interlaminar communication. As well, we observed cross-frequency coupling (CFC) between theta phase and high gamma amplitude similar to that observed in vivo. The high gamma "bursts" generated by such CFC varied in their frequency content, suggesting that this variability may underlie the broadband nature of high gamma activity.

Oscillatory hierarchy in a network of leaky integrate-and-fire neurons with short-term plasticity

Cross-frequency couplings between oscillatory modes have been observed in cortical and hippocampal local field potentials recorded from the brains of rodents, primates[1] and humans. Multi-unit activity recordings have shown that the highest amplitude of gamma and theta oscillations occur at the rising positive going part of theta and delta oscillations, respectively[1]. We show that a network of leaky integrate-and-fire neurons with short term plasticity between pyramidal cells and interneurons can exhibit a similar oscillatory hierarchy (see Figure ​Figure1).1). Moreover, the network exhibits alpha oscillations which amplitude is modulated by the phase of delta oscillations. There is no phase-amplitude coupling between alpha and theta. Finally, these oscillations are nested in a slower 0.2 Hz oscillation. We study the mechanisms of these oscillatory patterns reminiscent of spontaneous cortical activity[2]. Figure 1 Simulation in a network composed of 8000 pyramidal cells and 2000 interneurons. (A) Firing rates of pyramidal cells νE and interneurons νI in red and blue, respectively, smoothen within a window of 25 ms duration. The frequency of slow ...

Reduction in LFP cross-frequency coupling between theta and gamma rhythms associated with impaired STP and LTP in a rat model of brain ischemia

The theta-gamma cross-frequency coupling (CFC) in hippocampus was reported to reflect memory process. In this study, we measured the CFC of hippocampal local field potentials (LFPs) in a two-vessel occlusion (2VO) rat model, combined with both amplitude and phase properties and associated with short and long-term plasticity indicating the memory function. Male Wistar rats were used and a 2VO model was established. STP and LTP were recorded in hippocampal CA3-CA1 pathway after LFPs were collected in both CA3 and CA1. Based on the data of relative power spectra and phase synchronization, it suggested that both the amplitude and phase coupling of either theta or gamma rhythm were involved in modulating the neural network in 2VO rats. In order to determine whether the CFC was also implicated in neural impairment in 2VO rats, the coupling of CA3 theta–CA1 gamma was measured by both phase-phase coupling (n:m phase synchronization) and phase-amplitude coupling. The attenuated CFC strength in 2VO rats implied the impaired neural communication in the coordination of theta-gamma entraining process. Moreover, compared with modulation index (MI) a novel algorithm named cross frequency conditional mutual information (CF-CMI), was developed to focus on the coupling between theta phase and the phase of gamma amplitude. The results suggest that the reduced CFC strength probably attributed to the disruption of the phase of CA1 gamma envelop. In conclusion, it implied that the phase coupling and CFC of hippocampal theta and gamma played an important role in supporting functions of neural network. Furthermore, synaptic plasticity on CA3-CA1 pathway was reduced in line with the decreased CFC strength from CA3 to CA1. It partly supported our hypothesis that directional CFC indicator might probably be used as a measure of synaptic plasticity.

Coupling of prefrontal gamma amplitude and theta phase is strengthened in trace eyeblink conditioning

