Decreased coherence in higher frequency ranges (beta and gamma) between central and frontal EEG in patients with schizophrenia: A preliminary report

Resting-state electroencephalogram in drug-free subjects with at-risk mental states who later developed psychosis: a low-resolution electromagnetic tomography analysis

Background and objectives

Several studies have reported on the resting-state electroencephalogram (EEG) power in patients with schizophrenia, with a decrease in α (especially α2) and an increase in δ and β1 power compared with healthy control; however, reports on at-risk mental states (ARMS) are few. In this study, we measured the resting-state EEG power in ARMS, and investigated its features and the relationship between the power of the frequency bands and their diagnostic outcomes.

Methods

Patients with ARMS who were not on any psychotropic medication and met the Comprehensive Assessment of At-Risk Mental State criteria were included. Patients who developed psychotic disorders were labeled as the ARMS-P group, while patients with ARMS who were followed up prospectively for more than 2 years and did not develop psychotic disorders were classified as the ARMS-NP group. EEGs were measured in the resting state, and frequencies were analyzed using standardized low-resolution brain electromagnetic tomography (sLORETA). Seven bands (δ, θ, α1, α2, β1-3) underwent analysis. The sLORETA values (current source density [CSD]) were compared between the ARMS-P and ARMS-NP groups. Clinical symptoms were assessed at the time of EEG measurements using the Positive and Negative Syndrome Scale (PANSS).

Results

Of the 39 patients included (25 males, 14 females, 18.8 ± 4.5 years old), eight developed psychotic disorders (ARMS-P). The ARMS-P group exhibited significantly higher CSD in the β1 power within areas of the left middle frontal gyrus (MFG) compared with the ARMS-NP group (best match: X = -35, Y = 25, Z = 50 [MNI coordinates], Area 8, CSD = 2.33, p < 0.05). There was a significant positive correlation between the β1/α ratio of the CSD at left MFG and the Somatic concern score measured by the PANSS.

Discussion

Increased β1 power was observed in the resting EEG before the onset of psychosis and correlated with a symptom. This suggests that resting EEG power may be a useful marker for predicting future conversion to psychosis and clinical symptoms in patients with ARMS.

The Pathophysiological Underpinnings of Gamma-Band Alterations in Psychiatric Disorders

Investigating the biophysiological substrates of psychiatric illnesses is of great interest to our understanding of disorders' etiology, the identification of reliable biomarkers, and potential new therapeutic avenues. Schizophrenia represents a consolidated model of γ alterations arising from the aberrant activity of parvalbumin-positive GABAergic interneurons, whose dysfunction is associated with perineuronal net impairment and neuroinflammation. This model of pathogenesis is supported by molecular, cellular, and functional evidence. Proof for alterations of γ oscillations and their underlying mechanisms has also been reported in bipolar disorder and represents an emerging topic for major depressive disorder. Although evidence from animal models needs to be further elucidated in humans, the pathophysiology of γ-band alteration represents a common denominator for different neuropsychiatric disorders. The purpose of this narrative review is to outline a framework of converging results in psychiatric conditions characterized by γ abnormality, from neurochemical dysfunction to alterations in brain rhythms.

Automatic recognition of schizophrenia from brain-network features using graph convolutional neural network

Schizophrenia is a severe mental illness that imposes considerable economic burden on families and society. However, its clinical diagnosis primarily relies on scales and doctors' clinical experience and lacks an objective and accurate diagnostic approach. In recent years, graph convolutional neural networks (GCN) have been used to assist in psychiatric diagnosis owing to their ability to learn spatial-association information. Therefore, this study proposes a schizophrenia automatic recognition model based on graph convolutional neural network. Herein, the resting-state electroencephalography (EEG) data of 103 first-episode schizophrenia patients and 92 normal controls (NCs) were obtained. The automatic recognition model was trained with a nodal feature matrix that comprised the time and frequency-domain features of the EEG signals and local features of the brain network. The most significant regions that contributed to the model classification were identified, and the correlation between the node topological features of each significant region and clinical evaluation metrics was explored. Experiments were conducted to evaluate the performance of the model using 10-fold cross-validation. The best performance in the theta frequency band with a 6 s epoch length and phase-locked value. The recognition accuracy was 90.01%. The most significant region for identifying with first-episode schizophrenia patients and NCs was located in the parietal lobe. The results of this study verify the applicability of the proposed novel method for the identification and diagnosis of schizophrenia.

Alpha and gamma EEG coherence during on-task and mind wandering states in schizophrenia

OBJECTIVE

Electroencephalographic (EEG) coherence is one of the most relevant physiological measures used to detect abnormalities in patients with schizophrenia. The present study applies a task-related EEG coherence approach to understand cognitive processing in patients with schizophrenia and healthy controls.

METHODS

EEG coherence for alpha and gamma frequency bands was analyzed in a group of patients with schizophrenia and a group of healthy controls during the performance of an ecological task of sustained attention. We compared EEG coherence when participants presented externally directed cognitive states (On-Task) and when they presented cognitive distraction episodes (Mind-Wandering).

RESULTS

Results reflect cortical differences between groups (higher coherence for schizophrenia in the frontocentral and fronto-temporal regions, and higher coherence for healthy-controls in the postero-central regions), especially in the On-Task condition for the alpha band, compared to Mind-Wandering episodes. Few individual differences in gamma coherence were found.

CONCLUSIONS

The current study provides evidence of neurophysiological differences underlying different cognitive states in schizophrenia and healthy controls.

SIGNIFICANCE

Differences between groups may reflect inhibitory processes necessary for the successful processing of information, especially in the alpha band, given its role in cortical inhibition processes. Patients may activate compensatory inhibitory mechanisms when performing the task, reflected in increased coherence in fronto-temporal regions.

Non-rapid eye movement sleep and wake neurophysiology in schizophrenia

Motivated by the potential of objective neurophysiological markers to index thalamocortical function in patients with severe psychiatric illnesses, we comprehensively characterized key non-rapid eye movement (NREM) sleep parameters across multiple domains, their interdependencies, and their relationship to waking event-related potentials and symptom severity. In 72 schizophrenia (SCZ) patients and 58 controls, we confirmed a marked reduction in sleep spindle density in SCZ and extended these findings to show that fast and slow spindle properties were largely uncorrelated. We also describe a novel measure of slow oscillation and spindle interaction that was attenuated in SCZ. The main sleep findings were replicated in a demographically distinct sample, and a joint model, based on multiple NREM components, statistically predicted disease status in the replication cohort. Although also altered in patients, auditory event-related potentials elicited during wake were unrelated to NREM metrics. Consistent with a growing literature implicating thalamocortical dysfunction in SCZ, our characterization identifies independent NREM and wake EEG biomarkers that may index distinct aspects of SCZ pathophysiology and point to multiple neural mechanisms underlying disease heterogeneity. This study lays the groundwork for evaluating these neurophysiological markers, individually or in combination, to guide efforts at treatment and prevention as well as identifying individuals most likely to benefit from specific interventions.

