Event-related gamma band activity during passive and active oddball tasks

Predisposition to domain-wide maladaptive changes in predictive coding in auditory phantom perception

Tinnitus is hypothesised to be a predictive coding problem. Previous research indicates lower sensitivity to prediction errors (PEs) in tinnitus patients while processing auditory deviants corresponding to tinnitus-specific stimuli. However, based on research with patients with hallucinations and no psychosis we hypothesise tinnitus patients may be more sensitive to PEs produced by auditory stimuli that are not related to tinnitus characteristics. Specifically in patients with minimal to no hearing loss, we hypothesise a more top-down subtype of tinnitus that may be driven by maladaptive changes in an auditory predictive coding network. To test this, we use an auditory oddball paradigm with omission of global and local deviants, a measure that is previously shown to empirically characterise hierarchical prediction errors (PEs). We observe: (1) increased predictions characterised by increased pre-stimulus response and increased alpha connectivity between the parahippocampus, dorsal anterior cingulate cortex and parahippocampus, pregenual anterior cingulate cortex and posterior cingulate cortex; (2) increased PEs characterised by increased P300 amplitude and gamma activity and increased theta connectivity between auditory cortices, parahippocampus and dorsal anterior cingulate cortex in the tinnitus group; (3) increased overall feed-forward connectivity in theta from the auditory cortex and parahippocampus to the dorsal anterior cingulate cortex; (4) correlations of pre-stimulus theta activity to tinnitus loudness and alpha activity to tinnitus distress. These results provide empirical evidence of maladaptive changes in a hierarchical predictive coding network in a subgroup of tinnitus patients with minimal to no hearing loss. The changes in pre-stimulus activity and connectivity to non-tinnitus specific stimuli suggest that tinnitus patients not only produce strong predictions about upcoming stimuli but also may be predisposed to stimulus a-specific PEs in the auditory domain. Correlations with tinnitus-related characteristics may be a biomarker for maladaptive changes in auditory predictive coding.

Broadband Dynamics Rather than Frequency-Specific Rhythms Underlie Prediction Error in the Primate Auditory Cortex

Detection of statistical irregularities, measured as a prediction error response, is fundamental to the perceptual monitoring of the environment. We studied whether prediction error response is associated with neural oscillations or asynchronous broadband activity. Electrocorticography was conducted in three male monkeys, who passively listened to the auditory roving oddball stimuli. Local field potentials (LFPs) recorded over the auditory cortex underwent spectral principal component analysis, which decoupled broadband and rhythmic components of the LFP signal. We found that the broadband component captured the prediction error response, whereas none of the rhythmic components were associated with statistical irregularities of sounds. The broadband component displayed more stochastic, asymmetrical multifractal properties than the rhythmic components, which revealed more self-similar dynamics. We thus conclude that the prediction error response is captured by neuronal populations generating asynchronous broadband activity, defined by irregular dynamic states, which, unlike oscillatory rhythms, appear to enable the neural representation of auditory prediction error response.SIGNIFICANCE STATEMENT This study aimed to examine the contribution of oscillatory and asynchronous components of auditory local field potentials in the generation of prediction error responses to sensory irregularities, as this has not been directly addressed in the previous studies. Here, we show that mismatch negativity-an auditory prediction error response-is driven by the asynchronous broadband component of potentials recorded in the auditory cortex. This finding highlights the importance of nonoscillatory neural processes in the predictive monitoring of the environment. At a more general level, the study demonstrates that stochastic neural processes, which are often disregarded as neural noise, do have a functional role in the processing of sensory information.

Dissociation of Unit Activity and Gamma Oscillations during Vocalization in Primate Auditory Cortex

Vocal production is a sensory-motor process in which auditory self-monitoring is used to ensure accurate communication. During vocal production, the auditory cortex of both humans and animals is suppressed, a phenomenon that plays an important role in self-monitoring and vocal motor control. However, the underlying neural mechanisms of this vocalization-induced suppression are unknown. γ-band oscillations (>25 Hz) have been implicated a variety of cortical functions and are thought to arise from activity of local inhibitory interneurons, but have not been studied during vocal production. We therefore examined γ-band activity in the auditory cortex of vocalizing marmoset monkeys, of either sex, and found that γ responses increased during vocal production. This increase in γ contrasts with simultaneously recorded suppression of single-unit and multiunit responses. Recorded vocal γ oscillations exhibited two separable components: a vocalization-specific nonsynchronized ("induced") response correlating with vocal suppression, and a synchronized ("evoked") response that was also present during passive sound playback. These results provide evidence for the role of cortical γ oscillations during inhibitory processing. Furthermore, the two distinct components of the γ response suggest possible mechanisms for vocalization-induced suppression, and may correspond to the sensory-motor integration of top-down and bottom-up inputs to the auditory cortex during vocal production.SIGNIFICANCE STATEMENT Vocal communication is important to both humans and animals. In order to ensure accurate information transmission, we must monitor our own vocal output. Surprisingly, spiking activity in the auditory cortex is suppressed during vocal production yet maintains sensitivity to the sound of our own voice ("feedback"). The mechanisms of this vocalization-induced suppression are unknown. Here we show that auditory cortical γ oscillations, which reflect interneuron activity, are actually increased during vocal production, the opposite response of that seen in spiking units. We discuss these results with proposed functions of γ activity during inhibitory sensory processing and coordination of different brain regions, suggesting a role in sensory-motor integration.

