Phase-locked alpha and theta oscillations generate the P1–N1 complex and are related to memory performance

Parallelism in the brain's visual form system

We used magnetoencephalography (MEG) to determine whether increasingly complex forms constituted from the same elements (lines) activate visual cortex with the same or different latencies. Twenty right-handed healthy adult volunteers viewed two different forms, lines and rhomboids, representing two levels of complexity. Our results showed that the earliest responses produced by lines and rhomboids in both striate and prestriate cortex had similar peak latencies (40 ms) although lines produced stronger responses than rhomboids. Dynamic causal modeling (DCM) showed that a parallel multiple input model to striate and prestriate cortex accounts best for the MEG response data. These results lead us to conclude that the perceptual hierarchy between lines and rhomboids is not mirrored by a temporal hierarchy in latency of activation and thus that a strategy of parallel processing appears to be used to construct forms, without implying that a hierarchical strategy may not be used in separate visual areas, in parallel.

Oscillatory characteristics of the visual mismatch negativity: what evoked potentials aren't telling us

The visual mismatch negativity (vMMN) response is typically examined by subtracting the average response to a deviant stimulus from the response to the standard. This approach, however, can omit a critical element of the neural response, i.e., the non-phase-locked (“induced”) oscillatory activity. Recent investigations of the oscillatory characteristics of the auditory mismatch negativity (aMMN) identified a crucial role for theta phase locking and power. Oscillatory characteristics of the vMMN from 39 healthy young adults were investigated in order to establish whether theta phase locking plays a similar role in the vMMN response. We explored changes in phase locking, overall post-stimulus spectral power as well as non-phase-locked spectral power compared to baseline (−300 to 0 ms). These were calculated in the frequency range of 4–50 Hz and analysed using a non-parametric cluster based analysis. vMMN was found intermittently in a broad time interval 133–584 ms post-stimulus and was associated with an early increase in theta phase locking (75–175 ms post-stimulus) that was not accompanied by an increase in theta power. Theta phase locking in the absence of an increase in theta power has been associated with the distribution and flow of information between spatially disparate neural locations. Additionally, in the 450–600 ms post-stimulus interval, deviant stimuli yielded a stronger decrease in non-phase-locked alpha power than standard stimuli, potentially reflecting a shift in attentional resources following the detection of change. The examination of oscillatory activity is crucial to the comprehensive analysis of a neural response to a stimulus, and when combined with evoked potentials (EPs) provide a more complete picture of neurocognitive processing.

EEG correlates of self-referential processing

Self-referential processing has been principally investigated using functional magnetic resonance imaging (fMRI). However, understanding of the brain functioning is not possible without careful comparison of the evidence coming from different methodological domains. This paper aims to review electroencephalographic (EEG) studies of self-referential processing and to evaluate how they correspond, complement, or contradict the existing fMRI evidence. There are potentially two approaches to the study of EEG correlates of self-referential processing. Firstly, because simultaneous registration of EEG and fMRI has become possible, the degree of overlap between these two signals in brain regions related to self-referential processing could be determined. Second and more direct approach would be the study of EEG correlates of self-referential processing per se. In this review, I discuss studies, which employed both these approaches and show that in line with fMRI evidence, EEG correlates of self-referential processing are most frequently found in brain regions overlapping with the default network, particularly in the medial prefrontal cortex. In the time domain, the discrimination of self- and others-related information is mostly associated with the P300 ERP component, but sometimes is observed even earlier. In the frequency domain, different frequency oscillations have been shown to contribute to self-referential processing, with spontaneous self-referential mentation being mostly associated with the alpha frequency band.

The strength of attentional biases reduces as visual short-term memory load increases

Despite our visual system receiving irrelevant input that competes with task-relevant signals, we are able to pursue our perceptual goals. Attention enhances our visual processing by biasing the processing of the input that is relevant to the task at hand. The top-down signals enabling these biases are therefore important for regulating lower level sensory mechanisms. In three experiments, we examined whether we apply similar biases to successfully maintain information in visual short-term memory (VSTM). We presented participants with targets alongside distracters and we graded their perceptual similarity to vary the extent to which they competed. Experiments 1 and 2 showed that the more items held in VSTM before the onset of the distracters, the more perceptually distinct the distracters needed to be for participants to retain the target accurately. Experiment 3 extended these behavioral findings by demonstrating that the perceptual similarity between target and distracters exerted a significantly greater effect on occipital alpha amplitudes, depending on the number of items already held in VSTM. The trade-off between VSTM load and target-distracter competition suggests that VSTM and perceptual competition share a partially overlapping mechanism, namely top-down inputs into sensory areas.

EEG anomalies in adult ADHD subjects performing a working memory task

Functional imaging studies have revealed differential brain activation patterns in attention deficit hyperactivity disorder (ADHD) adult patients performing working memory (WM) tasks. The existence of alterations in WM-related cortical circuits during childhood may precede executive dysfunctions in this disorder in adults. To date, there is no study exploring the electrophysiological activation of WM-related neural networks in ADHD. To address this issue, we carried out an electroencephalographic (EEG) activation study associated with time-frequency (TF) analysis in 15 adults with ADHD and 15 controls performing two visual N-back WM tasks, as well as oddball detection and passive fixation tasks. Frontal transient (phasic) theta event-related synchronization (ERS, 0-500 msec) was significantly reduced in ADHD as compared to control subjects. Such reduction was equally present in a task-independent manner. In contrast, the power of the later sustained (∼500-1200 msec) theta ERS for all tasks was comparable in ADHD and control groups. In active WM tasks, ADHD patients displayed lower alpha event-related desynchronization (ERD, ∼200-900 msec) and higher subsequent alpha ERS (∼900-2400 msec) compared to controls. The time course of alpha ERD/ERS cycle was modified in ADHD patients compared to controls, suggesting that they are able to use late compensatory mechanisms in order to perform this WM task. These findings support the idea of an ADHD-related dysfunction of neural generators sub-serving attention directed to the incoming visual information. ADHD cases may successfully face WM needs depending on the preservation of sustained theta ERS and prolonged increase of alpha ERS at later post-stimulus time points.

