Oscillatory EEG correlates of episodic trace decay

Similarities in the neural signature for the processing of behaviorally categorized and uncategorized speech sounds

Recent human behavioral studies have shown semantic and/or lexical processing for stimuli presented below the auditory perception threshold. Here, we investigated electroencephalographic responses to words, pseudo-words and complex sounds, in conditions where phonological and lexical categorizations were behaviorally successful (categorized stimuli) or unsuccessful (uncategorized stimuli). Data showed a greater decrease in low-beta power at left-hemisphere temporal electrodes for categorized non-lexical sounds (complex sounds and pseudo-words) than for categorized lexical sounds (words), consistent with the signature of a failure in lexical access. Similar differences between lexical and non-lexical sounds were observed for uncategorized stimuli, although these stimuli did not yield evoked potentials or theta activity. The results of the present study suggest that behaviorally uncategorized stimuli were processed at the lexical level, and provide evidence of the neural bases of the results observed in previous behavioral studies investigating auditory perception in the absence of stimulus awareness.

The posterior medial cortex is involved in visual but not in verbal memory encoding processing: an intracerebral recording study

The objective is to study the involvement of the posterior medial cortex (PMC) in encoding and retrieval by visual and auditory memory processing. Intracerebral recordings were studied in two epilepsy-surgery candidates with depth electrodes implanted in the retrosplenial cingulate, precuneus, cuneus, lingual gyrus and hippocampus. We recorded the event-related potentials (ERP) evoked by visual and auditory memory encoding-retrieval tasks. In the hippocampus, ERP were elicited in the encoding and retrieval phases in the two modalities. In the PMC, ERP were recorded in both the encoding and the retrieval visual tasks; in the auditory modality, they were recorded in the retrieval task, but not in the encoding task. In conclusion, the PMC is modality dependent in memory processing. ERP is elicited by memory retrieval, but it is not elicited by auditory encoding memory processing in the PMC. The PMC appears to be involved not only in higher-order top-down cognitive activities but also in more basic, rather than bottom-up activities.

A short review of alpha activity in cognitive processes and in cognitive impairment

AIM OF THE REPORT: In the companion report (Başar, this volume), the physiological fundaments of alpha activity in integrative brain function are described. The present report is a review of the significant role of alpha activity in memory and cognitive processes in healthy subjects, and in cognitive impairment. The role of neurotransmitters is also described, briefly, in this context. TOWARDS AN UNDERSTANDING OF BRAIN ALPHA: Despite numerous experimental studies, it is indicated that the presented results are only appropriate to establish an ensemble of reasonings and suggestions for analyzing "alphas" in the whole brain. In turn, in the near future, these reasonings and suggestions may serve (or are indispensable to serve) as fundaments of more general and tenable hypotheses on the genesis and function of "alphas".

Eating breakfast enhances the efficiency of neural networks engaged during mental arithmetic in school-aged children

To determine the influence of a morning meal on complex mental functions in children (8-11 y), time-frequency analyses were applied to electroencephalographic (EEG) activity recorded while children solved simple addition problems after an overnight fast and again after having either eaten or skipped breakfast. Power of low frequency EEG activity [2 Hertz (Hz) bands in the 2-12 Hz range] was determined from recordings over frontal and parietal brain regions associated with mathematical thinking during mental calculation of correctly answered problems. Analyses were adjusted for background variables known to influence or reflect the development of mathematical skills, i.e., age and measures of math competence and math fluency. Relative to fed children, those who continued to fast showed greater power increases in upper theta (6-8 Hz) and both alpha bands (8-10 Hz; 10-12 Hz) across sites. Increased theta suggests greater demands on working memory. Increased alpha may facilitate task-essential activity by suppressing non-task-essential activity. Fasting children also had greater delta (2-4 Hz) and greater lower-theta (4-6 Hz) power in left frontal recordings-indicating a region-specific emphasis on both working memory for mental calculation (theta) and activation of processes that suppress interfering activity (delta). Fed children also showed a significant increase in correct responses while children who continued to fast did not. Taken together the findings suggest that neural network activity involved in processing numerical information is functionally enhanced and performance is improved in children who have eaten breakfast, whereas greater mental effort is required for this mathematical thinking in children who skip breakfast.

