Gamma-band synchronous oscillations: recent evidence regarding their functional significance

Neurophysiological Verbal Working Memory Patterns in Children: Searching for a Benchmark of Modality Differences in Audio/Video Stimuli Processing

Exploration of specific brain areas involved in verbal working memory (VWM) is a powerful but not widely used tool for the study of different sensory modalities, especially in children. In this study, for the first time, we used electroencephalography (EEG) to investigate neurophysiological similarities and differences in response to the same verbal stimuli, expressed in the auditory and visual modality during the n-back task with varying memory load in children. Since VWM plays an important role in learning ability, we wanted to investigate whether children elaborated the verbal input from auditory and visual stimuli through the same neural patterns and if performance varies depending on the sensory modality. Performance in terms of reaction times was better in visual than auditory modality (p = 0.008) and worse as memory load increased regardless of the modality (p < 0.001). EEG activation was proportionally influenced by task level and was evidenced in theta band over the prefrontal cortex (p = 0.021), along the midline (p = 0.003), and on the left hemisphere (p = 0.003). Differences in the effects of the two modalities were seen only in gamma band in the parietal cortices (p = 0.009). The values of a brainwave-based engagement index, innovatively used here to test children in a dual-modality VWM paradigm, varied depending on n-back task level (p = 0.001) and negatively correlated (p = 0.002) with performance, suggesting its computational effectiveness in detecting changes in mental state during memory tasks involving children. Overall, our findings suggest that auditory and visual VWM involved the same brain cortical areas (frontal, parietal, occipital, and midline) and that the significant differences in cortical activation in theta band were more related to memory load than sensory modality, suggesting that VWM function in the child's brain involves a cross-modal processing pattern.

Relationship of Metacognition and Insight to Neural Synchronization and Cognitive Function in Early Phase Psychosis

Metacognition is the process of thinking about one's own mental states. It involves a range of faculties that allow an individual to integrate information and form understanding of self and others, and use this understanding to respond to life challenges. Clinical insight is the awareness of one's mental illness, its consequences, and the need for treatment. Persons with psychotic disorders show impaired metacognition and insight, but the neurobiological bases for these impairments are not well characterized. We hypothesized that metacognition and insight may depend on capacity of neural circuits to synchronize at gamma frequencies, as well as the integrity of underlying cognitive processes. In order to test these hypotheses, 17 adults with early phase psychosis were evaluated. Metacognition was assessed with the Metacognition Assessment Scale-Abbreviated, and insight was assessed with the Scale of Unawareness of Illness-Abbreviated. The auditory steady state response (ASSR) to gamma range stimulation (40 Hz) was used as an index of neural synchronization. Cognitive function was assessed using the Brief Assessment of Cognition in Schizophrenia. Increases in ASSR power were associated with poorer metacognition and insight. Higher cognitive performance was associated with higher levels of metacognitive function and insight. These findings suggest that altered neural synchronization and constituent cognitive processes affect both metacognition and insight in early phase psychosis and may offer targets for both pharmacological and psychotherapeutic interventions.

EEG activity during the spatial span task in young men: Differences between short-term and working memory

Short-term memory and working memory are two closely-related concepts that involve the prefrontal and parietal areas. These two types of memory have been evaluated by means of the spatial span task in its forward and backward conditions, respectively. To determine possible neurofunctional differences between them, this study recorded electroencephalographic activity (EEG) in the frontopolar (Fp1, Fp2), dorsolateral (F3, F4), and parietal (P3 and P4) areas during performance of the forward and backward conditions of this task in young men. The backward condition (an indicator of working memory) was characterized by fewer correct answers, higher absolute power (AP) of the delta band in dorsolateral areas, and a lower correlation between frontopolar and dorsolateral regions in the fast bands (alpha, beta and gamma), mainly in the right hemisphere. The prefrontal EEG changes during backward performance may be associated with the higher attentional demands and inhibition processes required to invert the order of reproduction of a sequence. These data provide evidence that the forward and backward conditions of the spatial span task can be distinguished on the basis of neurofunctional activity and performance, and that each one is associated with a distinct pattern of electrical activity and synchronization between prefrontal areas. The higher AP of the delta band and lower correlation of the fast bands, particularly between right prefrontal areas during the backward condition of this visuospatial task, suggest greater participation by the right prefrontal areas in working memory.