Auditory-evoked alpha oscillations imply reduced anterior and increased posterior amplitudes in schizophrenia

OBJECTIVE

Most of the work on disturbed oscillatory activity during auditory tasks in schizophrenia has focused on reduced gamma oscillations at fronto-central sites. Recent studies of our group, however, indicate a more general disturbance affecting the spatial distribution of oscillatory brain activity of gamma as well as slow frequencies, such as alpha oscillations.

METHODS

During a passive auditory listening task, electroencephalography was recorded from healthy controls and patients with schizophrenia. Stimulus-locked alpha activity within the first 250 ms after stimulus onset was analyzed from midline electrodes.

RESULTS

Healthy controls showed the common fronto-central maximum of the early alpha response, while patients with schizophrenia showed lower fronto-central and larger parieto-occipital alpha activity than controls, leading to a more similar amplitude distribution across the midline electrode sites.

CONCLUSIONS

The present results indicate malfunctioning long-range inhibition of task-irrelevant cortical areas in schizophrenia, which may disturb functional integration of perception and attention. We emphasize the importance of the whole-brain network theory for the understanding of schizophrenia since it proposes that integrative brain function is based on the coexistence and cooperative action of many interwoven and interacting sub-mechanisms.

SIGNIFICANCE

Neuropsychiatric illnesses such as schizophrenia are marked by communication and coordination failures between different brain regions and different frequency bands.

Coherence, phase differences, phase shift, and phase lock in EEG/ERP analyses

Electroencephalogram (EEG) coherence is a mixture of phase locking interrupted by phase shifts in the spontaneous EEG. Average reference, Laplacian transforms, and independent component (ICA) reconstruction of time series can distort physiologically generated phase differences and invalidate the computation of coherence and phase differences as well as in the computation of directed coherence and phase reset. Time domain measures of phase shift and phase lock are less prone to artifact and are independent of volume conduction. Cross-frequency synchrony in the surface EEG and in Low Resolution Electromagnetic Tomography (LORETA) provides insights into dynamic functions of the brain.

Cortical oscillatory dynamics in a social interaction model

In this study we sought to investigate cortical oscillatory dynamics accompanying three major kinds of social behavior: aggressive, friendly, and avoidant. Behavioral and EEG data were collected in 48 participants during a computer game modeling social interactions with virtual 'persons'. 3D source reconstruction and independent component analysis were applied to EEG data. Results showed that social behavior was partly reactive and partly proactive with subject's personality playing an important role in shaping this behavior. Most salient differences were found between avoidance and approach behaviors, whereas the two kinds of approach behavior (i.e., aggression and friendship) did not differ from each other. Comparative to avoidance, approach behaviors were associated with higher induced responses in most frequency bands which were mostly observed in cortical areas overlapping with the default mode network. The difference between approach- and avoidance-related oscillatory dynamics was more salient in subjects predisposed to approach behaviors (i.e., in aggressive or sociable subjects) and was less pronounced in subjects predisposed to avoidance behavior (i.e., in high trait anxiety scorers). There was a trend to higher low frequency phase-locking in motor area in approach than in avoid condition. Results are discussed in light of the concept linking induced responses with top-down and evoked responses with bottom-up processes.

Genetic influences on composite neural activations supporting visual target identification

Behavior genetic studies of brain activity associated with complex cognitive operations may further elucidate the genetic and physiological underpinnings of basic and complex neural processing. In the present project, monozygotic (N=51 pairs) and dizygotic (N=48 pairs) twins performed a visual oddball task with dense-array EEG. Using spatial PCA, two principal components each were retained for targets and standards; wavelets were used to obtain time-frequency maps of eigenvalue-weighted event-related oscillations for each individual. Distribution of inter-trial phase coherence (ITC) and single trial power (STP) over time indicated that the early principal component was primarily associated with ITC while the later component was associated with a mixture of ITC and STP. Spatial PCA on point-by-point broad sense heritability matrices revealed data-derived frequency bands similar to those well established in EEG literature. Biometric models of eigenvalue-weighted time-frequency data suggest a link between physiology of oscillatory brain activity and patterns of genetic influence.

