Gamma Coherence Research Articles

Disturbance in long distance gamma coherence in bipolar disorder

The aim of this study was to investigate long distance event-related gamma (28–48 Hz) coherence in mania before and after valproate monotherapy. Gamma coherence in response to visual oddball paradigm in ten medication-free, manic patients was studied before and after six weeks of valproate monotherapy in comparison to ten controls. Inter-hemispheric F 3–F 4, C 3–C 4, T 3–T 4, T 5–T 6, P 3–P 4, O 1–O 2 and intra-hemispheric F 3–P 3, F 4–P 4, F 3–T 5, F 4–T 6, F 3–O 1, F 4–O 2, C 3–O 1, C 2–O 4 electrode pairs were included in the analysis. Repeated measures ANOVA revealed a significant difference between groups with regard to pre-treatment coherence values ( p: 0.018). The coherence to the target stimuli at the right fronto-temporal location was significantly reduced by 35.41% in the patients compared to controls ( p: 0.003). Patients showed significantly lower pre-treatment coherence values in response to non-target stimuli compared to controls at the right fronto-temporal (28.51%, p: 0.004), right fronto-occipital (23.71%, p: 0.024), and right centro-occipital (25.69%, p: 0.029) locations. After six weeks of valproate monotherapy, manic symptoms improved significantly. Post-treatment change in target and non-target coherence values was statistically non-significant. EEG coherence is a measure of functional connectivity in the brain. Event-related gamma oscillations are essential for brain electrical activity. The results show that acute mania presents right sided long distance connectivity disturbance, thus pointing to the potential importance of measuring oscillatory responses in the search for consistent neurobiological markers in such a complicated condition as bipolar disorder.

Synchronized Activity between the Ventral Hippocampus and the Medial Prefrontal Cortex during Anxiety

The ventral hippocampus, unlike its dorsal counterpart, is required for anxiety-like behavior. The means by which it acts are unknown. We hypothesized that the hippocampus synchronizes with downstream targets that influence anxiety, such as the medial prefrontal cortex (mPFC). To test this hypothesis, we recorded mPFC and hippocampal activity in mice exposed to two anxiogenic arenas. Theta-frequency activity in the mPFC and ventral, but not dorsal, hippocampus was highly correlated at baseline, and this correlation increased in both anxiogenic environments. Increases in mPFC theta power predicted avoidance of the aversive compartments of each arena and were larger in serotonin 1A receptor knockout mice, a genetic model of increased anxiety-like behavior. These results suggest a role for theta-frequency synchronization between the ventral hippocampus and the mPFC in anxiety. They are consistent with the notion that such synchronization is a general mechanism by which the hippocampus communicates with downstream structures of behavioral relevance.

The Model- and the Data-Gamma

Brain regions that are structurally connected might become functionally connected by synchronization in the gamma-frequency band. In a recent issue of Nature, Colgin et al. show spatially and temporally fine-grained gamma-band synchronization between different parts of the rat hippocampal formation, suggesting a theta-modulated switching of gamma-mediated communication.

Kindled seizure in the prefrontal cortex activated behavioral hyperactivity and increase in accumbens gamma oscillations through the hippocampus

In previous studies, we reported that a single afterdischarge (AD) or repeated ADs (kindling) in the hippocampus resulted in schizophrenia-like behaviors such as hyperactivity and loss of sensorimotor gating. Given that medial prefrontal cortex (PFC) dysfunction is also found in models of schizophrenia, we hypothesized that a single AD in the PFC induces postictal hyperactivity, and PFC kindling results in loss in prepulse inhibition (PPI). An AD was induced by stimulating the PFC with a 5 s stimulus train of 60 Hz frequency and 600–800 μA intensity. An initial AD evoked in the PFC was not accompanied by clear postictal behavioral change. After partial kindling (11 ± 2 ADs) of the PFC, the PFC-AD propagated into the hippocampus and nucleus accumbens (NAC) and postictal hyperactivity lasted >5 min. The postictal hyperactivity was accompanied by increased gamma EEG oscillations in both PFC and NAC. A single AD in hippocampal CA1 also induced >5 min of postictal hyperactivity and increased gamma oscillations in the NAC and the PFC, with a transient increase in hippocampus-NAC gamma coherence occurring 2–3 min after a hippocampal AD. Electrolytic lesion or inactivation of the dorsal hippocampus abolished the behavioral hyperactivity and the NAC/PFC gamma wave increase induced by a PFC-AD. Kindling of the PFC (21 ADs) but not of the lateral frontal cortex resulted in a deficit of PPI to the acoustic startle response tested 3 days after the last AD. In summary, gamma waves in the NAC were found to accompany postictal hyperactivity induced by an AD in the PFC. Postictal gamma and hyperactivity required an intact hippocampus, perhaps through the hippocampal-NAC pathway. PFC kindling, similar to hippocampal CA1 kindling, resulted in a prolonged deficit in PPI.

