Gamma frequency-range abnormalities to auditory stimulation in schizophrenia

Induced gamma activity and event-related coherence in schizophrenia

Evidence has been provided that high frequency oscillations within the gamma band reflect mechanisms of cortical integration. In the light of recently proposed pathophysiological models of schizophrenia, suggesting a disturbance of the functional connectivity within distributed neural networks, it has been hypothesized that abnormalities in the gamma band underlie perceptual and cognitive dysfunctions in patients with schizophrenia. In the present study we investigated evoked and induced 40-Hz gamma power as well as frontoparietal and frontotemporal event-related coherence in patients with deficit and nondeficit schizophrenia and in matched healthy controls. In patients, correlations between gamma oscillations and psychopathological dimensions were also investigated. A reduction of both induced gamma power and event-related coherence was observed in patients with nondeficit schizophrenia, but not in those with deficit schizophrenia. Our findings support the hypothesis that deficit and nondeficit schizophrenia represent separate disease entities, suggesting the presence of a poor integration of the neuronal activity within distributed neural network only in the subgroup of schizophrenic patients without primary and persistent negative symptoms. Associations between an excess of gamma oscillations and psychopathological dimensions were observed, suggesting that abnormal thoughts, behaviors and perceptions might be related to the formation of inappropriate neural connections.

Adjudicating neurocognitive endophenotypes for schizophrenia

Although genetic influences on schizophrenia are well established, localization of the genes responsible for this illness has proven extremely difficult. Given evidence that genes predisposing to schizophrenia may be transmitted without expression of the clinical phenotype, efforts have focused on developing endophenotypes. While several neuropsychological measures have been proposed to be endophenotypes, few studies have systematically assessed batteries of neurocognitive tests to determine which tests are most sensitive to liability for the illness. Two hundred sixty-nine Latino individuals were administered a standard neuropsychological battery. Two hundred fourteen of these were members of families with at least two siblings diagnosed with schizophrenia or schizoaffective disorder. The remaining were community controls without history of major psychiatric illness. Neurocognitive measures found to be heritable were entered into analyses designed to determine which tests covary with the degree of genetic relationship to affected individuals. Although five measures were found to uniquely model genetic liability for schizophrenia, digit symbol coding was the most sensitive. To assess the specificity of these endophenotypes, performance on these measures were compared to family members with bipolar and unipolar affective disorders. These markers clearly distinguished between individuals with psychotic illnesses and those with major depression. As measures contributed uniquely to discriminate individuals at varying risk for schizophrenia, our findings imply multiple independently inherited elements to the liability for the illness. We present a practical model for adjudicating endophenotypes and determining which measures are best suited for use in linkage analyses.

Increased inhibitory input to CA1 pyramidal cells alters hippocampal gamma frequency oscillations in the MK-801 model of acute psychosis

The phencyclidine compound MK-801 can induce psychosis with symptoms which closely resemble those observed in an acute schizophrenic episode. Here we used an in vitro model of psychosis after systemic administration of MK-801. We found that kainate-induced gamma frequency field oscillations in animals previously exposed to MK-801 have significantly higher power than in control animals. The intrinsic membrane properties of pyramidal cells, such as membrane input resistance and time constant, were not found to be different. In contrast, the MK-801 cells exhibited significantly more depolarized resting membrane potentials than control cells. We propose cellular alterations in Na+-K+-pump activity and increases in phasic inhibition in MK-801 cells to be the respective underlying mechanisms responsible for the more depolarized resting membrane potentials and the increased power of gamma frequency oscillations observed in MK-801 pretreated animals.

An EEG-based real-time cortical rhythmic activity monitoring system: a pilot study

This paper introduces an electroencephalography (EEG)-based, real-time, cortical rhythmic activity monitoring system which can monitor spatiotemporal changes of cortical rhythmic activity on a subject's cortical surface, with a high temporal resolution. In the monitoring system, a frequency domain inverse operator is preliminarily constructed, based on the subject's anatomical information and sensor arrangement, and then spectral current power at each cortical vertex is calculated for the Fourier transforms of successive sections of continuous data, when a particular frequency band is given. The first pilot system was applied to two human experiments: (1)cortical alpha rhythm changes induced by opening and closing eyes and (2) cortical mu rhythm changes originated from arm movements, demonstrating the feasibility of the system.

Impairments in frontal cortical gamma synchrony and cognitive control in schizophrenia

A critical component of cognitive impairments in schizophrenia can be characterized as a disturbance in cognitive control, or the ability to guide and adjust cognitive processes and behavior flexibly in accordance with one's intentions and goals. Cognitive control impairments in schizophrenia are consistently linked to specific disturbances in prefrontal cortical functioning, but the underlying neurophysiologic mechanisms are not yet well characterized. Synchronous gamma-band oscillations have been associated with a wide range of perceptual and cognitive processes, raising the possibility that they may also help entrain prefrontal cortical circuits in the service of cognitive control processes. In the present study, we measured induced gamma-band activity during a task that reliably engages cognitive control processes in association with prefrontal cortical activations in imaging studies. We found that higher cognitive control demands were associated with increases in induced gamma-band activity in the prefrontal areas of healthy subjects but that control-related modulation of prefrontal gamma-band activity was absent in schizophrenia subjects. Disturbances in gamma-band activity in patients correlated with illness symptoms, and gamma-band activity correlated positively with performance in control subjects but not in schizophrenia patients. Our findings may provide a link between previously reported postmortem abnormalities in thalamofrontocortical circuitry and alterations in prefrontal activity observed in functional neuroimaging studies. They also suggest that deficits in frontal cortical gamma-band synchrony may contribute to the cognitive control impairments in schizophrenia.

Gamma band oscillations reveal neural network cortical coherence dysfunction in schizophrenia patients

BACKGROUND

Gamma band activity has been associated with many sensory and cognitive functions, and is important for cortico-cortical transmission and the integration of information across neural networks. The aims of the present study were to determine if schizophrenia patients have deficits in the generation and maintenance of coherent, synchronized oscillations in response to steady-state stimulation, and to examine the clinical and cognitive correlates of the electroencephalography (EEG) oscillatory dynamics.