Functional Synchronization: The Emergence of Coordinated Activity in Human Systems

The topical landscape of psychology is highly compartmentalized, with distinct phenomena explained and investigated with recourse to theories and methods that have little in common. Our aim in this article is to identify a basic set of principles that underlie otherwise diverse aspects of human experience at all levels of psychological reality, from neural processes to group dynamics. The core idea is that neural, behavioral, mental, and social structures emerge through the synchronization of lower-level elements (e.g., neurons, muscle movements, thoughts and feelings, individuals) into a functional unit—a coherent structure that functions to accomplish tasks. The coherence provided by the formation of functional units may be transient, persisting only as long as necessary to perform the task at hand. This creates the potential for the repeated assembly and disassembly of functional units in accordance with changing task demands. This perspective is rooted in principles of complexity science and non-linear dynamical systems and is supported by recent discoveries in neuroscience and recent models in cognitive and social psychology. We offer guidelines for investigating the emergence of functional units in different domains, thereby honoring the topical differentiation of psychology while providing an integrative foundation for the field.

Temporal sampling in vision and the implications for dyslexia

It has recently been suggested that dyslexia may manifest as a deficit in the neural synchrony underlying language-based codes (Goswami, 2011), such that the phonological deficits apparent in dyslexia occur as a consequence of poor synchronisation of oscillatory brain signals to the sounds of language. There is compelling evidence to support this suggestion, and it provides an intriguing new development in understanding the aetiology of dyslexia. It is undeniable that dyslexia is associated with poor phonological coding, however, reading is also a visual task, and dyslexia has also been associated with poor visual coding, particularly visuo-spatial sensitivity. It has been hypothesized for some time that specific frequency oscillations underlie visual perception. Although little research has been done looking specifically at dyslexia and cortical frequency oscillations, it is possible to draw on converging evidence from visual tasks to speculate that similar deficits could occur in temporal frequency oscillations in the visual domain in dyslexia. Thus, here the plausibility of a visual correlate of the Temporal Sampling Framework is considered, leading to specific hypotheses and predictions for future research. A common underlying neural mechanism in dyslexia, may subsume qualitatively different manifestations of reading difficulty, which is consistent with the heterogeneity of the disorder, and may open the door for a new generation of exciting research.

High-frequency neural oscillations and visual processing deficits in schizophrenia

Visual information is fundamental to how we understand our environment, make predictions, and interact with others. Recent research has underscored the importance of visuo-perceptual dysfunctions for cognitive deficits and pathophysiological processes in schizophrenia. In the current paper, we review evidence for the relevance of high frequency (beta/gamma) oscillations towards visuo-perceptual dysfunctions in schizophrenia. In the first part of the paper, we examine the relationship between beta/gamma band oscillations and visual processing during normal brain functioning. We then summarize EEG/MEG-studies which demonstrate reduced amplitude and synchrony of high-frequency activity during visual stimulation in schizophrenia. In the final part of the paper, we identify neurobiological correlates as well as offer perspectives for future research to stimulate further inquiry into the role of high-frequency oscillations in visual processing impairments in the disorder.

The hypnotic zolpidem increases the synchrony of BOLD signal fluctuations in widespread brain networks during a resting paradigm

Networks of brain regions having synchronized fluctuations of the blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) time-series at rest, or "resting state networks" (RSNs), are emerging as a basis for understanding intrinsic brain activity. RSNs are topographically consistent with activity-related networks subserving sensory, motor, and cognitive processes, and studying their spontaneous fluctuations following acute drug challenge may provide a way to understand better the neuroanatomical substrates of drug action. The present within-subject double-blind study used BOLD fMRI at 3T to investigate the functional networks influenced by the non-benzodiazepine hypnotic zolpidem (Ambien). Zolpidem is a positive modulator of γ-aminobutyric acid(A) (GABA(A)) receptors, and engenders sedative effects that may be explained in part by how it modulates intrinsic brain activity. Healthy participants (n=12) underwent fMRI scanning 45 min after acute oral administration of zolpidem (0, 5, 10, or 20mg), and changes in BOLD signal were measured while participants gazed at a static fixation point (i.e., at rest). Data were analyzed using group independent component analysis (ICA) with dual regression and results indicated that compared to placebo, the highest dose of zolpidem increased functional connectivity within a number of sensory, motor, and limbic networks. These results are consistent with previous studies showing an increase in functional connectivity at rest following administration of the positive GABA(A) receptor modulators midazolam and alcohol, and suggest that investigating how zolpidem modulates intrinsic brain activity may have implications for understanding the etiology of its powerful sedative effects.