Cognit activation: a mechanism enabling temporal integration in working memory

Working memory is critical to the integration of information across time in goal-directed behavior, reasoning and language, yet its neural substrate is unknown. Based on recent research, we propose a mechanism by which the brain can retain working memory for prospective use, thereby bridging time in the perception/action cycle. The essence of the mechanism is the activation of 'cognits', which consist of distributed, overlapping and interactive cortical networks that in the aggregate encode the long-term memory of the subject. Working memory depends on the excitatory reentry between perceptual and executive cognits of posterior and frontal cortices, respectively. Given the pervasive role of working memory in the structuring of purposeful cognitive sequences, its mechanism looms essential to the foundation of behavior, reasoning and language.

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.

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.

Visual P2-N2 complex and arousal at the time of encoding predict the time domain characteristics of amnesia for multiple intravenous anesthetic drugs in humans

BACKGROUND

Intravenous anesthetics have marked effects on memory function, even at subclinical concentrations. Fundamental questions remain in characterizing anesthetic amnesia and identifying affected system-level processes. The authors applied a mathematical model to evaluate time-domain components of anesthetic amnesia in human subjects.

METHODS

Sixty-one volunteers were randomized to receive propofol (n = 12), thiopental (n = 13), midazolam (n = 12), dexmedetomidine (n = 12), or placebo (n = 12). With drug present, subjects encoded pictures into memory using a 375-item continuous recognition task, with subsequent recognition later probed with drug absent. Memory function was sampled at up to 163 time points and modeled over the time domain using a two-parameter, first-order negative power function. The parietal event-related P2-N2 complex was derived from electroencephalography, and arousal was repeatedly sampled. Each drug was evaluated at two concentrations.

RESULTS

The negative power function consistently described the course of amnesia (mean R = 0.854), but there were marked differences between drugs in the modulation of individual components (P < 0.0001). Initial memory strength was a function of arousal (P = 0.005), whereas subsequent decay was related to the reaction time (P < 0.0001) and the P2-N2 complex (P = 0.007/0.002 for discrete components).

CONCLUSIONS

In humans, the amnesia caused by multiple intravenous anesthetic drugs is characterized by arousal-related effects on initial trace strength, and a subsequent decay predicted by attenuation of the P2-N2 complex at encoding. The authors propose that the failure of normal memory consolidation follows drug-induced disruption of interregional synchrony critical for neuronal plasticity and discuss their findings in the framework of memory systems theory.

Low frequency oscillations in rat posterior parietal cortex are differentially activated by cues and distractors

The posterior parietal cortex (PPC) is hypothesized to detect visual cues among competing distractors. Anatomical and neurophysiologic evidence indicates that the rat PPC is part of a network of brain areas involved in directed attention, specifically when new task parameters or conditions are introduced. Here, we test the hypothesis that changes in the local field potential (LFP) of the PPC of rats performing a sustained attention task reflect aspects of detection. Two event-related potentials were observed during detection: the P300 response and the contingent negative variation (CNV). Spectrogram analysis also indicated a detection-specific increase in alpha power in the retention interval of this task. This is consistent with observations from human studies, which indicate that tasks requiring a subject to withhold a response produced a pronounced synchronization of alpha rhythms during the delay, and desynchronization during retrieval. We also found cycles of alpha synchrony and desynchrony in response to a periodic distractor. These cycles were most pronounced in the initial trial block of the distractor when the false alarm rate was highest, and as task performance improved these cycles significantly diminished. This result suggests that alpha cycling in the PPC represent neural activity critical for learning to inhibit distractors. The occurrence of alpha synchronization and desynchronization to attention-demanding stimuli, in addition to the P300 and CNV responses observed during detection, is evidence that rat PPC is involved in sustained attention, particularly in the presence of distractors.