METHODS

Schizophrenia patients (n = 100) and nonpsychiatric subjects (n = 80) underwent auditory steady-state event-related potential testing. Click trains varying in rate of stimulation (20, 30, and 40 Hz) were presented; EEG-evoked power and intertrial phase synchronization were obtained in response to each stimulation frequency. Subjects also underwent clinical and neurocognitive assessments.

RESULTS

Patients had reductions in both evoked power and phase synchronization in response to 30- and 40-Hz stimulation but normal responsivity to 20-Hz stimulation. Maximal deficits were detected in response to 40-Hz stimulation. A modest association of reduced working memory performance and 40-Hz intertrial phase synchronization was present in schizophrenia patients (r = .32, p <.01).

CONCLUSIONS

Schizophrenia patients have frequency-specific deficits in the generation and maintenance of coherent gamma-range oscillations, reflecting a fundamental degradation of basic integrated neural network activity.

Neural Synchrony in Brain Disorders: Relevance for Cognitive Dysfunctions and Pathophysiology

Following the discovery of context-dependent synchronization of oscillatory neuronal responses in the visual system, novel methods of time series analysis have been developed for the examination of task- and performance-related oscillatory activity and its synchronization. Studies employing these advanced techniques revealed that synchronization of oscillatory responses in the beta- and gamma-band is involved in a variety of cognitive functions, such as perceptual grouping, attention-dependent stimulus selection, routing of signals across distributed cortical networks, sensory-motor integration, working memory, and perceptual awareness. Here, we review evidence that certain brain disorders, such as schizophrenia, epilepsy, autism, Alzheimer's disease, and Parkinson's are associated with abnormal neural synchronization. The data suggest close correlations between abnormalities in neuronal synchronization and cognitive dysfunctions, emphasizing the importance of temporal coordination. Thus, focused search for abnormalities in temporal patterning may be of considerable clinical relevance.

Switch and maintenance of task set in schizophrenia

Task set maintenance and switching deficits are robust in schizophrenia. However, little is known about how these constructs are related to one another. The development of an improved understanding of set switching and maintenance deficits in schizophrenia requires that these constructs be explicated in terms of elementary cognitive processes rather than grouped into broad psychological concepts like executive functioning. A relevant dichotomy has been proposed in which sensory and perceptual ("attentional") processes are distinguished from decisional ("intentional") processes in task maintenance and switching; however, the contributions these processes make to performance deficits in schizophrenia is not known. In the present study, 30 participants with schizophrenia and 27 healthy comparisons completed a cued attentional set switching task. In addition to analyses of mean response times, the contributions of attentional and intentional processes to task performance were estimated using an ex-Gaussian distributional analysis. Schizophrenia was associated with a set maintenance deficit that was accounted for by an attentional, rather than intentional, dysfunction. Both groups showed significant switch costs that could be attributed to attentional processes, but there was no evidence for an attentional set switching deficit in schizophrenia. The findings suggest that set switching and set maintenance may reflect distinct cognitive deficits in schizophrenia and that they may be associated with unique information processing mechanisms.

Dysregulation of thalamic sensory "transmission" in schizophrenia: neurochemical vulnerability to hallucinations

Cholinergic arousal mechanisms predispose thalamic and cortical neurons to fire action potentials at gamma rhythms, which have a tendency to resonate in thalamocortical networks, thereby forming coherent assemblies under constraints of sensory input to specific thalamic nuclei, on the one hand, and prefrontal and limbic attentional mechanisms, on the other. Perception may be based on sustained assemblies of coherent gamma oscillations in thalamocortical circuits. In schizophrenia, the impact of sensory input on self-organization of thalamocortical activity may be generally reduced. As a result, processes underlying perception can become uncoupled from sensory input, particularly at times of hyperarousal, leading to domination of attentional mechanisms and the emergence of hallucinations. Evidence is reviewed that implicates excessive neuronal noise in specific thalamic nuclei in the generation of hallucinations in schizophrenia. Nicotinic receptor abnormalities, dopaminergic hyperactivity and glutamate-receptor hypofunction are reconciled within a model of psychotic symptom generation that places crucial emphasis on dysfunction of the reticular thalamic nucleus.

Cognitive disorganization in hippocampus: a physiological model of the disorganization in psychosis

Cognitive coordination refers to processes that organize the timing of activity among neurons without altering individual discharge properties. Coordinating processes allow neural networks to coactivate related representations and prevent the coactivation of unrelated representations. Impaired cognitive coordination, also called cognitive disorganization, is hypothesized to be the core deficit in the disorganized syndrome of schizophrenia (Phillips and Silverstein, 2003), a condition characterized by hallucinations, disorganization, and thought disorder. This disorganization hypothesis is based on the observation that schizophrenic subjects are impaired at segregating relevant and irrelevant stimuli and selectively using associations between relevant cues. We report that injecting the neural activity blocker tetrodotoxin (TTX) into one hippocampus persistently coactivated pyramidal cells in the uninjected hippocampus that initially discharged independently. In accord with the definition of cognitive disorganization, pyramidal cell firing rates only changed for 15 min and did not accompany the coactivation. The TTX-induced coactivity was maximal at gamma periods, consistent with altered gamma oscillations and disorganization in schizophrenia. A network model confirmed that increasing the coupling of weakly associated cells impairs the selective activation and inhibition of stored spatial representations. This TTX-induced cognitive disorganization correctly predicted that the same TTX injection selectively impaired the ability of rats to segregate relevant associations among distal spatial stimuli from irrelevant local stimuli (Wesierska et al., 2005). The TTX-induced coactivity of hippocampal pyramidal cell discharge has construct and predictive validity as a physiological model of psychosis-related disorganization.