Nucleus Accumbens Deep Brain Stimulation Produces Region-Specific Alterations in Local Field Potential Oscillations and Evoked ResponsesIn Vivo

Deep brain stimulation of the nucleus accumbens (NAC) region is an effective therapeutic avenue for several psychiatric disorders that are not responsive to traditional treatment strategies. Nonetheless, the mechanisms by which DBS achieves therapeutic effects remain unclear. We showed previously that high-frequency (HF) NAC DBS suppressed pyramidal cell firing and enhanced slow local field potential (LFP) oscillations in the orbitofrontal cortex (OFC) via antidromic activation of corticostriatal recurrent inhibition. Using simultaneous multisite LFP recordings in urethane-anesthetized rats, we now show that NAC DBS delivered for 90 min at high or low frequency (LF) selectively affects spontaneous and evoked LFP oscillatory power and coherence within and between the medial prefrontal cortex (mPFC), lateral OFC, mediodorsal thalamus (MD), and NAC. Compared with LF or sham DBS, HF DBS enhanced spontaneous slow oscillations and potentiated evoked LFP responses only in OFC. HF DBS also produced widespread increases in spontaneous beta and gamma power and enhanced coherent beta activity between MD and all other regions. In contrast, LF DBS elevated theta power in MD and NAC. Analysis of acute NAC-induced oscillations showed that HF DBS increased and LF DBS decreased induced relative gamma coherence compared with sham DBS. These data suggest that HF (therapeutic) and LF (possibly deleterious) NAC DBS produce distinct region-specific and frequency band-specific changes in LFP oscillations. NAC DBS may achieve therapeutic effects by enhancing rhythmicity and synchronous inhibition within and between afferent structures, thereby normalizing function of a neural circuit that shows aberrant activity in obsessive-compulsive disorder and depression.

Theta and Gamma Coherence Along the Septotemporal Axis of the Hippocampus

Theta and gamma rhythms synchronize neurons within and across brain structures. Both rhythms are widespread within the hippocampus during exploratory behavior and rapid-eye-movement (REM) sleep. How synchronous are these rhythms throughout the hippocampus? The present study examined theta and gamma coherence along the septotemporal (long) axis of the hippocampus in rats during REM sleep, a behavioral state during which theta signals are unaffected by external sensory input or ongoing behavior. Unilateral entorhinal cortical inputs are thought to play a prominent role in the current generation of theta, whereas current generation of gamma is primarily due to local GABAergic neurons. The septal 50% (4-5 mm) of the dentate gyrus (DG) receives a highly divergent, unilateral projection from any focal point along a lateral band of entorhinal neurons near the rhinal sulcus. We hypothesized that theta coherence in the target zone (septal DG) of this divergent entorhinal input would not vary, while gamma coherence would significantly decline with distance in this zone. However, both theta and gamma coherence decreased significantly along the long axis in the septal 50% of the hippocampus across both DG and CA1 electrode sites. In contrast, theta coherence between homotypic (e.g., DG to DG) sites in the contralateral hemisphere ( approximately 3-5 mm distant) were quite high ( approximately 0.7-0.9), much greater than theta coherence between homotypic sites 3-5 mm distant ( approximately 0.4-0.6) along the long axis. These findings define anatomic variation in both rhythms along the longitudinal axis of the hippocampus, indicate the bilateral CA3/mossy cell projections are the major determinant of theta coherence during REM, and demonstrate that theta coherence varies as a function of anatomical connectivity rather than physical distance. We suggest CA3 and entorhinal inputs interact dynamically to generate theta field potentials and advance the utility of theta and gamma coherence as indicators of hippocampal dynamics.