Cortical Microcircuit Mechanisms of Mismatch Negativity and Its Underlying Subcomponents

In the neocortex, neuronal processing of sensory events is significantly influenced by context. For instance, responses in sensory cortices are suppressed to repetitive or redundant stimuli, a phenomenon termed “stimulus-specific adaptation” (SSA). However, in a context in which that same stimulus is novel, or deviates from expectations, neuronal responses are augmented. This augmentation is termed “deviance detection” (DD). This contextual modulation of neural responses is fundamental for how the brain efficiently processes the sensory world to guide immediate and future behaviors. Notably, context modulation is deficient in some neuropsychiatric disorders such as schizophrenia (SZ), as quantified by reduced “mismatch negativity” (MMN), an electroencephalography waveform reflecting a combination of SSA and DD in sensory cortex. Although the role of NMDA-receptor function and other neuromodulatory systems on MMN is established, the precise microcircuit mechanisms of MMN and its underlying components, SSA and DD, remain unknown. When coupled with animal models, the development of powerful precision neurotechnologies over the past decade carries significant promise for making new progress into understanding the neurobiology of MMN with previously unreachable spatial resolution. Currently, rodent models represent the best tool for mechanistic study due to the vast genetic tools available. While quantifying human-like MMN waveforms in rodents is not straightforward, the “oddball” paradigms used to study it in humans and its underlying subcomponents (SSA/DD) are highly translatable across species. Here we summarize efforts published so far, with a focus on cortically measured SSA and DD in animals to maintain relevance to the classically measured MMN, which has cortical origins. While mechanistic studies that measure and contrast both components are sparse, we synthesize a potential set of microcircuit mechanisms from the existing rodent, primate, and human literature. While MMN and its subcomponents likely reflect several mechanisms across multiple brain regions, understanding fundamental microcircuit mechanisms is an important step to understand MMN as a whole. We hypothesize that SSA reflects adaptations occurring at synapses along the sensory-thalamocortical pathways, while DD depends on both SSA inherited from afferent inputs and resulting disinhibition of non-adapted neurons arising from the distinct physiology and wiring properties of local interneuronal subpopulations and NMDA-receptor function.

Search asymmetry in a serial auditory task: Neural source analyses of EEG implicate attention strategies

Here we report a detailed analysis of the fast network dynamics underlying P3a and P3b event-related potential (ERP) subcomponents generated during an unconventional serial auditory search paradigm. We dissect the electroencephalographic (EEG) data from an earlier study of ours, using a variety of advanced signal processing techniques, in order to discover how the brain is processing auditory targets differently when they possess a rare, salient, unpredictable feature not shared with distractors than when targets lack this feature but distractors have it. We find that brain regions associated with the Ventral Attention Network (VAN) are the primary neural generators of the P3a subcomponent in response to feature-present targets, whereas regions associated with the Dorsal Attention Network (DAN), as well as regions associated with detecting auditory oddball stimuli (ODD), may be the primary neural generators of the P3b, in the context of our study, and perhaps in search paradigms in general. Moreover, measurements of the time courses of oscillatory power changes and inter-regional synchronization in theta and low-gamma frequency bands were consistent with the early activation and synchronization within the VAN associated with the P3a subcomponent, and with the later activation and synchronization within the DAN and ODD networks associated with the P3b subcomponent. Implications of these finding for the mechanisms underlying search asymmetry phenomena are discussed.

Brain activity underlying auditory perceptual learning during short period training: simultaneous fMRI and EEG recording

BACKGROUND

There is an accumulating body of evidence indicating that neuronal functional specificity to basic sensory stimulation is mutable and subject to experience. Although fMRI experiments have investigated changes in brain activity after relative to before perceptual learning, brain activity during perceptual learning has not been explored. This work investigated brain activity related to auditory frequency discrimination learning using a variational Bayesian approach for source localization, during simultaneous EEG and fMRI recording. We investigated whether the practice effects are determined solely by activity in stimulus-driven mechanisms or whether high-level attentional mechanisms, which are linked to the perceptual task, control the learning process.

RESULTS

The results of fMRI analyses revealed significant attention and learning related activity in left and right superior temporal gyrus STG as well as the left inferior frontal gyrus IFG. Current source localization of simultaneously recorded EEG data was estimated using a variational Bayesian method. Analysis of current localized to the left inferior frontal gyrus and the right superior temporal gyrus revealed gamma band activity correlated with behavioral performance.

CONCLUSIONS

Rapid improvement in task performance is accompanied by plastic changes in the sensory cortex as well as superior areas gated by selective attention. Together the fMRI and EEG results suggest that gamma band activity in the right STG and left IFG plays an important role during perceptual learning.

Fractal analysis of EEG upon auditory stimulation during waking and hypnosis in healthy volunteers

The authors tested fluctuation analyses (DFA) of EEGs upon auditory stimulation in waking and hypnotic states as related to topography and hypnotizability. They administered the Hypnotic Induction Profile (HIP), Dissociation Experience Scale, and Tellegen Absorption Scale to 10 healthy volunteers and measured subjects' EEGs while the subjects listened to sounds, either selecting or ignoring tones of different decibels, in waking and hypnotic states. DFA scaling exponents were closest to 0.5 when subjects reported the tones in the hypnotic state. Different DFA values at C3 showed significant positive correlations with the HIP eye-roll sign. Adding to the literature supporting the state theory of hypnosis, the DFA values at F3 and C3 showed significant differences between waking and hypnotic states. Application of auditory stimuli is useful for understanding neurophysiological characteristics of hypnosis using DFA.