Stimulus-dependent gamma (30-50 Hz) oscillations in simple and complex fast rhythmic bursting cells in primary visual cortex

Oscillatory activity is generated by many neural systems. gamma band (approximately 40 Hz) oscillations in the thalamus and cortex occur spontaneously and in response to sensory stimuli. Fast rhythmic bursting (FRB) cells (also called chattering cells) comprise a unique class of cortical neurons that, during depolarization by current injection, intrinsically generate bursts of high-frequency action potentials with an interburst frequency between 30 and 50 Hz. In the present study, we show for the first time that FRB cells in the primary visual cortex can be either simple or complex and are distributed throughout all cortical layers. Strikingly, both simple and complex FRB cells generate spike bursts at gamma frequencies in response to depolarizing current pulses, but only simple FRB cells exhibit a selective, stimulus feature-dependent increase in gamma oscillations in response to visual stimulation. In addition, we find that hyperpolarization does not reduce the relative power of visually evoked gamma oscillations in the V(m) response of FRB cells. Our results thus indicate that visually evoked gamma activity in individual simple and complex FRB cells is generated in large part by rhythmic synaptic input, rather than by depolarization-dependent activation of intrinsic properties. Finally, the presence of FRB cells in layer 6 suggests a role for corticothalamic feedback in potentiating thalamic oscillations and facilitating the generation of a corticothalamocortical oscillatory loop. We propose that rather than functioning as pacemakers, FRB cells amplify and distribute stimulus-driven gamma oscillations in the neocortex.

Regional specificity of chandelier neuron axon terminal alterations in schizophrenia

BACKGROUND

The axon terminals of GABAergic chandelier cells form linear arrays, termed cartridges, that synapse on the axon initial segment of neocortical pyramidal cells. These cartridges are immunoreactive for the GABA membrane transporter-1, and the density of GABA membrane transporter-1-immunoreactive cartridges in the prefrontal cortex has been reported to be reduced in schizophrenia. The goal of this study was to determine if reductions in the density of GABA membrane transporter-1-immunoreactive cartridges in schizophrenia are restricted to the prefrontal cortex.

METHODS

Relative GABA membrane transporter-1-immunoreactive cartridge density was determined in auditory association area 42, a region previously implicated in the pathophysiology of schizophrenia, in 14 matched pairs of subjects with schizophrenia and normal comparison subjects. The results were compared with similar data from prefrontal area 46 in the same subjects.

RESULTS

Mean GABA membrane transporter-1-immunoreactive cartridge density in area 42 was decreased by 9.8% in layers II-IIIa, and by 11.9% in layer VI in subjects with schizophrenia, although these differences did not achieve statistical significance. However, the magnitude of the reductions in the density of GABA membrane transporter-1-immunoreactive cartridges in area 42 of the subjects with schizophrenia was not significantly smaller than those in area 46.

CONCLUSIONS

In subjects with schizophrenia, alterations in chandelier neuron axon cartridges appear to be more marked in the prefrontal cortex than in another cortical region implicated in the illness, although such changes might not be restricted to the prefrontal cortex.

Event-related gamma activity in schizophrenia patients during a visual backward-masking task

OBJECTIVE

Schizophrenia patients experience deficits in many aspects of cognition and perception. Abnormalities in gamma activity may underlie some of these deficits, including rapid processing of visual stimuli. This study examined event-related gamma range activity during a visual backward-masking task in schizophrenia patients and normal comparison subjects.

METHOD

Event-related gamma activity was recorded in 15 normal comparison subjects and 32 schizophrenia patients. Participants had event-related gamma activity recorded while viewing 60 unmasked visual targets and 240 trials of visual backward masking. Effects of group, accuracy (correct versus incorrect), stimulus-onset asynchrony, and regional activity (left versus right hemisphere, anterior versus posterior regions) were assessed.

RESULTS

Schizophrenia patients had significantly reduced gamma activity in relation to comparison subjects during the backward-masking task. Normal comparison subjects showed significantly greater gamma activity in the right hemisphere, whereas schizophrenia patients did not show this pattern of lateralization. For the unmasked target, there was no group effect and no significant interactions in gamma-band responses.

CONCLUSIONS

These results extend previous findings of abnormal gamma range activity in schizophrenia patients. Patients showed overall less gamma activity and failed to show lateralization of activity to the right hemisphere during masking, but they showed comparable levels of gamma activity to unmasked stimuli. Schizophrenia patients' poorer performance during a masking task may be partly influenced by this abnormal level and the distribution of gamma activity.

Contributions of subtype and spectral frequency analyses to the study of P50 ERP amplitude and suppression in schizophrenia

BACKGROUND

Poor suppression of P50 event-related potential (ERP) amplitudes to paired-click stimuli may indicate genetic liability for schizophrenia and weak "sensory gating." Evidence suggests, however, that P50 amplitude is selectively impaired in nonparanoid, but not paranoid, schizophrenia subtypes. Furthermore, paired-click suppression can appear deficient in schizophrenia due to smaller evoked responses to the first stimulus (S1), rather than larger, less effectively "gated" responses to the second (S2). Finally, the P50 ERP is comprised of activity from at least two frequency components that may be distinctly impaired: the gamma band, associated with sensory registration, and the low frequency response, associated with attention/encoding processes. P50 and related frequency subcomponents were examined as a function of illness subtype to further integrate these concepts.

METHOD

The standard paired-click paradigm was administered to 38 schizophrenia (27 paranoid, 11 nonparanoid) and 38 age-matched healthy control participants. P50 amplitudes and spectral power of gamma band (GBR; 20-50 Hz) and low frequency (LFR; 1-20 Hz) responses were analyzed.

RESULTS

P50 analyses revealed smaller S1 amplitude and normal S2 in schizophrenia participants collectively, but no differentiation of schizophrenia subtypes. Spectral analyses revealed smaller magnitude S1 and normal S2 responses in schizophrenia across both the GBR and LFR. The LFR, but not GBR, was found to distinguish nonparanoid from control groups, while paranoid participants evidenced no impairment in either frequency domain. LFR amplitude values correlated with clinical ratings of cognitive symptomatology.

CONCLUSIONS

ERP deficits in the dual-click paradigm were specific to S1 amplitudes and most prominent in the low frequency response. These results replicate previous findings and extend their relevance to schizophrenia subtype distinctions. Implications for the recurrent inhibition model of sensory gating are discussed.