Developmental changes of BOLD signal correlations with global human EEG power and synchronization during working memory

In humans, theta band (5–7 Hz) power typically increases when performing cognitively demanding working memory (WM) tasks, and simultaneous EEG-fMRI recordings have revealed an inverse relationship between theta power and the BOLD (blood oxygen level dependent) signal in the default mode network during WM. However, synchronization also plays a fundamental role in cognitive processing, and the level of theta and higher frequency band synchronization is modulated during WM. Yet, little is known about the link between BOLD, EEG power, and EEG synchronization during WM, and how these measures develop with human brain maturation or relate to behavioral changes. We examined EEG-BOLD signal correlations from 18 young adults and 15 school-aged children for age-dependent effects during a load-modulated Sternberg WM task. Frontal load (in-)dependent EEG theta power was significantly enhanced in children compared to adults, while adults showed stronger fMRI load effects. Children demonstrated a stronger negative correlation between global theta power and the BOLD signal in the default mode network relative to adults. Therefore, we conclude that theta power mediates the suppression of a task-irrelevant network. We further conclude that children suppress this network even more than adults, probably from an increased level of task-preparedness to compensate for not fully mature cognitive functions, reflected in lower response accuracy and increased reaction time. In contrast to power, correlations between instantaneous theta global field synchronization and the BOLD signal were exclusively positive in both age groups but only significant in adults in the frontal-parietal and posterior cingulate cortices. Furthermore, theta synchronization was weaker in children and was –in contrast to EEG power– positively correlated with response accuracy in both age groups. In summary we conclude that theta EEG-BOLD signal correlations differ between spectral power and synchronization and that these opposite correlations with different distributions undergo similar and significant neuronal developments with brain maturation.

Neural synchrony during response production and inhibition

Inhibition of irrelevant information (conflict monitoring) and/or of prepotent actions is an essential component of adaptive self-organized behavior. Neural dynamics underlying these functions has been studied in humans using event-related brain potentials (ERPs) elicited in Go/NoGo tasks that require a speeded motor response to the Go stimuli and withholding a prepotent response when a NoGo stimulus is presented. However, averaged ERP waveforms provide only limited information about the neuronal mechanisms underlying stimulus processing, motor preparation, and response production or inhibition. In this study, we examine the cortical representation of conflict monitoring and response inhibition using time-frequency analysis of electroencephalographic (EEG) recordings during continuous performance Go/NoGo task in 50 young adult females. We hypothesized that response inhibition would be associated with a transient boost in both temporal and spatial synchronization of prefrontal cortical activity, consistent with the role of the anterior cingulate and lateral prefrontal cortices in cognitive control. Overall, phase synchronization across trials measured by Phase Locking Index and phase synchronization between electrode sites measured by Phase Coherence were the highest in the Go and NoGo conditions, intermediate in the Warning condition, and the lowest under Neutral condition. The NoGo condition was characterized by significantly higher fronto-central synchronization in the 300-600 ms window, whereas in the Go condition, delta- and theta-band synchronization was higher in centro-parietal regions in the first 300 ms after the stimulus onset. The present findings suggest that response production and inhibition is supported by dynamic functional networks characterized by distinct patterns of temporal and spatial synchronization of brain oscillations.

Consolidation of temporal order in episodic memories

Even though it is known that sleep benefits declarative memory consolidation, the role of sleep in the storage of temporal sequences has rarely been examined. Thus we explored the influence of sleep on temporal order in an episodic memory task followed by sleep or sleep deprivation. Thirty-four healthy subjects (17 men) aged between 19 and 28 years participated in the randomized, counterbalanced, between-subject design. Parameters of interests were NREM/REM cycles, spindle activity and spindle-related EEG power spectra. Participants of both groups (sleep group/sleep deprivation group) performed retrieval in the evening, morning and three days after the learning night. Results revealed that performance in temporal order memory significantly deteriorated over three days only in sleep deprived participants. Furthermore our data showed a positive relationship between the ratios of the (i) first NREM/REM cycle with more REM being associated with delayed temporal order recall. Most interestingly, data additionally indicated that (ii) memory enhancers in the sleep group show more fast spindle related alpha power at frontal electrode sites possibly indicating access to a yet to be consolidated memory trace. We suggest that distinct sleep mechanisms subserve different aspects of episodic memory and are jointly involved in sleep-dependent memory consolidation.

Event-related delta, theta, alpha and gamma correlates to auditory oddball processing during Vipassana meditation

Long-term Vipassana meditators sat in meditation vs. a control (instructed mind wandering) states for 25 min, electroencephalography (EEG) was recorded and condition order counterbalanced. For the last 4 min, a three-stimulus auditory oddball series was presented during both meditation and control periods through headphones and no task imposed. Time-frequency analysis demonstrated that meditation relative to the control condition evinced decreased evoked delta (2-4 Hz) power to distracter stimuli concomitantly with a greater event-related reduction of late (500-900 ms) alpha-1 (8-10 Hz) activity, which indexed altered dynamics of attentional engagement to distracters. Additionally, standard stimuli were associated with increased early event-related alpha phase synchrony (inter-trial coherence) and evoked theta (4-8 Hz) phase synchrony, suggesting enhanced processing of the habituated standard background stimuli. Finally, during meditation, there was a greater differential early-evoked gamma power to the different stimulus classes. Correlation analysis indicated that this effect stemmed from a meditation state-related increase in early distracter-evoked gamma power and phase synchrony specific to longer-term expert practitioners. The findings suggest that Vipassana meditation evokes a brain state of enhanced perceptual clarity and decreased automated reactivity.

Event-related potentials and changes of brain rhythm oscillations during working memory activation in patients with first-episode psychosis

BACKGROUND

Earlier contributions have documented significant changes in sensory, attention-related endogenous event-related potential (ERP) components and θ band oscillatory responses during working memory activation in patients with schizophrenia. In patients with first-episode psychosis, such studies are still scarce and mostly focused on auditory sensory processing. The present study aimed to explore whether subtle deficits of cortical activation are present in these patients before the decline of working memory performance.