Dissociable mechanisms supporting awareness: the P300 and gamma in a linguistic attentional blink task

As demonstrated by the attentional blink (AB) phenomenon, awareness for attended stimuli is governed by sharp capacity limits. We used a linguistic AB task to investigate the neural mechanisms that underlie failures of awareness, examining both event-related potentials and oscillatory brain activity to correctly reported and missed second targets (T2s) presented after a correctly reported first target (T1) in a rapid visual stream of distractors. Correctly reported targets occurring at a short lag (250 ms) after T1-within the classic AB period-elicited enhanced late gamma activity relative to incorrectly reported targets but showed no P300 modulation relative to missed targets. In contrast, correctly reported targets presented at a long lag (830 ms)-outside the classic AB period-elicited a greater P300 component but did not significantly modulate oscillatory activity. This double dissociation suggests that there are multiple neural mechanisms supporting awareness that may operate in parallel. Either the P300 or the gamma can index impairment in the cascade of processing leading to a target's entry into awareness. We conclude that the P300 and gamma activity reflect functionally distinct neural mechanisms, each of which plays an independent role in awareness.

Relationships between pre-stimulus γ power and subsequent P300 and reaction time breakdown in schizophrenia

INTRODUCTION

Little is known about the relationship between gamma-band oscillations prior to the arrival of a target stimulus and subsequent sensory processing and response execution. Although schizophrenia has been associated with abnormalities in gamma-band oscillations, P300, and reaction time (RT), few studies have examined the possible correspondence between these three neurobiological and behavioral markers in schizophrenia. To characterize the relationship between preparatory processes, information processing, and subsequent behavioral performance in schizophrenia, the present study investigated the relationships between pre-stimulus gamma-band power, RT and P300 amplitude.

METHODS

EEG and behavioral data were collected from 18 schizophrenia patients and 21 healthy controls during a conventional auditory oddball task.

RESULTS

In controls, single-trial pre-stimulus gamma power was positively correlated with RT, and average P300 amplitude was positively correlated with average pre-stimulus gamma power.

DISCUSSION

We interpret these findings as evidence that gamma power enhancement reflects a state of greater pre-stimulus preparation resulting in fuller evaluation of the target stimulus and therefore slower RT, as proposed by Jokeit and Makeig (1994). Consistent with previous research, schizophrenia patients exhibited RT slowing and P300 amplitude reductions relative to controls. Importantly, neither RT nor P300 amplitude was related to pre-stimulus gamma power in schizophrenia, suggesting a breakdown in the preparatory brain state critical for stimulus processing and later motor execution. The present findings underscore the behavioral significance of gamma-band responses, and provide an additional link between gamma-band oscillations and information processing abnormalities in schizophrenia.

Dynamics of EEG power spectra at 1-60 Hz during the prestimulus periods at different stages of a cognitive set to a facial expression

Power spectra of cortical potentials at frequencies of 1-60 Hz were assessed in subjects in the state of resting with the eyes open, after listening to instructions, and during the prestimulus periods at the stages of forming and testing a cognitive set to a facial expression. Comparative assessment of power spectra in five frequency ranges (2-7, 8-13, 14-20, 21-40, and 41-60 Hz) showed significant decreases in the power of potentials in the 1-20 Hz frequency band in most subjects, along with a parallel increase in power in the 21-60 Hz band in all states, as compared with the resting state. Interregional differences in power were maximal in the 2-7 and 41-60 Hz bands and minimal in the 14-20 Hz band. In terms of power dynamics in the states being compared, intergroup differences were seen only at the test stage. Thus, a larger proportion of subjects with the plastic type of set had greater power in the gamma frequency range than was the case in subjects with the rigid type of set.

The dependence of P300 amplitude on gamma synchrony breaks down in schizophrenia

INTRODUCTION

Auditory P300 amplitude reduction in schizophrenia is canonical and may be explained by poor synchronization or reduced power of the underlying neural activity. We asked if patients have reduced synchrony and power, and whether together with P300 amplitude, they make unique or overlapping contributions to the discrimination between patients and controls. We also asked whether people who have large P300s have higher power and greater synchrony of neural activity, and if the relationships between P300 and power and synchrony are different in patients and healthy controls.

METHODS

We recorded EEG data from 22 controls and 21 patients with schizophrenia (DSM-IV) while they performed an auditory target detection task. We used wavelet analyses of the single trial data to estimate total power and synchrony of delta, theta, alpha, beta, and gamma activity in a 50ms window around the peak of the P300 to the target. We measured P300 amplitude from the average of the single trials, in a 50ms window around its peak.

RESULTS AND CONCLUSIONS

P300 amplitude and delta and theta synchrony were reduced in patients; delta power and synchrony better distinguished between groups than P300 amplitude. In healthy controls, but not patients, gamma synchrony predicted P300 amplitude. In patients, P300 and gamma synchrony are affected by independent factors; the relationship between them is attenuated by an additional pathophysiological process.

Stimulus intensity affects early sensory processing: Visual contrast modulates evoked gamma-band activity in human EEG

We studied the effect of different contrast levels on the visual evoked gamma-band response (GBR) in order to investigate whether the GBR is modulated in a similar manner as previously reported for visual evoked potentials. Previous studies showed that the GBR can be modulated by individual characteristics (age) and experimental conditions (task difficulty, attention). However, stimulus properties, such as size and spatial frequency, also have a large impact on the GBR, which necessitates identification and control of relevant stimulus properties for optimal experimental setups. Twenty-one healthy participants were investigated during a forced-choice discrimination task. Sinusoidal gratings were presented at three contrast levels with a constant spatial frequency of 5 cycles per degree visual arc (cpd). The present data replicate the results reported for visual evoked potentials and exhibit a contrast dependent modulation of the GBR. Gamma activity is increased for higher contrast levels. These results demonstrate the importance of stimulus contrast for evoked gamma activity. Thus, it appears meaningful to control the contrast of stimuli in experiments investigating the role of gamma activity in perception and information processing.