Increased Gamma Oscillatory Activity in the Subthalamic Nucleus During Tremor in Parkinson's Disease Patients

Rest tremor is one of the main symptoms in Parkinson's disease (PD), although in contrast to rigidity and akinesia, the severity of the tremor does not correlate well with the degree of dopamine deficiency or the progression of the disease. Studies suggest that akinesia in PD patients is related to abnormal increased beta (15-30 Hz) and decreased gamma (35-80 Hz) synchronous oscillatory activity in the basal ganglia. Here we investigated the dynamics of oscillatory activity in the subthalamic nucleus (STN) during tremor. We used two adjacent microelectrodes to simultaneously record neuronal firing and local field potential (LFP) activity in nine PD patients who exhibited resting tremor during functional neurosurgery. We found that neurons exhibiting oscillatory activity at tremor frequency are located in the dorsal region of STN, where neurons with beta oscillatory activity are observed, and that their activity is coherent with LFP oscillations in the beta frequency range. Interestingly, in 85% of the 58 sites examined, the LFP exhibited increased oscillatory activity in the low gamma frequency range (35-55 Hz) during periods with stronger tremor. Furthermore, in 17 of 26 cases where two LFPs were recorded simultaneously, their coherence in the gamma range increased with increased tremor. When averaged across subjects, the ratio of the beta to gamma coherence was significantly lower in periods with stronger tremor compared with periods of no or weak tremor. These results suggest that resting tremor in PD is associated with an altered balance between beta and gamma oscillations in the motor circuits of STN.

Integration of Bimodal Looming Signals through Neuronal Coherence in the Temporal Lobe

The ability to integrate information across multiple sensory systems offers several behavioral advantages, from quicker reaction times and more accurate responses to better detection and more robust learning. At the neural level, multisensory integration requires large-scale interactions between different brain regions--the convergence of information from separate sensory modalities, represented by distinct neuronal populations. The interactions between these neuronal populations must be fast and flexible, so that behaviorally relevant signals belonging to the same object or event can be immediately integrated and integration of unrelated signals can be prevented. Looming signals are a particular class of signals that are behaviorally relevant for animals and that occur in both the auditory and visual domain. These signals indicate the rapid approach of objects and provide highly salient warning cues about impending impact. We show here that multisensory integration of auditory and visual looming signals may be mediated by functional interactions between auditory cortex and the superior temporal sulcus, two areas involved in integrating behaviorally relevant auditory-visual signals. Audiovisual looming signals elicited increased gamma-band coherence between these areas, relative to unimodal or receding-motion signals. This suggests that the neocortex uses fast, flexible intercortical interactions to mediate multisensory integration.

Theta and Gamma Coordination of Hippocampal Networks during Waking and Rapid Eye Movement Sleep

Rapid eye movement (REM) sleep has been considered a paradoxical state because, despite the high behavioral threshold to arousing perturbations, gross physiological patterns in the forebrain resemble those of waking states. To understand how intrahippocampal networks interact during REM sleep, we used 96 site silicon probes to record from different hippocampal subregions and compared the patterns of activity during waking exploration and REM sleep. Dentate/CA3 theta and gamma synchrony was significantly higher during REM sleep compared with active waking. In contrast, gamma power in CA1 and CA3-CA1 gamma coherence showed significant decreases in REM sleep. Changes in unit firing rhythmicity and unit-field coherence specified the local generation of these patterns. Although these patterns of hippocampal network coordination characterized the more common tonic periods of REM sleep (approximately 95% of total REM), we also detected large phasic bursts of local field potential power in the dentate molecular layer that were accompanied by transient increases in the firing of dentate and CA1 neurons. In contrast to tonic REM periods, phasic REM epochs were characterized by higher theta and gamma synchrony among the dentate, CA3, and CA1 regions. These data suggest enhanced dentate processing, but limited CA3-CA1 coordination during tonic REM sleep. In contrast, phasic bursts of activity during REM sleep may provide windows of opportunity to synchronize the hippocampal trisynaptic loop and increase output to cortical targets. We hypothesize that tonic REM sleep may support off-line mnemonic processing, whereas phasic bursts of activity during REM may promote memory consolidation.

