Neuronal fast oscillations as a target site for psychoactive drugs

Pharmacological MRI approaches to understanding mechanisms of drug action

Functional neuroimaging is a novel technique for the study of drug action in the brain. The emerging role of this method is intimately tied to the unique challenges to advancing drug development for neuropsychiatric disorders. This chapter first presents a brief overview of the important treatment needs that remain to be met, which serve as clinical targets for drug development. Important factors that hinder progress in drug development, which arise from clinical, scientific and economic issues, are acknowledged. This sets the stage for the unique advantages of functional neuroimaging modalities such as functional MRI (fMRI) as a biomarker and drug development tool, in both clinical and preclinical phases. The physiological basis of the fMRI signal is briefly outlined, and aspects of neural signaling related to this signal change, with emphasis on implications for pharmacology studies. The utility of fMRI for evaluating the full anatomic extent of central neurotransmitter systems in a dynamic manner is then described. This is a critical advantage, and particularly important for studies of how systems such as the monoamines modulate distributed neural networks during cognitive processes in both health and illness, and how these actions are modified with pharmacological intervention. Central catecholamine systems are seen as paradigmatic targets amenable to pharmacologic fMRI. fMRI is observed to occupy a unique position in the armamentarium of methods available to the pharmacologist and the drug development process, and poised to play an expanding role in basic and clinical neuroscience.

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.

Attractor dynamics and thermodynamic analogies in the cerebral cortex: synchronous oscillation, the background EEG, and the regulation of attention

Ongoing changes in attention and cognition depend upon cortical/subcortical interactions, which select sequences of different spatial patterns of activation in the cortex. It is proposed that each pattern of cortical activation permits evolution of electrocortical wave activity toward statistically stationary states, analogous to thermodynamic equilibrium. In each steady-state, neurons fire with an intrinsic Poisson spike probability and also with a bursting pattern related to network oscillations. Excitatory cell dendrites act as a regenerative reservoir in which pulse generation is balanced against dissipations. Equilibria exhibit contrasting limits. One limit, at high cortical activation, generates widespread zero-lag synchrony among excitatory cells, with partial suppression of noise. Excitatory and inhibitory cells approach zero-lag local correlation, with 1/4 cycle lag-correlation at greater distances of separation. The high-activation limit defines a correlated system of attractor basins, capable of co-ordinating synaptic modifications and intracortical signal generation. Suppression of noise would enhance convergence about attractor basins in the manner of simulated annealing, while, conversely, the persistence of some noise prevents network paralysis by phase locking. At the opposite limit-that of low activation-spikes and waves have low cross- and auto-correlation, but have wide-spectrum sensitivity to inputs. It is hypothesised that cortical regions, transiently at equilibrium near these extremes, engage in interaction with each other and with subcortical systems, to generate ongoing sequences of attention and cognition. This account is compatible with classical and recently observed experimental phenomena. The principle features inferred from a simplified linear mathematical account are reproduced in a more physiologically realistic and non-linear numerical simulation.

Beta-like hippocampal network activity is differentially affected by amyloid beta peptides

Alzheimer disease (AD) patients show alterations in both neuronal network oscillations and the cognitive processes associated to them. Related to this clinical observation, it has been found that amyloid beta protein (Abeta) differentially affects some hippocampal network activities, reducing theta and gamma oscillations, without affecting sharp waves and ripples. Beta-like oscillations is another cognitive-related network activity that can be evoked in hippocampal slices by several experimental manipulations, including bath application of kainate and increasing extracellular potassium. Here, we tested whether or not different Abeta peptides differentially affect beta-like oscillatory patterns. We specifically tested the effects of fresh dissolved Abeta(25-35) and oligomerized Abeta(1-42) and found that kainate-induced oscillatory network activity was affected, in a slightly concentration dependent-manner, by both fresh dissolved (mostly monomeric) Abeta(25-35) and oligomeric Abeta(1-42). In contrast, potassium-induced oscillatory activity, which is reduced by oligomeric Abeta(1-42), is not affected by monomeric Abeta(25-35) at any of the concentrations tested. Our results support the idea that different amyloid peptides might alter specific cellular mechanisms related to the generation of specific neuronal network activities, instead of a generalized inhibitory effect of Abeta peptides on neuronal network function.

Disturbance in long distance gamma coherence in bipolar disorder

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

Steady state responses: electrophysiological assessment of sensory function in schizophrenia

Persons with schizophrenia experience subjective sensory anomalies and objective deficits on assessment of sensory function. Such deficits could be produced by abnormal signaling in the sensory pathways and sensory cortex or later stage disturbances in cognitive processing of such inputs. Steady state responses (SSRs) provide a noninvasive method to test the integrity of sensory pathways and oscillatory responses in schizophrenia with minimal task demands. SSRs are electrophysiological responses entrained to the frequency and phase of a periodic stimulus. Patients with schizophrenia exhibit pronounced auditory SSR deficits within the gamma frequency range (35-50 Hz) in response to click trains and amplitude-modulated tones. Visual SSR deficits are also observed, most prominently in the alpha and beta frequency ranges (7-30 Hz) in response to high-contrast, high-luminance stimuli. Visual SSR studies that have used the psychophysical properties of a stimulus to target specific visual pathways predominantly report magnocellular-based deficits in those with schizophrenia. Disruption of both auditory and visual SSRs in schizophrenia are consistent with neuropathological and magnetic resonance imaging evidence of anatomic abnormalities affecting the auditory and visual cortices. Computational models suggest that auditory SSR abnormalities at gamma frequencies could be secondary to gamma-aminobutyric acid-mediated or N-methyl-D-aspartic acid dysregulation. The pathophysiological process in schizophrenia encompasses sensory processing that probably contributes to alterations in subsequent encoding and cognitive processing. The developmental evolution of these abnormalities remains to be characterized.

Cell type-specific development of NMDA receptors in the interneurons of rat prefrontal cortex

In the prefrontal cortex, N-methyl-D-aspartic acid (NMDA) receptors (NMDARs) are critical not only for normal prefrontal functions but also for the pathological processes of schizophrenia. Little is known, however, about the developmental properties of NMDARs in the functionally diverse sub-populations of interneurons. We investigated the developmental changes of NMDARs in rat prefrontal interneurons using patch clamp recording in cortical slices. We found that fast-spiking (FS) interneurons exhibited properties of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA currents distinct from those in regular spiking (RS) and low-threshold spiking (LTS) interneurons, particularly during the adolescent period. In juvenile animals, most (73%) of the FS cells demonstrated both AMPA and NMDA currents. The NMDA currents, however, gradually became undetectable during cortical development, with most (74%) of the FS cells exhibiting no NMDA current in adults. In contrast, AMPA and NMDA currents in RS and LTS interneurons were relatively stable, without significant changes from juveniles to adults. Moreover, even in FS cells with NMDA currents, the NMDA/AMPA ratio dramatically decreased during the adolescent period but returned to juvenile level in adults, compared with the relatively stable ratios in RS and LTS interneurons. These data suggest that FS interneurons in the prefrontal cortex undergo dramatic changes in glutamatergic receptors during the adolescent period. These properties may make FS cells particularly sensitive and vulnerable to epigenetic stimulation, thus contributing to the onset of many psychiatric disorders, including schizophrenia.