Binding Symbols and Sounds: Evidence from Event-Related Oscillatory Gamma-Band Activity

The present study intended to examine the neural basis of audiovisual integration, hypothetically achieved by synchronized gamma-band oscillations (30-80 Hz) that have been suggested to integrate stimulus features and top-down information. To that end, we studied the impact of visual symbolic information on early auditory sensory processing of upcoming sounds. In particular, we used a symbol-to-sound-matching paradigm in which simple score-like patterns predict corresponding sound patterns. Occasionally, a single sound is incongruent with the corresponding element of the visual pattern. In response to expected sounds congruent with the corresponding visual symbol, a power increase of phase-locked (evoked) activity in the 40-Hz band was observed peaking 42-ms poststimulus onset. Thus, for the first time, we demonstrated that the comparison process between a neural model, the expectation, and the current sensory input is implemented at very early levels of auditory processing. Subsequently, expected congruent sounds elicited a broadband power increase of non-phase-locked (induced) activity peaking 152-ms poststimulus onset, which might reflect the formation of a unitary event representation including both visual and auditory aspects of the stimulation. Gamma-band responses were not present for unexpected incongruent sounds. A model explaining the anticipatory activation of cortical auditory representations and the match of experience against expectation is presented.

Evoked gamma band response in male adolescent subjects at high risk for alcoholism during a visual oddball task

This study investigates early evoked gamma band activity in male adolescent subjects at high risk for alcoholism (HR; n=68) and normal controls (LR; n=27) during a visual oddball task. A time-frequency representation method was applied to EEG data in order to obtain stimulus related early evoked (phase-locked) gamma band activity (29-45 Hz) and was analyzed within a 0-150 ms time window range. Significant reduction of the early evoked gamma band response in the frontal and parietal regions during target stimulus processing was observed in HR subjects compared to LR subjects. Additionally, the HR group showed less differentiation between target and non-target stimuli in both frontal and parietal regions compared to the LR group, indicating difficulty in early stimulus processing, probably due to a dysfunctional frontoparietal attentional network. The results indicate that the deficient early evoked gamma band response may precede the development of alcoholism and could be a potential endophenotypic marker of alcoholism risk.

Region-specific reduction in entorhinal gamma oscillations and parvalbumin-immunoreactive neurons in animal models of psychiatric illness

Psychiatric illnesses, particularly schizophrenia, are associated with disrupted markers for interneuronal function and interneuron-mediated brain rhythms such as gamma frequency oscillations. Here we investigate a possible link between these two observations in the entorhinal cortex and hippocampus by using a genetic and an acute model of psychiatric illness. Lysophosphatidic acid 1 receptor-deficient (LPA1-deficient) mice show psychomotor-gating deficits and neurochemical changes resembling those seen in postmortem schizophrenia studies. Similar deficits are seen acutely with antagonism of the NMDA subtype of glutamate receptor. Neither model induced any change in power or frequency of gamma rhythms generated by kainate in hippocampal slices. In contrast, a dramatic decrease in the power of gamma oscillations was seen in superficial, but not deep, medial entorhinal cortex layers in both models. Immunolabeling for GABA, parvalbumin, and calretinin in medial entorhinal cortex from LPA1-deficient mice showed an approximately 40% reduction in total GABA- and parvalbumin-containing neurons, but no change in the number of calretinin-positive neurons. This deficit was specific for layer II (LII). No change in the number of neurons positive for these markers was seen in the hippocampus. Acute NMDA receptor blockade, which selectively reduces synaptic drive to LII entorhinal interneurons, also disrupted gamma rhythms in a similar manner in superficial entorhinal cortex, but not in hippocampus. These data demonstrate an area-specific deficit in gamma rhythmogenesis in animal models of psychiatric illness and suggest that loss, or reduction in function, of interneurons having a large NMDA receptor expression may underlie the network dysfunction that is seen.

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

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

EEG gamma-band activity in rapid serial visual presentation

Evidence is available that oscillatory activity in the gamma frequency range (>30 Hz) might be related to the attentional selection of target items. Rapid serial visual presentation (RSVP) paradigms are instrumental in addressing cognitive functions such as visual attention, and they are increasingly combined with the measurement of electrical brain activity. In the present study, gamma-band responses for target and standard stimuli were investigated in an RSVP oddball paradigm. In a first study, stimuli were presented at a frequency of 10 Hz, the stimulus sequence consisted of rare colored letters (targets) and frequent black letters (standards). In addition, stimulus size was varied across experimental blocks. Significant target modulations were observed for the P3 ERP and induced (i.e., not phase-locked) gamma-band responses. Besides this late activation, no further gamma-band responses were observed. A second study aimed at replicating these findings by employing a reduced stimulus presentation rate of 7.1 Hz. Again, besides the P3 ERP a late increase in induced gamma-band activity was observed. However, as compared to Study 1, this induced response was less pronounced. The induced gamma-band response observed in the present studies might reflect utilization of information derived from previous processing steps for behavioral performance or memory storage as suggested in the 'match-and-utilization-model' of gamma activity.

Neurophysiological markers of contextual processing: the relationship between P3b and Gamma synchrony and their modulation by arousal, performance and individual differences

The ability to identify and respond to significant events in the environment is a vital aspect of human cognition and yet is poorly understood as a dynamic neural process. While the response to a contextually-relevant stimulus involves a number of complimentary processes, including selective attention and neural binding, it is also subject to modulation by factors like arousal, age and sex. Adopting an integrative approach, we investigated contextual processing (as indexed by P3b and Gamma phase synchrony) in 120 healthy subjects performing an auditory oddball task while controlling for these other modulating factors. Results suggest a relationship between P3b and Gamma-2 synchrony in posterior regions only, with phasic anterior processing seemingly unrelated to that in posterior regions. However, only the P3b was significantly correlated to central and autonomic arousal. Further, while age and sex were associated with variation in individual measures, they did not strongly affect the relationship between the measures. We concluded that, in simple contextual processing, global and local elements of target stimuli are processed in parallel with little variation being shown between the sexes or resulting from increasing age.