Gamma oscillations dynamically couple hippocampal CA3 and CA1 regions during memory task performance

The hippocampal formation is believed to be critical for the encoding, consolidation, and retrieval of episodic memories. Yet, how these processes are supported by the anatomically diverse hippocampal networks is still unknown. To examine this issue, we tested rats in a hippocampus-dependent delayed spatial alternation task on a modified T maze while simultaneously recording local field potentials from dendritic and somatic layers of the dentate gyrus, CA3, and CA1 regions by using high-density, 96-site silicon probes. Both the power and coherence of gamma oscillations exhibited layer-specific changes during task performance. Peak increases in the gamma power and coherence were found in the CA3-CA1 interface on the maze segment approaching the T junction, independent of motor aspects of task performance. These results show that hippocampal networks can be dynamically coupled by gamma oscillations according to specific behavioral demands. Based on these findings, we propose that gamma oscillations may serve as a physiological mechanism by which CA3 output can coordinate CA1 activity to support retrieval of hippocampus-dependent memories.

On the formation of gamma-coherent cell assemblies by oriens lacunosum-moleculare interneurons in the hippocampus.

Gamma frequency (30-80 Hz) network oscillations have been observed in the hippocampus during several behavioral paradigms in which they are often modulated by a theta frequency (4-12 Hz) oscillation. Interneurons of the hippocampus have been shown to be crucially involved in rhythms generation, and several subtypes with distinct anatomy and physiology have been described. In particular, the oriens lacunosum-moleculare (O-LM) interneurons were shown to synapse on distal apical dendrites of pyramidal cells and to spike preferentially at theta frequency, even in the presence of gamma-field oscillations. O-LM cells have also recently been shown to present higher axonal ramification in the longitudinal axis of the hippocampus. By using a hippocampal network model composed of pyramidal cells and two types of interneurons (O-LM and basket cells), we show here that the O-LM interneurons lead to gamma coherence between anatomically distinct cell modules. We thus propose that this could be a mechanism for coupling longitudinally distant cells excited by entorhinal cortex inputs into gamma-coherent assemblies.

Induced 49 Hz gamma and event-related coherence in deficit and nondeficit schizophrenia

Induced 49 Hz gamma and event-related coherence in deficit and nondeficit schizophrenia

Functional frontoparietal connectivity during encoding and retrieval processes follows HERA model: A high-resolution study

Recent neuroimaging studies of long-term episodic memory have suggested that left prefrontal cortex predominates in encoding condition, whereas right prefrontal cortex predominates in retrieval condition (hemispheric encoding and retrieval asymmetry, HERA model). The present electroencephalographic (EEG) study investigated the functional coupling of fronto-parietal regions during long-term memorization of visuo-spatial contents (i.e. landscapes, interiors of apartments), to test the predictions of the HERA model. Global fronto-parietal coupling was estimated by spectral coherence, whereas the “direction” of the fronto-parietal information flow was estimated by directed transfer function (DTF). The EEG rhythms of interest were theta (4–7 Hz), alpha (8–13 Hz), beta (14–30 Hz), and gamma (30–45 Hz). Statistically significant coherence in line with the HERA model was obtained at the gamma band. Namely, the fronto-parietal gamma coherence prevailed in the left hemisphere during the encoding condition and in the right hemisphere during the retrieval condition. The DTF estimates of the gamma band showed a dominant parietal-to-frontal directional flow in the right hemisphere during the encoding condition and in the left hemisphere during the retrieval condition (i.e. hemisphere-condition combination not involved by the HERA model). In contrast, a balanced bidirectional flow of the fronto-parietal coupling was observed in the left hemisphere during the encoding condition and in the right hemisphere during the retrieval condition (i.e. hemisphere-condition combination involved by the HERA model). In conclusion, the present encoding-retrieval conditions induced maximal fronto-parietal gamma coupling with bidirectional information flow in the hemisphere-condition combination predicted by the HERA model.