GluK2-mediated excitability within the superficial layers of the entorhinal cortex

Recent analysis of genetically modified mice deficient in different kainate receptor (KAR) subunits have strongly pointed to a role of the GluK2 subunit, mediating the vulnerability of the brain towards seizures. Research concerning this issue has focused mainly on the hippocampus. However, several studies point to a potential role of other parts of the hippocampal formation, in particular the entorhinal cortex, in the development of epileptic seizures. There is extensive cell death after such seizures in layer III of the medial entorhinal cortex (LIII mEC), making this region of special interest for investigation into related pathological conditions. We therefore characterized KAR mediated currents in LIII mEC pyramidal neurons by several different approaches. Using patch-clamp technique, in combination with glutamate uncaging in horizontal brain slices, we show that LIII mEC neurons exhibit KAR currents. Use of genetically modified mice reveal that these currents are mediated by GluK2 containing KARs. The IV curve indicates the predominant presence of a Ca²⁺ impermeable and edited form of the KAR. Finally, we show that GluK2 containing kainate receptors are essential for kainate-induced gamma oscillations within the entorhinal cortex.

Synchronized gamma oscillations (30-50 Hz) in the amygdalo-hippocampal network in relation with seizure propagation and severity

In this study, we demonstrate that gamma oscillations (30-50 Hz) recorded in the local field potentials (LFP) of the hippocampus are a marker of temporal lobe seizure propagation and that the level of LFP synchrony in the amygdalo-hippocampal network, during these oscillations, is related to the severity of seizures. Sprague-Dawley rats were given a single systemic dose of kainic acid (KA; 6 mg/kg, i.p.) and local field potential activity (1-475 Hz) of the dorsal hippocampus, the amygdala and the neocortex was recorded. Of 135 ictal discharges, 55 (40.7%) involved both limbic structures. We demonstrated that 78.2% of seizures involving both the hippocampus and amygdala showed hippocampal gamma oscillations. Seizure duration was also significantly correlated with the frequency of hippocampal gamma oscillations (r2=0.31, p<0.01) and LFP synchrony in the amygdalo-hippocampal network (r2=0.21, p<0.05). These results suggest that gamma oscillations in the amygdalo-hippocampal network could facilitate long-range synchrony and participate in the propagation of seizures.

Driving fast-spiking cells induces gamma rhythm and controls sensory responses

Cortical gamma oscillations (20-80 Hz) predict increases in focused attention, and failure in gamma regulation is a hallmark of neurological and psychiatric disease. Current theory predicts that gamma oscillations are generated by synchronous activity of fast-spiking inhibitory interneurons, with the resulting rhythmic inhibition producing neural ensemble synchrony by generating a narrow window for effective excitation. We causally tested these hypotheses in barrel cortex in vivo by targeting optogenetic manipulation selectively to fast-spiking interneurons. Here we show that light-driven activation of fast-spiking interneurons at varied frequencies (8-200 Hz) selectively amplifies gamma oscillations. In contrast, pyramidal neuron activation amplifies only lower frequency oscillations, a cell-type-specific double dissociation. We found that the timing of a sensory input relative to a gamma cycle determined the amplitude and precision of evoked responses. Our data directly support the fast-spiking-gamma hypothesis and provide the first causal evidence that distinct network activity states can be induced in vivo by cell-type-specific activation.

Steady state and induced auditory gamma deficits in schizophrenia

Steady state auditory evoked potentials (SSAEPs) in the electroencephalogram (EEG) and magnetoencephalogram (MEG) have been reported to be reduced in schizophrenia, most consistently to frequencies in the gamma range (40 Hz and greater). The current study evaluated the specificity of this deficit over a broad range of stimulus frequencies and harmonics, the relationship between phase locking and signal power, and whether induced 40 Hz activity was also affected. SSAEPs to amplitude modulated tones from 5 to 50 Hz were obtained from subjects with schizophrenia (SZ) and healthy control subjects in 5 Hz steps. Time-frequency spectral analysis was used to differentiate EEG activity synchronized in phase across trials using Phase Locking Factor (PLF) and Mean Power (MP) change from baseline activity. In the SSAEP frequency response condition, patients with SZ showed broad band reductions in both PLF and MP. In addition, the control subjects showed a more pronounced increase in PLF with increases in power compared to SZ subjects. A noise pulse embedded in 40 Hz stimuli resulted in a transient reduction of PLF and MP at 40 Hz in control subjects, while SZ showed diminished overall PLF. Finally, induced gamma (around 40 Hz) response to unmodulated tone stimuli was also reduced in SZ, indicating that disturbances in this oscillatory activity are not confined to SSAEPs. In summary, SZ subjects show impaired oscillatory responses in the gamma range across a wide variety of experimental conditions. Reduction of PLF along with reduced MP may reflect abnormalities in the auditory cortical circuits, such as a reduction in pyramidal cell volume, spine density and alterations in GABAergic neurons.

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.

Brain oscillatory responses in patients with bipolar disorder manic episode before and after valproate treatment

BACKGROUND

GABA/Glutamatergic dysfunction and neural circuits which regulate cognitive processing are involved in the underlying pathology of bipolar disorder. Event related oscillatory neuroelectrical activity reflects integrative brain functioning, different frequency bands representing different cognitive functions.

METHODS

Event Related Potentials to visual odd-ball paradigm in ten manic/hypomanic medication free, DSM-IV bipolar patients were measured before and after six weeks of valproate monotherapy in comparison to ten sex and age matched healthy controls. Different frequency band responses were obtained by digital filtration of ERPs. Young mania rating scale (YMRS) was used to assess clinical response. Repeated measures ANOVA, Wilcoxon and Mann Whitney U tests were used for statistical analysis.