METHODS

We assessed exogenous and endogenous ERPs and frontal θ event-related synchronization (ERS) in patients with first-episode psychosis and healthy controls who successfully performed an adapted 2-back working memory task, including 2 visual n-backworking memory tasks as well as oddball detection and passive fixation tasks.

RESULTS

We included 15 patients with first-episode psychosis and 18 controls in this study. Compared with controls, patients with first-episode psychosis displayed increased latencies of early visual ERPs and phasic θ ERS culmination peak in all conditions. However, they also showed a rapid recruitment of working memory-related neural generators, even in pure attention tasks, as indicated by the decreased N200 latency and increased amplitude of sustained θ ERS in detection compared with controls.

LIMITATIONS

Owing to the limited sample size, no distinction was made between patients with first-episode psychosis with positive and negative symptoms. Although we controlled for the global load of neuroleptics, medication effect cannot be totally ruled out.

CONCLUSION

The present findings support the concept of a blunted electroencephalographic response in patients with first-episode psychosis who recruit the maximum neural generators in simple attention conditions without being able to modulate their brain activation with increased complexity of working memory tasks.

Event-Related Oscillations in Alcoholism Research: A Review

Alcohol dependence is characterized as a multi-factorial disorder caused by a complex interaction between genetic and environmental liabilities across development. A variety of neurocognitive deficits/dysfunctions involving impairments in different brain regions and/or neural circuitries have been associated with chronic alcoholism, as well as with a predisposition to develop alcoholism. Several neurobiological and neurobehavioral approaches and methods of analyses have been used to understand the nature of these neurocognitive impairments/deficits in alcoholism. In the present review, we have examined relatively novel methods of analyses of the brain signals that are collectively referred to as event-related oscillations (EROs) and show promise to further our understanding of human brain dynamics while performing various tasks. These new measures of dynamic brain processes have exquisite temporal resolution and allow the study of neural networks underlying responses to sensory and cognitive events, thus providing a closer link to the physiology underlying them. Here, we have reviewed EROs in the study of alcoholism, their usefulness in understanding dynamical brain functions/dysfunctions associated with alcoholism as well as their utility as effective endophenotypes to identify and understand genes associated with both brain oscillations and alcoholism.

Spatial attention boosts short-latency neural responses in human visual cortex

In a previous study of visual-spatial attention, Martinez et al. (2007) replicated the well-known finding that stimuli at attended locations elicit enlarged early components in the averaged event-related potential (ERP), which were localized to extrastriate visual cortex. The mechanisms that underlie these attention-related ERP modulations in the latency range of 80-200 ms, however, remain unclear. The main question is whether attention produces increased ERP amplitudes in time-domain averages by augmenting stimulus-triggered neural activity, or alternatively, by increasing the phase-locking of ongoing EEG oscillations to the attended stimuli. We compared these alternative mechanisms using Morlet wavelet decompositions of event-related EEG changes. By analyzing single-trial spectral amplitudes in the theta (4-8 Hz) and alpha (8-12 Hz) bands, which were the dominant frequencies of the early ERP components, it was found that stimuli at attended locations elicited enhanced neural responses in the theta band in the P1 (88-120 ms) and N1 (148-184 ms) latency ranges that were additive with the ongoing EEG. In the alpha band there was evidence for both increased additive neural activity and increased phase-synchronization of the EEG following attended stimuli, but systematic correlations between pre- and post-stimulus alpha activity were more consistent with an additive mechanism. These findings provide the strongest evidence to date in humans that short-latency neural activity elicited by stimuli within the spotlight of spatial attention is boosted or amplified at early stages of processing in extrastriate visual cortex.

Alpha amplitude and phase locking in obsessive-compulsive disorder during working memory

Alpha event-related desynchronization (ERD) and synchronization are known to reflect brain activation and inhibition, respectively. Alpha phase locking seems to reflect the timing in the cortical process. In a previous study, lower alpha ERD was related to working memory in obsessive-compulsive disorder (OCD) patients than in controls during the retention and retrival phases, but not in the encoding phase. However, memory deficits in OCD patients are known to be related to executive failure during the encoding phase. Thus, focusing on the encoding phase, we tested the level of alpha amplitude and phase locking in OCD patients according to memory load. The EEGs of fifteen OCD patients and fifteen controls were recorded during a Sternberg working memory task. The behavioral performance of the OCD patients was normal. However, the OCD group yielded significantly lower ERD and stronger phase locking. As memory load rose, ERD and phase locking significantly increased in both groups. A difference in event-related alpha oscillation was observed in the encoding phase. Lower alpha modulation in the OCD patient simplied abnormality of the excitatory/inhibitory process in the brain, and increased phase locking might reflect excessive attentional excitability.