Trace eyeblink conditioning requires animals to associate a neutral stimulus (CS) and an aversive periorbital shock (US) that occurred moment later. Acquisition in this conditioning depends on several forebrain regions including the hippocampus, medial prefrontal, and entorhinal cortices in addition to the cerebellum. Activities of single and population neurons in these regions show several patterns of change with the conditioning. For example, the power and synchronization of theta oscillations are correlated with the rate of acquisition. Yet, little is known about how neuronal oscillations at other frequency bands change with the conditioning. The present study examined changes in gamma oscillations, which are typically associated with spiking activity of individual cells. We found that after CS offset the power of gamma oscillations at 35–45Hz fluctuated at about 7Hz. This rhythmic fluctuation of gamma power was observed in all three regions and locked to local theta oscillations at 4–8Hz. Furthermore, over the course of 10days of acquisition sessions, the coupling of gamma power and theta phase became stronger in the medial prefrontal cortex while it did not change in the hippocampus or lateral entorhinal cortex. Neither theta nor gamma power in any of the regions significantly changed across sessions, rejecting a possibility that the observed increase in prefrontal theta–gamma coupling was secondary to an increase in theta or gamma power. The theta–gamma coupling between different regions did not significantly change across sessions. These results suggest that prefrontal gamma oscillations become more effectively coordinated with concurrent theta oscillations in trace eyeblink conditioning. This may result in a stronger impact of prefrontal neuronal firing responses to the CS on the processing in down-stream regions, such as the lateral entorhinal cortex and/or pontine nuclei.

Layer-Specific Entrainment of Gamma-Band Neural Activity by the Alpha Rhythm in Monkey Visual Cortex

Although the mammalian neocortex has a clear laminar organization, layer-specific neuronal computations remain to be uncovered. Several studies suggest that gamma band activity in primary visual cortex (V1) is produced in granular and superficial layers and is associated with the processing of visual input. Oscillatory alpha band activity in deeper layers has been proposed to modulate neuronal excitability associated with changes in arousal and cognitive factors. To investigate the layer-specific interplay between these two phenomena, we characterized the coupling between alpha and gamma band activity of the local field potential in V1 of the awake macaque. Using multicontact laminar electrodes to measure spontaneous signals simultaneously from all layers of V1, we found a robust coupling between alpha phase in the deeper layers and gamma amplitude in granular and superficial layers. Moreover, the power in the two frequency bands was anticorrelated. Taken together, these findings demonstrate robust interlaminar cross-frequency coupling in the visual cortex, supporting the view that neuronal activity in the alpha frequency range phasically modulates processing in the cortical microcircuit in a top-down manner.

Cross‐frequency dynamics of neuromagnetic oscillatory activity: Two mechanisms of emotion regulation

Hemodynamic and electromagnetic neuroimaging suggests a sequence of intraregional and interregional activity during emotion processing. Oscillatory activity within trials may offer insight into neural mechanisms involved in this process. MEG was measured in 24 subjects during passive viewing of neutral and unpleasant pictures and emotion regulation by cognitive reappraisal. Regulation prompted more gamma increase and alpha decrease to picture onset than did passive viewing. Source analysis confirmed differential modulation of these power changes in occipitoparietal regions during passive viewing and larger power changes in prefrontal regions during regulation. Local coupling of alpha phase to gamma amplitude within a medial prefrontal region and long-range synchrony of medial prefrontal, occipital, and temporoparietal regions index mechanisms of prefrontal top-down contribution to emotion processing.

A novel method for reducing the effect of tonic muscle activity on the gamma band of the scalp EEG.

Neural oscillations in the gamma band are of increasing interest, but separating them from myogenic electrical activity has proved difficult. A novel algorithm has been developed to reduce the effect of tonic scalp and neck muscle activity on the gamma band of the EEG. This uses mathematical modelling to fit individual muscle spikes and then subtracts them from the data. The method was applied to the detection of motor associated gamma in two separate groups of eight subjects using different sampling rates. A reproducible increase in high gamma (65–85 Hz) magnitude occurred immediately after the motor action in the left central area (p = 0.02 and p = 0.0002 for the two cohorts with individually optimized algorithm parameters, compared to p = 0.03 and p = 0.16 before correction). Whilst the magnitude of this event-related gamma synchronisation was not reduced by the application of the EMG reduction algorithm, the baseline left central gamma magnitude was significantly reduced by an average of 23 % with a faster sampling rate (p < 0.05). In comparison, at left and right temporo-parietal locations the gamma amplitude was reduced by 60 and 54 % respectively (p < 0.05). The reduction of EMG contamination by fitting and subtraction of individual spikes shows promise as a method of improving the signal to noise ratio of high frequency neural oscillations in scalp EEG.