RESULTS

Patients showed significantly higher baseline occipital beta (18-30 Hz) (p: 0.014) response than healthy controls. They were devoid of the occipito-frontal alpha (8-13 Hz) dominance presented by the control group. Occipital beta response reduced significantly (p: 0.009) and became similar to controls after treatment. Post-treatment alpha responses were significantly lower than baseline in anterior temporal (p: 0.038) and occipital (p: 0.027) locations. Healthy controls displayed a significantly increased frontal alpha response at the second assessment but the patients did not. Mean YMRS score reduced significantly compared to baseline at the end of six weeks (p: 0.004).

CONCLUSIONS

Alpha response is the universal operator in the brain. Increased occipital beta response in mania may be compensatory to the dysfunctional alpha operation. Its reduction after valproate may be through modulation of glutamatergic and GABAergic mechanisms and indicate medication's corrective effect on the underlying pathogenesis.

P3 and delta band responses in visual oddball paradigm in schizophrenia

Amplitude reduction of the oddball P3 wave is a well-replicated but non-specific finding of schizophrenia. The time-frequency analysis of single-trial ERP data allows to specify in a reliable manner whether the P3 reduction in schizophrenia is due to the decreased P3 response in single trials or due to the inter-trial variability in the timing of the response. Since the delta response most strongly contributes to the P3 amplitude, we focused to the low frequency range of the time-frequency transformed data. EEG was recorded from chronic schizophrenia patients and matched healthy controls during a simple visual oddball task. The wavelet transforms of the averaged ERP and the single trials were computed to investigate the amplitudes of the evoked (phase-locked) and total (phase-locked+non-phase-locked) delta (1-3 Hz) responses, respectively. Evoked delta activity and P3 amplitude to target stimuli were both reduced significantly in patients with schizophrenia, whereas no such difference was obtained for the total delta activity. The significant reduction of the evoked delta response and the absence of such a difference in the total delta response of schizophrenia patients reveals that the delta band response is weakly phase-locked to stimulus in schizophrenia. This result suggests that the reduced P3 amplitudes in the averaged ERPs of schizophrenia patients result from a temporal jitter in the activation of neural circuits engaged in P3 generation.

Gamma oscillations and spontaneous network activity in the hippocampus are highly sensitive to decreases in pO2 and concomitant changes in mitochondrial redox state

Gamma oscillations have been implicated in higher cognitive processes and might critically depend on proper mitochondrial function. Using electrophysiology, oxygen sensor microelectrode, and imaging techniques, we investigated the interactions of neuronal activity, interstitial pO2, and mitochondrial redox state [NAD(P)H and FAD (flavin adenine dinucleotide) fluorescence] in the CA3 subfield of organotypic hippocampal slice cultures. We find that gamma oscillations and spontaneous network activity decrease significantly at pO2 levels that do not affect neuronal population responses as elicited by moderate electrical stimuli. Moreover, pO2 and mitochondrial redox states are tightly coupled, and electrical stimuli reveal transient alterations of redox responses when pO2 decreases within the normoxic range. Finally, evoked redox responses are distinct in somatic and synaptic neuronal compartments and show different sensitivity to changes in pO2. We conclude that the threshold of interstitial pO2 for robust CA3 network activities and required mitochondrial function is clearly above the "critical" value, which causes spreading depression as a result of generalized energy failure. Our study highlights the importance of a functional understanding of mitochondria and their implications on activities of individual neurons and neuronal networks.

Neurotech for neuroscience: unifying concepts, organizing principles, and emerging tools

The ability to tackle analysis of the brain at multiple levels simultaneously is emerging from rapid methodological developments. The classical research strategies of "measure," "model," and "make" are being applied to the exploration of nervous system function. These include novel conceptual and theoretical approaches, creative use of mathematical modeling, and attempts to build brain-like devices and systems, as well as other developments including instrumentation and statistical modeling (not covered here). Increasingly, these efforts require teams of scientists from a variety of traditional scientific disciplines to work together. The potential of such efforts for understanding directed motor movement, emergence of cognitive function from neuronal activity, and development of neuromimetic computers are described by a team that includes individuals experienced in behavior and neuroscience, mathematics, and engineering. Funding agencies, including the National Science Foundation, explore the potential of these changing frontiers of research for developing research policies and long-term planning.

Functional maps of neocortical local circuitry

This review aims to summarize data obtained with different techniques to provide a functional map of the local circuit connections made by neocortical neurones, a reference for those interested in cortical circuitry and the numerical information required by those wishing to model the circuit. A brief description of the main techniques used to study circuitry is followed by outline descriptions of the major classes of neocortical excitatory and inhibitory neurones and the connections that each layer makes with other cortical and subcortical regions. Maps summarizing the projection patterns of each class of neurone within the local circuit and tables of the properties of these local circuit connections are provided.This review relies primarily on anatomical studies that have identified the classes of neurones and their local and long distance connections and on paired intracellular and whole-cell recordings which have documented the properties of the connections between them. A large number of different types of synaptic connections have been described, but for some there are only a few published examples and for others the details that can only be obtained with paired recordings and dye-filling are lacking. A further complication is provided by the range of species, technical approaches and age groups used in these studies. Wherever possible the range of available data are summarised and compared. To fill some of the more obvious gaps for the less well-documented cases, data obtained with other methods are also summarized.

At clinically relevant concentrations the anaesthetic/amnesic thiopental but not the anticonvulsant phenobarbital interferes with hippocampal sharp wave-ripple complexes

Background

Many sedative agents, including anesthetics, produce explicit memory impairment by largely unknown mechanisms. Sharp-wave ripple (SPW-R) complexes are network activity thought to represent the neuronal substrate for information transfer from the hippocampal to neocortical circuits, contributing to the explicit memory consolidation. In this study we examined and compared the actions of two barbiturates with distinct amnesic actions, the general anesthetic thiopental and the anticonvulsant phenobarbital, on in vitro SPW-R activity.

Results

Using an in vitro model of SPW-R activity we found that thiopental (50–200 μM) significantly and concentration-dependently reduced the incidence of SPW-R events (it increased the inter-event period by 70–430 %). At the concentration of 25 μM, which clinically produces mild sedation and explicit memory impairment, thiopental significantly reduced the quantity of ripple oscillation (it reduced the number of ripples and the duration of ripple episodes by 20 ± 5%, n = 12, P < 0.01), and suppressed the rhythmicity of SPWs by 43 ± 15% (n = 6, P < 0.05). The drug disrupted the synchrony of SPWs within the CA1 region at 50 μM (by 19 ± 12%; n = 5, P < 0.05). Similar effects of thiopental were observed at higher concentrations. Thiopental did not affect the frequency of ripple oscillation at any of the concentrations tested (10–200 μM). Furthermore, the drug significantly prolonged single SPWs at concentrations ≥50 μM (it increased the half-width and the duration of SPWs by 35–90 %). Thiopental did not affect evoked excitatory synaptic potentials and its results on SPW-R complexes were also observed under blockade of NMDA receptors. Phenobarbital significantly accelerated SPWs at 50 and 100 μM whereas it reduced their rate at 200 and 400 μM. Furthermore, it significantly prolonged SPWs, reduced their synchrony and reduced the quantity of ripples only at the clinically very high concentration of 400 μM, reported to affect memory.