Corollary discharge dysfunction in schizophrenia: Can it explain auditory hallucinations?

Failure of corollary discharge, a mechanism for distinguishing self-generated from externally generated percepts, has been posited to underlie certain positive symptoms of schizophrenia, including auditory hallucinations. Although originally described in the visual system, corollary discharge may exist in the auditory system, whereby signals from motor speech commands prepare auditory cortex for self-generated speech. While associated with sensorimotor systems, it might also apply to inner speech or thought, regarded as our most complex motor act. In this paper, we describe the results of a series of studies in which we have shown that: (1) event-related brain potentials (ERPs) can be used to demonstrate the corollary discharge phenomenon during talking, (2) corollary discharge is abnormal in patients with schizophrenia, (3) EEG gamma band coherence between frontal and temporal lobes is greater during talking than listening and is disrupted by distorted feedback during talking in normals, and (4) patients with schizophrenia do not show this pattern for EEG gamma coherence. While these studies have identified ERPs and EEG gamma coherence indices of the efference copy/corollary discharge system and documented abnormalities in these systems in patients with schizophrenia, we have so far had limited success in establishing a relationship between these neurobiologic indicators of corollary discharge abnormality and reports of hallucinations in patients.

Reduced gamma-band coherence to distorted feedback during speech when what you say is not what you hear

Background Communication between the frontal lobes, where speech is generated, and the temporal lobes, where it is perceived, may occur through the action of an efference copy/corollary discharge mechanism that prepares the temporal lobes for the expected sound. We suggest that coherence of EEG in gamma-band between frontal and temporal lobes may reflect the successful action of such a mechanism. We tested the hypothesis that there would be a disruption of gamma-band coherence when the expected auditory consequence of speech does not match the auditory experience. Method EEG was recorded from 21 healthy adult subjects as they uttered the sound [a:] (Talking condition) and as they heard these recorded sounds played back (Listening condition). As they spoke, subjects heard real-time feedback of the sounds that were: (1) pitch-shifted down one semi-tone, (2) pitch-shifted down one-half of a semi-tone, or (3) not pitch-shifted (veridical), each in separate runs. Event-related gamma coherence between frontal and temporal sites was calculated relative to the onset of the sound, as it was being spoken during Talking, and as it was being played back during Listening. Results Frontal–temporal gamma-band coherence spanning 33–43 Hz was greater during Talking than Listening and greater when speech was veridical than when it was distorted a whole semi-tone. Conclusions Gamma-band fronto-temporal synchrony may reflect a “binding of expectation with experience.” Disruption of this synchrony may provide feedback to the frontal lobes, particularly regions subserving vocalization, to implement sensorimotor adaptations to either adjust motor programs for speech production in the short run, or to reorganize expectations in the long run.

EEG Gamma Coherence and Other Correlates of Subjective Reports During Ayahuasca Experiences

The current study examined QEEG power and coherence of ayahuasca experiences with two experienced participants in a Brazilian jungle setting. An exploratory case series design was adopted for naturalistic field research. EEGs recorded during visual imagery was compared to eyesclosed baselines. The most important findings were increases in global EEG coherence in the 36–44 Hz and 50–64 Hz frequency bands for both subjects. Widely distributed cortical hyper-coherence seems reasonable given the intense synesthesia during ayahuasca experiences. Other findings include increased modal EEG alpha frequency and global power decreases across the cortex in most frequency bands, which concur with the EEG of psychedelics literature. Exploratory analysis revealed the usefulness of analyzing single Hz bins over the standard wide-band analysis. The discovery-oriented naturalistic approach developed for this study resulted in potentially important findings. We believe that finding increases in global gamma coherence during peak psychedelic experiences might contribute to the discussion of binding theory. Also, in light of recent research with gamma coherence during advanced meditative conditions, our findings might further the comparison of shamanic psychedelic practices with meditation.