Steady state visual evoked potential abnormalities in schizophrenia

OBJECTIVE

The steady state visual evoked potential (SSVEP) can be used to test the frequency response function of neural circuits. Previous studies have shown reduced SSVEPs to alpha and lower frequencies of stimulation in schizophrenia. We investigated SSVEPs in schizophrenia at frequencies spanning the theta (4Hz) to gamma (40Hz) range.

METHODS

The SSVEPs to seven different frequencies of stimulation (4, 8, 17, 20, 23, 30 and 40Hz) were obtained from 18 schizophrenia subjects and 33 healthy control subjects. Power at stimulating frequency (signal power) and power at frequencies above and below the stimulating frequency (noise power) were used to quantify the SSVEP responses.

RESULTS

Both groups showed an inverse relationship between power and frequency of stimulation. Schizophrenia subjects showed reduced signal power compared to healthy control subjects at higher frequencies (above 17Hz), but not at 4 and 8Hz at occipital region. Noise power was higher in schizophrenia subjects at frequencies between 4 and 20Hz over occipital region and at 4, 17 and 20Hz over frontal region.

CONCLUSIONS

SSVEP signal power at beta and gamma frequencies of stimulation were reduced in schizophrenia. Schizophrenia subjects showed higher levels of EEG noise during photic stimulation at beta and lower frequencies.

SIGNIFICANCE

Inability to generate or maintain oscillations in neural networks may contribute to deficits in visual processing in schizophrenia.

Sudarshan Kriya yogic breathing in the treatment of stress, anxiety, and depression: part I-neurophysiologic model

Mind-body interventions are beneficial in stress-related mental and physical disorders. Current research is finding associations between emotional disorders and vagal tone as indicated by heart rate variability. A neurophysiologic model of yogic breathing proposes to integrate research on yoga with polyvagal theory, vagal stimulation, hyperventilation, and clinical observations. Yogic breathing is a unique method for balancing the autonomic nervous system and influencing psychologic and stress-related disorders. Many studies demonstrate effects of yogic breathing on brain function and physiologic parameters, but the mechanisms have not been clarified. Sudarshan Kriya yoga (SKY), a sequence of specific breathing techniques (ujjayi, bhastrika, and Sudarshan Kriya) can alleviate anxiety, depression, everyday stress, post-traumatic stress, and stress-related medical illnesses. Mechanisms contributing to a state of calm alertness include increased parasympathetic drive, calming of stress response systems, neuroendocrine release of hormones, and thalamic generators. This model has heuristic value, research implications, and clinical applications.

Delta-9-tetrahydrocannabinol effects in schizophrenia: implications for cognition, psychosis, and addiction

BACKGROUND

Recent advances in the neurobiology of cannabinoids have renewed interest in the association between cannabis and psychotic disorders.

METHODS

In a 3-day, double-blind, randomized, placebo-controlled study, the behavioral, cognitive, motor, and endocrine effects of 0 mg, 2.5 mg, and 5 mg intravenous Delta-9-tetrahydrocannabinol (Delta-9-THC) were characterized in 13 stable, antipsychotic-treated schizophrenia patients. These data were compared with effects in healthy subjects reported elsewhere.

RESULTS

Delta-9-tetrahydrocannabinol transiently increased 1) learning and recall deficits; 2) positive, negative, and general schizophrenia symptoms; 3) perceptual alterations; 4) akathisia, rigidity, and dyskinesia; 5) deficits in vigilance; and 6) plasma prolactin and cortisol. Schizophrenia patients were more vulnerable to Delta-9-THC effects on recall relative to control subjects. There were no serious short- or long-term adverse events associated with study participation.

CONCLUSIONS

Delta-9-tetrahydrocannabinol is associated with transient exacerbation in core psychotic and cognitive deficits in schizophrenia. These data do not provide a reason to explain why schizophrenia patients use or misuse cannabis. Furthermore, Delta-9-THC might differentially affect schizophrenia patients relative to control subjects. Finally, the enhanced sensitivity to the cognitive effects of Delta-9-THC warrants further study into whether brain cannabinoid receptor dysfunction contributes to the pathophysiology of the cognitive deficits associated with schizophrenia.

Language disorder in schizophrenia as a developmental learning disorder

Receptive language disorder in schizophrenia is hypothesized to represent a learning disorder that involves a neurodevelopmental etiology. It is argued that a preexisting developmental language disorder may characterize a subset of schizophrenia patients. A primary deficit in the temporal dynamics of brain function is assumed to cause receptive language disorder in schizophrenia. This hypothesized core deficit includes both disturbance in the processing of rapid, sequential information and disruptions to patterns of brain activation and synchronization. These timing deficits may alter the way associative connections are formed and/or accessed in semantic memory. It is suggested that abnormalities in second-messenger pathways of subcortical-cortical circuitry offer an etiological nexus for language dysfunction in schizophrenia and developmental dyslexia.

Spectral spatiotemporal imaging of cortical oscillations and interactions in the human brain

This paper presents a computationally efficient source estimation algorithm that localizes cortical oscillations and their phase relationships. The proposed method employs wavelet-transformed magnetoencephalography (MEG) data and uses anatomical MRI to constrain the current locations to the cortical mantle. In addition, the locations of the sources can be further confined with the help of functional MRI (fMRI) data. As a result, we obtain spatiotemporal maps of spectral power and phase relationships. As an example, we show how the phase locking value (PLV), that is, the trial-by-trial phase relationship between the stimulus and response, can be imaged on the cortex. We apply the method to spontaneous, evoked, and driven cortical oscillations measured with MEG. We test the method of combining MEG, structural MRI, and fMRI using simulated cortical oscillations along Heschl's gyrus (HG). We also analyze sustained auditory gamma-band neuromagnetic fields from MEG and fMRI measurements. Our results show that combining the MEG recording with fMRI improves source localization for the non-noise-normalized wavelet power. In contrast, noise-normalized spectral power or PLV localization may not benefit from the fMRI constraint. We show that if the thresholds are not properly chosen, noise-normalized spectral power or PLV estimates may contain false (phantom) sources, independent of the inclusion of the fMRI prior information. The proposed algorithm can be used for evoked MEG/EEG and block-designed or event-related fMRI paradigms, or for spontaneous MEG data sets. Spectral spatiotemporal imaging of cortical oscillations and interactions in the human brain can provide further understanding of large-scale neural activity and communication between different brain regions.