Activity-State Dependent Reversal of Ketamine-Induced Resting State EEG Effects by Clozapine and Naltrexone in the Freely Moving Rat

Ketamine is a non-competitive N-Methyl-D-aspartate receptor (NMDAR) antagonist used in the clinic to initiate and maintain anaesthesia; it induces dissociative states and has emerged as a breakthrough therapy for major depressive disorder. Using local field potential recordings in freely moving rats, we studied resting state EEG profiles induced by co-administering ketamine with either: clozapine, a highly efficacious antipsychotic; or naltrexone, an opioid receptor antagonist reported to block the acute antidepressant effects of ketamine. As human electroencephalography (EEG) is predominantly recorded in a passive state, head-mounted accelerometers were used with rats to determine active and passive states at a high temporal resolution to offer the highest translatability. In general, pharmacological effects for the three drugs were more pronounced in (or restricted to) the passive state. Specifically, during inactive periods clozapine induced increases in delta (0.1-4 Hz), gamma (30-60 Hz) and higher frequencies (>100 Hz). Importantly, it reversed the ketamine-induced reduction in low beta power (10-20 Hz) and potentiated ketamine-induced increases in gamma and high frequency oscillations (130-160 Hz). Naltrexone inhibited frequencies above 50 Hz and significantly reduced the ketamine-induced increase in high frequency oscillations. However, some frequency band changes, such as clozapine-induced decreases in delta power, were only seen in locomoting rats. These results emphasise the potential in differentiating between activity states to capture drug effects and translate to human resting state EEG. Furthermore, the differential reversal of ketamine-induced EEG effects by clozapine and naltrexone may have implications for the understanding of psychotomimetic as well as rapid antidepressant effects of ketamine.

Differentiating ictal/subclinical spikes and waves in childhood absence epilepsy by spectral and network analyses: A pilot study

OBJECTIVE

Childhood absence epilepsy (CAE) is a disease with distinct seizure semiology and electroencephalographic (EEG) features. Differentiating ictal and subclinical generalized spikes and waves discharges (GSWDs) in the EEG is challenging, since they appear to be identical upon visual inspection. Here, spectral and functional connectivity (FC) analyses were applied to routine EEG data of CAE patients, to differentiate ictal and subclinical GSWDs.

METHODS

Twelve CAE patients with both ictal and subclinical GSWDs were retrospectively selected for this study. The selected EEG epochs were subjected to frequency analysis in the range of 1-30 Hz. Further, FC analysis based on the imaginary part of coherency was used to determine sensor level networks.

RESULTS

Delta, alpha and beta band frequencies during ictal GSWDs showed significantly higher power compared to subclinical GSWDs. FC showed significant network differences for all frequency bands, demonstrating weaker connectivity between channels during ictal GSWDs.

CONCLUSION

Using spectral and FC analyses significant differences between ictal and subclinical GSWDs in CAE patients were detected, suggesting that these features could be used for machine learning classification purposes to improve EEG monitoring.

SIGNIFICANCE

Identifying differences between ictal and subclinical GSWDs using routine EEG, may improve understanding of this syndrome and the management of patients with CAE.

The association of polygenic risk for schizophrenia, bipolar disorder, and depression with neural connectivity in adolescents and young adults: examining developmental and sex differences

Neurodevelopmental abnormalities in neural connectivity have been long implicated in the etiology of schizophrenia (SCZ); however, it remains unclear whether these neural connectivity patterns are associated with genetic risk for SCZ in unaffected individuals (i.e., an absence of clinical features of SCZ or a family history of SCZ). We examine whether polygenic risk scores (PRS) for SCZ are associated with functional neural connectivity in adolescents and young adults without SCZ, whether this association is moderated by sex and age, and if similar associations are observed for genetically related neuropsychiatric PRS. One-thousand four-hundred twenty-six offspring from 913 families, unaffected with SCZ, were drawn from the Collaborative Study of the Genetics of Alcoholism (COGA) prospective cohort (median age at first interview = 15.6 (12-26), 51.6% female, 98.1% European American, 41% with a family history of alcohol dependence). Participants were followed longitudinally with resting-state EEG connectivity (i.e., coherence) assessed every two years. Higher SCZ PRS were associated with elevated theta (3-7 Hz) and alpha (7-12 Hz) EEG coherence. Associations differed by sex and age; the most robust associations were observed between PRS and parietal-occipital, central-parietal, and frontal-parietal alpha coherence among males between ages 15-19 (B: 0.15-0.21, p < 10^-4). Significant associations among EEG coherence and Bipolar and Depression PRS were observed, but differed from SCZ PRS in terms of sex, age, and topography. Findings reveal that polygenic risk for SCZ is robustly associated with increased functional neural connectivity among young adults without a SCZ diagnosis. Striking differences were observed between men and women throughout development, mapping onto key periods of risk for the onset of psychotic illness and underlining the critical importance of examining sex differences in associations with neuropsychiatric PRS across development.

EEG-Based Measures in At-Risk Mental State and Early Stages of Schizophrenia: A Systematic Review

Introduction: Electrophysiological (EEG) abnormalities in subjects with schizophrenia have been largely reported. In the last decades, research has shifted to the identification of electrophysiological alterations in the prodromal and early phases of the disorder, focusing on the prediction of clinical and functional outcome. The identification of neuronal aberrations in subjects with a first episode of psychosis (FEP) and in those at ultra high-risk (UHR) or clinical high-risk (CHR) to develop a psychosis is crucial to implement adequate interventions, reduce the rate of transition to psychosis, as well as the risk of irreversible functioning impairment. The aim of the review is to provide an up-to-date synthesis of the electrophysiological findings in the at-risk mental state and early stages of schizophrenia. Methods: A systematic review of English articles using Pubmed, Scopus, and PsychINFO was undertaken in July 2020. Additional studies were identified by hand-search. Electrophysiological studies that included at least one group of FEP or subjects at risk to develop psychosis, compared to healthy controls (HCs), were considered. The heterogeneity of the studies prevented a quantitative synthesis. Results: Out of 319 records screened, 133 studies were included in a final qualitative synthesis. Included studies were mainly carried out using frequency analysis, microstates and event-related potentials. The most common findings included an increase in delta and gamma power, an impairment in sensory gating assessed through P50 and N100 and a reduction of Mismatch Negativity and P300 amplitude in at-risk mental state and early stages of schizophrenia. Progressive changes in some of these electrophysiological measures were associated with transition to psychosis and disease course. Heterogeneous data have been reported for indices evaluating synchrony, connectivity, and evoked-responses in different frequency bands. Conclusions: Multiple EEG-indices were altered during at-risk mental state and early stages of schizophrenia, supporting the hypothesis that cerebral network dysfunctions appear already before the onset of the disorder. Some of these alterations demonstrated association with transition to psychosis or poor functional outcome. However, heterogeneity in subjects' inclusion criteria, clinical measures and electrophysiological methods prevents drawing solid conclusions. Large prospective studies are needed to consolidate findings concerning electrophysiological markers of clinical and functional outcome.

Resting-state functional EEG connectivity in salience and default mode networks and their relationship to dissociative symptoms during NMDA receptor antagonism

N-methyl-d-aspartate receptor (NMDAR) antagonists administered to healthy humans results in schizophrenia-like symptoms, which are thought in part to be related to glutamatergically altered electrophysiological connectivity in large-scale intrinsic functional brain networks. Here, we examine resting-state source electroencephalographic (EEG) connectivity within and between the default mode (DMN: for self-related cognitive activity) and salience networks (SN: for detection of salient stimuli in internal and external environments) in 21 healthy volunteers administered a subanesthetic dose of the dissociative anesthetic and NMDAR antagonist, ketamine. In addition to provoking symptoms of dissociation, which are thought to originate from an altered sense of self that is common to schizophrenia, ketamine induces frequency-dependent increases and decreases in connectivity within and between DMN and SN. These altered interactive network couplings together with emergent dissociative symptoms tentatively support an NMDAR-hypofunction hypothesis of disturbed electrophysiologic connectivity in schizophrenia.