EEG spectral dynamics during discrimination of auditory and visual targets

This study measured the changes in the spectrum of the EEG (electroencephalogram) and in the event-related potentials (ERPs) as subjects detected an improbable target in a train of standard stimuli. The intent was to determine how these measurements are related, and to what extent the ERPs might represent phase-locked changes in EEG rhythms. The experimental manipulations were the stimulus modality (auditory or visual), the discriminability of the target, and the presence or absence of distraction. The ERPs showed sensory-evoked potentials that were specific to the modality and a target-evoked P300 wave that was later in the visual modality than in the auditory, and later and smaller when the discrimination was more difficult. The averaged EEG spectrograms showed that targets increased the frontal theta activity, decreased posterior and central alpha and beta activity, and decreased the central gamma activity. The scalp topography of the changes in the alpha and beta activity indicated a posterior desynchronization specific for the visual task and occurring with both targets and standards and a more widespread desynchronization for targets in either modality. Increased phase synchronization occurred during the event-related potentials, but modeling demonstrated that this can be seen when an evoked potential waveform is simply added to the background EEG. However, subtracting the spectrogram of the average ERP from the average spectrogram of the single trials indicated that phase-resetting of the background EEG rhythms can occur during the ERP. The idea that the ERPs and the EEG rhythms "share generators" can explain these findings.

Identifying neural correlates of consciousness: The state space approach

This article sketches an idealized strategy for the identification of neural correlates of consciousness. The proposed strategy is based on a state space approach originating from the analysis of dynamical systems. The article then focuses on one constituent of consciousness, phenomenal awareness. Several rudimentary requirements for the identification of neural correlates of phenomenal awareness are suggested. These requirements are related to empirical data on selective attention, on completely intrinsic selection and on globally unconscious states. As an example, neuroscientific findings on synchronized gamma activity are categorized according to these requirements.

The BOLD response and the gamma oscillations respond differently than evoked potentials: an interleaved EEG-fMRI study

BACKGROUND

The integration of EEG and fMRI is attractive because of their complementary precision regarding time and space. But the relationship between the indirect hemodynamic fMRI signal and the more direct EEG signal is uncertain. Event-related EEG responses can be analyzed in two different ways, reflecting two different kinds of brain activity: evoked, i.e. phase-locked to the stimulus, such as evoked potentials, or induced, i.e. non phase-locked to the stimulus such as event-related oscillations. In order to determine which kind of EEG activity was more closely related with fMRI, EEG and fMRI signals were acquired together, while subjects were presented with two kinds of rare events intermingled with frequent distractors. Target events had to be signaled by pressing a button and Novel events had to be ignored.

RESULTS

Both Targets and Novels triggered a P300, of larger amplitude in the Novel condition. On the opposite, the fMRI BOLD response was stronger in the Target condition. EEG event-related oscillations in the gamma band (32-38 Hz) reacted in a way similar to the BOLD response.

CONCLUSIONS

The reasons for such opposite differential reactivity between oscillations / fMRI on the one hand, and evoked potentials on the other, are discussed in the paper. Those results provide further arguments for a closer relationship between fast oscillations and the BOLD signal, than between evoked potentials and the BOLD signal.

Is synchronized neuronal gamma activity relevant for selective attention?

Today, much evidence exists that sensory feature binding is accomplished by phase synchronization of induced neuronal gamma activity (30-80 Hz). Recent studies furthermore suggest that phase synchronization of induced gamma activity may represent a general mechanism enabling transient associations of neural assemblies and thus may play a central role in cortical information processing. Here, we describe findings indicating that synchronized gamma activity is moreover specifically involved in selective attention. While feature binding appears to depend primarily on induced gamma synchronization, attentional processes seem to involve both induced and evoked gamma oscillations. Yet it is still an open question, as to which top-down and bottom-up processes are associated with attentional modulation of gamma activity. A possible mechanism to project influences from attentional control structures to areas concerned with stimulus representation and vice versa, may be neuronal synchronization and the resulting firing rate changes of coincidence-detecting neurons in target areas.

Stimulus-related gamma oscillations in primate auditory cortex

With a multielectrode system, we explored neuronal activity in the gamma range (>40 Hz) in the primary and caudomedial auditory cortex of six anesthetized macaque monkeys. Stimuli were tone bursts of 100- to 500-ms duration that were presented at sound pressure levels of 40-60 dB and were varied over a wide range of frequencies. These stimuli induced gamma oscillations, not phase-locked to the onset of stimulation, in 465 of 616 multiunit clusters and at 321 of 422 sites at which field potentials were recorded. Occurrence of gamma activity was stimulus dependent. It was mostly seen when the stimulus was at the units' preferred frequency. The incidence of gamma activity decreased with increasing difference between stimulus frequency and preferred frequency. gamma activity emerged 100-900 ms after stimulus onset with highest incidence ~120 ms. Amplitudes of stimulus-induced gamma oscillations in field potentials were, on average, almost twice the amplitude of spontaneously occurring gamma oscillations. gamma activity at different sites within the primary and the caudomedial auditory field could be synchronized at near-zero phase. Synchrony depended on the spatial distance and on the receptive fields similarity of pairs of units. It decreased with increasing distance between recording sites and increased with similarity of preferred frequencies of the pairs of units. The results indicate that stimulus-induced gamma oscillations originate from sources in the auditory cortex. They further suggest that gamma oscillations may provide a mechanism utilized in many parts of the sensory cortex, including the auditory cortex, to integrate neurons according to the similarity of their receptive fields.