Event-related activity and phase locking during a psychomotor vigilance task over the course of sleep deprivation

There is profound knowledge that sleep restriction increases tonic (event-unrelated) electroencephalographic (EEG) activity. In the present study we focused on time-locked activity by means of phasic (event-related) EEG analysis during a psychomotor vigilance task (PVT) over the course of sleep deprivation. Twenty healthy subjects (10 male; mean age ± SD: 23.45 ± 1.97 years) underwent sleep deprivation for 24 h. Subjects had to rate their sleepiness hourly (Karolinska Sleepiness Scale) and to perform a PVT while EEG was recorded simultaneously. Tonic EEG changes in the δ (1-4 Hz), θ (4-8 Hz) and α (8-12 Hz) frequency range were investigated by power spectral analyses. Single-trial (phase-locking index, PLI) and event-related potential (ERP) analyses (P1, N1) were used to examine event-related changes in EEG activity. Subjective sleepiness, PVT reaction times and tonic EEG activity (delta and theta spectral power) significantly increased over the night. In contrast, event-related EEG parameters decreased throughout sleep deprivation. Specifically, the ERP component P1 diminished in amplitude, and delta and theta PLI estimates decreased progressively over the night. It is suggested that event-related EEG measures (such as the amplitude of the P1 and especially delta/theta phase-locking) serve as a complimentary method to track the deterioration of attention and performance during sleep loss. As these measures actually reflect the impaired response to specific events rather than tonic changes during sleep deprivation they are a promising tool for future sleep research.

Evoked alpha and early access to the knowledge system: the P1 inhibition timing hypothesis

In this article, a theory is presented which assumes that the visual P1 reflects the same cognitive and physiological functionality as alpha (with a frequency of about 10 Hz).Whereas alpha is an ongoing process, the P1 is the manifestation of an event-related process. It is suggested that alpha and the P1 reflect inhibition that is effective during early access to a complex knowledge system (KS). Most importantly, inhibition operates in two different ways. In potentially competing and task irrelevant networks, inhibition is used to block information processing. In task relevant neural networks, however, inhibition is used to increase the signal to noise ratio (SNR) by enabling precisely timed activity in neurons with a high level of excitation but silencing neurons with a comparatively low level of excitation. Inhibition is increased to modulate the SNR when processing complexity and network excitation increases and when certain types of attentional demands - such as top-down control, expectancy or reflexive attention - increase. A variety of findings are reviewed to demonstrate that they can well be interpreted on the basis of the suggested theory. One interesting aspect thereby is that attentional benefits (reflected e.g., by a larger P1 for attended as compared to unattended items at contralateral sites) and costs (reflected e.g., by a larger P1 at ipsilateral sites) can both be interpreted in terms of inhibition. In the former case an increased P1 is associated with a more effective processing of the presented item (due to an inhibition modulated increase in SNR), in the latter case, however, with a suppression of item processing (due to inhibition that blocks information processing).

Pre-stimulus alpha phase-alignment predicts P1-amplitude

Since years there is a hotly discussed dispute whether event-related potentials are either generated by an evoked component or by resetting of ongoing phase. We argue that phase-reset must not be proven in order to accept the general involvement of phase in ERP-generation as it is only one of several possible mechanisms influencing or generating certain ERP-components. Supporting data are presented showing that positive peaks of ongoing pre-stimulus alpha activity are not randomly distributed in time across trials. Most importantly, we found that a certain kind of pre-stimulus phase concentration that represents a continuous development of an alpha wave up to the time window where the P1 is generated is associated with an enlarged event-related component. We conclude that ongoing oscillations cannot be considered random background noise (even before stimulus onset) and that there are probably more phase-mechanisms that can contribute to the ERP-generation.

Resting state cortical electroencephalographic rhythms in subjects with normal and abnormal body weight

It is well known that resting state regional cerebral blood flow is abnormal in obese when compared to normal-weight subjects but the underlying neurophysiological mechanisms are poorly known. To address this issue, we tested the hypothesis that amplitude of resting state cortical electroencephalographic (EEG) rhythms differ among underweight, normal-weight, and overweight/obese subjects as a reflection of the relationship between cortical neural synchronization and regulation of body weight. Eyes-closed resting state EEG data were recorded in 16 underweight subjects, 25 normal-weight subjects, and 18 overweight/obese subjects. All subjects were psychophysically healthy (no eating disorders or major psychopathologies). EEG rhythms of interest were delta (2-4Hz), theta (4-8Hz), alpha 1 (8-10.5Hz), alpha 2 (10.5-13Hz), beta 1 (13-20Hz), beta 2 (20-30Hz), and gamma (30-40Hz). EEG cortical sources were estimated by low-resolution brain electromagnetic tomography (LORETA). Statistical results showed that parietal and temporal alpha 1 sources fitted the pattern underweight>normal-weight>overweight/obese (p<0.004), whereas occipital alpha 1 sources fitted the pattern normal-weight>underweight>overweight/obese (p<0.00003). Furthermore, amplitude of the parietal, occipital, and temporal alpha 2 sources was stronger in the normal-weight subjects than in the underweight and overweight/obese subjects (p<0.0007). These results suggest that abnormal weight in healthy overweight/obese subjects is related to abnormal cortical neural synchronization at the basis of resting state alpha rhythms and fluctuation of global brain arousal.

Abnormal neural oscillations in schizotypy during a visual working memory task: support for a deficient top-down network?

Neural oscillatory deficits have been proposed to be a core feature of schizophrenia spectrum disorders. In this study we aimed to confirm this by examining early evoked oscillatory patterns in the EEG theta, beta and gamma bands in individuals with high schizotypal personality trait scores. We carried out an event-related experiment using a computerised delayed matching to sample working memory (WM) task on a sample of volunteers scoring high or low on the Schizotypal Personality Questionnaire (SPQ). Phase-locking factor (PLF), a measure of network synchronisation, was reduced in the beta and gamma bands in two distinct topographical regions (fronto-central and centraloccipital). In addition, signal power in the beta band was decreased in the high schizotypy group in the same fronto-occipital network. These findings suggest that abnormalities in functional connectivity, already described in schizophrenia, extend to schizotypy. Further, the pattern and latency of the altered neural oscillations in the high schizotypy group suggests a deficient modulation of the sensory processing by higher-order structures. Such top-down deficits have been reported in schizophrenia and this data supports the idea that top-down dysfunction is a vulnerability trait that is independent of disease course, medication or symptom severity.