Conclusion

We hypothesize that thiopental, by interfering with SPW-R activity, through enhancement of the GABA_A receptor-mediated transmission, affects memory processes which involve hippocampal circuit activation. The quantity but not the frequency of ripple oscillation was affected by the drug.

Application of electroencephalography to the study of cognitive and brain functions in schizophrenia

The electroencephalogram (EEG) recorded from the human scalp is widely used to study cognitive and brain functions in schizophrenia. Current research efforts are primarily devoted to the assessment of event-related potentials (ERPs) and event-related oscillations (EROs), extracted from the ongoing EEG, in patients with schizophrenia and in clinically unaffected individuals who, due to their family history and current mental status, are at high risk for developing schizophrenia. In this article, we discuss the potential usefulness of ERPs and EROs as genetic vulnerability markers, as pathophysiological markers, and as markers of possible ongoing progressive cognitive and cortical deterioration in schizophrenia. Our main purpose is to illustrate that these neurophysiological measures can offer valuable quantitative biological markers of basic pathophysiological mechanisms and cognitive dysfunctions in schizophrenia, yet they may not be specific to current psychiatry's diagnosis and classification. These biological markers can provide unique information on the nature and extent of cognitive and brain dysfunction in schizophrenia. Moreover, they can be utilized to gain deeper theoretical insights into illness etiology and pathophysiology and may lead to improvements in early detection and more effective and targeted treatment of schizophrenia. We conclude by addressing several key methodological, conceptual, and interpretative issues involved in this research field and by suggesting future research directions.

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.

Molecular regulation of cognitive functions and developmental plasticity: impact of GABAA receptors

By controlling spike timing and sculpting neuronal rhythms, inhibitory interneurons play a key role in regulating neuronal circuits and behavior. The pronounced diversity of GABAergic (gamma-aminobutyric acid) interneurons is paralleled by an extensive diversity of GABAA receptor subtypes. The region- and domain-specific location of these receptor subtypes offers the opportunity to gain functional insights into the role of defined neuronal circuits. These developments are reviewed with regard to the regulation of sleep, anxiety, memory, sensorimotor processing and post-natal developmental plasticity.

Definitions of state variables and state space for brain-computer interface : Part 2. Extraction and classification of feature vectors

The hypothesis is proposed that the central dynamics of the action-perception cycle has five steps: emergence from an existing macroscopic brain state of a pattern that predicts a future goal state; selection of a mesoscopic frame for action control; execution of a limb trajectory by microscopic spike activity; modification of microscopic cortical spike activity by sensory inputs; construction of mesoscopic perceptual patterns; and integration of a new macroscopic brain state. The basis is the circular causality between microscopic entities (neurons) and the mesoscopic and macroscopic entities (populations) self-organized by axosynaptic interactions. Self-organization of neural activity is bidirectional in all cortices. Upwardly the organization of mesoscopic percepts from microscopic spike input predominates in primary sensory areas. Downwardly the organization of spike outputs that direct specific limb movements is by mesoscopic fields constituting plans to achieve predicted goals. The mesoscopic fields in sensory and motor cortices emerge as frames within macroscopic activity. Part 1 describes the action-perception cycle and its derivative reflex arc qualitatively. Part 2 describes the perceptual limb of the arc from microscopic MSA to mesoscopic wave packets, and from these to macroscopic EEG and global ECoG fields that express experience-dependent knowledge in successive states. These macroscopic states are conceived to embed and control mesoscopic frames in premotor and motor cortices that are observed in local ECoG and LFP of frontoparietal areas. The fields sampled by ECoG and LFP are conceived as local patterns of neural activity in which trajectories of multiple spike activities (MSA) emerge that control limb movements. Mesoscopic frames are located by use of the analytic signal from the Hilbert transform after band pass filtering. The state variables in frames are measured to construct feature vectors by which to describe and classify frame patterns. Evidence is cited to justify use of linear analysis. The aim of the review is to enable researchers to conceive and identify goal-oriented states in brain activity for use as commands, in order to relegate the details of execution to adaptive control devices outside the brain.

Acute clozapine suppresses synchronized pyramidal synaptic network activity by increasing inhibition in the ferret prefrontal cortex

Recent studies have indicated that impaired neural circuitry in the prefrontal cortex is a prominent feature of the neuropathology of schizophrenia. Clozapine is one of the most effective antipsychotic drugs used for this debilitating disease. Despite its effectiveness, the mechanism by which clozapine acts on prefrontal cortical circuitry remains poorly understood. In this study, in vitro multiple whole cell recordings were performed in slices of the ferret prefrontal cortex. Clozapine, which effectively inhibited the spontaneous synchronized network activities in the prefrontal neurons, achieved the suppressive effect by decreasing the recurrent excitation among pyramidal neurons and by enhancing the inhibitory inputs onto pyramidal cells through a likely network mechanism. Indeed, under the condition of disinhibition, the depressing effects were reversed and clozapine enhanced the recurrent excitation. These results suggest that the therapeutic actions of clozapine in alleviating the positive symptoms of schizophrenia are achieved, at least partially, through the readjustment of synaptic balance between the excitation and inhibition in the prefrontal cortical circuitry.

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.

Gamma band activity and its synchronization reflect the dysfunctional emotional processing in alexithymic persons

In the present study, we investigated the gamma band response and its phase synchrony between electrodes in alexithymia, which is characterized by a disability in identifying and describing feelings. Individuals with high and low alexithymia scores were selected according to the scores on the 20-item Toronto Alexithymia Scale. EEG was recorded from alexithymic and nonalexithymic persons viewing emotionally negative or neutral stimuli. Nonalexithymic persons exhibited increased gamma band power and phase synchronization at the 400-450-ms time window when processing emotionally negative stimuli. Neither enhanced gamma band power nor phase synchronization was observed in alexithymic persons in the negative emotion condition. These results suggest that gamma band activity reflects emotional processing, and alexithymic persons may have a deficit in communication between brain regions or in the utilization of memory or emotional information during the processing of emotional stimuli.