SCN1A mutations in Dravet syndrome: impact of interneuron dysfunction on neural networks and cognitive outcome

Dravet syndrome (DS) is a childhood disorder associated with loss-of-function mutations in SCN1A and is characterized by frequent seizures and severe cognitive impairment. Animal studies have revealed new insights into the mechanisms by which mutations in this gene, encoding the type I voltage-gated sodium channel (Na(v)1.1), may lead to seizure activity and cognitive dysfunction. In this review, we further consider the function of fast-spiking GABAergic neurons, one cell type particularly affected by these mutations, in the context of the temporal coordination of neural activity subserving cognitive functions. We hypothesize that disruptions in GABAergic firing may directly contribute to the poor cognitive outcomes in children with DS, and discuss the therapeutic implications of this possibility.

Adaptive affective response identification for hearing threshold detection

Emotional arousal, or affective patterns, can be probed using observable bioelectric signals, in particular using the fluctuations of electroencephalographic potentials from the human scalp. Hearing impairment related to increased threshold of audio tone detection may cause the loss of intelligibility of speech resulting in an innate automatic emotional response. An adaptive support vector machine can be trained to identify a subject's unique affective response based upon an audiogram hearing test. This paper presents the efficacy of our model, initial SVM classification data, and discusses potential application.

Adaptation and maladaptation insights from brain plasticity

Evolutionary concepts such as adaptation and maladaptation have been used by neuroscientists to explain brain properties and mechanisms. In particular, one of the most compelling characteristics of the brain, known as neuroplasticity, denotes the ability of the brain to continuously adapt its functional and structural organization to changing requirements. Although brain plasticity has evolved to favor adaptation, there are cases in which the same mechanisms underlying adaptive plasticity can turn into maladaptive changes. Here, we will consider brain plasticity and its functional and structural consequences from an evolutionary perspective, discussing cases of adaptive and maladaptive plasticity and using examples from typical and atypical development.

Occipital gamma activation during Vipassana meditation

Long-term Vipassana meditators sat in meditation vs. a control rest (mind-wandering) state for 21 min in a counterbalanced design with spontaneous EEG recorded. Meditation state dynamics were measured with spectral decomposition of the last 6 min of the eyes-closed silent meditation compared to control state. Meditation was associated with a decrease in frontal delta (1-4 Hz) power, especially pronounced in those participants not reporting drowsiness during meditation. Relative increase in frontal theta (4-8 Hz) power was observed during meditation, as well as significantly increased parieto-occipital gamma (35-45 Hz) power, but no other state effects were found for the theta (4-8 Hz), alpha (8-12 Hz), or beta (12-25 Hz) bands. Alpha power was sensitive to condition order, and more experienced meditators exhibited no tendency toward enhanced alpha during meditation relative to the control task. All participants tended to exhibit decreased alpha in association with reported drowsiness. Cross-experimental session occipital gamma power was the greatest in meditators with a daily practice of 10+ years, and the meditation-related gamma power increase was similarly the strongest in such advanced practitioners. The findings suggest that long-term Vipassana meditation contributes to increased occipital gamma power related to long-term meditational expertise and enhanced sensory awareness.

Increased absolute magnitude of gamma synchrony in first-episode psychosis

OBJECTIVES

Recent studies have explored a model of the disconnection hypothesis of schizophrenia through the demonstration of abnormal stimulus induced gamma phase synchrony (GPS). These studies have principally examined synchrony in the 40 Hz band elicited in post-stimulus time periods, relative to a pre-stimulus baseline. In this study we examined the absolute magnitude of GPS elicited by a selective attention task, in first-episode psychosis (FEP). We hypothesized that FEP would be associated with abnormalities in absolute GPS, particularly when required to selectively attend to task-relevant stimuli.

METHODS

Fifty-five first-episode psychosis (FEP) subjects and one hundred and ten matched healthy control subjects underwent an auditory oddball selective attention task during EEG recording. The absolute magnitude of GPS was extracted for the range 35-45 Hz, and time-locked to stimulus onset. GPS averaged were computed for oddball 'target' (task-relevant) and 'non-target' (task-irrelevant) stimuli, for each subject.

RESULTS

FEP subjects showed a significant elevation in absolute GPS relative to controls, apparent across the 35-45 Hz range. This elevation was most marked in the left centro-temporal region, across the 800 ms post-stimulus period. In FEP subjects, the elevation in GPS was also greater for target compared to non-target stimuli, while healthy controls did not show a stimulus effect.