Oscillatory brain activity in the time frequency domain associated to change blindness and change detection awareness

Despite the importance of change detection (CD) for visual perception and for performance in our environment, observers often miss changes that should be easily noticed. In the present study, we employed time-frequency analysis to investigate the neural activity associated with CD and change blindness (CB). Observers were presented with two successive visual displays and had to look for a change in orientation in any one of four sinusoid gratings between both displays. Theta power increased widely over the scalp after the second display when a change was consciously detected. Relative to no-change and CD, CB was associated with a pronounced theta power enhancement at parietal-occipital and occipital sites and broadly distributed alpha power suppression during the processing of the prechange display. Finally, power suppressions in the beta band following the second display show that, even when a change is not consciously detected, it might be represented to a certain degree. These results show the potential of time-frequency analysis to deepen our knowledge of the temporal curse of the neural events underlying CD. The results further reveal that the process resulting in CB begins even before the occurrence of the change itself.

Alpha oscillations and early stages of visual encoding

For a long time alpha oscillations have been functionally linked to the processing of visual information. Here we propose an new theory about the functional meaning of alpha. The central idea is that synchronized alpha reflects a basic processing mode that controls access to information stored in a complex long-term memory system, which we term knowledge system in order to emphasize that it comprises not only declarative memories but any kind of knowledge comprising also procedural information. Based on this theoretical background, we assume that during early stages of perception, alpha "directs the flow of information" to those neural structures which represent information that is relevant for encoding. The physiological function of alpha is interpreted in terms of inhibition. We assume that alpha enables access to stored information by inhibiting task-irrelevant neuronal structures and by timing cortical activity in task relevant neuronal structures. We discuss a variety findings showing that evoked alpha and phase locking reflect successful encoding of global stimulus features in an early post-stimulus interval of about 0-150 ms.

Magnetoencephalographic signatures of visual form and motion binding

This study investigates neural magneto-encephalographic (MEG) correlates of visual form and motion binding. Steady-state visual evoked fields (SSVEF) were recorded in MEG while observers reported their bound or unbound perception of moving bars arranged in a square shape. By using pairs of oscillating vertical and horizontal bars, "frequency-tagged" at f1 and f2, we identified a region with enhanced sustained power at 2f1+2f2 intermodulation frequency correlated with perceptual reports. Intermodulation power is more important during perceptual form/motion integration than during the perceptual segmentation of the stimulus into individual component motions, indicating that intermodulation frequency power is a neuromarker of form/motion integration. Source reconstruction of cortical activities at the relevant frequencies further reveals well segregated activity in the occipital lobe at the fundamental of the stimulation, f1 and f2, widely spread activity at 2f1 and 2f2 and a focal activity in the medial part of the right precentral sulcus region at the intermodulation component, 2f1+2f2. The present findings indicate that motion tagging provides a powerful way of investigating the processes underlying visual form/motion binding non-invasively in humans.

The default mode network and EEG α oscillations: an independent component analysis

The default mode network (DMN) has been principally investigated using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) and has received mixed support in electroencephalographic (EEG) studies. In particular, the existing evidence is too inconsistent to allow formulation of specific hypotheses linking DMN activity to traditional EEG frequency bands. In this study, we aimed to test whether blind decomposition methods are able to identify in EEG data spatial patterns resembling the DMN as it is described in PET and fMRI studies. Further we aimed to test a degree of task-relatedness of DMN patterns identified in the traditional EEG frequency bands. To answer these questions we collected data both in a resting state and during performance of two experimental tasks: an explicit judgment of facial affect and a social game task. Individual differences in amount of self-referential thoughts during the resting state were measured by a short self-report scale. Only alpha band spatial patterns simultaneously showed a considerable overlap with the DMN and high correlations with presumptive DMN function-related outcomes both in the resting state and during the social game task. Spontaneous self-referential thoughts were associated with enhanced alpha activity in the posterior DMN hub, whereas processing of DMN function-related external stimuli disrupted this activity and simultaneously caused partial alpha phase-locking to external events. This evidence implies that synchronization of internal mental processes, as opposed to the processing of external stimuli, might be the primary function of alpha oscillations which is bound to be related to activity of the DMN.

Auditory development between 7 and 11 years: an event-related potential (ERP) study

BACKGROUND

There is considerable uncertainty about the time-course of central auditory maturation. On some indices, children appear to have adult-like competence by school age, whereas for other measures development follows a protracted course.

METHODOLOGY

We studied auditory development using auditory event-related potentials (ERPs) elicited by tones in 105 children on two occasions two years apart. Just over half of the children were 7 years initially and 9 years at follow-up, whereas the remainder were 9 years initially and 11 years at follow-up. We used conventional analysis of peaks in the auditory ERP, independent component analysis, and time-frequency analysis.