High frequency EEG activity during sleep: characteristics in schizophrenia and depression

Previous studies indicate that high frequency power (>20Hz) in the electroencephalogram (EEG) are associated with feature binding and attention. It has been hypothesized that hallucinations and perceptual abnormalities might be linked to irregularities in fast frequency activity. This study examines the power and distribution of high frequency activity (HFA) during sleep in healthy control subjects and unmedicated patients with schizophrenia and depression. This is a post-hoc analysis of an archival database collected under identical conditions. Groups were compared using multivariate analyses of covariance (MANCOVA) using group frequency by stage analysis. A multiple regression analyzed the association between HFA power and clinical symptoms. Schizophrenic (SZ) and major depressive disorder (MDD) patients showed significantly greater high frequency (HF) power than healthy controls (HC) in all sleep stages (p<0.0001). SZs also exhibited significantly greater HF power than MDD patients in all sleep stages except wakefulness (W) (p<0.05). In all groups, gamma (35-45Hz) power was greater in W, decreased during slow wave sleep (SWS) and decreased further during rapid eye movement (REM). Beta 2 (20-35 Hz) power was greater in W and REM than in SWS. Only positive symptoms exhibited an association with HF power. Elevated HFA during sleep in unmedicated patients with SZ and MDD is associated with positive symptoms of illness. It is not clear how HFA would change in relation to clinical improvement, and further study is needed to clarify the association of HFA to the state/trait characteristics of SZ and MDD.

Neural synchrony indexes disordered perception and cognition in schizophrenia

Current views of schizophrenia suggest that it results from abnormalities in neural circuitry, but empirical evidence in the millisecond range of neural activity has been difficult to obtain. In pursuit of relevant evidence, we previously demonstrated that schizophrenia is associated with abnormal patterns of stimulus-evoked phaselocking of the electroencephalogram in the gamma band (30-100 Hz). These patterns may reflect impairments in neural assemblies, which have been proposed to use gamma-band oscillations as a mechanism for synchronization. Here, we report the unique finding that, in both healthy controls and schizophrenia patients, visual Gestalt stimuli elicit a gamma-band oscillation that is phase-locked to reaction time and hence may reflect processes leading to conscious perception of the stimuli. However, the frequency of this oscillation is lower in schizophrenics than in healthy individuals. This finding suggests that, although synchronization must occur for perception of the Gestalt, it occurs at a lower frequency because of a reduced capability of neural networks to support high-frequency synchronization in the brain of schizophrenics. Furthermore, the degree of phase locking of this oscillation is correlated with visual hallucinations, thought disorder, and disorganization in the schizophrenia patients. These data provide support for linking dysfunctional neural circuitry and the core symptoms of schizophrenia.

Reduced oscillatory gamma-band responses in unmedicated schizophrenic patients indicate impaired frontal network processing

OBJECTIVE

Integration of sensory information by cortical network binding appears to be crucially involved in target detection. Studies in schizophrenia using functional and diffusion tensor 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 40 Hz) is essential for cortical network binding. Studies in medicated schizophrenia provide evidence for a reduced gamma activity in the context of auditory stimulus processing. This is the first investigation of oscillatory activations in the gamma-band in an auditory oddball paradigm in unmedicated schizophrenic patients.

METHODS

EEG gamma-band responses (GBRs) of 15 drug-free schizophrenic patients and 15 age- and gender-matched healthy controls were compared. A wavelet transform based on Morlet wavelets was employed for the calculation of oscillatory GBRs.

RESULTS

In response to standard stimuli, early evoked GBRs (20-100 ms), which are supposed to reflect auditory cortex activation, did not show significant group differences. However, schizophrenic patients showed reduced evoked GBRs in a late latency range (220-350 ms), particularly after target stimuli. This deficit occurred over right frontal scalp regions. Furthermore, significant correlations were observed between oscillatory GBRs and clinical parameters in schizophrenic patients.

CONCLUSIONS

The results are consistent with a relative preserved stimulus processing in the auditory cortex as reflected by the early GBR. The reduced late GBR is compatible with an abnormal interaction within a frontal lobe network, as was postulated by previous neuroimaging studies.

SIGNIFICANCE

The present study provides evidence for disturbed processing within frontal cortical regions in unmedicated schizophrenic patients as indicated by reduced evoked EEG GBRs.

Neural synchronization deficits to auditory stimulation in bipolar disorder

Patients with bipolar disorder show cognitive deficits and disorganized behavior, which may reflect a disturbance in neural synchronization. We tested whether EEG measures of auditory neural synchronization were abnormal in bipolar disorder. Nineteen symptomatic patients with bipolar disorder and 32 non-psychiatric control subjects were evaluated. Click trains (500 ms duration) presented at 20, 30, 40 and 50 Hz were used to evoke EEG synchronization. Patients with bipolar disorder showed reduced power across the frequencies of stimulation. Phase-locking across trials was also disturbed in bipolar disorder, consistent with poor phase synchronization between the stimulus and EEG. Abnormal high frequency neural synchronization may contribute to cognitive deficits in bipolar disorder.