Changes in neural network connectivity in mice brain following exposures to palatable food

Previously, satiated animals or human subjects can still be motivated to eat by palatable food-associated cues. However, neural circuitries of hedonic hunger have not been well investigated. This study identified neural network connectivities between major brain areas in response to chocolate-associated cues following repeated exposures to chocolate. Adult male Swiss albino ICR mice were anesthetized and implanted with intracranial electrodes in the lateral hypothalamus (LHa), nucleus accumbens (NAc), olfactory bulb (OB) and hippocampus (HP) for local field potential (LFP) recording. LFP oscillations were recorded before and after repeated exposures to chocolate for chocolate experienced group whereas control group was not exposed to chocolate. On testing days, satiated animals were individually put into a place preference-like apparatus with two opposite chambers of chocolate and normal chow scent cues, separately. The results showed that chocolate experienced group significantly increased time spent in chocolate chamber whereas control group did not. One-way ANOVA revealed significant influence of chocolate sessions on LFP spectral powers of multiple frequencies in the LHa (delta, low gamma and high gamma) and NAc (high gamma). Moreover, coherence function analyses also highlighted significant increases in LHa-NAc and LHa-OB, and decrease in LHa-HP coherent activities in response to olfactory cues of chocolate. This study demonstrated modifications of neural network connectivity and associative learning following multiple exposures to palatable food. These findings might explain why energy homeostatic hunger is overridden by hedonic hunger.

The mGluR2/3 agonist LY379268 reverses NMDA receptor antagonist effects on cortical gamma oscillations and phase coherence, but not working memory impairments, in mice

BACKGROUND

Abnormalities in neural oscillations that occur in the gamma frequency range (30-80 Hz) may underlie cognitive deficits in schizophrenia. Both cognitive impairments and gamma oscillatory disturbances can be induced in healthy people and rodents by administration of N-methyl-D-aspartate receptor (NMDAr) antagonists.

AIMS

We studied relationships between cognitive impairment and gamma abnormalities following NMDAr antagonism, and attempted to reverse deficits with the metabotropic glutamate receptor type 2/3 (mGluR_2/3) agonist LY379268.

METHODS

C57/Bl6 mice were trained to perform the Trial-Unique Nonmatching to Location (TUNL) touchscreen test for working memory. They were then implanted with local field potential (LFP) recording electrodes in prefrontal cortex and dorsal hippocampus. Mice were administered either LY379268 (3 mg/kg) or vehicle followed by the NMDAr antagonist MK-801 (0.3 or 1 mg/kg) or vehicle prior to testing on the TUNL task, or recording LFPs during the presentation of an auditory stimulus.

RESULTS

MK-801 impaired working memory and increased perseveration, but these behaviours were not improved by LY379268 treatment. MK-81 increased the power of ongoing gamma and high gamma (130-180 Hz) oscillations in both brain regions and regional coherence between regions, and these signatures were augmented by LY379268. However, auditory-evoked gamma oscillation deficits caused by MK-801 were not affected by LY379268 pretreatment.

CONCLUSIONS

NMDA receptor antagonism impairs working memory in mice, but this is not reversed by stimulation of mGluR_2/3. Since elevations in ongoing gamma power and regional coherence caused by MK-801 were improved by LY379268, it appears unlikely that these specific oscillatory abnormalities underlie the working memory impairment caused by NMDAr antagonism.

Gamma band oscillations in the early phase of psychosis: A systematic review

Abnormal gamma oscillations, measured by electroencephalography (EEG), have been associated with chronic psychotic disorders, but their prevalence in the early phase of psychosis is less clear. We sought to address this by systematically reviewing the relevant literature. We searched for EEG studies of gamma band oscillations in subjects at high risk for psychosis and in patients with first episode psychosis. The following measures of gamma oscillations were extracted: resting power, evoked power, induced power, connectivity and peak frequency. Forty-five studies with a total of 3099 participants were included. There were potential sources of bias in the study designs and potential artefacts. Although there were few consistent findings, several studies reported decreased evoked or induced power in both high risk subjects and first episode patients. Studies using larger samples with serial EEG measurements, and designs that minimise artefacts that occur at the gamma frequency may advance work in this area.

EEG 40 Hz Coherence Decreases in REM Sleep and Ketamine Model of Psychosis

Cognitive processes are carried out during wakefulness by means of extensive interactions between cortical and subcortical areas. In psychiatric conditions, such as psychosis, these processes are altered. Interestingly, REM sleep where most dreams occurs, shares electrophysiological, pharmacological, and neurochemical features with psychosis. Because of this fact, REM sleep is considered a natural model of psychosis. Ketamine is a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist that at sub-anesthetic dose induces psychotomimetic-like effects in humans and animals, and is employed as a pharmacological model of psychosis. Oscillations in the gamma frequency band of the electroencephalogram (EEG), mainly at about 40 Hz, have been involved in cognitive functions. Hence, the present study was conducted to analyze the EEG low gamma (30-45 Hz) band power and coherence of the cat, in natural (REM sleep) and pharmacological (sub-anesthetic doses of ketamine) models of psychosis. These results were compared with the gamma activity during alert (AW) and quiet wakefulness (QW), as well as during non-REM (NREM) sleep. Five cats were chronically prepared for polysomnographic recordings, with electrodes in different cortical areas. Basal recordings were obtained and ketamine (5, 10, and 15 mg/kg, i.m.) was administrated. Gamma activity (power and coherence) was analyzed in the abovementioned conditions. Compared to wakefulness and NREM sleep, following ketamine administration gamma coherence decreased among all cortical regions studied; the same coherence profile was observed during REM sleep. On the contrary, gamma power was relatively high under ketamine, and similar to QW and REM sleep. We conclude that functional interactions between cortical areas in the gamma frequency band decrease in both experimental models of psychosis. This uncoupling of gamma frequency activity may be involved in the cognitive features shared by dreaming and psychosis.

Neuronal desynchronization as marker of an impaired brain network

Synchronization is a central key feature of neural information processing and communication between different brain areas. Disturbance of oscillatory brain rhythms and decreased synchronization have been associated with different disorders including schizophrenia. The aim of this study was to investigate whether synchronization (in relaxed conditions with no stimuli) between different brain areas within the delta, theta, alpha (alpha1, alpha2), beta (beta1, beta2), and gamma bands is altered in patients with a neurological disorder in order to generate significant cortical enhancements. To achieve this, we investigated schizophrenic patients (SZO; N=17, 37.5±10.4 years, 15 males) and compared them to healthy subjects (CON; N=21, 36.7±13.4 years, 15 males) applying the phase locking value (PLV). We found significant differences between SZO and CON in different brain areas of the theta, alpha1, beta2 and gamma bands. These areas are related to the central and parietal lobes for the theta band, the parietal lobe for the alpha1, the parietal and frontal for the beta2 and the frontal-central for the gamma band. The gamma band revealed the most significant differences between CON and SZO. PLV were 61.7% higher on average in SZO in most of the clusters when compared to CON. The related brain areas are directly related to cognition skills which are proved to be impaired in SZO. The results of this study suggest that synchronization in SZO is also altered when the patients were not asked to perform a task that requires their cognitive skills (i.e., no stimuli are applied - in contrast to other findings).