CONCLUSION

These findings complement previous evidence for reductions in peak gamma synchrony, calculated relative to a pre-stimulus baseline, in schizophrenia. The results an excess of absolute GPS in schizophrenia may contribute to an inability to effectively integrate task-relevant information, which underlie psychotic symptoms.

EEG oscillations induced by contour closure in a noisy visual field

This study was designed to separate the neural response associated with awareness of a visual object from the neural responses to the visual input without awareness. Two classes of stimuli were presented to subjects: (1) a field of randomly positioned and oriented Gabor patches (no contour) or (2) the same field of Gabor patches in which a subset of the Gabor patches were oriented in such a way as to give rise to the percept of an ellipse (contour). To measure the electroencephalograph (EEG) responses to the contour without awareness, trials during which a contour was present but not perceived were compared to trials where no contour was present and its absence was correctly indicated. At beta frequencies, the presence of the contour reduced the evoked potential (EP) power approximately 130-230 ms post-stimulus. Coherence in the theta band were also reduced when a contour was present but not perceived. Thus, modulations of the EP take place when a contour is presented even without the subjects' awareness. To explore the correlates of awareness, trials during which the subject correctly indicated the presence of a contour were compared to trials in which the object was not detected. Consistent across subjects, awareness caused an increase in the EP power in the beta range around 130-230 ms post-stimulus. These results suggest that the neural response to an object and awareness of that object are dissociable in EEG recordings.

Does any aspect of mind survive brain damage that typically leads to a persistent vegetative state? Ethical considerations

Recent neuroscientific evidence brings into question the conclusion that all aspects of consciousness are gone in patients who have descended into a persistent vegetative state (PVS). Here we summarize the evidence from human brain imaging as well as neurological damage in animals and humans suggesting that some form of consciousness can survive brain damage that commonly causes PVS. We also raise the issue that neuroscientific evidence indicates that raw emotional feelings (primary-process affects) can exist without any cognitive awareness of those feelings. Likewise, the basic brain mechanisms for thirst and hunger exist in brain regions typically not damaged by PVS. If affective feelings can exist without cognitive awareness of those feelings, then it is possible that the instinctual emotional actions and pain "reflexes" often exhibited by PVS patients may indicate some level of mentality remaining in PVS patients. Indeed, it is possible such raw affective feelings are intensified when PVS patients are removed from life-supports. They may still experience a variety of primary-process affective states that could constitute forms of suffering. If so, withdrawal of life-support may violate the principle of nonmaleficence and be tantamount to inflicting inadvertent "cruel and unusual punishment" on patients whose potential distress, during the process of dying, needs to be considered in ethical decision-making about how such individuals should be treated, especially when their lives are ended by termination of life-supports. Medical wisdom may dictate the use of more rapid pharmacological forms of euthanasia that minimize distress than the de facto euthanasia of life-support termination that may lead to excruciating feelings of pure thirst and other negative affective feelings in the absence of any reflective awareness.

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

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

Correlation between EEG rhythms during sleep: surface versus mediotemporal EEG

We compared surface and intracranial electroencephalogram recordings of mediotemporal structures. These structures are critically involved in declarative memory formation and memory consolidation during sleep. As memory processing is suggested to involve the interplay between fast and slow oscillations, we hypothesized different correlations between frequency bands in surface versus mediotemporal electroencephalogram recordings. Polysomnographic recordings obtained in 10 patients with unilateral temporal lobe epilepsy were analyzed. In accordance with earlier studies, we observed that power density in surface electroencephalogram is organized reciprocally between delta/theta and fast frequencies above 16 Hz during non-rapid-eye-movement sleep (negative correlations). In contrast, we found that within the hippocampus delta/theta power alternated in parallel with fast oscillations above 16 Hz during non-rapid-eye-movement sleep (positive correlations).

Somatosensory cortex stimulation for deafferentation pain

Functional neuroimaging has demonstrated that a relationship exists between the intensity of deafferentation pain and the degree of deafferentation-related reorganization of the primary somatosensory cortex. It has also revealed that this cortical reorganization can be reversed after the attenuation of pain. Deafferentation pain is also associated with hyperactivity of the somatosensory thalamus and cortex. Therefore, in order to suppress pain, it seems logical to attempt to modify this deafferentation-related somatosensory cortex hyperactivity and reorganization. This can be achieved using neuronavigation-guided transcranial magnetic stimulation (TMS), a technique that is capable of modulating cortical activity. If TMS is capable of suppressing deafferentation pain, this benefit should be also obtained by the implantation of epidural stimulating electrodes over the area of electrophysiological signal abnormality in the primary somatosensory cortex. The first studies demonstrated a statistically significant pain suppression in all patients and a clinically significant pain suppression in 80% of them. This clinical experience suggests that somatosensory cortex stimulation may become a neurophysiology-based new approach for treating deafferentation pain in selected patients. In this chapter, we review the relevant recent reports and describe our studies in this field.