Control mechanisms in working memory: a possible function of EEG theta oscillations

Neural correlates of control mechanisms in human working memory are discussed at two levels in this review: (i) at 'item level', where in multi-item working memory information needs to be organized into sequential memory representations, and (ii) at a 'process level', indicating the integration and control of a variety of cognitive functions involved in working memory, independent of item representations per se. It will be discussed that at both levels electroencephalographic theta activity is responsible for control of working memory functions. On item level, exact phase coding, e.g., approached by coupling between theta and gamma oscillations or phase resetting of theta frequency, is suggested to integrate information into working memory representations. At process level interregional theta synchronization is discussed to integrate brain structures necessary for working memory. When discussing the specificity of theta activity for control of working memory processes it will be suggested that theta oscillations might play an important general integrative role in organization of brain activity. And as working memory often involves a variety of cognitive processes which need to be coordinated there is particular need for an integrative brain mechanism like theta activity as suggested in this review.

Theta oscillations during holeboard training in rats: different learning strategies entail different context-dependent modulations in the hippocampus

A functional connection between theta rhythms, information processing, learning and memory formation is well documented by studies focusing on the impact of theta waves on motor activity, global context or phase coding in spatial learning. In the present study we analyzed theta oscillations during a spatial learning task and assessed which specific behavioral contexts were connected to changes in theta power and to the formation of memory. Therefore, we measured hippocampal dentate gyrus theta modulations in male rats that were allowed to establish a long-term spatial reference memory in a holeboard (fixed pattern of baited holes) in comparison to rats that underwent similar training conditions but could not form a reference memory (randomly baited holes). The first group established a pattern specific learning strategy, while the second developed an arbitrary search strategy, visiting increasingly more holes during training. Theta power was equally influenced during the training course in both groups, but was significantly higher when compared to untrained controls. A detailed behavioral analysis, however, revealed behavior- and context-specific differences within the experimental groups. In spatially trained animals theta power correlated with the amounts of reference memory errors in the context of the inspection of unbaited holes and exploration in which, as suggested by time frequency analyses, also slow wave (delta) power was increased. In contrast, in randomly trained animals positive correlations with working memory errors were found in the context of rearing behavior. These findings indicate a contribution of theta/delta to long-lasting memory formation in spatially trained animals, whereas in pseudo trained animals theta seems to be related to attention in order to establish trial specific short-term working memory. Implications for differences in neuronal plasticity found in earlier studies are discussed.

Event-related desynchronization/synchronization during discrimination task conditions in patients with Parkinson's disease

Parkinson's disease is a neurodegenerative disorder with symptoms, which include movement disturbances and changes of cognitive information processing. The aim of the present study was to investigate the functional relationships between oscillatory electroencephalographic (EEG) dominant components with event-related desynchronization/synchronization (ERD/ERS) method for idiopathic non-demented Parkinson's patients (PP) and control subjects (CS) during auditory discrimination tasks within two post-stimulus intervals of 0-250 and 250-600 ms. When comparing the CS and PP during the first post-stimulus period, we found delta- and theta-ERS significantly pronounced in CS for both tone types (low--800, high--1,000 Hz) with the following exceptions: at Fz, PP displayed higher delta-ERS, while at C3' theta-ERD in response to a high tone. Alpha-ERS was found in PP in response to either tone at all electrodes and mainly alpha-ERD in CS. In the second post-stimulus interval, the significant differences between the groups were: (i) delta-ERS in CS and delta-ERD in PP in response to the low tone and (ii) delta-ERS for both groups in answer to the high tone, more prominent in CS at Cz and Pz, except for delta-ERD in PP at C3'. For both groups, we detected predominantly theta-ERD and alpha-ERD following both tone types within this second interval. PP showed more expressed theta-ERD at Fz and parietal theta-ERS. Alpha-ERD was significantly higher in CS, while frontal alpha-ERD was more prominent in the PP in response to both tones. The data obtained showed specific functional differences of event-related oscillatory activity in cognitive and sensory-motor information processing between the PP and CS.

