Biomedical subjects

W Klimesch

Publications and source records attributed to W Klimesch.

At least 19 recordsLinked to original sources

PubMed2006

The significance of sigma neurofeedback training on sleep spindles and aspects of declarative memory.

The functional significance of sleep spindles for overnight memory consolidation and general learning aptitude as well as the effect of four 10-minute sessions of spindle frequency (11.6-16 Hz, sigma) neurofeedback-training on subsequent sleep spindle activity and overnight performance change was investigated. Before sleep, subjects were trained on a paired-associate word list task after having received either neurofeedback training (NFT) or pseudofeedback training (PFT). Although NFT had no significant impact on subsequent spindle activity and behavioral outcomes, there was a trend for enhanced sigma band-power during NREM (stage 2 to 4) sleep after NFT as compared to PFT. Furthermore, a significant positive correlation between spindle activity during slow wave sleep (in the first night half) and overall memory performance was revealed. The results support the view that the considerable inter-individual variance in sleep spindle activity can at least be partly explained by differences in the ability to acquire new declarative information. We conclude that the short NFT before sleep was not sufficient to efficiently enhance phasic spindle activity and/or to influence memory processing. NFT was, however, successful in increasing sigma power, presumably because sigma NFT effects become more easily evident in actually trained frequency bands than in associated phasic spindle activity.

Adolescent↗

PubMed2006

Sleep spindle-related activity in the human EEG and its relation to general cognitive and learning abilities.

Stage 2 sleep spindles have been previously viewed as useful markers for the development and integrity of the CNS and were more currently linked to 'offline re-processing' of implicit as well as explicit memory traces. Additionally, it had been discussed if spindles might be related to a more general learning or cognitive ability. In the present multicentre study we examined the relationship of automatically detected slow (< 13 Hz) and fast (> 13 Hz) stage 2 sleep spindles with: (i) the Raven's Advanced Progressive Matrices (testing 'general cognitive ability'); as well as (ii) the Wechsler Memory scale-revised (evaluating memory in various subdomains). Forty-eight healthy subjects slept three times (separated by 1 week) for a whole night in a sleep laboratory with complete polysomnographic montage. Whereas the first night only served adaptation and screening purposes, the two remaining nights were preceded either by an implicit mirror-tracing or an explicit word-pair association learning or (corresponding) control task. Robust relationships of slow and fast sleep spindles with both cognitive as well as memory abilities were found irrespectively of whether learning occurred before sleep. Based on the present findings we suggest that besides being involved in shaping neuronal networks after learning, sleep spindles do reflect important aspects of efficient cortical-subcortical connectivity, and are thereby linked to cognitive- and memory-related abilities alike.

Adult↗

PubMed2005

Relevance of EEG alpha and theta oscillations during task switching.

In a task switching design, we investigated the question whether long-range theta coupling primarily reflects top-down control processes. Switch and stay trials did not differ with respect to memory load or global working memory (WM) demands. The results revealed significantly stronger theta coupling (in a range of 4-7 Hz) between prefrontal and posterior regions during switch as compared to stay trials. Power differences, reflecting more local effects, were largest in the upper alpha band (10-13 Hz) and over posterior brain areas, possibly reflecting long-term memory activation. The conclusion of the present study is that long-range coherent oscillatory activity in the theta band reflects top-down activation rather than global WM functions.

Adult↗

PubMed2005

Intelligence related upper alpha desynchronization in a semantic memory task.

Recent evidence shows that event-related (upper) alpha desynchronization (ERD) is related to cognitive performance. Several studies observed a positive, some a negative relationship. The latter finding, interpreted in terms of the neural efficiency hypothesis, suggests that good performance is associated with a more 'efficient', smaller extent of cortical activation. Other studies found that ERD increases with semantic processing demands and that this increase is larger for good performers. Studies supporting the neural efficiency hypothesis used tasks that do not specifically require semantic processing. Thus, we assume that the lack of semantic processing demands may at least in part be responsible for the reduced ERD. In the present study we measured ERD during a difficult verbal-semantic task. The findings demonstrate that during semantic processing, more intelligent (as compared to less intelligent) subjects exhibited a significantly larger upper alpha ERD over the left hemisphere. We conclude that more intelligent subjects exhibit a more extensive activation in a semantic processing system and suggest that divergent findings regarding the neural efficiency hypotheses are due to task specific differences in semantic processing demands.

Adolescent↗

PubMed2005

Intelligence related differences in EEG-bandpower.