Increases of theta–low gamma coupling in rat medial prefrontal cortex during working memory task

Cross-frequency coupling (CFC) between the theta (4-12Hz) phase and the amplitude of gamma (30-100Hz) oscillations occurs frequently in brain. However, the function of theta-gamma coupling in rat medial prefrontal cortex (mPFC) in working memory remains unclear. To address this issue, we studied 16-channel CFC in local field potentials (LFPs) recorded from the mPFC of over-trained rats as they performed a Y-maze working memory task. During this course, the amplitude of the low gamma subband (30-60Hz) was more strongly modulated by theta phase during the correct trials than in the error ones. Meanwhile, more channels showed higher theta-low gamma coupling levels during the correct performance. By contrast, the strength of theta-high gamma coupling did not significantly increase during the working memory task in both correct and error trials, indicating an insignificant correlation with the performance validity. These findings suggest a role of mPFC theta-low gamma coupling in working memory.

A Computational Study on Altered Theta-Gamma Coupling during Learning and Phase Coding

There is considerable interest in the role of coupling between theta and gamma oscillations in the brain in the context of learning and memory. Here we have used a neural network model which is capable of producing coupling of theta phase to gamma amplitude firstly to explore its ability to reproduce reported learning changes and secondly to memory-span and phase coding effects. The spiking neural network incorporates two kinetically different GABAA receptor-mediated currents to generate both theta and gamma rhythms and we have found that by selective alteration of both NMDA receptors and GABAA,slow receptors it can reproduce learning-related changes in the strength of coupling between theta and gamma either with or without coincident changes in theta amplitude. When the model was used to explore the relationship between theta and gamma oscillations, working memory capacity and phase coding it showed that the potential storage capacity of short term memories, in terms of nested gamma-subcycles, coincides with the maximal theta power. Increasing theta power is also related to the precision of theta phase which functions as a potential timing clock for neuronal firing in the cortex or hippocampus.

Hierarchical Organization of Gamma and Theta Oscillatory Dynamics in Schizophrenia

Schizophrenia patients have deficits across a broad range of important cognitive and clinical domains. Synchronization of oscillations in the gamma frequency range (~40 Hz) is associated with many normal cognitive functions and underlies at least some of the deficits observed in schizophrenia patients. Recent studies have demonstrated that gamma oscillations are modulated by the phase of theta waves, and this cross-frequency coupling indicates that a complex and hierarchical organization governs neural oscillatory dynamics. The aims of the present study were to determine if schizophrenia patients have abnormalities in the amplitude, synchrony, and cross-frequency coupling of gamma and theta oscillations in response to gamma-frequency steady-state stimulation and if abnormal neural oscillatory dynamics are associated with cognitive deficits in schizophrenia. Schizophrenia patients (n = 234) and healthy control subjects (n = 188) underwent electroencephalography testing in response to 40-Hz auditory steady-state stimulation. Cognitive functions were assessed with a battery of neuropsychological tests. Schizophrenia patients had significantly reduced gamma intertrial phase coherence, increased theta amplitude, and intact cross-frequency coupling relative to healthy control subjects. In schizophrenia patients, increased theta amplitude was associated with poor verbal memory performance. Results suggest that schizophrenia patients have specific alterations in both gamma and theta oscillations, but these deficits occur in the context of an intact hierarchical organization of their cross-frequency modulation in response to 40-Hz steady-state stimulation. Cortical oscillatory dynamics may be useful for understanding the neural mechanisms that underlie the disparate cognitive and functional impairments of schizophrenia.