Oscillatory correlates of retrieval-induced forgetting in recognition memory

Retrieval practice on a subset of previously studied material enhances later memory for practiced material but can inhibit memory for related unpracticed material. The present study examines the effects of prior retrieval practice on evoked (ERPs) and induced (oscillatory power) measures of electrophysiological activity underlying recognition of practiced and unpracticed words. Compared to control material, recognition of unpracticed words was characterized by reduced amplitudes of the P2 ERP component and by reduced early (200-400 msec) oscillatory theta power. The reduction in P2 amplitude was associated with decreased evoked theta power but not with decreased theta phase locking (phase-locking index). Recognition of unpracticed material was further accompanied by a reduction in occipital gamma power (>250 msec). In contrast, the beneficial effects of retrieval practice on practiced words were reflected by larger parietal ERP positivity (>500 msec) and by a stronger decrease in oscillatory alpha power in a relatively late time window (>700 msec). The results suggest that the beneficial and detrimental effects of retrieval practice are mediated by different processes. In particular, they suggest that reduced theta (4-7 Hz) and gamma (60-90 Hz) power reflect the specific effects of inhibitory processes on the unpracticed material's memory representation.

Repetition suppression and reactivation in auditory-verbal short-term recognition memory

The neural response to stimulus repetition is not uniform across brain regions, stimulus modalities, or task contexts. For instance, it has been observed in many functional magnetic resonance imaging (fMRI) studies that sometimes stimulus repetition leads to a relative reduction in neural activity (repetition suppression), whereas in other cases repetition results in a relative increase in activity (repetition enhancement). In the present study, we hypothesized that in the context of a verbal short-term recognition memory task, repetition-related "increases" should be observed in the same posterior temporal regions that have been previously associated with "persistent activity" in working memory rehearsal paradigms. We used fMRI and a continuous recognition memory paradigm with short lags to examine repetition effects in the posterior and anterior regions of the superior temporal cortex. Results showed that, consistent with our hypothesis, the 2 posterior temporal regions consistently associated with working memory maintenance, also show repetition increases during short-term recognition memory. In contrast, a region in the anterior superior temporal lobe showed repetition suppression effects, consistent with previous research work on perceptual adaptation in the auditory-verbal domain. We interpret these results in light of recent theories of the functional specialization along the anterior and posterior axes of the superior temporal lobe.

Theta and Gamma Coherence Along the Septotemporal Axis of the Hippocampus

Theta and gamma rhythms synchronize neurons within and across brain structures. Both rhythms are widespread within the hippocampus during exploratory behavior and rapid-eye-movement (REM) sleep. How synchronous are these rhythms throughout the hippocampus? The present study examined theta and gamma coherence along the septotemporal (long) axis of the hippocampus in rats during REM sleep, a behavioral state during which theta signals are unaffected by external sensory input or ongoing behavior. Unilateral entorhinal cortical inputs are thought to play a prominent role in the current generation of theta, whereas current generation of gamma is primarily due to local GABAergic neurons. The septal 50% (4–5 mm) of the dentate gyrus (DG) receives a highly divergent, unilateral projection from any focal point along a lateral band of entorhinal neurons near the rhinal sulcus. We hypothesized that theta coherence in the target zone (septal DG) of this divergent entorhinal input would not vary, while gamma coherence would significantly decline with distance in this zone. However, both theta and gamma coherence decreased significantly along the long axis in the septal 50% of the hippocampus across both DG and CA1 electrode sites. In contrast, theta coherence between homotypic (e.g., DG to DG) sites in the contralateral hemisphere (∼3–5 mm distant) were quite high (∼0.7–0.9), much greater than theta coherence between homotypic sites 3–5 mm distant (∼0.4–0.6) along the long axis. These findings define anatomic variation in both rhythms along the longitudinal axis of the hippocampus, indicate the bilateral CA3/mossy cell projections are the major determinant of theta coherence during REM, and demonstrate that theta coherence varies as a function of anatomical connectivity rather than physical distance. We suggest CA3 and entorhinal inputs interact dynamically to generate theta field potentials and advance the utility of theta and gamma coherence as indicators of hippocampal dynamics.