Synchronization of gamma oscillations increases functional connectivity of human hippocampus and inferior‐middle temporal cortex during repetitive visuomotor events

Do recency processes associated with repetitive sensorimotor events modulate the magnitude and functional coupling of brain rhythmicity in human temporal cortex? Intracranial stereo electroencephalographic activity (SEEG; 256 Hz sampling rate) was recorded from hippocampus, and inferior (BA20) and middle (BA21) temporal cortex in four epilepsy patients. The repetitive events were represented by predicted imperative somatosensory stimuli (CNV paradigm) triggering hand movements ("repetitive visuomotor") or counting ("repetitive counting"). The non-repetitive events were "rare" (P3 paradigm) somatosensory stimuli triggering hand movements ("non-repetitive visuomotor") or counting ("non-repetitive counting"). Brain rhythmicity was indexed by event-related desynchronization/synchronization (ERD/ERS) of SEEG data, whereas the functional coupling was evaluated by spectral SEEG coherence between pairs of the mentioned areas. The frequency bands of interest were theta (4-8 Hz), alpha (8-12 Hz), beta (14-30 Hz), and gamma (32-46 Hz). Compared to the non-repetitive events, the "repetitive visuomotor" events showed a significant beta and gamma ERS in the hippocampus and a significant theta ERD in the inferior temporal cortex. Furthermore, the "repetitive visuomotor" events induced a task-specific significant gamma coherence among the examined areas. These results suggest that recency processes do modulate the magnitude and functional coupling of brain rhythmicity (especially gamma) in the human temporal cortex.

Gamma EEG dynamics in neocortex and hippocampus during human wakefulness and sleep

Little is known about the neurophysiological mechanisms underlying the human sleep–wake cycle. Using intracranial electrodes in humans, we investigated changes in topographic distribution of gamma power and local- and long-range gamma EEG coherence in neocortex and hippocampus during different cerebral states. We report significantly greater variability in gamma power across cortical regions during wakefulness than during either slow wave or rapid eye movement (REM) sleep. In addition, local (within cortical regions) and long-range (between cortical regions) gamma coherence was significantly higher during wakefulness than during sleep, and functional gamma-range coupling between the neocortex and hippocampus was seen during wakefulness, but not during sleep. These findings demonstrate a functional link between different stages of conscious awareness and the level of coupling of gamma-band oscillations in the human brain.

Gamma EEG dynamics in neocortex and hippocampus during human wakefulness and sleep

Little is known about the neurophysiological mechanisms underlying the human sleep–wake cycle. Using intracranial electrodes in humans, we investigated changes in topographic distribution of gamma power and local- and long-range gamma EEG coherence in neocortex and hippocampus during different cerebral states. We report significantly greater variability in gamma power across cortical regions during wakefulness than during either slow wave or rapid eye movement (REM) sleep. In addition, local (within cortical regions) and long-range (between cortical regions) gamma coherence was significantly higher during wakefulness than during sleep, and functional gamma-range coupling between the neocortex and hippocampus was seen during wakefulness, but not during sleep. These findings demonstrate a functional link between different stages of conscious awareness and the level of coupling of gamma-band oscillations in the human brain.

Electroencephalographic Power and Coherence Analyses Suggest Altered Brain Function in Abstinent Male Heroin-Dependent Patients

Previous studies have shown that drug abuse is associated with altered brain function. However, studies of heroin abuse-related brain dysfunctions are scarce. Electroencephalographic (EEG) power and coherence analyses are two important tools for examining the effects of drugs on brain function. In the present study, we compared EEG power and coherence measures of 18 abstinent heroin-dependent subjects with those of 12 healthy control subjects. Furthermore, within the heroin group, associations between heroin use in the past, heroin craving and these EEG measures were studied. The results show that heroin-dependent subjects have increased relative beta-2 power and increased left intrahemispheric gamma coherence compared with control subjects. Furthermore, coherence measures showed correlations with clinical variables. These EEG abnormalities may reflect underlying changes in brain function due to long-term drug abuse and premorbid characteristics.