Meditation states and traits: EEG, ERP, and neuroimaging studies

Neuroelectric and imaging studies of meditation are reviewed. Electroencephalographic measures indicate an overall slowing subsequent to meditation, with theta and alpha activation related to proficiency of practice. Sensory evoked potential assessment of concentrative meditation yields amplitude and latency changes for some components and practices. Cognitive event-related potential evaluation of meditation implies that practice changes attentional allocation. Neuroimaging studies indicate increased regional cerebral blood flow measures during meditation. Taken together, meditation appears to reflect changes in anterior cingulate cortex and dorsolateral prefrontal areas. Neurophysiological meditative state and trait effects are variable but are beginning to demonstrate consistent outcomes for research and clinical applications. Psychological and clinical effects of meditation are summarized, integrated, and discussed with respect to neuroimaging data.

Conjugate eye movements and gamma power modulation of the EEG in persistent vegetative state

BACKGROUND

Power in the gamma band EEG increases during saccades in normal subjects.

OBJECTIVE

To develop a potential method to quantify signs of cortical responsiveness in persistent vegetative state (PVS) we quantified gamma range EEG in association with conjugate slow ballistic eye movements (SBEM).

METHODS

The EEG and the simultaneous electro-oculogram were recorded in 14 (8F/6M) PVS patients. Clinical scoring was based on the Glasgow Coma Scale (GCS) and Coma Rating Scale (CRS). The Wavelet Transform, followed by Hilbert transform was applied to the EEG and gamma power distribution was quantified relative to the timing of an eye movement. We correlated the clinical and the neurophysiological measures.

RESULTS

Gamma activity was present in all PVS patients. Its power was modulated in association with eye movements only in less severely affected patients, with minimum power prior to, and maximum power during the eye movement. In severely affected patients there was no evidence of a temporal relationship between gamma power and the phase of the eye movement.

CONCLUSIONS

Detecting changes in the time course of gamma power in relation to conjugate ballistic eye movements provides a quantitative neurophysiological method for prospective longitudinal studies to explore if the preservation of this CNS function relates to the potential for recovery in PVS patients.

Intracellular activity of cortical and thalamic neurones during high-voltage rhythmic spike discharge in Long-Evans rats in vivo

Spontaneous high-voltage rhythmic spike (HVRS) discharges at 6-12 Hz have been widely described in the electrocorticogram (EcoG) of Long-Evans rats. These ECoG oscillations have been proposed to reflect a state of attentive immobility allowing the optimization of sensory integration within the corticothalamic pathway. This hypothesis has been challenged by recent studies emphasizing similarities between HVRS discharges and spike-and-wave discharges (SWDs) in well-established rat genetic models of absence epilepsy. Here, we made in vivo intracellular recordings to determine, for the first time, the cellular mechanisms responsible for the synchronized oscillations in the corticothalamic loop during HVRS discharges in the Long-Evans rats. We show that HVRS discharges are associated in corticothalamic neurones with rhythmic suprathreshold synaptic depolarizations superimposed on a tonic hyperpolarization, likely due to a process of synaptic disfacilitation. Simultaneously, thalamocortical neurones exhibit a large-amplitude 'croissant'-shaped membrane hyperpolarization with a voltage sensitivity suggesting a potassium-dependent mechanism. This thalamic hyperpolarizing envelope was associated with a membrane oscillation resulting from interactions between excitatory synaptic inputs, a chloride-dependent inhibitory conductance and voltage-gated intrinsic currents. These cortical and thalamic cellular mechanisms underlying HVRS activity in Long-Evans rats are remarkably similar to those previously described in the thalamocortical networks during SWDs. Thus, the present study provides an additional support to the hypothesis that HVRS activity in Long-Evans rats is an absence-like seizure activity.