Theta oscillations during the processing of monetary loss and gain: a perspective on gender and impulsivity

Event-related oscillations (EROs) have proved to be very useful in the understanding of a variety of neurocognitive processes including reward/outcome processing. In the present study, theta power (4.0-7.0 Hz) following outcome stimuli in the time window of the N2-P3 complex (200-500 ms) was analyzed in healthy normals (20 males and 20 females) while performing a gambling task that involved monetary loss and gain. The main aim was to analyze outcome processing in terms of event-related theta power in the context of valence, amount, gender, and impulsivity. The S-transform was used for the signal processing of the ERO data in terms of time-frequency-power. Results from filtered waveforms showed a partially consistent phase-alignment of the increased theta activity corresponding to N2 and P3 components following the outcome stimuli. Gain conditions produced more theta power than loss conditions. While there was anterior involvement in both gain and loss, posterior activation was stronger during gain conditions than during loss conditions. Females exhibited posterior maxima during gain conditions while males had an anterior maxima during both loss and gain conditions. The current source density of theta activity in females involved larger areas with a bilateral frontal activity while males predominantly had a frontal midline activity. Theta power was significantly higher in females than males across all conditions. Low theta (4.0-5.5 Hz) predominantly contributed to the posterior activity during gain conditions. High theta (5.5-7.0 Hz) was more associated with impulsivity measures than low theta activity. These findings may offer valuable clues to understand outcome processing, impulsivity, and gender differences.

Event-related delta oscillatory responses of Alzheimer patients

BACKGROUND AND PURPOSE

Alzheimer type of dementia (AD) is the most common neuropsychiatric morbidity in elderly individuals. Event-related oscillations (ERO) provide an useful tool for detecting subtle abnormalities of cognitive processes with high temporal resolution.

METHODS

In the present report, event-related oscillations of patients with AD were analyzed by using a visual oddball paradigm. A total of 22 mild probable AD subjects according to NINCDS-ADRDA criteria and 20 age-, gender-, and education-matched healthy control subjects were compared. AD group consisted from 11 untreated patients and 11 patients treated with cholinesterase inhibitor. Oscillatory responses were recorded from 13 scalp electrodes.

RESULTS

Significant differences in delta frequency range were seen between the groups by using repeated measures of anova analysis [F(9.120) = 2.228; P = 0.022]. Post-hoc analyses using Wilcoxon test showed that at mid- and left central regions, (Cz, C3) peak amplitudes of delta responses of healthy subjects were significantly higher than either group. Also cholinesterase inhibitors did not have effect on delta oscillatory responses.

CONCLUSIONS

Our findings imply that the delta oscillatory responses at central locations are highly instable in mild probable AD patients regardless of treatment when compared to the healthy aged controls. This study supports the importance of oscillatory event-related potentials for investigating AD brain dynamics.

The electrophysiological dynamics of interference during the Stroop task

If subjects are required to name the color of the word red printed in blue ink, interference between word meaning and ink color occurs, which slows down reaction time. This effect is well known as the Stroop effect. It is still an unresolved issue how the brain deals with interference in this type of task. To explore this question, an electroencephalogram (EEG) study was carried out. By analyzing several measures of EEG activity, two main findings emerged. First, the event-related potential (ERP) showed increased fronto-central negativity in a time window around 400 msec for incongruent items in contrast to congruent and neutral items. Source localization analysis revealed that a source in the anterior cingulate cortex (ACC) contributed most to the difference. Second, time-frequency analysis showed that theta oscillations (4-7 Hz) in the ACC increased linearly with increasing interference and that phase coupling between the ACC and the left prefrontal cortex was longer persistent for incongruent items compared to congruent and neutral items. These effects occurred at a time window around 600 msec. We conclude that interference between color naming and word meaning in the Stroop task manifests itself at around 400 msec and mainly activates the ACC. Thereafter, sustained phase coupling between the ACC and the prefrontal cortex occurs, which most likely reflects the engagement of cognitive control mechanisms.

Dissociation between MEG alpha modulation and performance accuracy on visual working memory task in obsessive compulsive disorder

Oscillatory brain activity in the alpha band (8-13 Hz) is modulated by cognitive events. Such modulation is reflected in a decrease of alpha (event-related desynchronization; ERD) with high cognitive load, or an increase (event-related synchronization) with low cognitive demand or with active inhibition of distractors. We used magnetoencephalography to investigate the pattern of prefrontal and parieto-occipital alpha modulation related to two variants of visual working memory task (delayed matching-to-sample) with and without a distractor. We tested nonmedicated, nondepressed patients suffering obsessive-compulsive disorder (OCD), and pair-matched healthy controls. The level of event-related alpha as a function of time was estimated using the temporal-spectral evolution technique. The results in OCD patients indicated: (1) a lower level of prestimulus (reference) alpha when compared to controls, (2) a task-phase specific reduction in event-related alpha ERD in particular for delayed matching-to-sample task with distractor, (3) no significant correlations between the pattern of modulation in prefrontal and parietal-occipital alpha oscillatory activity. Despite showing an abnormally low alpha modulation, the OCD patients' performance accuracy was normal. The results suggest a relationship of alpha oscillations and the underlying thalamocortical network to etiology of OCD and an involvement of a compensatory mechanism related to effortful inhibition of extrinsic and intrinsic interference.