Sleep and mental disorders: A meta-analysis of polysomnographic research

Investigating sleep in mental disorders has the potential to reveal both disorder-specific and transdiagnostic psychophysiological mechanisms. This meta-analysis aimed at determining the polysomnographic (PSG) characteristics of several mental disorders. Relevant studies were searched through standard strategies. Controlled PSG studies evaluating sleep in affective, anxiety, eating, pervasive developmental, borderline and antisocial personality disorders, attention-deficit-hyperactivity disorder (ADHD), and schizophrenia were included. PSG variables of sleep continuity, depth, and architecture, as well as rapid-eye movement (REM) sleep were considered. Calculations were performed with the "Comprehensive Meta-Analysis" and "R" software. Using random effects modeling, for each disorder and each variable, a separate meta-analysis was conducted if at least 3 studies were available for calculation of effect sizes as standardized means (Hedges' g). Sources of variability, that is, sex, age, and mental disorders comorbidity, were evaluated in subgroup analyses. Sleep alterations were evidenced in all disorders, with the exception of ADHD and seasonal affective disorders. Sleep continuity problems were observed in most mental disorders. Sleep depth and REM pressure alterations were associated with affective, anxiety, autism and schizophrenia disorders. Comorbidity was associated with enhanced REM sleep pressure and more inhibition of sleep depth. No sleep parameter was exclusively altered in 1 condition; however, no 2 conditions shared the same PSG profile. Sleep continuity disturbances imply a transdiagnostic imbalance in the arousal system likely representing a basic dimension of mental health. Sleep depth and REM variables might play a key role in psychiatric comorbidity processes. Constellations of sleep alterations may define distinct disorders better than alterations in 1 single variable. (PsycINFO Database Record

Schizophrenia and reelin: a model based on prenatal stress to study epigenetics, brain development and behavior

Schizophrenia is a severe psychiatric disorder that results in a significant disability for the patient. The disorder is characterized by impairment of the adaptive orchestration of actions, a cognitive function that is mainly dependent on the prefrontal cortex. This behavioral deficit, together with cellular and neurophysiological alterations in the prefrontal cortex, as well as reduced density of GABAergic cells and aberrant oscillatory activity, all indicate structural and functional deficits of the prefrontal cortex in schizophrenia. Among the several risk factors for the development of schizophrenia, stress during the prenatal period has been identified as crucial. Thus, it is proposed that prenatal stress induces neurodevelopmental alterations in the prefrontal cortex that are expressed as cognitive impairment observed in schizophrenia. However, the precise mechanisms that link prenatal stress with the impairment of prefrontal cortex function is largely unknown. Reelin is an extracellular matrix protein involved in the development of cortical neural connectivity at embryonic stages, and in synaptic plasticity at postnatal stages. Interestingly, down-regulation of reelin expression has been associated with epigenetic changes in the reelin gene of the prefrontal cortex of schizophrenic patients. We recently showed that, similar to schizophrenic patients, prenatal stress induces down-expression of reelin associated with the methylation of its promoter in the rodent prefrontal cortex. These alterations were paralleled with altered prefrontal cortex functional connectivity and impairment in prefrontal cortex-dependent behavioral tasks. Therefore, considering molecular, cellular, physiological and behavioral evidence, we propose a unifying framework that links prenatal stress and prefrontal malfunction through epigenetic alterations of the reelin gene.

Cellular and circuit models of increased resting-state network gamma activity in schizophrenia

Schizophrenia (SCZ) is a disorder characterized by positive symptoms (hallucinations, delusions), negative symptoms (blunted affect, alogia, reduced sociability, and anhedonia), as well as persistent cognitive deficits (memory, concentration, and learning). While the biology underlying subjective experiences is difficult to study, abnormalities in electroencephalographic (EEG) measures offer a means to dissect potential circuit and cellular changes in brain function. EEG is indispensable for studying cerebral information processing due to the introduction of techniques for the decomposition of event-related activity into its frequency components. Specifically, brain activity in the gamma frequency range (30-80Hz) is thought to underlie cognitive function and may be used as an endophenotype to aid in diagnosis and treatment of SCZ. In this review we address evidence indicating that there is increased resting-state gamma power in SCZ. We address how modeling this aspect of the illness in animals may help treatment development as well as providing insights into the etiology of SCZ.

A study on validity of cortical alpha connectivity for schizophrenia

Abnormalities in schizophrenia are thought to be associated with functional disconnections between different brain regions. Most previous studies on schizophrenia have considered high-band connectivity in preference to the Alpha band, as there has been some uncertainty correlating the latter to the condition. In this paper we attempt to clarify this correlation using an Electroencephalogram (EEG) analysis of the Alpha band from schizophrenic patients. Global, regional Omega and dimensional complexity and local Omega complexity differentials (LCD) of single channel are calculated using 16 channels of resting EEG data from 31 adult patients with schizophrenia and 31 age/sex matched control subjects. It was found that, compared to the controls, anterior alpha Omega and dimensional complexity are higher in schizophrenia patients (p<0.05) with the single channel LCD also increasing at FP1, FP2, F7 and F8 electrodes. Furthermore, higher left hemisphere dimensional complexity and LCD at T3 point was also found. The results suggest there is lower connectivity in the pre-frontal and left temporal regions with respect to the alpha band in schizophrenia patients.

Schneiderian first rank symptoms and gamma oscillatory activity in neuroleptic naïve first episode schizophrenia: a 192 channel EEG study

OBJECTIVE

Schneiderian first-rank symptoms (FRS) and abnormal EEG gamma activity in schizophrenia have been reported independently to have a neurodevelopmental basis. We aimed to investigate spontaneous gamma power in two groups of first episode schizophrenia patients (those who experience FRS and those who do not).

METHODS

A comparative hospital based study having 37neuroleptic naïve male patients with schizophrenia divided into two groups-FRS(+) and FRS(-) groups based on the presence of FRS. Thirty age, sex, education and handedness matched individuals served as controls (N). All participants underwent a 192-channel resting Electroencephalography (EEG) recording. Gamma spectral power was calculated for low- (30-50 Hz) and high-gamma 1 & 2 (51-70 and 71-100 Hz) bands. Spectral power was compared between three groups using MANOVA and supplementary one-way ANOVA with Bonferroni test controlling for multiple comparisons. Linear regression was used to identifying predictor variables for FRS. Pearson correlation coefficient was computed between spectral power parameters and various clinical variables.

RESULTS

Significantly higher high gamma band-1 power was observed over right frontal (p<0.05), parietal (p<0.05) and temporal (p<0.05) regions in FRS(+) than FRS(-) group and normal controls. Right parietal high gamma-1 power and paranoid cluster on PANSS significantly predicted number of FRS in total schizophrenia patients; paranoid cluster on PANSS showed significant correlation with number of FRS in FRS(+) group.

CONCLUSION

Findings of our study add to the evidence that areas contained within the hetero modal association cortex are associated with FRS. The study findings also strengthen the neurodevelopmental basis of FRS in schizophrenia.