PRINCIPAL FINDINGS

We demonstrated maturational changes in the auditory ERP between 7 and 11 years, both using conventional peak measurements, and time-frequency analysis. The developmental trajectory was different for temporal vs. fronto-central electrode sites. Temporal electrode sites showed strong lateralisation of responses and no increase of low-frequency phase-resetting with age, whereas responses recorded from fronto-central electrode sites were not lateralised and showed progressive change with age. Fronto-central vs. temporal electrode sites also mapped onto independent components with differently oriented dipole sources in auditory cortex. A global measure of waveform shape proved to be the most effective method for distinguishing age bands.

CONCLUSIONS/SIGNIFICANCE

The results supported the idea that different cortical regions mature at different rates. The ICC measure is proposed as the best measure of 'auditory ERP age'.

Topography, power, and current source density of θ oscillations during reward processing as markers for alcohol dependence

Recent studies have linked alcoholism with a dysfunctional neural reward system. Although several electrophysiological studies have explored reward processing in healthy individuals, such studies in alcohol-dependent individuals are quite rare. The present study examines theta oscillations during reward processing in abstinent alcoholics. The electroencephalogram (EEG) was recorded in 38 abstinent alcoholics and 38 healthy controls as they performed a single outcome gambling task, which involved outcomes of either loss or gain of an amount (10 or 50¢) that was bet. Event-related theta band (3.0-7.0 Hz) power following each outcome stimulus was computed using the S-transform method. Theta power at the time window of the outcome-related negativity (ORN) and positivity (ORP) (200-500 ms) was compared across groups and outcome conditions. Additionally, behavioral data of impulsivity and task performance were analyzed. The alcoholic group showed significantly decreased theta power during reward processing compared to controls. Current source density (CSD) maps of alcoholics revealed weaker and diffuse source activity for all conditions and weaker bilateral prefrontal sources during the Loss 50 condition when compared with controls who manifested stronger and focused midline sources. Furthermore, alcoholics exhibited increased impulsivity and risk-taking on the behavioral measures. A strong association between reduced anterior theta power and impulsive task-performance was observed. It is suggested that decreased power and weaker and diffuse CSD in alcoholics may be due to dysfunctional neural reward circuitry. The relationship among alcoholism, theta oscillations, reward processing, and impulsivity could offer clues to understand brain circuitries that mediate reward processing and inhibitory control.

Inter-individual variability of oscillatory responses to subject's own name. A single-subject analysis

In previous studies event-related potentials and oscillations in response to subject's own name have been analyzed extensively on group-level in healthy subjects and in patients with a disorder of consciousness. Subject's own name as a deviant produces a P3. With equiprobable stimuli, non-phase-locked alpha oscillations are smaller in response to subject's own name compared to other names or subject's own name backwards. However, little is known about replicability on a single-subject level. Seventeen healthy subjects were assessed in an own-name paradigm with equiprobable stimuli of subject's own name, another name, and subject's own name backwards. Event-related potentials and non-phase locked oscillations were analyzed with single-subject, non-parametric statistics. No consistent results were found either for ERPs or for the non-phase locked changes of oscillatory activities. Only 4 subjects showed a robust effect as expected, that is, a lower activity in the alpha-beta range to subject's own name compared to other conditions. Four subjects elicited a higher activity for subject's own name. Thus, analyzing the EEG reactivity in the own-name paradigm with equiprobable stimuli on a single-subject level yields a high variance between subjects. In future research, single-subject statistics should be applied for examining the validity of physiologic measurements in other paradigms and for examining the pattern of reactivity in patients.

Dissociating the effect of noise on sensory processing and overall decision difficulty

It has been proposed that perceptual decision making involves a task-difficulty component, which detects perceptual uncertainty and guides allocation of attentional resources. It is thought to take place immediately after the early extraction of sensory information and is specifically reflected in a positive component of the event related potentials, peaking at ∼ 220 ms after stimulus onset. However, in the previous research, neural processes associated with the monitoring of overall task difficulty were confounded by those associated with the increased sensory processing demands as a result of adding noise to the stimuli. Here we dissociated the effect of phase noise on sensory processing and overall decision difficulty using a face gender categorization task. Task difficulty was manipulated either by adding noise to the stimuli or by adjusting the female/male characteristics of the face images. We found that it is the presence of noise and not the increased overall task difficulty that affects the electrophysiological responses in the first 300 ms following stimulus onset in humans. Furthermore, we also showed that processing of phase-randomized as compared to intact faces is associated with increased fMRI responses in the lateral occipital cortex. These results revealed that noise-induced modulation of the early electrophysiological responses reflects increased visual cortical processing demands and thus failed to provide support for a task-difficulty component taking place between the early sensory processing and the later sensory accumulation stages of perceptual decision making.

What is the Functional Relevance of Prefrontal Cortex Entrainment to Hippocampal Theta Rhythms?