Computational neuropharmacology: dynamical approaches in drug discovery

Computational approaches that adopt dynamical models are widely accepted in basic and clinical neuroscience research as indispensable tools with which to understand normal and pathological neuronal mechanisms. Although computer-aided techniques have been used in pharmaceutical research (e.g. in structure- and ligand-based drug design), the power of dynamical models has not yet been exploited in drug discovery. We suggest that dynamical system theory and computational neuroscience--integrated with well-established, conventional molecular and electrophysiological methods--offer a broad perspective in drug discovery and in the search for novel targets and strategies for the treatment of neurological and psychiatric diseases.

Suppression of early evoked gamma band response in male alcoholics during a visual oddball task

We investigated the early evoked gamma frequency band activity in alcoholics (n=122) and normal controls (n=72) during a visual oddball task. A time-frequency representation method was applied to EEG data in order to obtain phase-locked gamma band activity (29-45 Hz) and was analyzed within a 0-150 ms time window range. Significant reduction of the gamma band response in the frontal region during target stimulus processing was observed in alcoholic compared to control subjects. In contrast, significantly higher gamma band response for the non-target stimulus was observed in alcoholics compared to controls. It is suggested that the reduction in early evoked frontal gamma band response to targets may be associated with frontal lobe dysfunction commonly observed in alcoholics. This perhaps can be characterized by a deficient top-down processing mechanism.

Beyond the DSM: defining endophenotypes for genetic studies of substance abuse

Although substance-related disorders are heritable, the genetic factors contributing to vulnerability to these disorders are expected to be complex. Nonetheless, identifying genes underlying this vulnerability and understanding their relationship with environmental factors and behavior holds the promise of dramatic advances in diagnosis, prevention, and treatment. The search is complicated by a number of factors, however, including the weak validity of psychiatric diagnosis for identifying gene carriers, the complexity of the brain and behavior, and the numerous intervening variables between genetic transcription and its behavioral consequences. One strategy for bridging this theoretical gap is to study endophenotypes--biologic correlates of disorders that precede their overt development, may have higher reliability than behavioral measures, and present simpler relationships with a smaller number of genes. This article reviews research suggesting the usefulness of several putative endophenotypes for substance-related disorders, including 1) reduced P3 amplitude of the visual event-related potential, 2) increased EEG beta power, 3) a lowered level of response to an alcohol challenge, and 4) the inability to modulate autonomic nervous system reactivity under the stress of anticipating a predictable aversive stimulus.

Origin, structure, and role of background EEG activity. Part 4: Neural frame simulation

OBJECTIVE

To develop a method for simulating background EEG based on the premise that the self-organized activity from synaptic interaction among populations of neurons creates sustained fluctuations that can be modeled with the filtered output of a random number generator.

METHODS

The logarithm of the amplitude of activity was weighted in accordance with 1/f, the log frequency in both temporal (PSD(T)) and spatial (PSD(X)) power spectral densities. The activity was spatially smoothed by volume conduction. Further deviation from full randomness was by sustained spatial coherence averaging 25% of total power. The departure from the background state to an active state, as seen in the awake EEG, was simulated by adding segments that were 90% correlated while attenuating by 50% the uncorrelated background activity in those segments. Spatial amplitude modulation was imposed on the correlated noise to create signals that simulated AM patterns.

RESULTS

The statistical properties of the EEG that were replicated (Freeman, 2004a,b, 2005) included the PSD(T), PSD(X), point spread function (PSF), partitioning of the variance with PCA, and the percentages of correct classification of AM patterns.

CONCLUSIONS

The origin of background EEG was traced to self-sustaining mutual excitation among pyramidal cells creating stable noise that was filtered by self-organized criticality to give 1/f(2) PSD, by inhibitory feedback to give oscillations in the classic clinical bands, and by volume conduction to give smoothing. The essential change that identified a frame in EEG was transient synchrony by phase transition among cortical populations in beta and gamma bands of the PSD(T).

SIGNIFICANCE

This simulation can provide test data with which to optimize techniques for noninvasively extracting information from the EEG for diagnosis and treatment evaluation of neuropsychiatric disorders and for operation by paraplegics of prosthetic devices.

Clinical correlates of quantitative EEG alterations in alcoholic patients

OBJECTIVE

To evaluate the incidence and clinical correlations of abnormal QEEG features in alcoholic patients.

METHODS

Quantitative EEG (frequency analysis, absolute and relative powers of the four classical bands) was assessed in 191 male alcoholic patients admitted in our facility for detoxification process. All underwent psychiatric, medical and neurological examination prior to the EEG recording, in search for specific clinical or paraclinical findings. The presence or absence of relevant clinical features was codified as nominal dichotomic variables to be related to specific QEEG features.

RESULTS

Only 7 patients had normal QEEG. The most frequent alteration (81 cases) was decreased power in slow (delta and theta) bands with a concurrent increase in beta band, followed by decreased power only in slow bands (33), increase only in beta band (29), decrease in both slow and alpha bands without beta alterations (28), decrease only in alpha band (6) and others. Alterations in slow and beta bands were uncorrelated. However, a significant correlation was found between decreased power in slow bands and cortical atrophy as revealed by MRI (especially in patients with early onset of alcoholism), time elapsed from the beginning of alcoholic habits (but only in younger or early onset subjects) and in a lesser degree arterial hypertension, but neither with age nor any other clinical or psychiatric feature. On the other hand, increased power in beta band correlated mainly with the use of benzodiazepines, sensoperceptual alterations (hallucinations, illusions), clinical seizures and family history of alcoholism. The effects of those variables were strongly interrelated.

CONCLUSIONS

Decreased power in slow bands in alcoholic patients may be an indicator of brain atrophy or chronic brain damage, while increase in beta band is related to medication use, family history of alcoholism, hallucinations and seizures, suggesting a state of cortical hyperexcitability.

SIGNIFICANCE

This study show the relation of specific QEEG alterations to certain clinical features found in alcoholics, in a further attempt to elucidate the semiological value of those alterations in individual patients.