Increased phase synchronization during continuous face integration measured simultaneously with EEG and fMRI

ObjectiveGamma zero-lag phase synchronization has been measured in the animal brain during visual binding. Human scalp EEG studies used a phase locking factor (trial-to-trial phase-shift consistency) or gamma amplitude to measure binding but did not analyze common-phase signals so far. This study introduces a method to identify networks oscillating with near zero-lag phase synchronization in human subjects. MethodsWe presented unpredictably moving face parts (NOFACE) which – during some periods – produced a complete schematic face (FACE). The amount of zero-lag phase synchronization was measured using global field synchronization (GFS). GFS provides global information on the amount of instantaneous coincidences in specific frequencies throughout the brain. ResultsGamma GFS was increased during the FACE condition. To localize the underlying areas, we correlated gamma GFS with simultaneously recorded BOLD responses. Positive correlates comprised the bilateral middle fusiform gyrus and the left precuneus. ConclusionsThese areas may form a network of areas transiently synchronized during face integration, including face-specific as well as binding-specific regions and regions for visual processing in general. SignificanceThus, the amount of zero-lag phase synchronization between remote regions of the human visual system can be measured with simultaneously acquired EEG/fMRI.

The decay $ \overline B \to \overline K {\ell^{ + }}{\ell^{ - }} $ at low hadronic recoil and model-independent ∆B = 1 constraints

We study the decay B --> K l^+ l^- for l = e,mu,tau with a softly recoiling kaon, that is, for high dilepton invariant masses sqrt{q^2} of the order of the b-quark mass. This kinematic region can be treated within an operator product expansion and simplified using heavy quark symmetry, leading to systematic predictions for heavy-to-light processes such as B --> K^(*) l^+ l^-. We show that the decay rates of both B --> K^* l^+ l^- and B --> K l^+ l^- decays into light leptons depend on a common combination of short-distance coefficients. The corresponding CP-asymmetries are hence identical. Furthermore we present low recoil predictions for B --> K l^+ l^- observables, including the flat term in the angular distribution which becomes sizable for taus. We work out model-independently the constraints on Delta B = 1 operators using the most recent data from the experiments BaBar, Belle, CDF and LHCb. For constructive interference with the standard model, generic new physics is pushed up to scales above 44 TeV at 95% CL. Assuming none or small CP-violation we obtain a lower bound on the position of the zero of the forward-backward asymmetry of B^0 --> K^*0 l^+ l^- decays as q_0^2 > 1.7 GeV^2, which improves to q_0^2 > 2.6 GeV^2 for a standard model-like sign b --> s gamma amplitude.

Resting oscillations and cross-frequency coupling in the human posteromedial cortex.

Using rare intracranial recordings from the posterior interhemispheric region of the human brain, we explored the oscillatory properties of the posteromedial cortex (PMC) during rest. The PMC is a core structure of the default mode network, which is known for its higher activity during the resting state. We found that resting PMC spectral power peaked in the theta band range (4-7 Hz) and was clearly distinguishable from adjacent cortical sites in the occipital lobe displaying peaks in the alpha band range (8-12 Hz). Additionally, the phase of PMC theta oscillations modulated the amplitude of ongoing high gamma (70-180 Hz) activity during the resting state. The magnitude of this cross-frequency modulation was shown to fluctuate at time scales comparable to those observed in functional neuroimaging studies of intrinsic functional connectivity networks (~0.1 Hz). The difference of canonical oscillations in the PMC compared to its adjacent cortical sites conforms to functional specialization across anatomical boundaries. Such differences may reflect separate oscillatory preferences between networks that are functionally connected.

Roles for the pre-supplementary motor area and the right inferior frontal gyrus in stopping action: Electrophysiological responses and functional and structural connectivity