Aberrant brain dynamics in schizophrenia: delayed buildup and prolonged decay of the visual steady-state response

In schizophrenia, aberrant brain activity has been reported both during stimulus processing and at rest. Evoked response amplitude is a function of both the number and synchronization of neurons firing in relation to a stimulus. It is at present unclear whether schizophrenia patients have normal synchronization of neural activity in relation to stimulus processing, and whether the amount and time course of synchronization is related to their evoked response amplitudes. EEG brain dynamics in response to visual steady-state stimulation were assessed in 12 schizophrenia and 12 healthy subjects at three stimulation durations (2, 4, and 6 s). Group differences in the visual evoked potential, the visual steady-state response, and the local coherence of the visual steady-state response were evaluated over time. Schizophrenia patients had smaller and delayed event-related potentials. Moreover, they had a slower buildup of steady-state amplitude following stimulation onset and a prolonged decrease after stimulation offset. Groups did not differ during mid-segments of steady-state stimulation. Increase in coherence to stimulation onset did not differ between-groups, but coherence decay of the visual steady-state response following stimulus offset was delayed in schizophrenia patients. The initial response to visual stimulation among schizophrenia subjects, therefore, may be reduced in amplitude due to weak signal strength, not poor coordination between distant cortical regions. The prolonged recovery function of schizophrenia patients' visual system may indicate abnormal nonlinearity in neural response. These findings have implications understanding the nature of evoked response differences between schizophrenia and normal groups especially in repetitive stimulus paradigms.

Phase shift detection in thalamocortical oscillations using magnetoencephalography in humans

Magnetoencephalography was used to investigate exogenously stimulated oscillatory activity between cortex and thalamus resulting from clicks presented binaurally at the rate of 40 Hz. Analysis of the responses demonstrated activation of left and right auditory cortex, medial parietal cortex, thalamus, and cerebellum. Cross-correlations of the source waveforms revealed synchronicity between the auditory cortex sources (r > 0.9), auditory cortex and thalamic sources (r > 0.7), and thalamic and parietal sources (r > 0.7). The 40 Hz response in auditory cortex occurred 6 ms after thalamic activation. Supporting earlier findings, the results demonstrate the networks involved in the maintenance of 40 Hz auditory steady-state response and will prove useful for the interrogation of dysfunction in disorders demonstrating thalamocortical dysrhythmia, such as schizophrenia, Parkinson's disease, and depression.

Contribution of different EEG frequencies to auditory evoked potential abnormalities in schizophrenia

OBJECTIVE

We have shown previously [Clin Neurophysiol 2003;114:79] that phase reorganization of the ongoing electroencephalogram (EEG) plays an important role in the generation of auditory evoked potential (EP) components with a latency between 50 and 200 ms. In the present study, we investigate whether schizophrenia patients suffer from phase synchronization deficits as compared to normal subjects.

METHODS

The auditory EPs from 20 normal subjects and 19 schizophrenia patients were analyzed. EPs were obtained using a double stimulus paradigm, in which two identical tone bursts (S1 and S2) were delivered with an average inter-stimulus interval of 500 ms and an inter-pair interval of 8 s. The Piecewise Prony Method (PPM) was used to decompose single trial auditory evoked potentials into different frequency bands. Pre- and post-stimulus phase histograms were compared for each frequency band to determine the degree of phase synchronization produced by auditory stimulation in the two populations.

RESULTS

The S1 stimulus produced significantly less (P < 0.05) phase synchronization in schizophrenia patients than in normal subjects in the 2-12 Hz frequency range. Far fewer and smaller inter-population phase synchronization differences were seen for the S2 stimulus. Both populations showed more phase synchronization for S1 than S2. A significant correlation (P < 0.01) between N100 amplitude and phase synchronization 100 ms post S1 was observed for the normal population but not for the schizophrenia group. The correlation between P200 amplitude and phase synchronization 200 ms post S1 was significant for the normal group (P < 0.01) and the schizophrenia group (P < 0.03).

CONCLUSIONS

Schizophrenia patients have a phase synchronization deficiency, as compared to a normal control group, especially for the first stimulus, in the 2-12 Hz frequency range. This deficiency explains the lower EP amplitudes and may be a significant factor contributing to reduced sensory gating reported in schizophrenic subjects.

SIGNIFICANCE

The research presented here contributes to the understanding of the mechanism underlying sensory gating in health and gating deficiencies in schizophrenia.

Pupillary responses and attentional allocation problems on the backward masking task in schizophrenia

Early visual information processing impairment has consistently been found on the backward masking task in patients with schizophrenia, but the nature of this impairment remains unclear. Pupillometry was used to measure attentional allocation during visual backward masking task performance in patients with schizophrenia (n=16) and nonpsychiatric controls (n=16). The extent of pupil dilation recorded during a cognitive task reflects the processing load placed on the nervous system by the task. Schizophrenia patients detected significantly fewer targets than controls only when the stimulus onset asynchrony (SOA) between targets and masks reached 317 ms. For both groups, peak pupil dilation responses were also significantly larger in the 317 ms SOA condition relative to a no-mask condition, suggesting that the processing load of the 317 SOA masking condition was greater than the no-mask condition. In addition, a principal components analysis of pupillary response waveforms identified time-related factors that appeared to differentially index attentional allocation to targets vs. masks. Patients with schizophrenia showed less dilation than controls on a middle factor that appeared to index attentional allocation to targets, but patients showed greater dilation than controls on a late factor that appeared to index attentional allocation to masks. That is, controls attended more to targets than to masks, but patients attended more to masks than to targets. These findings suggest that masking impairments at SOA intervals greater than 100-200 ms may be due abnormalities in attentional allocation mechanisms.

The relationship between membrane pathology and language disorder in schizophrenia

Receptive language disorder in schizophrenia has been hypothesized to involve a fundamental deficit in the temporal (time-based) dynamics of brain function that includes disruptions to patterns of activation and synchronization. In this paper, candidate mechanisms and pathways that could account for this basic deficit are discussed. Parallels are identified between the patterns of language dysfunction observed for schizophrenia and dyslexia, two separate clinical disorders that may share a common abnormality in cell membrane phospholipids. A heuristic is proposed which details a trajectory involving an interaction of brain fatty acids and second-messenger function that modulates synaptic efficacy, and, in turn, influences language processing in schizophrenia patients. It is additionally hypothesized that a primary deficit of functional excitation originating in the cerebellum, in combination with a compensatory decrease of functional inhibition in the prefrontal cortex, influences receptive language dysfunction in schizophrenia.