Source-retrieval requirements influence late ERP and EEG memory effects

The present study examined whether event-related potential (ERP) memory effects and measures of ongoing EEG activity (power and phase locking) are sensitive to varying source retrieval requirements in recognition memory. ERP old/new effects were obtained in two distinct source-memory tasks. Functionally related EEG power and phase locking effects were found in the delta and theta frequency range. A late posterior negativity (LPN) was larger for old than new responses irrespective of source accuracy. It was also larger when participants were required to judge how they had previously interacted with a recognized picture as compared to judging its study location. This result is consistent with the view that the LPN reflects processes in the service of reconstructing previous episodes by integrating recognized items with task-relevant contextual attributes, and that LPN amplitude is related to the amount of contextual features available for forming such an integrated representation. Phase locking of ongoing delta and theta activity (but not EEG power) was functionally equivalent to LPN amplitude modulations, suggesting that stimulus-induced concentration of delta and theta phases without stimulus-induced power changes may be the neural mechanism of LPN generation. In addition, sustained enhancements of phase-locking precision in the theta range were observed for erroneous and delayed source judgments, suggesting that theta-phase locking is related to the coordination of multiple cortical assemblies in highly demanding task situations.

Updating P300: an integrative theory of P3a and P3b

The empirical and theoretical development of the P300 event-related brain potential (ERP) is reviewed by considering factors that contribute to its amplitude, latency, and general characteristics. The neuropsychological origins of the P3a and P3b subcomponents are detailed, and how target/standard discrimination difficulty modulates scalp topography is discussed. The neural loci of P3a and P3b generation are outlined, and a cognitive model is proffered: P3a originates from stimulus-driven frontal attention mechanisms during task processing, whereas P3b originates from temporal-parietal activity associated with attention and appears related to subsequent memory processing. Neurotransmitter actions associating P3a to frontal/dopaminergic and P3b to parietal/norepinephrine pathways are highlighted. Neuroinhibition is suggested as an overarching theoretical mechanism for P300, which is elicited when stimulus detection engages memory operations.

Updating P300: an integrative theory of P3a and P3b

The empirical and theoretical development of the P300 event-related brain potential (ERP) is reviewed by considering factors that contribute to its amplitude, latency, and general characteristics. The neuropsychological origins of the P3a and P3b subcomponents are detailed, and how target/standard discrimination difficulty modulates scalp topography is discussed. The neural loci of P3a and P3b generation are outlined, and a cognitive model is proffered: P3a originates from stimulus-driven frontal attention mechanisms during task processing, whereas P3b originates from temporal-parietal activity associated with attention and appears related to subsequent memory processing. Neurotransmitter actions associating P3a to frontal/dopaminergic and P3b to parietal/norepinephrine pathways are highlighted. Neuroinhibition is suggested as an overarching theoretical mechanism for P300, which is elicited when stimulus detection engages memory operations.

Theta activity attenuation correlates with avoidance learning progress in gerbils

Changes of cortical activity were examined with electrocorticograms sampled from gerbils during learning. Animals were subjected to tone-conditioned avoidance training in a shuttle-box. Electrocorticograms were recorded from an electrode placed over medial prefrontal cortex. Temporal patterns of theta activity were analyzed across 180 successive trials. With the start of conditioning strong theta activity occurred in each trial in the phase immediately after hurdle crossing. With reliable occurrence of conditioned responses, that is at the stage of retrieval of the avoidance response, the theta activity became reduced. A negative correlation exists between the theta power and the development of learning progress. Theta reduction thus could reflect decreasing demands on information processing in the course of avoidance success monitoring across trials.