Brain state-dependent abnormal LFP activity in the auditory cortex of a schizophrenia mouse model

In schizophrenia, evoked 40-Hz auditory steady-state responses (ASSRs) are impaired, which reflects the sensory deficits in this disorder, and baseline spontaneous oscillatory activity also appears to be abnormal. It has been debated whether the evoked ASSR impairments are due to the possible increase in baseline power. GABAergic interneuron-specific NMDA receptor (NMDAR) hypofunction mutant mice mimic some behavioral and pathophysiological aspects of schizophrenia. To determine the presence and extent of sensory deficits in these mutant mice, we recorded spontaneous local field potential (LFP) activity and its click-train evoked ASSRs from primary auditory cortex of awake, head-restrained mice. Baseline spontaneous LFP power in the pre-stimulus period before application of the first click trains was augmented at a wide range of frequencies. However, when repetitive ASSR stimuli were presented every 20 s, averaged spontaneous LFP power amplitudes during the inter-ASSR stimulus intervals in the mutant mice became indistinguishable from the levels of control mice. Nonetheless, the evoked 40-Hz ASSR power and their phase locking to click trains were robustly impaired in the mutants, although the evoked 20-Hz ASSRs were also somewhat diminished. These results suggested that NMDAR hypofunction in cortical GABAergic neurons confers two brain state-dependent LFP abnormalities in the auditory cortex; (1) a broadband increase in spontaneous LFP power in the absence of external inputs, and (2) a robust deficit in the evoked ASSR power and its phase-locking despite of normal baseline LFP power magnitude during the repetitive auditory stimuli. The “paradoxically” high spontaneous LFP activity of the primary auditory cortex in the absence of external stimuli may possibly contribute to the emergence of schizophrenia-related aberrant auditory perception.

Enhanced synchronization of gamma activity between frontal lobes during REM sleep as a function of REM sleep deprivation in man

Studies have shown that synchrony or temporal coupling of gamma activity is involved in processing and integrating information in the brain. Comparing rapid eye movement (REM) sleep to waking and non-REM (NREM) sleep, interhemispheric temporal coupling is higher, but lower between the frontal and posterior association areas of the same hemisphere. However, the homeostatic response of REM sleep temporal coupling after selective REM sleep deprivation (REMD) has not been studied. This study proposed exploring the effect of one night of selective REMD on the temporal coupling of cortical gamma activity during recovery REM sleep. Two groups of healthy subjects were subjected to either REMD by awakening them at each REM sleep onset, or to NREM sleep interruptions. Subjects slept four consecutive nights in the laboratory: first for adaptation, second as baseline, third for sleep manipulation, and fourth for recovery. Interhemispheric and intrahemispheric EEG correlations were analyzed during tonic REM (no eye movements) for the first three REM sleep episodes during baseline sleep, and recovery sleep after one night of selective REMD. Temporal coupling between frontal lobes showed a significant homeostatic rebound that increased during recovery REM sleep relative to baseline and controls. Results showed a rebound in temporal coupling between the two frontal lobes after REM sleep deprivation, indicating that the enhanced gamma temporal coupling that occurs normally during REM sleep has functional consequences.

CONCLUSION

results suggest that synchronized activity during REM sleep may play an important role in integrating and reprocessing information.

Prefrontal cortical minicolumn: from executive control to disrupted cognitive processing

The prefrontal cortex of the primate brain has a modular architecture based on the aggregation of neurons in minicolumnar arrangements having afferent and efferent connections distributed across many brain regions to represent, select and/or maintain behavioural goals and executive commands. Prefrontal cortical microcircuits are assumed to play a key role in the perception to action cycle that integrates relevant information about environment, and then selects and enacts behavioural responses. Thus, neurons within the interlaminar microcircuits participate in various functional states requiring the integration of signals across cortical layers and the selection of executive variables. Recent research suggests that executive abilities emerge from cortico-cortical interactions between interlaminar prefrontal cortical microcircuits, whereas their disruption is involved in a broad spectrum of neurologic and psychiatric disorders such as autism, schizophrenia, Alzheimer's and drug addiction. The focus of this review is on the structural, functional and pathological approaches involving cortical minicolumns. Based on recent technological progress it has been demonstrated that microstimulation of infragranular cortical layers with patterns of microcurrents derived from supragranular layers led to an increase in cognitive performance. This suggests that interlaminar prefrontal cortical microcircuits are playing a causal role in improving cognitive performance. An important reason for the new interest in cortical modularity comes from both the impressive progress in understanding anatomical, physiological and pathological facets of cortical microcircuits and the promise of neural prosthetics for patients with neurological and psychiatric disorders.

Resting state EEG power and coherence abnormalities in bipolar disorder and schizophrenia

Resting state electroencephalogram (EEG) abnormalities in schizophrenia and bipolar disorder patients suggest alterations in neural oscillatory activity. However, few studies directly compare these anomalies between patient groups, and none have examined EEG coherence. Therefore, this study investigated whether these electrophysiological characteristics differentiate clinical populations from one another, and from non-psychiatric controls. To address this question, resting EEG power and coherence were assessed in 76 bipolar patients (BP), 132 schizophrenia patients (SZ), and 136 non-psychiatric controls (NC). We conducted separate repeated-measures ANOVAs to examine group differences within seven frequency bands across several brain regions. BP showed significantly greater power relative to SZ at higher frequencies including Beta and Gamma across all regions. In terms of intra-hemispheric coherence, while SZ generally exhibited higher coherence at Delta compared to NC and BP, both SZ and BP showed higher coherence at Alpha1 and Alpha2. In contrast, BP and HC showed higher coherence within hemispheres compared to SZ at Beta 1. In terms of inter-hemispheric coherence, SZ displayed higher coherence compared to NC at temporal sites at both Alpha1 and Alpha2. Taken together, BP exhibited increased high frequency power with few disruptions in neural synchronization. In contrast, SZ generally exhibited enhanced synchronization within and across hemispheres. These findings suggest that resting EEG can be a sensitive measure for differentiating between clinical disorders.

A role for the precuneus in thought-action fusion: evidence from participants with significant obsessive-compulsive symptoms

Likelihood thought-action fusion (TAF-L) refers to a cognitive bias in which individuals believe that the mere thought of a negative event increases its likelihood of occurring in reality. TAF-L is most commonly associated with obsessive-compulsive disorder (OCD) but is also present in depression, generalized anxiety disorder and psychosis. We induced TAF-L in individuals with high (High-OC, N = 23) and low (Low-OC, N = 24) levels of OC traits, and used low resolution electromagnetic tomography (LORETA) to localise the accompanying electrical brain activity patterns. The results showed greater TAF-L in the High-OC than in the Low-OC group (p < .005), which was accompanied by significantly greater upper beta frequency (19-30 Hz) activity in the precuneus (p < .05). Further, the precuneus activity was positively correlated with self-reported magnitude of TAF-L (p < .01), suggesting a specific role of this region in this cognitive bias. Results are discussed with reference to self-referential processing and the default-mode network.