Induced gamma-band oscillations correlate with awareness in hemianopic patient GY

In normal vision gamma oscillations are involved in object perception, are modulated by attention, and have been linked to awareness by way of their putative role in perceptual integration, arguably as a mechanism for synchronizing activity in separate neural assemblies. We tested the hypothesis that the presence of gamma oscillations (approximately 30-80 Hz) signal the entry of a neural representation into awareness (as indexed by direct report), while attempting to control for other measures of neural information processing such as discrimination accuracy and reaction time. Hemianopic patient GY sometimes reports an awareness "that something happened" in his blind visual hemifield, in response to stimuli of sufficiently high contrast, although he may deny "seeing" anything. At lower contrast levels GY denies any awareness, but may continue to exhibit greater-than-chance accuracy (blindsight). Using a near-threshold level of contrast offers a unique way to test hypotheses concerning correlates of perceptual awareness, since GY's accuracy on certain tasks is independent of awareness. We tested GY on an orientation-discrimination task using stationary stimuli at a fixed near-threshold level of contrast, to which GY sometimes responded "aware" and sometimes "unaware". We recorded brain activity using magnetoencephalography (MEG) in order to determine the relationship between local induced gamma-band oscillations and awareness. GY's accuracy was significantly greater than expected by chance and no different whether or not he reported awareness of the stimulus. Oscillatory activity in the gamma band (44-66 Hz) over the left occipito-parietal region correlated significantly with awareness (but not accuracy), whereas activity in the alpha band (8-12 Hz) did not.

Age-dependent change in executive function and gamma 40 Hz phase synchrony

Decline in cognitive function is well recognized, yet few neurophysiological correlates of age-related cognitive decline have been identified. In this study we examined the impact of age on neurocognitive function and Gamma phase synchrony among 550 normal subjects (aged 11-70). Gamma phase synchrony was acquired to targets in the auditory oddball paradigm. The two tasks of executive function were switching of attention and an electronic maze. Subjects were divided into four age groups, which were balanced for sex. We hypothesized that reduced cognitive performance among older healthy individuals would be associated with age-related changes in gamma phase synchrony. Results showed a significant decrease in executive function in the oldest (51-70 years) age group. ANOVAs of age-by-frontal Gamma synchrony also showed a significant effect of age on Gamma phase synchrony in the left frontal region that corresponded modestly to the age effect found on executive task performance, with reduced performance associated with increased gamma synchrony. The results indicate that age-related changes in cognitive function evident among elderly individuals may in part be related to decreased ability to integrate information and this may be reflected as a compensatory increase in gamma synchrony in frontal regions of the brain.

Single-neuron theory of consciousness

By most accounts, the mind arises from the integrated activity of large populations of neurons distributed across multiple brain regions. A contrasting model is presented in the present paper that places the mind/brain interface not at the whole brain level but at the level of single neurons. Specifically, it is proposed that each neuron in the nervous system is independently conscious, with conscious content corresponding to the spatial pattern of a portion of that neuron's dendritic electrical activity. For most neurons, such as those in the hypothalamus or posterior sensory cortices, the conscious activity would be assumed to be simple and unable to directly affect the organism's macroscopic conscious behavior. For a subpopulation of layer 5 pyramidal neurons in the lateral prefrontal cortices, however, an arrangement is proposed to be present such that, at any given moment: (i) the spatial pattern of electrical activity in a portion of the dendritic tree of each neuron in the subpopulation individually manifests a complexity and diversity sufficient to account for the complexity and diversity of conscious experience; (ii) the dendritic trees of the neurons in the subpopulation all contain similar spatial electrical patterns; (iii) the spatial electrical pattern in the dendritic tree of each neuron interacts non-linearly with the remaining ambient dendritic electrical activity to determine the neuron's overall axonal response; (iv) the dendritic spatial pattern is reexpressed at the population level by the spatial pattern exhibited by a synchronously firing subgroup of the conscious neurons, thereby providing a mechanism by which conscious activity at the neuronal level can influence overall behavior. The resulting scheme is one in which conscious behavior appears to be the product of a single macroscopic mind, but is actually the integrated output of a chorus of minds, each associated with a different neuron.

The coincidence between late non-phase-locked gamma synchronization response and saccadic eye movements

The event-related response in the gamma (30-45 Hz) frequency band was studied in healthy subjects (n=45) viewing sequentially presented pictures from the International Affective Picture System. The distinct non-phase-locked gamma response was obtained in characteristic time window (200-400 ms) with clear-cut centro-parietal location. The strong coincidence between induced gamma oscillations and saccadic eye movements was revealed. We suggest that saccade-related gamma increase is another manifestation of the phenomenon known as presaccadic spike potential, which is commonly registered over parietal scalp leads at 10-20 ms prior to saccade onset. It is hypothesized that late non-phase-locked gamma synchronization mainly reflects activity of a system responsible for attentional tuning and motor planning/execution of saccadic eye movements.