Suppression of EEG gamma activity may cause the attentional blink

The attentional blink (AB) is an impairment of attention, which occurs when subjects have to report a target stimulus (T2) following a previous target (T1) with a short delay (up to 600 ms). Theories explaining the AB assume that processing of T2 is more vulnerable to decay or substitution, as long as attention is allocated to T1. Existing models of the AB, however, do not account for the fact that T2 detection accuracy reaches the minimum when T2 is presented after about 300 ms and not immediately following T1. Therefore, a new model is suggested, which is based on chronometrical considerations together with recent neurophysiological findings concerning the relation between the P3 event-related potential and the AB, the interaction between P3 and gamma oscillations, and the significance of the early evoked gamma band response. We hypothesize that suppression of the early gamma response to T2, accompanying the P3 related to T1, causes the AB.

Band-pass specific contributions of multiple generators to the auditory 40-Hz steady state potentials

OBJECTIVE

The purpose of this study was to separate the composite components of the auditory 40 Hz steady-state potentials (40 Hz SSP), by differentially augmenting them with filtering at different low passes, and to compare them with their counterparts in the transient-evoked auditory middle-latency evoked potentials (AMEP).

METHODS

Transient-evoked AMEP to 3.3/sec clicks and 40 Hz SSP to 40/sec clicks were recorded from 18 subjects using three orthogonally positioned electrode pairs. Each type of potentials was filtered with a 100 Hz and with a 50 Hz low pass. Equivalent dipoles of components were estimated using Three-channel Lissajous' Trajectories and compared between filter settings (50 and 100 Hz low pass) and between the transient-evoked and the steady-state potentials.

RESULTS

With a band pass of 3 to 100 each period of the 40 Hz SSP consisted of a brain stem (V) and four cortical (P0 Na, Pa1 Pa2, and Nb) components. The lower-frequency components of the 40-Hz response corresponded in latency and equivalent dipole orientation to the later transient-evoked cortical AMEP components, whereas the higher-frequency components corresponded to the earlier, brain stem and primary cortical components of transient-evoked AMEP. Band-pass filtering at 3 to 50 Hz resulted in fewer components, as early brain stem and primary cortical components diminished.

CONCLUSIONS

A band pass of 3 to 100 Hz for recording the 40 Hz SSP results in a composite waveform comprising of distinct brain stem and cortical generators with different orientations of their equivalent dipoles. The relative contributions of the multiple constituents are affected by the acquisition filter low pass: brain stem and primary cortical generators mostly contribute the high frequencies and later cortical contributions dominate the lower frequencies.

Time dynamics of stimulus- and event-related gamma band activity: contrast-VEPs and the visual P300 in man

OBJECTIVES

To investigate the time dynamics and phase relationship with the stimulus of the onset/offset visual evoked potentials (VEPs), P300 and gamma band oscillatory responses to visual (contrast) stimulation. Gamma band oscillatory activity mediates in sensory and cognitive operations, with a role in stimulus-related cortical synchronization, but is reportedly reduced in the time window of the P300 response.

METHODS

Ten healthy volunteers were studied. VEPs and P300 were obtained in a stimulus condition combining standard contrast stimulation and a visual odd-ball paradigm. Visual stimuli were gratings with a sinusoidal luminance profile (9.0 degrees central retina; 1.3 cycles/degree; 70% contrast) that were presented monocularly in onset/offset mode, with vertical orientation (frequent stimulus; 80%) or with a 15 degrees rotation to the right (infrequent, target stimulus). The total signal activity (temporal spectral evolution), the activity phase-locked to the stimulus onset (rectified integrated average), and the 'locking index' (ratio of the activity phase-locked to the stimulus to the total signal activity) were computed over time and across frequencies on the signals recorded at occipital (visual responses) and central locations (P300).

RESULTS

Oscillatory activity centered around approximately 20.0-35.0 Hz and phase-locked to the stimulus was recorded at occipital locations with time dynamics anticipating the conventional VEPs. Phase-locking was higher after frequent than in response to target stimuli and after the stimulus offset compared to onset, while the phase-locking of the VEP frequency components was higher after the stimulus onset. The low frequency components of the P300 recorded at Cz (below approximately 8.0-10.0 Hz) were almost totally phase-locked to the stimulus, while the gamma band activity at the P300 location did not vary over time in amplitude or phase-locking and was mostly non-locked to the target stimulus.

CONCLUSIONS

These observations add to the evidence of a role of the gamma band oscillatory responses (centered at approximately 20.0-35.0 Hz) in visual information processing and suggest that the increment in gamma band activity during cognitive operations also depends on task characteristics, vigilance or selective attention, and brain functional state. The visual P300 appears to reflect low frequency synchronization mechanisms.

Mechanisms of human attention: event-related potentials and oscillations

Electrophysiological and hemodynamical responses of the brain allow investigation of the neural origins of human attention. We review attention-related brain responses from auditory and visual tasks employing oddball and novelty paradigms. Dipole localization and intracranial recordings as well as functional magnetic resonance imaging reveal multiple areas involved in generating and modulating attentional brain responses. In addition, the influence of brain lesions of circumscribed areas of the human cortex onto attentional mechanisms are reviewed. While it is obvious that damaged brain tissue no longer functions properly, it has also been shown that functions of non-lesioned brain areas are impaired due to loss of modulatory influence of the lesioned area. Both early (P1 and N1) and late (P3) event-related potentials are modulated by excitatatory and inhibitory mechanisms. Oscillatory EEG-correlates of attention in the alpha and gamma frequency range also show attentional modulation.