Dopamine, cognitive function, and gamma oscillations: role of D4 receptors

Cognitive deficits in individuals with schizophrenia (SCZ) are considered core symptoms of this disorder, and can manifest at the prodromal stage. Antipsychotics ameliorate positive symptoms but only modestly improve cognitive symptoms. The lack of treatments that improve cognitive abilities currently represents a major obstacle in developing more effective therapeutic strategies for this debilitating disorder. While D4 receptor (D4R)-specific antagonists are ineffective in the treatment of positive symptoms, animal studies suggest that D4R drugs can improve cognitive deficits. Moreover, recent work from our group suggests that D4Rs synergize with the neuregulin/ErbB4 signaling pathway, genetically identified as risk factors for SCZ, in parvalbumin (PV)-expressing interneurons to modulate gamma oscillations. These high-frequency network oscillations correlate with attention and increase during cognitive tasks in healthy subjects, and this correlation is attenuated in affected individuals. This finding, along with other observations indicating impaired GABAergic function, has led to the idea that abnormal neural activity in the prefrontal cortex (PFC) in individuals with SCZ reflects a perturbation in the balance of excitation and inhibition. Here we review the current state of knowledge of D4R functions in the PFC and hippocampus, two major brain areas implicated in SCZ. Special emphasis is given to studies focusing on the potential role of D4Rs in modulating GABAergic transmission and to an emerging concept of a close synergistic relationship between dopamine/D4R and neuregulin/ErbB4 signaling pathways that tunes the activity of PV interneurons to regulate gamma frequency network oscillations and potentially cognitive processes.

Increased spontaneous gamma power and synchrony in schizophrenia patients having higher minor physical anomalies

The higher frequency of minor physical anomalies (MPAs) in schizophrenia provides morphological evidence for the neurodevelopmental theory. Abnormal gamma oscillations (>30 Hz) seen in the electroencephalogram (EEG) in schizophrenia have been hypothesized to result from developmental insults. This study investigated spontaneous gamma oscillations in schizophrenia patients having higher and lower number of MPAs. Forty drug naïve/free schizophrenia patients and 20 matched healthy controls were assessed for MPAs on the Extended Waldrop Scale (EWS). All participants underwent an awake, resting 192-channel EEG recording. Spontaneous gamma spectral power and coherence were estimated in the low- (30-50 Hz) and high-gamma (51-70 and 71-100 Hz) bands. Significantly higher power was observed in high-MPA than healthy control group in low-gamma band over right frontal, parietal and temporal regions. Spectral power in the high-gamma band (71-100 Hz) was also significantly higher in the high-MPA schizophrenia subgroup than in the healthy control group over left frontal, parietal and temporal regions. Additionally, regional intra-hemispheric and inter-hemispheric coherence in the low-gamma band was significantly higher in the high-MPA schizophrenia subgroup than on the healthy control group. This study is the first to provide evidence of increased spontaneous gamma power and synchrony in schizophrenia patients having higher MPAs, supporting the idea that it may represent a distinct subgroup of schizophrenia with a neurodevelopmental basis.

Brain oscillations in bipolar disorder in search of new biomarkers

This report presents six cardinal results obtained with methods of oscillatory brain dynamics in euthymic and manic bipolar patients in comparison to healthy controls. Measurements include changes in oscillatory response activities in the theta, alpha, beta, and gamma frequency ranges. The analysis shows that spontaneous and response activities in the alpha range are highly reduced in euthymic and manic patients, respectively; conversely, beta responses are increased in euthymic and manic patients. Lithium use seems to be associated with further and significant increase in the beta frequency range in euthymic patients. Theta responses to auditory target stimulus during odd-ball paradigm appeared in two different frequency bands (4-6 and 6-8 Hz) in healthy participants. However, only fast theta responses were highly reduced under cognitive load in drug-free euthymic patients. The analysis of connectivity was performed by assessment of long-distance coherence function in the gamma frequency range. Both manic and euthymic patients presented significantly decreased fronto-temporal coherence function during visual odd-ball task, indicating a selective reduction in connectivity during cognitive processing. The present report also discusses that these six oscillatory parameters may serve as an ensemble of biomarkers for diagnostic purposes and tracking treatment response in bipolar disorder.

Reduced sleep spindles and spindle coherence in schizophrenia: mechanisms of impaired memory consolidation?

BACKGROUND

Sleep spindles are thought to induce synaptic changes and thereby contribute to memory consolidation during sleep. Patients with schizophrenia show dramatic reductions of both spindles and sleep-dependent memory consolidation, which may be causally related.

METHODS

To examine the relations of sleep spindle activity to sleep-dependent consolidation of motor procedural memory, 21 chronic, medicated schizophrenia outpatients and 17 healthy volunteers underwent polysomnography on two consecutive nights. On the second night, participants were trained on the finger-tapping motor sequence task (MST) at bedtime and tested the following morning. The number, density, frequency, duration, amplitude, spectral content, and coherence of stage 2 sleep spindles were compared between groups and examined in relation to overnight changes in MST performance.

RESULTS

Patients failed to show overnight improvement on the MST and differed significantly from control participants who did improve. Patients also exhibited marked reductions in the density (reduced 38% relative to control participants), number (reduced 36%), and coherence (reduced 19%) of sleep spindles but showed no abnormalities in the morphology of individual spindles or of sleep architecture. In patients, reduced spindle number and density predicted less overnight improvement on the MST. In addition, reduced amplitude and sigma power of individual spindles correlated with greater severity of positive symptoms.

CONCLUSIONS

The observed sleep spindle abnormalities implicate thalamocortical network dysfunction in schizophrenia. In addition, the findings suggest that abnormal spindle generation impairs sleep-dependent memory consolidation in schizophrenia, contributes to positive symptoms, and is a promising novel target for the treatment of cognitive deficits in schizophrenia.

Gamma oscillation in schizophrenia

Dysfunctional neural circuitry has been found to be involved in abnormalities of perception and cognition in patients with schizophrenia. Gamma oscillations are essential for integrating information within neural circuits and have therefore been associated with many perceptual and cognitive processes in healthy human subjects and animals. This review presents an overview of the neural basis of gamma oscillations and the abnormalities in the GABAergic interneuronal system thought to be responsible for gamma-range deficits in schizophrenia. We also review studies of gamma activity in sensory and cognitive processes, including auditory steady state response, attention, object representation, and working memory, in animals, healthy humans and patients with schizophrenia.

Electroencephalographic spectral and coherence analysis of ketamine in rats: correlation with behavioral effects and pharmacokinetics

AIMS

This study was designed to evaluate the changes in EEG power spectra and EEG coherence in a ketamine model of psychosis in rats. Analyses of behavioral measurements--locomotion and sensorimotor gating--and the pharmacokinetics of ketamine and norketamine were also conducted.

METHODS

Ketamine and norketamine levels in rat sera and brains were analyzed by gas chromatography-mass spectrometry after ketamine 30 mg/kg (i.p.). Ketamine 9 and 30 mg/kg (i.p.) were used in the behavioral and EEG experiments. Locomotor effects in an open field test and deficits in prepulse inhibition of acoustic startle reaction (PPI ASR) were evaluated in the behavioral experiments. EEG signals were simultaneously recorded from 12 implanted active electrodes; subsequently, an EEG power spectral and coherence analysis was performed.

RESULTS

Ketamine had a rapid penetration into the brain; the peak concentrations of the drug were reached within 15 min after administration. Ketamine induced marked hyperlocomotion and deficits in the PPI ASR. EEG spectral analysis mainly showed increases in EEG power as well as coherence. These were most robust at 10-15 min after the administration and influenced all parts of the spectrum with ketamine 30 mg/kg.

CONCLUSIONS

Ketamine at behaviorally active doses induces a robust increase in EEG power spectra and coherence. The maximum levels of change correlated with the kinetics of ketamine.