Adaptation through minimization of the phase lag in coupled nonidentical systems

We show that the internal control of adaptation can be obtained from the properties of the phase lag that results from phase synchronization of two nonidentical chaotic oscillators. The direction and magnitude of the phase lag depend upon the relative internal properties of the coupled units, and they can be used as indicators during the adjustment of dynamics, i.e., adaptation of the target unit to match that of the control. The properties of the phase lag are obtained using a method based on the estimation of properties of the distributions of relative event times of both (target and control) units. The phase lag dependent mechanism to control the adaptation process was applied to a system of nonidentical Rössler oscillators and a system of nonidentical Lorenz oscillators. We also elucidate its importance as a control mechanism of the changes of neuronal activity showing its application to neural adaptation.

BCI Competition 2003—Data Set Ia: Combining Gamma-Band Power With Slow Cortical Potentials to Improve Single-Trial Classification of Electroencephalographic Signals

In one type of brain-computer interface (BCI), users self-modulate brain activity as detected by electroencephalography (EEG). To infer user intent, EEG signals are classified by algorithms which typically use only one of the several types of information available in these signals. One such BCI uses slow cortical potential (SCP) measures to classify single trials. We complemented these measures with estimates of high-frequency (gamma-band) activity, which has been associated with attentional and intentional states. Using a simple linear classifier, we obtained significantly greater classification accuracy using both types of information from the same recording epochs compared to using SCPs alone.

Task-related coupling from high- to low-frequency signals among visual cortical areas in human subdural recordings

Cortical cooperativity during cognitive demands includes high- and low-frequency activities, which raises the question whether there are interdependencies between fast and slow processes and how they are reflected in electrical brain signals. We had the opportunity to record signals intracranially from occipital visual areas in an epileptic patient and quantified inter-areal signal coupling while the patient performed a visual delayed-match-to-sample task. We computed coherence, phase consistency and amplitude envelope correlation and we also determined inter-frequency coupling through correlation between low-frequency signal components and amplitude envelopes of high-frequency components. There was a pronounced task-related increase of correlation between gamma-band (28-70 Hz) signal envelopes from a superior (occipital) and low-frequency (0-3.5 Hz) signals from an inferior (occipital) visual area, lasting for approximately 1 s and possibly reflecting a short-term memory encoding process. The correlational delay between envelopes and low-frequency components was 40 ms. In contrast, coherence, phase consistency and envelope correlation showed event-, but no task-related changes of intra-areal and no changes of inter-areal coupling. Our data suggest a specific effect of gamma-activity in the superior onto low-frequency activity in the inferior area. We argue that temporal dispersion of conduction delays might prevent coherent transmission of high-frequency signals and thus account for the absence of gamma-coherence. As such dispersion is a general property of long-range projections, envelope-to-signal correlation possibly reflects a general neuronal mechanism. Hence, our method provides a powerful tool for detecting such inter-areal interactions not visible with conventional linear coupling measures.

Is perception discrete or continuous?

How does conscious perception evolve following stimulus presentation? The idea that perception relies on discrete processing epochs has been often considered, but never widely accepted. The alternative, a continuous translation of the external world into explicit perception, although more intuitive and subjectively appealing, cannot satisfactorily account for a large body of psychophysical data. Cortical and thalamocortical oscillations in different frequency bands could provide a neuronal basis for such discrete processes, but are rarely analyzed in this context. This article reconciles the unduly abandoned topic of discrete perception with current views and advances in neuroscience.

Functional connectivity of gamma EEG activity is modulated at low frequency during conscious recollection

We examined two subjectively distinct memory states that are elicited during recognition memory in humans and compared them in terms of the gamma oscillations (20-60 Hz) in the electroencepahalogram (EEG) that they induced. These subjective states, 'recollection' and 'familiarity' both entail correct recognition but one involves a clear and conscious recollection of the event including memory for contextual detail whilst the other involves a sense of familiarity without clear recollection. Here we show that during a verbal recognition memory test, the subjective experience of 'recollection' induced higher amplitude gamma oscillations than the subjective experience of 'familiarity' in the time period 300-500 ms after stimulus presentation. Recollection, but not familiarity, was also associated with greater functional connectivity in the gamma frequency range between frontal and parietal sites. Furthermore, the magnitude of the gamma functional connectivity varied over time and was modulated at 3 Hz. Previous studies in animals have shown local theta frequency modulation (3-7 Hz) of gamma-oscillations but this is the first time that a similar effect has been reported in the human EEG.