There has been considerable interest in the importance of oscillations in the brain and in how these oscillations relate to the firing of single neurons. Recently a number of studies have shown that the spiking of individual neurons in the medial prefrontal cortex (mPFC) become entrained to the hippocampal (HPC) theta rhythm. We recently showed that theta-entrained mPFC cells lost theta-entrainment specifically on error trials even though the firing rates of these cells did not change (Hyman et al., 2010). This implied that the level of HPC theta-entrainment of mPFC units was more predictive of trial outcome than differences in firing rates and that there is more information encoded by the mPFC on working memory tasks than can be accounted for by a simple rate code. Nevertheless, the functional meaning of mPFC entrainment to HPC theta remains a mystery. It is also unclear as to whether there are any differences in the nature of the information encoded by theta-entrained and non-entrained mPFC cells. In this review we discuss mPFC entrainment to HPC theta within the context of previous results as well as provide a more detailed analysis of the Hyman et al. (2010) data set. This re-analysis revealed that theta-entrained mPFC cells selectively encoded a variety of task-relevant behaviors and stimuli while never theta-entrained mPFC cells were most strongly attuned to errors or the lack of expected rewards. In fact, these error responsive neurons were responsible for the error representations exhibited by the entire ensemble of mPFC neurons. A theta reset was also detected in the post-error period. While it is becoming increasingly evident that mPFC neurons exhibit correlates to virtually all cues and behaviors, perhaps phase-locking directs attention to the task-relevant representations required to solve a spatially based working memory task while the loss of theta-entrainment at the start of error trials may represent a shift of attention away from these representations. The subsequent theta reset following error commission, when coupled with the robust responses of never theta-entrained cells, could produce a potent error-evoked signal used to alert the rat to changes in the relationship between task-relevant cues and reward expectations.

Aberrant EEG responses to gamma-frequency visual stimulation in schizophrenia

Disturbance in the integration of visual information is one of the hallmarks of schizophrenia. In the spatial domain, visual integration is compromised, resulting in impaired perceptual grouping and contour integration. In the time domain, in contrast, visual integration is enhanced, as manifested by increased backward masking and lower ability of patients to detect successively presented visual stimuli as asynchronous. There is much evidence that integrative processes in the brain are supported by dynamic synchronization, or phase-locking, of neural firing. In particular, synchrony in the gamma band (>30 Hz) has been related to local visual information binding whereas synchrony in lower frequencies has been linked to global-scale integration. We recorded EEG signals evoked by steady-state gamma-frequency (40 Hz) photic stimulation in order to directly test the phase-locking of neural responses in schizophrenia. Compared with healthy control subjects, patients showed higher phase-locking of early evoked activity in the gamma band (36-44 Hz) over the posterior cortex, but lower phase-locking in theta (4-8 Hz), alpha (8-13 Hz) and beta (13-24 Hz) frequencies over the anterior cortex. Phase-locking of evoked responses separated schizophrenia and control subjects with accuracy of 86%. This result suggests that schizophrenia is associated with an enhanced early low-level integration in the visual cortex but a deficient high-level integration of visual information within the brain global workspace.

Sensory evoked and event related oscillations in Alzheimer's disease: a short review

Diagnosis and treatment of Alzheimer's disease (AD) depend on clinical evaluation and there is a strong need for an objective tool as a biomarker. Our group has investigated brain oscillatory responses in a small group of AD subjects. We found that the de novo (untreated) AD group differs from both the cholinergically-treated AD group and aged-matched healthy controls in theta and delta responses over left frontal-central areas after cognitive stimulation. On the contrary, the difference observed in AD groups upon a sensory visual stimulation includes response increase over primary or secondary visual sensorial areas compared to controls. These findings imply at least two different neural networks, depending on type of stimulation (i.e. cognitive or sensory). The default mode defined as activity in resting state in AD seems to be affected electrophysiologically. Coherences are also very valuable in observing the group differences, especially when a cognitive stimulus is applied. In healthy controls, higher coherence values are elicited after a cognitive stimulus than after a sensory task. Our findings support the notion of disconnectivity of cortico-cortical connections in AD. The differences in comparison of oscillatory responses upon sensory and cognitive stimulations and their role as a biomarker in AD await further investigation in series with a greater number of subjects.

Rhythmic pulsing: linking ongoing brain activity with evoked responses

The conventional assumption in human cognitive electrophysiology using EEG and MEG is that the presentation of a particular event such as visual or auditory stimuli evokes a “turning on” of additional brain activity that adds to the ongoing background activity. Averaging multiple event-locked trials is thought to result in the cancellation of the seemingly random phased ongoing activity while leaving the evoked response. However, recent work strongly challenges this conventional view and demonstrates that the ongoing activity is not averaged out due to specific non-sinusoidal properties. As a consquence, systematic modulations in ongoing activity can produce slow cortical evoked responses reflecting cognitive processing. In this review we introduce the concept of “rhythmic pulsing” to account for this specific non-sinusoidal property. We will explain how rhythmic pulsing can create slow evoked responses from a physiological perspective. We will also discuss how the notion of rhythmic pulsing provides a unifying framework linking ongoing oscillations, evoked responses and the brain's capacity to process incoming information.

Attention protects the fidelity of visual memory: behavioral and electrophysiological evidence

Recall from visual memory is vulnerable to the influence of task-irrelevant information, including the remembered, prototypical value of stimuli seen previously. Wilken and Ma (2004) proposed that this distortion of recall was actually adaptive, with the task-irrelevant information compensating for imperfections of memory. We tested their proposal by using trial-by-trial oscillations in the electroencephalogram's alpha band (8-14 Hz) collected from human subjects as a marker for the strength of visual attention. Subjects' recall of stimulus spatial frequency showed a systematic error, namely a shift toward the prototypical value of previously seen stimuli. The magnitude of this prototype effect was strongly related to the amplitude of alpha band oscillations recorded at posterior sensor locations during the first 100 ms after stimulus onset. Our results support the hypothesis that the prototype effect is compensatory for imprecision in memory. Moreover, the attentional modulation of alpha activity during the encoding of the target stimulus is consistent with the view that attention sharpens the neural responses that are elicited by the task-relevant stimulus.