Spontaneous gamma coherence as a possible trait marker of schizophrenia-An explorative study

OBJECTIVES

Integration of sensory information by cortical network binding appears to be crucially involved in sensory processing activity. Studies in schizophrenia using functional neuroimaging, event-related potentials and EEG coherence indicate an impairment of cortical network coupling in this disorder. Previous electrophysiological investigations in animals and humans suggested that gamma activity (oscillations at around 30-100Hz) is essential for cortical network binding. This is the first investigation of spontaneous gamma activity in schizophrenics and their first degree relatives as compared to normal controls.

METHOD

Resting EEG was recorded in 20 drug naïve/drug free male schizophrenic patients, their pair matched male first degree relatives and 20 age-, sex-, education- and handedness-matched normal controls. Power spectrum and coherence of gamma band activity was analysed using MATLAB 6.5.

RESULTS

Schizophrenic patients had significantly less interhemispheric spontaneous gamma coherence and increased gamma power compared to normal controls. But there was no significant difference in gamma coherence between patients and their first degree relatives. Spontaneous gamma coherence had significant correlation with few PANSS subscale scores.

CONCLUSIONS

There is cortical hyperactivation and a reduced spontaneous and induced gamma coherence abnormality in schizophrenia. The abnormal gamma coherence appears explaining the psychopathology and poor performance on cognitive task in schizophrenia. This study has also generated hypotheses that the gamma band abnormality may be a trait abnormality in schizophrenics as seen by the similarity between the patient and their clinically asymptomatic first degree relatives.

Reduced long distance gamma (28-48 Hz) coherence in euthymic patients with bipolar disorder

BACKGROUND

EEG coherence represents the brain's functional connectivity. Synchronous neural gamma oscillations are critical for cortico-cortical communication and large-scale integration of distributed sets of neurons. We investigated long distance gamma (28-48 Hz) coherence in bipolar disorder.

METHODS

Sensory evoked coherence (EC) and event related coherence (ERC) values for the gamma frequency band during simple light stimulation and visual odd-ball paradigm was assessed in 20 drug-free euthymic bipolar patients in comparison to healthy controls. Groups were compared for the coherence values of the left (F(3)-T(3), F(3)-TP(7), F(3)-P(3), F(3)-O(1)) and right (F(4)-T(4), F(4)-TP(8), F(4)-P(4), F(4)-O(2)) intra-hemispheric electrode pairs by means of a repeated measure analysis of variance (ANOVA) and t-tests.

RESULTS

Patients showed significantly lower gamma coherence values in response to target stimuli than the healthy controls between left and right fronto-temporal, as well as between frontal and temporo-parietal electrode pairs. Coherence values for the non-target stimuli were significantly lower in the patients than the healthy controls between frontal and temporo-parietal regions on both right and left sides. EP coherence values did not differ significantly between the groups.

LIMITATIONS

A relatively small sample size is the major limitation of the study.

CONCLUSIONS

Bipolar patients present disturbance in functional long-range connectivity between the frontal and temporal as well as temporo-parietal brain structures during a cognitive paradigm requiring attention and immediate recall. The location of the connectivity disturbance corresponds to the underlying neurobiology of executive function, memory and attention impairments in bipolar disorder and raises the question of whether gamma coherence reduction may be a candidate biomarker for bipolar disorder.

Frontal areas contribute to reduced global coordination of resting-state gamma activities in drug-naïve patients with schizophrenia

Schizophrenia has been postulated to involve impaired neuronal cooperation in large-scale neural networks, including cortico-cortical circuitry. Alterations in gamma band oscillations have attracted a great deal of interest as they appear to represent a pathophysiological process of cortical dysfunction in schizophrenia. Gamma band oscillations reflect local cortical activities, and the synchronization of these activities among spatially distributed cortical areas has been suggested to play a central role in the formation of networks. To assess global coordination across spatially distributed brain regions, Omega complexity (OC) in multichannel EEG was proposed. Using OC, we investigated global coordination of resting-state EEG activities in both gamma (30-50 Hz) and below-gamma (1.5-30 Hz) bands in drug-naïve patients with schizophrenia and investigated the effects of neuroleptic treatment. We found that gamma band OC was significantly higher in drug-naïve patients with schizophrenia compared to control subjects and that a right frontal electrode (F3) contributed significantly to the higher OC. After neuroleptic treatment, reductions in the contribution of frontal electrodes to global OC in both bands correlated with the improvement of schizophrenia symptomatology. The present study suggests that frontal brain processes in schizophrenia were less coordinated with activity in the remaining brain. In addition, beneficial effects of neuroleptic treatment were accompanied by improvement of brain coordination predominantly due to changes in frontal regions. Our study provides new evidence of improper intrinsic brain integration in schizophrenia by investigating the resting-state gamma band activity.

Gamma oscillatory power is impaired during cognitive control independent of medication status in first-episode schizophrenia

Schizophrenia is characterized by impaired cognitive control associated with prefrontal cortex dysfunction, but the underlying pathophysiological mechanisms remain unknown. Higher cognitive processes are associated with cortical oscillations in the gamma range, which are also impaired in chronic schizophrenia. We tested whether cognitive control-related gamma deficits are observed in first-episode patients, and whether they are associated with antipsychotic medication exposure. Fifty-three first-episode schizophrenia patients (21 without antipsychotic medication treatment) and 29 healthy control subjects underwent electroencephalography (EEG) during performance of a preparatory cognitive control task (preparing to overcome prepotency or POP task). The first-episode schizophrenia patient group was impaired (relative to the control group) on task performance and on delay-period gamma power at each of the three subgroups of frontal electrodes. The unmedicated patient subgroup was similarly impaired compared with controls, and was not different on these measures compared with the medicated patient subgroup. In contrast, delay-period theta power was not impaired in the full patient group nor in the unmedicated patient subgroup. Impaired cognitive control-related gamma cortical oscillatory activity is present at the first psychotic episode in schizophrenia, and is independent of medication status. This suggests that altered local circuit function supporting high-frequency oscillatory activity in prefrontal cortex ensembles may serve as the pathophysiological substrate of cognitive control deficits in schizophrenia.

Ketamine Modulates Theta and Gamma Oscillations

Ketamine, an N-methyl-d-aspartate (NMDA) receptor glutamatergic antagonist, has been studied as a model of schizophrenia when applied in subanesthetic doses. In EEG studies, ketamine affects sensory gating and alters the oscillatory characteristics of neuronal signals in a complex manner. We investigated the effects of ketamine on in vivo recordings from the CA3 region of mouse hippocampus referenced to the ipsilateral frontal sinus using a paired-click auditory gating paradigm. One issue of particular interest was elucidating the effect of ketamine on background network activity, poststimulus evoked and induced activity. We find that ketamine attenuates the theta frequency band in both background activity and in poststimulus evoked activity. Ketamine also disrupts a late, poststimulus theta power reduction seen in control recordings. In the gamma frequency range, ketamine enhances both background and evoked power, but decreases relative induced power. These findings support a role for NMDA receptors in mediating the balance between theta and gamma responses to sensory stimuli, with possible implications for dysfunction in schizophrenia.