Stereoselective effects of the novel anticonvulsant lacosamide against 4-AP induced epileptiform activity in rat visual cortex in vitro

We examined effects of the novel anticonvulsant lacosamide and its inactive isomer (SPM 6953) in an in vitro model of epileptiform activity. Focal field potential recordings (34+/-0.2 degrees C) were obtained from 17 to 22 day old rat brain slices. Physiological synaptic transmission (fEPSP amplitude and duration) in CA1 of rat hippocampus was not significantly altered (P > 0.05, n = 4) by lacosamide (1 microM-1 mM). Recording from visual cortex during application of 4-aminopyridine (4-AP; 100 microM) revealed both spontaneous and evoked 'ictal like' discharges. Spontaneous ictal like discharges in the visual cortex were blocked by 100 microM carbamazepine (CBZ), 100 microM pentobarbital and 200 microM phenobarbital (PHB) but were insensitive to the anti-absence drug ethosuximide (750 microM; n = 4, P > 0.05). Lacosamide reduced tonic duration and maximal firing frequency with EC(50)s of 41 and 71 microM, respectively. In contrast, the S stereoisomer (100-320 microM) produced no significant effect on spontaneous ictal activity (n = 3-4, P > 0.05). Seizures induced by high frequency (100 Hz, 1s) stimulation were selectively reduced in amplitude by PHB (200 microM) and frequency by CBZ (100 microM; n = 6) and lacosamide (100 microM; n = 4). GABAergic negative going potentials were attenuated by CBZ (irreversible with washing) and lacosamide (reversible) but not by PHB. We conclude that lacosamide blocks 4-AP induced epileptiform activity in the visual cortex. This novel anticonvulsant drug appears to inhibit epileptogenesis (seizure spread) by interacting with a stereoselective, but as yet unidentified, target site in rodent neocortex in the mid-micromolar range.

Dopamine depletion increases the power and coherence of β-oscillations in the cerebral cortex and subthalamic nucleus of the awake rat

Local field potentials (LFPs) recorded from the subthalamic nucleus (STN) of untreated patients implanted with stimulation electrodes for the treatment of Parkinson's disease (PD) demonstrate strong coherence with the cortical electroencephalogram over the beta-frequency range (15-30 Hz). However, studies in animal models of PD emphasize increased temporal coupling in cortico-basal ganglia circuits at substantially lower frequencies, undermining the potential usefulness of these models. Here we show that 6-hydroxydopamine (6-OHDA) lesions of midbrain dopamine neurons are associated with significant increases in the power and coherence of beta-frequency oscillatory activity present in LFPs recorded from frontal cortex and STN of awake rats, as compared with the healthy animal. Thus, the pattern of synchronization between population activity in the STN and cortex in the 6-OHDA-lesioned rodent model of PD closely parallels that seen in the parkinsonian human. The peak frequency of coherent activity in the beta-frequency range was increased in lesioned animals during periods of spontaneous and sustained movement. Furthermore, administration of the dopamine receptor agonist apomorphine to lesioned animals suppressed beta-frequency oscillations, and increased coherent activity at higher frequencies in the cortex and STN, before producing the rotational behaviour indicative of successful lesion. Taken together, these results support a crucial role for dopamine in the modulation of population activity in cortico-basal ganglia circuits, whereby dopaminergic mechanisms effectively filter out synchronized, rhythmic activity at beta-frequencies at the systems level, and shift temporal couplings in these circuits to higher frequencies. These changes may be important in regulating movement.

Distinct properties of carbachol- and DHPG-induced network oscillations in hippocampal slices

The aim of this study was to compare and contrast the properties of gamma oscillations induced by activation of muscarinic acetylcholine or metabotropic glutamate receptors in the CA3 region of rat hippocampal slices. Both carbachol and the group I metabotropic glutamate receptor agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG), induced network oscillations in the gamma-frequency range (30-100 Hz). The M1 muscarinic receptor antagonist, pirenzepine, blocked carbachol-, but enhanced DHPG-induced oscillations, whereas LY 341495, an antagonist at metabotropic glutamate receptors, abolished DHPG-, but left carbachol-induced oscillations unchanged. There were significant differences in the peak frequency, maximal power, and spectral width of the two oscillations. Pharmacological experiments showed that both types of oscillation depend on fast excitatory and inhibitory synaptic transmission. Interestingly, activation of neurokinin-1 receptors by substance P fragment or enhancement of inhibitory synaptic currents by the benzodiazepine ligand, zolpidem, boosted DHPG-, but reduced the power of carbachol-induced oscillations. These results suggest that, although carbachol and DHPG might activate similar conductances in individual pyramidal cells, the oscillations they induce in slices involve different network mechanisms, most likely by recruiting distinct types of GABAergic interneuron.

Layer-specific pyramidal cell oscillations evoked by tetanic stimulation in the rat hippocampal area CA1 in vitro and in vivo

Tetanic stimulation of axons terminating in the CA1 region of the hippocampus induces oscillations in the gamma-to-beta frequency band (13-100 Hz) and can induce long-term potentiation (LTP). The rapid pyramidal cell discharge is driven by a mainly GABA(A)-receptor-mediated slow depolarization and entrained mainly through ephaptic interactions. This study tests whether cellular compartmentalization can explain how cells, despite severely reduced input resistance, can still fire briskly and have IPSPs superimposed on the slow GABAergic depolarization, and whether this behaviour occurs in vivo. Oscillations induced in CA1 in vitro by tetanic stimulation of the stratum radiatum or oriens were analysed using intracellular and multichannel field potentials along the cell axis. Layer-specific effects of focal application of bicuculline indicate that the GABAergic depolarization is concentrated on tetanized dendrites. Current-source density analysis and characteristics of partial spikes indicate that early action potentials are initiated in the proximal nontetanized dendrite but cannot invade the tetanized dendrite, where recurrent EPSPs and evoked IPSPs were largely suppressed. As the oscillation progresses, IPSPs recover and slow the neuronal firing to beta frequencies, with a small subpopulation of neurons continuing to fire at gamma frequency. Carbonic anhydrase dependence, threshold intensity, frequency, field strength and spike initiation/propagation of tetanus-evoked oscillations in urethane-anaesthetized rats, validate our observations in vitro, and show that these mechanisms operate in healthy tissue. However, the disrupted electrophysiology of the tetanized dendrites will disable normal information processing, has implications for LTP induction and is likely to play a role in pathological synchronization as found during epileptic discharges.

Multiple GABAA receptor subtypes regulate hippocampal ripple oscillations

High-frequency oscillations (140-200 Hz) were recorded in behaving rats from the CA1 area of the hippocampus. As generation of these synchronous patterns is assumed to depend on coordinated interneuronal inhibition, we studied the interference of benzodiazepines with the fine structure and occurrence of ripple oscillations. The nonselective GABAA receptor alpha-subunit agonist, diazepam, lowered the frequency of ripple oscillations and reduced their occurrence, amplitude and duration. Zolpidem, an alpha1-subunit selective benzodiazepine elevated ripple duration but acted similar to diazepam in other respects. The nonselective alpha-subunit benzodiazepine antagonist, flumazenil, reduced ripple numbers, amplitude and duration. Wavelet based analysis of the dynamics of intraripple frequency revealed a dramatic decay within a ripple. Only diazepam (1 mg/kg) accelerated this intraripple frequency accommodation. The effects were not due to increased behavioural activity and alertness as evident from vigilance state control. The results suggest a differential role of GABAA receptor subtype specific inhibitory mechanisms in the mediation and fine-tuning of the network synchronization during approximately 200 Hz hippocampal oscillations.