Several studies on the relationship between event-related desynchronization/synchronization (ERD/ERS) and cognitive performance revealed contradictory results particularly for the alpha band. Studies from our laboratory have shown that good performers show a larger upper alpha ERD (interpreted in terms of larger cortical activation) than bad performers. In contrast, other researchers found evidence for the neural efficiency hypothesis, which states that more intelligent subjects exhibit a smaller extent of cortical activation, which is assumed to be reflected by a smaller upper alpha ERD. Here we address the question whether these divergent results may be due to differences in general task difficulty. Using a modified version of the RAVEN, individually divided into easy and difficult tasks, a group of average and a group of highly intelligent subjects (IQ- and IQ+) have been investigated. While in the theta frequency IQ+ subjects generally exhibited a significantly stronger activation, we found a significant interaction of task difficulty and IQ group in the upper alpha band, indicating both, a weaker activation for the high IQ group during the easy tasks, and a significant increase from easy to difficult tasks for IQ+ only.

Adolescent↗

PubMed2005

Oscillatory EEG correlates of episodic trace decay.

Recent studies suggest that human theta oscillations appear to be functionally associated with memory processes. It is less clear, however, to what type of memory sub-processes theta is related. Using a continuous word recognition task with different repetition lags, we investigate whether theta reflects the strength of an episodic memory trace or general processing demands, such as task difficulty. The results favor the episodic trace decay hypothesis and show that during the access of an episodic trace in a time window of approximately 200-400 ms, theta power decreases with increasing lag (between the first and second presentation of an item). LORETA source localization of this early theta lag effect indicates that parietal regions are involved in episodic trace processing, whereas right frontal regions may guide the process of retrieval. We conclude that episodic encoding can be characterized by two different stages: traces are first processed at parietal sites at approximately 300 ms, then further processing takes place in regions of the medial temporal lobe at approximately 500 ms. Only the first stage is related to theta, whereas the second is reflected by a slow wave with a frequency of approximately 2.5 Hz.

Adult↗

PubMed2005

Phase synchronization between theta and upper alpha oscillations in a working memory task.

Motivated by findings that theta and upper alpha oscillations respond selectively to different types of memory demands, we investigated the role of phase synchronization in a memory scanning task. During retention, we found a load dependent increase in upper alpha power at O2 and P4 and a significant upper alpha:theta phase synchronization between right posterior, central and left anterior sites. During retrieval, a load dependent increase in upper alpha phase locking was observed at O2 and an increase in upper alpha:theta phase synchronization between right posterior and left anterior sites. We suggest that theta reflects central executive functions whereas upper alpha may be important for the reactivation of long-term memory codes in short-term memory. The interplay between theta and upper alpha may be reflected by phase synchronization between these frequencies.

Adult↗

PubMed2005

A shift of visual spatial attention is selectively associated with human EEG alpha activity.

Event-related potentials and ongoing oscillatory electroencephalogram (EEG) activity were measured while subjects performed a cued visual spatial attention task. They were instructed to shift their attention to either the left or right visual hemifield according to a cue, which could be valid or invalid. Thereafter, a peripheral target had to be evaluated. At posterior parietal brain areas early components of the event-related potential (P1 and N1) were higher when the cue had been valid compared with invalid. An anticipatory attention effect was found in EEG alpha magnitude at parieto-occipital electrode sites. Starting 200 ms before target onset alpha amplitudes were significantly stronger suppressed at sites contralateral to the attended visual hemifield than ipsilateral to it. In addition, phase coupling between prefrontal and posterior parietal electrode sites was calculated. It was found that prefrontal cortex shows stronger phase coupling with posterior sites that are contralateral to the attended hemifield than ipsilateral sites. The results suggest that a shift of attention selectively modulates excitability of the contralateral posterior parietal cortex and that this posterior modulation of alpha activity is controlled by prefrontal regions.

Adult↗

PubMed2002

Frequency characteristics of evoked and oscillatory electroencephalic activity in a human memory scanning task.

Research on memory scanning tasks indicates that oscillatory activity increases with load during retention, whereas evoked power (EP, comprising the P3) decreases during retrieval. We investigate the question, whether both phenomena are primarily related to theta oscillations. However, we found that during retention alpha oscillations increased with load and that exactly this frequency exhibits increased phase locking - measured by a specially developed phase locking index (PLI) - during retrieval. The decreased P3 amplitude was related to decreased delta EP and PLI. The P3 coincides with the last of three evoked alpha peaks. Thus, alpha may be important for the timing of the scanning and the evaluation of the read out process that most likely is manifested by the P3.

Adult↗

PubMed2002

The interplay between theta and alpha oscillations in the human electroencephalogram reflects the transfer of information between memory systems.