Gamma band activity in an auditory oddball paradigm studied with the wavelet transform

OBJECTIVES

To examine the characteristics of evoked and induced gamma band oscillatory responses occurring during P300 development in an auditory oddball paradigm.

METHODS

A time-frequency analysis method was applied to an auditory oddball paradigm in 7 healthy subjects. This method combines a multiresolution wavelet algorithm for signal extraction and the Gabor transform to represent the temporal evolution of the selected frequency components. Phase-locked or evoked activity and also non-phase-locked activity were computed for both standard and target stimuli.

RESULTS

The gamma band frequency components differed between target and non-target stimuli processing. The study showed an early and mainly phase-locked oscillatory response appearing around 26--28 ms after both standard and target stimuli onset. This response showed a spectral peak around 44 Hz for both stimuli. A late oscillatory activity peaking at 37 Hz with a latency around 360 ms was observed appearing only for target stimuli. The latency of this late oscillatory activity had a high correlation (P=0.002) to the latency of the P300 wave.

CONCLUSIONS

EEG signal analysis with wavelet transform allows the identification of an early oscillatory cortical response in the gamma frequency range, as well as a late P300-related response.

Abnormal early stages of task stimulus processing in children with attention-deficit hyperactivity disorder – evidence from event-related gamma oscillations

OBJECTIVES

Attention-related differences in early stages of stimulus processing were assessed in healthy controls and children with attention-deficit hyperactivity disorder (ADHD) by analyzing phase-locked gamma band (31-63 Hz) responses to auditory stimuli in a selective-attention task.

METHODS

A total of 28 children aged 9-12 years (ADHD and matched healthy controls) pressed a button in response to each target stimulus presented at the attended side (right or left). Auditory gamma band responses (GBRs) within 0-120 ms were analyzed at 8 electrodes with wavelet transform. Effects of attended channel, stimulus type, and group were evaluated for GBR power and phase-locking.

RESULTS

For both groups, GBRs had a frontal-central distribution, were significantly larger and more strongly phase-locked to target than to non-target stimuli, and did not differentiate the attended from the unattended channel. ADHD children produced larger and more strongly phase-locked GBRs than controls only to right-side stimuli, irrespective of whether these were the attended or the ignored stimuli.

CONCLUSIONS

The association between auditory GBR and motor task stimulus in children suggests that phase-locked gamma oscillations may reflect processes of sensory-motor integration. ADHD-related deviations of GBRs indicate that early mechanisms of auditory stimulus processing are altered in ADHD, presumably as a result of impaired motor inhibition.

Oscillatory gamma activity in humans: a possible role for object representation

The coherent representation of an object has been suggested to be established by the synchronization in the gamma range (20-100 Hz) of a distributed neural network. So-called '40-Hz' activity in humans could reflect such a mechanism. We have presented here experimental evidence supporting this hypothesis, both in the visual and auditory modalities. However, different types of gamma activity should be distinguished, mainly the evoked 40-Hz response and the induced gamma activities. Only induced gamma activities seem to be related to coherent object representations. In addition, their topography depends on sensory modality and task, which is in line with the idea that they reflect the oscillatory synchronization of task-dependent networks. They can also be functionally and topographically distinguished from the classical evoked potentials and from the alpha rhythm. It was also proposed that the functional role of gamma oscillations is not restricted to object representation established through bottom-up mechanisms of feature binding, but also extends to the cases of internally driven representations and to the maintenance of information in memory.

Flash-related synchronization and desynchronization revealed by a multiple band frequency analysis

The fast Fourier transform (FFT) is a good method to estimate power spectral density (PSD), but the frequency resolution is limited to the sampling window, and thus the precise characteristics of PSD for short signals are not clear. To relax the limitation, a multiple band-pass filter was introduced to estimate the precise course of PSDs for flash visual evoked potentials (VEPs). Signals were recorded during -200 and 600 ms using balanced noncephalic electrodes, and sampled at 1,000 Hz in 12 bits. With 1 Hz and 10 ms resolutions, PSDs were estimated between 10 and 100 Hz. Background powers at the alpha- and beta-bands were high over the posterior scalp, and powers around 200 ms were evoked at the same bands over the same region, corresponding to P110 and N165 of VEPs. Normalized PSDs showed evoked powers around 200 ms and suppressed powers following the evoked powers over the posterior scalp. The evoked powers above the 20 Hz band were not statistically significant, however, the gamma band was significantly evoked intra-individually; details in the gamma bands were varied among the subjects. Details of PSDs were complicated even for a simple task such as watching flashes; both synchronization and desynchronization occurred with different distributions and different time courses.

Gamma band response in children is related to task-stimulus processing

The present study described the functional characteristics of early phase-locked electroencephalographic gamma band (30-60 Hz) responses (GBRs) in 9- to 12-year-old healthy children. GBRs were elicited in an auditory selective-attention task. Target stimuli required motor responding when presented to the right or to the left (attended side). Effects of stimulus type relevance and attended side were evaluated for GBR power and phase-locking within 0-120 ms. GBRs in children were frontally distributed, were larger and better phase-locked to targets relative to non-targets, but did not depend on the attended side. These results demonstrate that the auditory GBR is related to task-stimulus processing and imply that early target selection in children is guided by a sensory or sensorimotor preparatory set, rather than by an internal attentional focus to the side of stimulation.