Gamma oscillation deficits and the onset and early progression of schizophrenia

A fascinating convergence of evidence in recent years has implicated the disturbances of neural synchrony in the gamma frequency band (30-100 Hz) as a major pathophysiologic feature of schizophrenia. Evidence suggests that reduced glutamatergic neurotransmission via the N-methyl-D-aspartate (NMDA) receptors that are localized to inhibitory interneurons, perhaps especially the fast-spiking cells that contain the calcium-binding protein parvalbumin (PV), may contribute to gamma band synchrony deficits. These deficits may underlie the brain's failure to integrate information and hence the manifestations of many symptoms and deficits of schizophrenia. Furthermore, because gamma oscillations are thought to provide the temporal structure that is necessary for synaptic plasticity, gamma oscillation deficits may disturb the developmental synaptic reorganization process that is occurring during the period of late adolescence and early adulthood. This disturbance may contribute to the onset of schizophrenia and the functional deterioration that is characteristic of the early stage of the illness. Finally, reduced NMDA neurotransmission on inhibitory interneurons, including the PV-containing cells, may inflict excitotoxic or oxidative injury to downstream pyramidal neurons, leading to further loss of synapses and dendritic branchings. Hence, a key element in the conceptualization of rational early-intervention and prevention strategies for schizophrenia may involve correcting the abnormal NMDA neurotransmission on inhibitory interneurons-possibly that on the PV-containing neurons, in particular-thereby normalizing gamma oscillation deficits and attenuating downstream neuronal pathology.

On the recording reference contribution to EEG correlation, phase synchrony, and coherence

The degree of synchronization in electroencephalography (EEG) signals is commonly characterized by the time-series measures, namely, correlation, phase synchrony, and magnitude squared coherence (MSC). However, it is now well established that the interpretation of the results from these measures are confounded by the recording reference signal and that this problem is not mitigated by the use of other EEG montages, such as bipolar and average reference. In this paper, we analyze the impact of reference signal amplitude and power on EEG signal correlation, phase synchrony, and MSC. We show that, first, when two nonreferential signals have negative correlation, the phase synchrony and the absolute value of the correlation of the two referential signals may have two regions of behavior characterized by a monotonic decrease to zero and then a monotonic increase to one as the amplitude of the reference signal varies in [0, +∞). It is notable that even a small change of the amplitude may lead to significant impact on these two measures. Second, when two nonreferential signals have positive correlation, the correlation and phase-synchrony values of the two referential signals can monotonically increase to one (or monotonically decrease to some positive value and then monotonically increase to one) as the amplitude of the reference signal varies in [0, +∞). Third, when two nonreferential signals have negative cross-power, the MSC of the two referential signals can monotonically decrease to zero and then monotonically increase to one as reference signal power varies in [0, +∞). Fourth, when two nonreferential signals have positive cross-power, the MSC of the two referential signals can monotonically increase to one as the reference signal power varies in [0, +∞). In general, the reference signal with small amplitude or power relative to the signals of interest may decrease or increase the values of correlation, phase synchrony, and MSC. However, the reference signal with high relative amplitude or power will always increase each of the three measures. In our previous paper, we developed a method to identify and extract the reference signal contribution to intracranial EEG (iEEG) recordings. In this paper, we apply this approach to referential iEEG recorded from human subjects and directly investigate the contribution of recording reference on correlation, phase synchrony, and MSC. The experimental results demonstrate the significant impact that the recording reference may have on these bivariate measures.

Methodology for combined TMS and EEG

The combination of transcranial magnetic stimulation (TMS) with simultaneous electroencephalography (EEG) provides us the possibility to non-invasively probe the brain's excitability, time-resolved connectivity and instantaneous state. Early attempts to combine TMS and EEG suffered from the huge electromagnetic artifacts seen in EEG as a result of the electric field induced by the stimulus pulses. To deal with this problem, TMS-compatible EEG systems have been developed. However, even with amplifiers that are either immune to or recover quickly from the pulse, great challenges remain. Artifacts may arise from the movement of electrodes, from muscles activated by the pulse, from eye movements, from electrode polarization, or from brain responses evoked by the coil click. With careful precautions, many of these problems can be avoided. The remaining artifacts can be usually reduced by filtering, but control experiments are often needed to make sure that the measured signals actually originate in the brain. Several studies have shown the power of TMS-EEG by giving us valuable information about the excitability or connectivity of the brain.

NR1 knockdown mice as a representative model of the glutamate hypothesis of schizophrenia

N-methyl d-aspartate (NMDA) receptor subunit NR1 knockdown (NR1-KD) mice have a global reduction of NMDA receptors, enabling their use as a genetic model to study the role of NMDA receptors in the pathophysiology of schizophrenia. This targeted mutation results in a spectrum of altered behaviors that are similar to those induced by NMDA receptor antagonists, which have long been used to model schizophrenia in animals. NR1-KD mice serve as a complementary tool to pharmacological models, providing insight into the consequences of sustained NMDA receptor dysfunction in early brain development and throughout the life of the animal. Though in many respects the phenotype of NR1-KD mice mimics that of acute NMDA receptor antagonism, there are also notable differences. In this chapter we highlight some of the molecular, behavioral, and neurophysiological phenotypes of NR1-KD mice and compare these to pharmacological models of NMDA receptor dysfunction. Through the study of these models, our improved understanding of how the brain adapts to persistent NMDA receptor hypofunction may eventually suggest new therapeutic strategies for schizophrenia.

Hyperdopaminergia and NMDA receptor hypofunction disrupt neural phase signaling

Neural phase signaling has gained attention as a putative coding mechanism through which the brain binds the activity of neurons across distributed brain areas to generate thoughts, percepts, and behaviors. Neural phase signaling has been shown to play a role in various cognitive processes, and it has been suggested that altered phase signaling may play a role in mediating the cognitive deficits observed across neuropsychiatric illness. Here, we investigated neural phase signaling in two mouse models of cognitive dysfunction: mice with genetically induced hyperdopaminergia [dopamine transporter knock-out (DAT-KO) mice] and mice with genetically induced NMDA receptor hypofunction [NMDA receptor subunit-1 knockdown (NR1-KD) mice]. Cognitive function in these mice was assessed using a radial-arm maze task, and local field potentials were recorded from dorsal hippocampus and prefrontal cortex as DAT-KO mice, NR1-KD mice, and their littermate controls engaged in behavioral exploration. Our results demonstrate that both DAT-KO and NR1-KD mice display deficits in spatial cognitive performance. Moreover, we show that persistent hyperdopaminergia alters interstructural phase signaling, whereas NMDA receptor hypofunction alters interstructural and intrastructural phase signaling. These results demonstrate that dopamine and NMDA receptor dependent glutamate signaling play a critical role in coordinating neural phase signaling, and encourage further studies to investigate the role that deficits in phase signaling play in mediating cognitive dysfunction.

A review of brain oscillations in cognitive disorders and the role of neurotransmitters

The analysis of the functional correlates of "brain oscillations" has become an important branch of neuroscience. Although research on the functional correlates of brain oscillation has progressed to a high level, studies on cognitive disorders are rare and mainly limited to schizophrenia patients. The present review includes the results of the changes in brain oscillations in patients with Alzheimer's, schizophrenia, bipolar disorders, mild cognitive impairment, attention-deficit hyperactivity disorder (ADHD), alcoholism and those with genetic disorders. Furthermore, the effects of pharmaca and the influence of neurotransmitters in patients with cognitive disorders are also reviewed. Following the review, a short synopsis is given related to the analysis of brain oscillations.