Both the pre-supplementary motor area (preSMA) and the right inferior frontal gyrus (rIFG) are important for stopping action outright. These regions are also engaged when preparing to stop. We aimed to elucidate the roles of these regions by harnessing the high spatio-temporal resolution of electrocorticography (ECoG), and by using a task that engages both preparing to stop and stopping outright. First, we validated the task using fMRI in 16 healthy control participants to confirm that both the preSMA and the rIFG were active. Next, we studied a rare patient with intracranial grid coverage of both these regions, using macrostimulation, diffusion tractography, cortico-cortical evoked potentials (CCEPs) and task-based ECoG. Macrostimulation of the preSMA induced behavioral motor arrest. Diffusion tractography revealed a structural connection between the preSMA and rIFG. CCEP analysis showed that stimulation of the preSMA evoked strong local field potentials within 30 ms in rIFG. During the task, when preparing to stop, there was increased high gamma amplitude (~70-250 Hz) in both regions, with preSMA preceding rIFG by ~750 ms. For outright stopping there was also a high gamma amplitude increase in both regions, again with preSMA preceding rIFG. Further, at the time of stopping, there was an increase in beta band activity (~16 Hz) in both regions, with significantly stronger inter-regional coherence for successful vs. unsuccessful stop trials. The results complement earlier reports of a structural/functional action control network between the preSMA and rIFG. They go further by revealing between-region timing differences in the high gamma band when preparing to stop and stopping outright. They also reveal strong between-region coherence in the beta band when stopping is successful. Implications for theories of action control are discussed.

Speech enhancement using super gauss mixture model of speech spectral amplitude

Most speech enhancement algorithms are derived by applying gauss hypothesis based on decor related speech samples. And recently it is accepted that the real pdf of speech spectral amplitude lies between the Laplace and Gamma amplitude approximation. This paper presents a supper gauss mixture model of speech spectral amplitude which can accurately approximates real speech spectral amplitude distribution. With this model, a new speech enhancement algorithm on the basis of MMSE estimator is derived. Simulations show that this algorithm has a better noise reduction performance and improve output SNR in the sense of speech segmentation.

Temporal dynamics of primary motor cortex gamma oscillation amplitude and piper corticomuscular coherence changes during motor control

In recent years, the use of non-invasive techniques (EEG/MEG) to measure the ~80Hz ("gamma") oscillations generated by the primary motor cortex during motor control has been well validated. However, primary motor cortex gamma oscillations have yet to be systematically compared with lower frequency (30-50Hz, 'piper') corticomuscular coherence in the same tasks. In this paper, primary cortex gamma oscillations and piper corticomuscular coherence are compared for three types of movements: simple abductions of the index finger, repetitive abductions of the index finger of different extents and frequencies and static abduction of the index finger at two different force levels. For simple movements, piper coherence and gamma amplitude followed very similar time courses with coherence appearing at approximately half the frequency of cortical gamma oscillations. No evidence of 2:1 phase-phase coupling was observed. A similar pattern of results was observed for repetitive movements varying in size and frequency; however, during the production of static force, the time courses became dissociated. During these movements, EMG piper amplitude was sustained for the entire contraction; gamma power showed a burst at onset but no piper corticomuscular coherence was observed. For these data, this dissociation suggests that while primary motor cortex gamma oscillations and piper corticomuscular coherence may often co-occur during the production of dynamic movements, they probably reflect different functional processes in motor control.

Speed Controls the Amplitude and Timing of the Hippocampal Gamma Rhythm

Cortical and hippocampal gamma oscillations have been implicated in many behavioral tasks. The hippocampus is required for spatial navigation where animals run at varying speeds. Hence we tested the hypothesis that the gamma rhythm could encode the running speed of mice. We found that the amplitude of slow (20–45 Hz) and fast (45–120 Hz) gamma rhythms in the hippocampal local field potential (LFP) increased with running speed. The speed-dependence of gamma amplitude was restricted to a narrow range of theta phases where gamma amplitude was maximal, called the preferred theta phase of gamma. The preferred phase of slow gamma precessed to lower values with increasing running speed. While maximal fast and slow gamma occurred at coincident phases of theta at low speeds, they became progressively more theta-phase separated with increasing speed. These results demonstrate a novel influence of speed on the amplitude and timing of the hippocampal gamma rhythm which could contribute to learning of temporal sequences and navigation.