Event-related phase reorganization may explain evoked neural dynamics

The traditional view holds that event-related potentials (ERPs) reflect fixed latency, fixed polarity evoked responses that appear superimposed on the 'background EEG'. The validity of the evoked model has been questioned by studies arguing that ERPs are generated at least in part by a reset of ongoing oscillations. But a proof of phase reset that is distinct from the 'artificial' influence of evoked components on EEG phase-has been proven difficult for a variety of methodological reasons. We argue that a theoretical analysis of the assumptions and empirical evaluation of predictions of the evoked and oscillatory ERP model offer a promising way to shed new light on mechanisms generating ERPs that goes well beyond attempts to prove phase reset. Research on EEG oscillations documents that oscillations are task relevant and show a common operating principle, which is the control of the timing of neural activity. Both findings suggest that phase reorganization of task relevant oscillations is a theoretical necessity. We further argue and show evidence that (i) task relevant oscillations exhibit a typical interactive and task relevant relationship between pre- and poststimulus power in the theta and alpha frequency range in a way that small prestimulus power is related to large poststimulus power and vice versa, (ii) ERP (interpeak) latencies and (iii) ERP amplitudes reflect frequency characteristics of alpha and theta oscillations. We emphasize that central assumptions of the evoked model cannot be substantiated and conclude that the ERPR model offers a new way for an integrative interpretation of ongoing and event-related EEG phenomena.

Modality matters: the effects of stimulus modality on the 4- to 30-Hz brain electric oscillations during a lexical decision task

The aim of the current study was to assess modality-specific brain oscillatory responses during cognitive processing. Brain oscillatory ERD/ERS responses of the 4- to 30-Hz EEG frequency bands were examined during lexical decision where the task is to identify whether the presented stimulus is a word or a pseudoword. Seven subjects performed the task with visual stimuli and twelve subjects with auditory stimuli. Visual stimuli elicited greater theta ERS responses as compared to the auditory stimuli. Both stimulus modalities elicited alpha and beta frequency ERD, these being greater for the auditory stimuli. Auditory stimuli elicited also later emerging beta ERS responses, absent for the visual stimuli. The lexicality effects (words vs. pseudowords) were greater for the auditory than for the visual stimuli. When studying brain oscillatory correlates of cognitive processing, the stimulus modality matters. Some effects may arise and some vanish depending on in which modality a cognitive experiment is being conducted.

EEG alpha oscillations: the inhibition-timing hypothesis

The traditional belief is that the event-related alpha response can solely be described in terms of suppression or event-related desynchronization (ERD). Recent research, however, has shown that under certain conditions alpha responds reliably with an increase in amplitudes (event-related synchronization or ERS). ERS is elicited in situations, where subjects withhold or control the execution of a response and is obtained over sites that probably are under, or exert top-down control. Thus, we assume that alpha ERS reflects top-down, inhibitory control processes. This assumption leads over to the timing aspect of our hypothesis. By the very nature of an oscillation, rhythmic amplitude changes reflect rhythmic changes in excitation of a population of neurons. Thus, the time and direction of a change - described by phase - is functionally related to the timing of neuronal activation processes. A variety of findings supports this view and shows, e.g., that alpha phase coherence increases between task-relevant sites and that phase lag lies within a time range that is consistent with neuronal transmission speed. Another implication is that phase reset will be a powerful mechanism for the event-related timing of cortical processes. Empirical evidence suggests that the extent of phase locking is a functionally sensitive measure that is related to cognitive performance. Our general conclusion is that alpha ERS plays an active role for the inhibitory control and timing of cortical processing whereas ERD reflects the gradual release of inhibition associated with the emergence of complex spreading activation processes.

EEG oscillations and recognition memory: Theta correlates of memory retrieval and decision making

Studies of memory retrieval have identified electroencephalographic (EEG) correlates of a test item's old-new status, reaction time, and memory load. In the current study, we used a multivariate analysis to disentangle the effects of these correlated variables. During retrieval, power of left-parietal theta (4-8 Hz) oscillations increased in proportion to how well a test item was remembered, and theta in central regions correlated with decision making. We also studied how these oscillatory dynamics complemented event-related potentials. These findings are the first to demonstrate that distinct patterns of theta oscillations can simultaneously relate to different aspects of behavior.