Peeling the onion: NMDA dysfunction as a unifying model in schizophrenia

N-methyl-d-aspartate receptor (NMDAR) dysfunction plays a crucial role in schizophrenia, leading to impairments in cognitive coordination. NMDAR agonists (e.g., glycine) ameliorate negative and cognitive symptoms, consistent with NMDAR models. However, not all types of cognitive coordination use NMDAR. Further, not all aspects of cognitive coordination are impaired in schizophrenia, suggesting the need for specificity in applying the cognitive coordination construct.

High-frequency synchronisation in schizophrenia: Too much or too little?

Phillips & Silverstein's focus on schizophrenia as a failure of “cognitive coordination” is welcome. They note that a simple hypothesis of reduced Gamma synchronisation subserving impaired coordination does not fully account for recent observations. We suggest that schizophrenia reflects a dynamic compensation to a core deficit of coordination, expressed either as hyper- or hyposynchronisation, with neurotransmitter systems and arousal as modulatory mechanisms.

No blind schizophrenics: Are NMDA-receptor dynamics involved?

Numerous searches have failed to identify a single co-occurrence of total blindness and schizophrenia. Evidence that blindness causes loss of certain NMDA-receptor functions is balanced by reports of compensatory gains. Connections between visual and anterior cingulate NMDA-receptor systems may help to explain how blindness could protect against schizophrenia.

Setting domain boundaries for convergence of biological and psychological perspectives on cognitive coordination in schizophrenia

The claim that the disorganized subtype of schizophrenia results from glutamate hypofunction is enhanced by consideration of current subtypology of schizophrenia, symptom definition, interdependence of neurotransmitters, and the nature of the data needed to support the hypothesis. Careful specification clarifies the clinical reality of disorganization as a feature of schizophrenia and increases the utility of the subtype.

Schizophrenia: Putting context in context

Although context-processing deficits may be core features of schizophrenia, context remains a poorly defined concept. To test Phillips & Silverstein's model, we need to operationalize context more precisely. We offer several useful ways of framing context and discuss enhancing or facilitating schizophrenic patients' performance under different contextual situations. Furthermore, creativity may be a byproduct of cognitive uncoordination.

Phenomenology, context, and self-experience in schizophrenia

Impairments in cognitive coordination in schizophrenia are supported by phenomenological data that suggest deficits in the processing of visual context. Although the target article is sympathetic to such a phenomenological perspective, we argue that the relevance of phenomenological data for a wider understanding of consciousness in schizophrenia is not sufficiently addressed by the authors.

Guarding against over-inclusive notions of “context”: Psycholinguistic and electrophysiological studies of specific context functions in schizophrenia

Phillips & Silverstein offer an exciting synthesis of ongoing efforts to link the clinical and cognitive manifestations of schizophrenia with cellular accounts of its pathophysiology. We applaud their efforts but wonder whether the highly inclusive notion of “context” adequately captures some important details regarding schizophrenia and NMDA/glutamate function that are suggested by work on language processing and cognitive electrophysiology.

Mechanisms of disrupted language comprehension in schizophrenia

Mechanisms that contribute to perceptual processing dysfunction in schizophrenia were examined by Phillips & Silverstein, and formulated as involving disruptions in both local and higher-level coordination of signals. We agree that dysfunction in the coordination of cognitive functions (disconnection) is also indicated for many of the linguistic processing deficits documented for schizophrenia. We suggest, however, that it may be necessary to add a timing mechanism to the theoretical account.

Theory of mind in schizophrenia: Damaged module or deficit in cognitive coordination?

Schizophrenics exhibit a deficit in theory of mind (ToM), but an intact theory of biology (ToB). One explanation is that ToM relies on an independent module that is selectively damaged. Phillips & Silverstein's analyses suggest an alternative: ToM requires the type of coordination that is impaired in schizophrenia, whereas ToB is spared because this type of coordination is not involved.

Cognitive coordination and its neurobiological bases: A new continent to explore

The additional arguments and evidence supplied by the commentaries strengthen the hypothesis that underactivity of NMDA receptors produces impaired cognitive coordination in schizophrenia. This encourages the hope that though the distance from molecules to mind is great, it can nevertheless be traversed. We therefore predict that in this decade or the next molecular psychology will be seen to be as fundamental to our understanding of mind as molecular biology is to our understanding of life.

Context rules

It is proposed that cortical activity is normally coordinated across synaptically connected areas and that this coordination supports cognitive coherence relations. This view is consistent with the NMDA- hypoactivity hypothesis of the target article in regarding disorganization symptoms in schizophrenia as arising from disruption of normal interareal coordination. This disruption may produce abnormal contextual effects in the cortex that lead to anomalous cognitive coherence relations.

The ketamine model for schizophrenia

This commentary compares clinical aspects of ketamine with the amphetamine model of schizophrenia. Hallucinations and loss of insight, associated with amphetamine, seem more schizophrenia-like. Flat affect encountered with ketamine is closer to the clinical presentation in schizophrenia. We argue that flat affect is not a sign of schizophrenia, but rather, arisk factorfor chronic schizophrenia.

Cortical connectivity in high-frequency beta-rhythm in schizophrenics with positive and negative symptoms

In chronic schizophrenic patients with both positive and negative symptoms (see Table 1), interhemispheric connections at the high frequency beta2-rhythm are absent during cognitive tasks, in contrast to normal controls, who have many interhemispheric connections at this frequency in the same situation. Connectivity is a fundamental brain feature, evidently greatly promoted by the NMDA system. It is a more reliable measure of brain function than the spectral power of this rhythm.

Hallucinations: synchronisation of thalamocortical gamma oscillations underconstrained by sensory input

What we perceive is the product of an intrinsic process and not part of external physical reality. This notion is consistent with the philosophical position of transcendental idealism but also agrees with physiological findings on the thalamocortical system. gamma-Frequency rhythms of discharge activity from thalamic and cortical neurons are facilitated by cholinergic arousal and resonate in thalamocortical networks, thereby transiently forming assemblies of coherent gamma oscillations under constraints of sensory input and prefrontal attentional mechanisms. Perception and conscious experience may be based on such assemblies and sensory input to thalamic nuclei plays merely a constraining role in their formation. In schizophrenia, the ability of sensory input to modulate self-organisation of thalamocortical gamma activity may be generally reduced. If during arousal thalamocortical self-organisation is underconstrained by sensory input, then attentional mechanisms alone may determine the content of perception and hallucinations may arise.