Gamma and beta frequency oscillations in response to novel auditory stimuli: A comparison of human electroencephalogram (EEG) data with in vitro models

Investigations using hippocampal slices maintained in vitro have demonstrated that bursts of oscillatory field potentials in the gamma frequency range (30-80 Hz) are followed by a slower oscillation in the beta 1 range (12-20 Hz). In this study, we demonstrate that a comparable gamma-to-beta transition is seen in the human electroencephalogram (EEG) in response to novel auditory stimuli. Correlations between gamma and beta 1 activity revealed a high degree of interdependence of synchronized oscillations in these bands in the human EEG. Evoked (stimulus-locked) gamma oscillations preceded beta 1 oscillations in response to novel stimuli, suggesting that this may be analogous to the gamma-to-beta shift observed in vitro. Beta 1 oscillations were the earliest discriminatory responses to show enhancement to novel stimuli, preceding changes in the broad-band event-related potential (mismatch negativity). Later peaks of induced beta activity over the parietal cortex were always accompanied by an underlying gamma frequency oscillation as seen in vitro. A further analogy between in vitro and human recordings was that both gamma and beta oscillations habituated markedly after the initial novel stimulus presentation.

Peak gamma latency correlated with reaction time in a conventional oddball paradigm

OBJECTIVE

To determine whether a relationship existed across trials between evoked gamma and reaction time in a conventional cognitive ERP paradigm.

METHODS

Auditory oddball data in 40 normal subjects was examined. A moving Welch window and short time FFT were employed to elucidate the time course of gamma (29 to 45 Hz) activity in each single-trial target ERP. The time point with maximum total gamma amplitude (across all sites) was identified automatically, and the associated amplitude, power and latency of this peak was determined. Spearman's rank correlation was employed to examine the relationship of these measures with reaction time.

RESULTS

A highly significant and robust correlation between peak gamma latency and reaction time was found (P = 0.000001). Further frequency analysis revealed that this relationship was primarily confined to the 37 to 41 Hz band. This narrow band finding, coupled with the fact that the finding remained after exclusion of possibly EMG contaminated epochs, indicated that the effect was not due to EMG contamination. No relationship between peak gamma amplitude or power and reaction time was found.

CONCLUSIONS

Peak evoked gamma latency was correlated with reaction time and seems likely to have functional significance in relation to stimulus processing.

The phase-locking of auditory gamma band responses in humans is sensitive to task processing

The present study assessed the effects of stimulus task-relevance and certainty on early and late 40 Hz (gamma) band responses (GBRs) in humans. Auditory GBRs of nine young adults were recorded in passive listening, simple reaction task, and choice-reaction task (target probability = 0.5) conditions and evaluated in three consecutive post-stimulus periods (0-120, 120-250, 250-400 ms) corresponding to the serial occurrence of gamma oscillation bursts. Amplitude and phase-locking of GBRs within these bursts were analysed separately at the level of single sweeps by applying a method that allows the independent quantification of between-sweep synchronization. Major results showed that the effects of stimulus certainty and task relevance on single response amplitude were specific and different from the effects on the phase-locking. The functional involvement of the early and late auditory gamma responses was distinct: early auditory gamma band responses appear primarily associated with focused attention, while the late gamma responses vary with motor-task relevance. It is concluded that along with power measures, the stability of phase-locking of gamma band responses should be regarded as a functionally meaningful parameter that varies with processing demands and recording site.

Synchronization of neuronal activity during stimulus expectation in a direction discrimination task

The dorsal pathway of the primate brain, especially the middle temporal area (MT or V5) and the superior middle temporal area (MST or V5a), is strongly involved in motion detection. The relation between neural firing rates and psychophysical performance has led to the assumption that the neural code used by these areas consists of the relative discharge rates of neuronal populations. As an additional neural code, temporal correlation of neural activity has been suggested. Our study addresses the involvement of such a code in awake monkeys performing a motion discrimination task. We found significant temporal correlations between simultaneously recorded pairs of units in areas MT and MST and other extrastriate cortical areas. Units recorded from the same electrode were more frequently synchronized than units recorded from different electrodes placed within the same or different cortical areas. Activity synchronization was present in the expectation period before stimulus presentation and could not be induced de novo by the stimulus. Rather, we found a contrast-dependent reduction of correlation strength on stimulus onset. Correlation strength did not vary systematically with stimulus directions. We conclude that under the conditions of this study, temporal decorrelation of MT and MST neurons could be used to detect the stimulus, but synchronization does not convey specific information about its direction of motion and therefore is unlikely to contribute to performance in our direction discrimination task. Activity synchronization in the period before stimulus onset could be related to attentive expectation.

Effects of task variables on the amplitude and phase-locking of auditory gamma band responses in human

The present study was designed to assess the effects of stimulus certainty and motor task-relevance on auditory transient 40-Hz or gamma band responses. To study the effects of these factors in a balanced design, auditory event-related potentials (ERPs) of 9 young adults were recorded in a passive listening, simple reaction task, and choice-reaction task (target tone probability = 0.5) conditions. Amplitude and phase-locking of event-related gamma activity were analyzed separately at the level of single sweeps by applying a method that allows the independent quantification of phase synchronization between consecutive single responses. Major results demonstrated that (1) During auditory stimulus processing discernible gamma oscillation bursts were observed in three time windows of the poststimulus epoch: early (0-120) ms, middle (120-250 ms) and late (250-400 ms). (2) Early gamma response amplitudes were significantly largest for highly expected motor-task stimuli, whereas the phase-locking did not depend on either of the two variables. (3) The phase-locking of late gamma responses, however, was significantly stronger to targets than to nontargets. These results indicate that auditory gamma responses are functionally involved in the processing of task variables such as stimulus certainty and motor-task relevance. It is also demonstrated that single gamma response amplitude and phase-locking have independent functional significance as being affected in a different manner by different task conditions.