Bispectral index monitoring during electroconvulsive therapy under propofol anaesthesia

BACKGROUND

The accuracy of the bispectral index (BIS) as a monitor of consciousness has not been well studied in patients who have abnormal electroencephalograms (EEG).

METHODS

We studied the changes in BIS, its subparameters, and spectral entropy of the EEG during 18 electroconvulsive treatments under propofol and succinylcholine anaesthesia. A single bifrontal EEG, and second subocular channel (for eye movement estimation) was recorded.

RESULTS

The median (interquartile range) BIS value at re-awakening was only 57 (47-78)--thus more than a quarter of the patients woke at BIS values of less than 50. The changes in spectral entropy values were similar: 0.84 (0.68-0.99) at the start, 0.65 (0.42-0.88) at the point of loss-of-consciousness, 0.63 (0.47-0.79) during the seizures, and 0.58 (0.31-0.85) at awakening.

CONCLUSIONS

Post-ictal slow-wave activity in the EEG (acting via the SynchFastSlow subparameter) may cause low BIS values that do not correspond to the patient's clinical level of consciousness. This may be important in the interpretation of the BIS in other groups of patients who have increased delta-band power in their EEG.

Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling

In humans, distinct processes within the hippocampus and rhinal cortex support declarative memory formation. But do these medial temporal lobe (MTL) substructures directly cooperate in encoding new memories? Phase synchronization of gamma-band electroencephalogram (EEG) oscillations (around 40 Hz) is a general mechanism of transiently connecting neural assemblies. We recorded depth-EEG from within the MTL of epilepsy patients performing a memorization task. Successful as opposed to unsuccessful memory formation was accompanied by an initial elevation of rhinal-hippocampal gamma synchronization followed by a later desynchronization, suggesting that effective declarative memory formation is accompanied by a direct and temporarily limited cooperation between both MTL substructures.

Biological timing and the clock metaphor: oscillatory and hourglass mechanisms

Living organisms have developed a multitude of timing mechanisms--"biological clocks." Their mechanisms are based on either oscillations (oscillatory clocks) or unidirectional processes (hourglass clocks). Oscillatory clocks comprise circatidal, circalunidian, circadian, circalunar, and circannual oscillations--which keep time with environmental periodicities--as well as ultradian oscillations, ovarian cycles, and oscillations in development and in the brain, which keep time with biological timescales. These clocks mainly determine time points at specific phases of their oscillations. Hourglass clocks are predominantly found in development and aging and also in the brain. They determine time intervals (duration). More complex timing systems combine oscillatory and hourglass mechanisms, such as the case for cell cycle, sleep initiation, or brain clocks, whereas others combine external and internal periodicities (photoperiodism, seasonal reproduction). A definition of a biological clock may be derived from its control of functions external to its own processes and its use in determining temporal order (sequences of events) or durations. Biological and chemical oscillators are characterized by positive and negative feedback (or feedforward) mechanisms. During evolution, living organisms made use of the many existing oscillations for signal transmission, movement, and pump mechanisms, as well as for clocks. Some clocks, such as the circadian clock, that time with environmental periodicities are usually compensated (stabilized) against temperature, whereas other clocks, such as the cell cycle, that keep time with an organismic timescale are not compensated. This difference may be related to the predominance of negative feedback in the first class of clocks and a predominance of positive feedback (autocatalytic amplification) in the second class. The present knowledge of a compensated clock (the circadian oscillator) and an uncompensated clock (the cell cycle), as well as relevant models, are briefly re viewed. Hourglass clocks are based on linear or exponential unidirectional processes that trigger events mainly in the course of development and aging. An important hourglass mechanism within the aging process is the limitation of cell division capacity by the length of telomeres. The mechanism of this clock is briefly reviewed. In all clock mechanisms, thresholds at which "dependent variables" are triggered play an important role.

Dimension of interaction dynamics

A method allowing one to distinguish interacting from noninteracting systems based on available time series is proposed and investigated. Some facts concerning generalized Rényi dimensions that form the basis of our method are proved. We show that one can find the dimension of the part of the attractor of the system connected with interaction between its parts. We use our method to distinguish interacting from noninteracting systems on the examples of logistic and Hénon maps. A classification of all possible interaction schemes is given.