Effects of diazepam and zolpidem on EEG beta frequencies are behavior-specific in rats

A pharmacological dissociation of the relation between electroencephalographic (EEG) activity and behavior has been described for the benzodiazepines. While a decrease in high frequency EEG activity is associated with a decrease in arousal in drug-free conditions, sedative benzodiazepines increase beta activity. Non-benzodiazepine GABA(A) receptor modulators can increase beta activity as well. To further study the relationship between rat behavior and EEG under GABA(A) receptor modulation, EEG effects of diazepam (2.5 mg/kg) and zolpidem (2.5 mg/kg) were studied during different behaviors. Both drugs modulate the GABA(A) receptor, albeit that zolpidem shows alpha(1) subunit selectivity while diazepam is non-selective. A detailed analysis of rat open field behavior was made with a distinction of 25 behavioral elements. The EEG was segmented according to each behavioral element and a corresponding power spectrum calculated. Both diazepam and zolpidem increased EEG beta frequencies, characteristic for the benzodiazepines. However, the beta and gamma increase was specific for active behavior and not for inactivity. Interestingly, diazepam and zolpidem seemed to amplify, rather than dissociate, the relation between behavior and the EEG. It is hypothesized that the large increase in beta-3/gamma activity caused by diazepam and zolpidem is a compensatory mechanism that allows for behavioral activation, despite pharmacologically induced sedation.

Modulation of septo-hippocampal Theta activity by GABAA receptors: an experimental and computational approach

Theta frequency oscillation of the septo-hippocampal system has been considered as a prominent activity associated with cognitive function and affective processes. It is well documented that anxiolytic drugs diminish septo-hippocampal oscillatory Theta activity contributing to their either therapeutic or unwanted side effects. In the present experiments we applied a combination of computational and physiological techniques to explore the functional role of GABAA receptors in Theta oscillation. In electrophysiological experiments extracellular single unit recordings were performed from medial septum/diagonal band of Broca with simultaneous hippocampal (CA1) electroencephalogram (EEG) recordings from anesthetized rats. Neurotransmission at GABAA receptors were modulated by means of pharmacological tools: the actions of the GABAA receptor positive allosteric modulator diazepam and inverse agonist/negative allosteric modulator FG-7142 were evaluated on septo-hippocampal activity. Systemic administration of diazepam inhibited, whereas FG-7142 enhanced Theta oscillation of septal neurons and hippocampal EEG Theta activity. In parallel to these experimental observations, a computational model has been constructed by implementing a septal GABA neuron model with a CA1 hippocampal model containing three types of neurons (including oriens and basket interneurons and pyramidal cells; latter modeled by multicompartmental techniques; for detailed model description with network parameters see online addendum: http://geza.kzoo.edu/theta). This connectivity made the network capable of simulating the responses of the septo-hippocampal circuitry to the modulation of GABAA transmission, and the presently described computational model proved suitable to reveal several aspects of pharmacological modulation of GABAA receptors. In addition, computational findings indicated different roles of distinctively located GABAA receptors in theta generation.

Origin, structure, and role of background EEG activity. Part 1. Analytic amplitude

OBJECTIVE

To explain the neural mechanisms of spontaneous EEG by measuring the spatiotemporal patterns of synchrony among beta-gamma oscillations during perception.

METHODS

EEGs were measured from 8 x 8 (5.6 x 5.6 mm2) arrays fixed on the surfaces of primary sensory areas in rabbits that were trained to discriminate visual, auditory or tactile conditioned stimuli (CSs) eliciting conditioned responses (CRs). EEG preprocessing was by (i) bandpass filtering to extract the beta-gamma range (deleting theta-alpha); (ii) low-pass spatial filtering (not high-pass Laplacians used for localization), (iii) spatial averaging (not time averaging used for evoked potentials), and (iv) close spacing of 64 electrodes for simultaneous recording in each area (not sampling single signals from several areas); (v) novel algorithms were devised to measure synchrony and spatial pattern stability by calculating variances among patterns in 64-space derived from the 8 x 8 arrays (not by fitting equivalent dipoles). These methodological differences are crucial for the proposed new perspective on EEG.

RESULTS

Spatial patterns of beta-gamma EEG emerged following sudden jumps in cortical activity called 'state transitions'. Each transition began with an abrupt phase re-setting to a new value on every channel, followed sequentially by re-synchronization, spatial pattern stabilization, and a dramatic increase in pattern amplitude. State transitions recurred at varying intervals in the theta range. A novel parameter was devised to estimate the perceptual information in the beta-gamma EEG, which disclosed 2-4 patterns with high information content in the CS-CR interval on each trial; each began with a state transition and lasted approximately 0.1 s.

CONCLUSIONS

The function of each primary sensory neocortex was discontinuous; discrete spatial patterns occurred in frames like those in cinema. The frames before and after the CS-CR interval had low content.

SIGNIFICANCE

Derivation and interpretation of unit data in studies of perception might benefit from using multichannel EEG recordings to define distinctive epochs that are demarcated by state transitions of neocortical dynamics in the CS-CR intervals, particularly in consideration of the possibility that EEG may reveal recurring episodes of exchange and sharing of perceptual information among multiple sensory cortices. Simultaneously recorded, multichannel beta-gamma EEG might assist in the interpretation of images derived by fMRI, since high beta-gamma EEG amplitudes imply high rates of energy utilization. The spatial pattern intermittency provides a tag to distinguish gamma bursts from contaminating EMG activity in scalp recording in order to establish beta-gamma recording as a standard clinical tool. Finally, EEG cannot fail to have a major impact on brain theory.

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.

MODELING DISCOORDINATION OF CORTICAL NEUROACTIVITY: RELEVANCE FOR THE EXECUTIVE CONTROL OF ATTENTION IN ALZHEIMER'S DISEASE

Executive deficits due to Alzheimer's disease (AD) seriously compromise patients' ability for concurrent manipulation of information. Understanding such deficits must integrate neurophysiological findings, results from dual-task experiments and successful psychological models of the matching of ascending and descending information to direct attention. We considered attention as dependent on an oscillatory matching between what is looked for and what is perceived. Hence we implemented a model of coordination between oscillatory neuroactivity of interconnected cortical units. We simulated executive deficits evident during dual-tasks as a breakdown of intercortical oscillatory coordination. We investigated the hypothesis that this breakdown is due to functional disconnection between cortical areas, by measuring the effect of interfering tasks in 'control' and 'lesioned' models. 'Control' models successfully reproduced many features of attention. Several neuropathological mechanisms in AD were found likely to cause functional disconnection. Functional disconnection resulted in much greater impairment of coordination during dual rather than single tasks. This could account for key neuropsychological data from the literature. Executive deficits in AD may thus be partly explained by oscillatory discoordination. Oscillatory coordination phenomena are likely to reflect large scale network interactions in the brain that are concerned with integrative function beyond the specific example considered in this study.