NMDA receptor antagonist effects, cortical glutamatergic function, and schizophrenia: toward a paradigm shift in medication development

There is an urgent need to improve the pharmacotherapy of schizophrenia despite the introduction of important new medications. New treatment insights may come from appreciating the therapeutic implications of model psychoses. In particular, basic and clinical studies have employed the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, ketamine, as a probe of NMDA receptor contributions to cognition and behavior. These studies illustrate a translational neuroscience approach for probing mechanistic hypotheses related to the neurobiology and treatment of schizophrenia and other disorders. Two particular pathophysiologic themes associated with schizophrenia, the disturbance of cortical connectivity and the disinhibition of glutamatergic activity may be modeled by the administration of NMDA receptor antagonists. The purpose of this review is to consider the possibility that agents that attenuate these two components of NMDA receptor antagonist response may play complementary roles in the treatment of schizophrenia.

Abnormal neural synchrony in schizophrenia

Schizophrenia has been conceptualized as a failure of cognitive integration, and abnormalities in neural circuitry (particularly inhibitory interneurons) have been proposed as a basis for this disorder. We used measures of phase locking and phase coherence in the scalp-recorded electroencephalogram to examine the synchronization of neural circuits in schizophrenia. Compared with matched control subjects, schizophrenia patients demonstrated: (1) absence of the posterior component of the early visual gamma band response to Gestalt stimuli; (2) abnormalities in the topography, latency, and frequency of the anterior component of this response; (3) delayed onset of phase coherence changes; and (4) the pattern of anterior-posterior coherence increases in response to Gestalt stimuli found in controls was replaced by a pattern of interhemispheric coherence decreases in patients. These findings support the hypothesis that schizophrenia is associated with impaired neural circuitry demonstrated as a failure of gamma band synchronization, especially in the 40 Hz range.

Abnormal neural synchrony in schizophrenia

Schizophrenia has been conceptualized as a failure of cognitive integration, and abnormalities in neural circuitry (particularly inhibitory interneurons) have been proposed as a basis for this disorder. We used measures of phase locking and phase coherence in the scalp-recorded electroencephalogram to examine the synchronization of neural circuits in schizophrenia. Compared with matched control subjects, schizophrenia patients demonstrated: (1) absence of the posterior component of the early visual gamma band response to Gestalt stimuli; (2) abnormalities in the topography, latency, and frequency of the anterior component of this response; (3) delayed onset of phase coherence changes; and (4) the pattern of anterior-posterior coherence increases in response to Gestalt stimuli found in controls was replaced by a pattern of interhemispheric coherence decreases in patients. These findings support the hypothesis that schizophrenia is associated with impaired neural circuitry demonstrated as a failure of gamma band synchronization, especially in the 40 Hz range.

Visual masking as a probe for abnormal gamma range activity in schizophrenia

BACKGROUND

Visual masking procedures assess very early stages of visual perception. Patients with schizophrenia consistently show deficits on visual masking tasks, and these deficits likely reflect vulnerability to schizophrenia. We conducted two experiments to determine whether visual masking procedures can reveal underlying abnormalities in gamma range oscillations in schizophrenia.

METHODS

In the first experiment, we conducted nonlinear modeling of visual masking performance data from 89 male schizophrenic patients and 20 male comparison subjects. In the second experiment, electrophysiological recordings of event-related gamma activity were taken during a visual masking task in a subset of eight patients and seven control subjects.

RESULTS

In the first experiment, nonlinear modeling of the performance data revealed evidence of oscillations in the gamma range (30 and 35 Hz) for the comparison group but not patients. In the second experiment, the comparison group, but not the patients, showed a burst of gamma range activity 200-400 msec following target presentation. The difference between patients and comparison subjects in this time period was significant (p <.05).

CONCLUSIONS

Visual masking procedures can serve as a probe for underlying gamma range activity, which appears to be aberrant in schizophrenia. Perceptual problems in schizophrenia may, at least in part, be due to a failure to establish and/or maintain gamma range oscillations.

Theory, methods and new directions in the psychophysiology of the schizophrenic process and schizotypy

Theoretical and methodological issues in the psychophysiology of the schizophrenic process are reviewed. These include the importance of schizotypy with its compensatory abilities as well as deficits for elucidating the processes of development and prevention of schizophrenia. The importance of individual differences, syndromes and single case studies. The recognition that this is a dynamic and fluctuating illness and hence the relevance of functional neurophysiology, including the role of imbalances in hemispheric activation in ontogeny, developmental course, expression of symptoms, the effects of neuroleptics and recovery process, and the influence of stress a precipitant of breakdown. The role of thalamo-cortical activation systems. The particular value of electrocortical measures including the interrelations of electroencephalographic rhythms throughout the spectrum, and relations of gamma, dynamic core neuronal complexity, connectivity and sensory gating with experiences of unreality and disturbances of consciousness.

Schizophrenia, neurodevelopment and corpus callosum

The Zeitgeist favors an interpretation of schizophrenia as a condition of abnormal connectivity of cortical neurons, particularly in the prefrontal and temporal cortex. The available evidence points to reduced connectivity, a possible consequence of excessive synaptic pruning in development. A decreased thalamic input to the cerebral cortex appears likely, and developmental studies predict that this decrease should entail a secondary loss of both long- and short-range cortico-cortical connections, including connections between the hemispheres. Indeed, morphological, electrophysiological and neuropsychological studies over the last two decades suggest that the callosal connections are altered in schizophrenics. However, the alterations are subtle and sometimes inconsistent across studies, and need to be investigated further with new methodologies.

Where the rubber meets the road: The importance of implementation

Phillips & Silverstein argue that a range of cognitive disturbances in schizophrenia result from a deficit in cognitive coordination attributable to NMDA receptor dysfunction. We suggest that the viability of this hypothesis would be further supported by explicit implementation in a computational framework that can produce quantitative estimates of the behavior of both healthy individuals and individuals with schizophrenia.