Learning alters theta amplitude, theta-gamma coupling and neuronal synchronization in inferotemporal cortex

BackgroundHow oscillatory brain rhythms alone, or in combination, influence cortical information processing to support learning has yet to be fully established. Local field potential and multi-unit neuronal activity recordings were made from 64-electrode arrays in the inferotemporal cortex of conscious sheep during and after visual discrimination learning of face or object pairs. A neural network model has been developed to simulate and aid functional interpretation of learning-evoked changes.ResultsFollowing learning the amplitude of theta (4-8 Hz), but not gamma (30-70 Hz) oscillations was increased, as was the ratio of theta to gamma. Over 75% of electrodes showed significant coupling between theta phase and gamma amplitude (theta-nested gamma). The strength of this coupling was also increased following learning and this was not simply a consequence of increased theta amplitude. Actual discrimination performance was significantly correlated with theta and theta-gamma coupling changes. Neuronal activity was phase-locked with theta but learning had no effect on firing rates or the magnitude or latencies of visual evoked potentials during stimuli. The neural network model developed showed that a combination of fast and slow inhibitory interneurons could generate theta-nested gamma. By increasing N-methyl-D-aspartate receptor sensitivity in the model similar changes were produced as in inferotemporal cortex after learning. The model showed that these changes could potentiate the firing of downstream neurons by a temporal desynchronization of excitatory neuron output without increasing the firing frequencies of the latter. This desynchronization effect was confirmed in IT neuronal activity following learning and its magnitude was correlated with discrimination performance.ConclusionsFace discrimination learning produces significant increases in both theta amplitude and the strength of theta-gamma coupling in the inferotemporal cortex which are correlated with behavioral performance. A network model which can reproduce these changes suggests that a key function of such learning-evoked alterations in theta and theta-nested gamma activity may be increased temporal desynchronization in neuronal firing leading to optimal timing of inputs to downstream neural networks potentiating their responses. In this way learning can produce potentiation in neural networks simply through altering the temporal pattern of their inputs.

Phase–amplitude coupling between theta and gamma oscillations during nociception in rat electroencephalography

In electroencephalography (EEG) study, gamma oscillations were reported to participate in pain processing; theta oscillations were also involved in pain processing. Moreover, theta always modulated gamma activity by phase–amplitude coupling in event-related oscillations. Whether theta modulate gamma by phase–amplitude coupling in pain processing is of interest. In the present study, using EEG of rats after laser nociceptive stimulation, we investigated gamma activity and phase–amplitude coupling between theta and gamma. It was found that induced gamma power increased starting 200ms after nociceptive stimulation onset. Moreover, significant coupling between theta phase and gamma amplitude was found over frontal and parietal region after nociceptive stimulation. Our results for the first time suggest that coupling between theta and gamma is involved in nociception processing.

Neurofeedback-Enhanced Gamma Brainwaves from the Prefrontal Cortical Region of Meditators and Non-Meditators and Associated Subjective Experiences

This study had two aims: (1) to explore the inner experiences associated with increased production of gamma brainwaves in an initial neurofeedback experience; and (2) to measure and compare neurofeedback-enhanced increased output from the prefrontal cortical region of meditators and non-meditators, using the Peak Brain Happiness Trainer(™) neurofeedback system. This was a controlled pilot study; it involved a single session per subject. The research was conducted in a nonprofit laboratory in the United States. There were 12 adults in 2 groups (N = 12): 6 practitioners of Transcendental Meditation(™) and six controls. The measures were self-assessed inner experiences and measurements of clarified gamma output at the prefrontal cortical region. (1) Self-assessed descriptions were comparable for both groups; (2) the associations of 16 supplied descriptors with the initial neurofeedback experience were comparable for both groups and showed highest scores for "happy" (p < 0.0001) and "loving" (p < 0.0001), and lowest scores for "stressed" (p < 0.0001) and "disappointed" (p < 0.0001); (3) baseline measures were comparable for both groups; (4) both groups were able to increase gamma brainwaves using neurofeedback (p < 0.01); and (5) meditators produced greater increases over controls (p = 0.02). The inner experience associated with increased clarified gamma amplitude from the prefrontal cortex apparently involves positive emotions of happiness and love, along with reduced stress. Meditators achieved greater increases in the gamma band from the prefrontal cortical region over controls during an initial neurofeedback session.