Resting EEG in offspring of male alcoholics: beta frequencies

This study examines the differences in beta (12-28 Hz) band power in offspring of male alcoholics from densely affected alcoholic families. We have attempted to investigate if the increase in beta power is a 'state' or 'trait' marker for alcoholism. This study also explores the gender differences in the expression of this potential risk marker. Absolute beta power in three bands-beta 1(12-16 Hz), beta 2 (16-20 Hz), and beta 3 (20-28 Hz)-in the eyes closed EEG of 171 high risk (HR) subjects who were offspring of male alcoholics and 204 low risk (LR) subjects with no family history of alcoholism, were compared for each gender separately using a repeated measures analysis of variance design. Alcoholic and non-alcoholic subjects within the high risk group were compared using a repeated measures design as a follow-up analysis. The present study demonstrated increased beta power in the resting EEG of offspring of male alcoholics. Male HR subjects had higher beta 1 (12-16 Hz) power and female HR subjects had increased power in beta 2 (16-20 Hz) and beta 3 (20-28 Hz) as compared with low risk participants. Female HR subjects also showed significantly increased beta 2 and beta 3 power if they had two or more alcoholic first-degree relatives when compared with HR females having only an affected father. Risk characteristics are expressed differentially in males and females and may be an index of differential vulnerability to alcoholism. The results indicate that increased EEG beta power can be considered as a likely marker of risk for developing alcoholism and may be used as a predictive endophenotype.

A neural mass model for MEG/EEG: coupling and neuronal dynamics

Although MEG/EEG signals are highly variable, systematic changes in distinct frequency bands are commonly encountered. These frequency-specific changes represent robust neural correlates of cognitive or perceptual processes (for example, alpha rhythms emerge on closing the eyes). However, their functional significance remains a matter of debate. Some of the mechanisms that generate these signals are known at the cellular level and rest on a balance of excitatory and inhibitory interactions within and between populations of neurons. The kinetics of the ensuing population dynamics determine the frequency of oscillations. In this work we extended the classical nonlinear lumped-parameter model of alpha rhythms, initially developed by Lopes da Silva and colleagues [Kybernetik 15 (1974) 27], to generate more complex dynamics. We show that the whole spectrum of MEG/EEG signals can be reproduced within the oscillatory regime of this model by simply changing the population kinetics. We used the model to examine the influence of coupling strength and propagation delay on the rhythms generated by coupled cortical areas. The main findings were that (1) coupling induces phase-locked activity, with a phase shift of 0 or pi when the coupling is bidirectional, and (2) both coupling and propagation delay are critical determinants of the MEG/EEG spectrum. In forthcoming articles, we will use this model to (1) estimate how neuronal interactions are expressed in MEG/EEG oscillations and establish the construct validity of various indices of nonlinear coupling, and (2) generate event-related transients to derive physiologically informed basis functions for statistical modelling of average evoked responses.

Desynchronisation of spontaneously recurrent experimental seizures proceeds with a single rhythm

Here we investigate the temporal properties of recurrent seizure-like events (SLEs) in a low-[Mg(2+)] model of experimental epilepsy. Simultaneous intra- and extracellular electric signals were recorded in the CA3 region of rat hippocampal slices whereby cytosolic [Ca(2+)] transients were imaged by fluorescence detection. Recurrence pattern analysis was applied to give a measure of synchrony of simultaneously recorded intra- and extracellular electric signals and the SLE frequencies were extracted by complex wavelet analysis. Slices from the juvenile, but not the young adult rats, displayed several high-amplitude triplets of electric and [Ca(2+)] transients, termed paroxysmal spikes, followed by an SLE. Occurrence of the full-blown SLE was associated with decreased synaptic activity between the paroxysmal spikes that were seen as incomplete SLE starting sequences. The time series of recurrent SLEs provide evidence for a single SLE rhythm as continuously declining from about 200 Hz to below 1 Hz at the onset and termination of SLE, respectively, with an intermediate spectral discontinuity, tentatively identified with the tonic/clonic transition. All other frequency components were the harmonics of the fundamental rhythm, whereby the gamma and the theta band oscillations were not detected as separate activities. Recurrence showed decreasing temporal synchrony of intra- and extracellular signals during the SLE, suggesting that coincidence is destroyed by the SLE. Blockade of gap junctions with 200 microM carbenoxolone ceased recurrent SLEs. Release of gap junction blockade shortened both SLEs and their tonic phase indicating that persistent changes occurred via an altered gap junction coupling. We conclude that the initially precise temporal synchrony is gradually destroyed during ictal events with a single rhythm of continuously decreasing frequency. Blockade of gap junction coupling might prevent epileptic synchronisation.

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.

THE WAVE PACKET: AN ACTION POTENTIAL FOR THE 21st CENTURY

Sensing and perceiving involve enormous numbers of widely distributed dendritic and action potentials in cortex, before, during and after stimulus arrival but with differing spatiotemporal patterns. Stimulus-activated receptors drive cortical neurons directly (olfactory) or indirectly through thalamocortical relays. The driven activity induces hemisphere-wide, self-organized patterns of neural activity called wave packets. Three levels of brain function are hypothesized to mediate transition from sensation and perception. Microscopic activity expressed by action potentials is sensory. Macroscopic activity of the whole forebrain expressed by behavior is perceptual. Mesoscopic activity bridges the gap by the formation of wave packets. They form when sensory input destabilizes the primary receiving areas by local state transitions. The sensory-driven action potentials condense into mesoscopic wave packets like molecules forming raindrops from vapor. The condensation disks sustain 2D spatial patterns of phase and amplitude of carrier waves in the beta and gamma EEG. The AM patterns correlate not with features but with the context and value of sensory stimuli for the subjects, in a word, their meaning. The wave packets from all sensory areas are broadly transmitted through the forebrain. They induce the formation of macroscopic patterns that coalesce like scintillating pools over much and perhaps all of each hemisphere. The prediction is made for clinical testing that wave packets are precursor to states of awareness. They are not by themselves accessible to experience, as may be the macroscopic states initiated by global state transitions.