The exchange of information between the working and long-term memory system (WMS and LTMS) was investigated. We analyzed evoked theta and upper alpha desynchronization in a special memory task, designed to study the transfer of information between both memory systems. The results show that during attempts to retrieve information from the LTMS, evoked theta oscillations spread from anterior to posterior recording sites. When information actually is retrieved, the direction reverses and theta spreads to frontal sites. This time point--when direction reverses--varies between subjects to a large extent but is significantly correlated with memory performance and the onset of upper alpha desynchronization. We conclude that this phenomenon reflects the transfer of information between the WMS and LTMS

Adult↗

PubMed2001

The role of theta and alpha oscillations for language comprehension in the human electroencephalogram.

The hypothesis is tested, whether increasing language processing demands draw on the capacity of working memory thereby leading to an increase in theta band power. Previous research has shown that theta reflects working memory whereas upper alpha semantic memory demands. Sentences were presented in four chunks in a reading and a semantic task. In the latter, subjects had to find a superordinate concept to a noun presented in the third chunk. The data show an increase in theta during sentence processing which was significantly smaller in the semantic task. In contrast, the upper alpha band exhibited a significantly larger change in band power during the semantic task and that time window in which subjects searched for the superordinate concept. Thus, we conclude that semantic processing does not draw selectively on the capacity of working memory and that different linguistic processes have no direct influence on theta oscillations.

Adult↗

PubMed2001

Episodic retrieval is reflected by a process specific increase in human electroencephalographic theta activity.

Is an increase in theta during retrieval due (primarily) to the access of a stored code or to more general processes? The electroencephalogram was recorded while subjects performed a recognition task with pictures. According to the event-related desynchronization/synchronization method, the percentage of band power changes was calculated during encoding and retrieval for a theta and three alpha bands. Significant results were obtained (with minor exceptions) only in the theta band. The increase in theta was significantly larger during retrieval than during encoding but did not differ significantly between new and successfully retrieved old pictures. Because a memory trace is lacking for new pictures, the increase in theta during retrieval reflects primarily general processing demands of a complex episodic memory system.

Adult↗

PubMed2001

Theta synchronization during episodic retrieval: neural correlates of conscious awareness.

The neural correlates of conscious awareness during successful memory retrieval were examined. In a recognition test, subjects indicated whether they consciously recalled the event in which a word was earlier presented (Remembering), or whether they recognized it on the basis that it was familiar in the absence of recollection (Knowing). An early EEG synchronization in the theta band predicted knowing, and a later remembering. Moreover, early and late event-related potentials were also found to predict knowing and remembering, respectively. The results indicate that the temporal dynamics of theta synchronization are related to the particular conscious experiences associated with memory retrieval.

Adult↗

PubMed2001

Alpha and beta band power changes in normal and dyslexic children.

OBJECTIVE: Previous research with healthy subjects suggests that the lower alpha band reflects attentional whereas the upper alpha band semantic processes. The aim of the present study was to investigate whether dyslexics show deficits in attentional control and/or semantic encoding. METHOD: The EEG was recorded while subjects were reading numbers, words and pseudowords and analyzed in a lower and upper alpha and two beta bands (spanning a range of about 8--16 Hz). A phasic response is measured in terms of a decrease in event related band power during reading with respect to a reference interval. Tonic power is measured in terms of (log) band power during a reference interval. RESULTS: In the lower alpha band dyslexics show an increased phasic response to words and pseudowords at right hemispheric sites but a lack to respond to words at O1. The upper alpha band exhibits a highly selective phasic response to words at left frontal sites but for controls only, whereas dyslexics show a general increase in tonic upper alpha power. Whereas the low frequency beta band (beta-1a) exhibits a rather diffuse pattern, a highly selective finding was obtained for the beta-1b band. CONCLUSIONS: Dyslexics have a lack of attentional control during the encoding of words at left occipital sites and a lack of a selective topographic activation pattern during the semantic encoding of words. Because only in controls reading of words is associated with a strong beta-1b desynchronization at those recording sites which correspond to Broca's area (FC5) and the angular gyrus (CP5, P3), we may conclude that this frequency band reflects the graphemicphonetic encoding of words.

Alpha Rhythm↗

PubMed2001

Theta band power changes in normal and dyslexic children.

OBJECTIVE: Tonic and phasic (event-related) theta band power changes were analyzed in a sample of 8 dyslexic and 8 control children. Previous research with healthy subjects suggests that electroencephalograph (EEG) theta activity reflects the encoding of new information into working memory. The aim of the present study was to investigate whether the processing deficits of dyslexics are related to a reduced phasic theta response during reading. METHOD: The EEG was recorded while subjects were reading numbers, words and pseudowords and analyzed in a lower and upper theta band (4--8 Hz). A phasic response is measured in terms of an increase in event related band power during reading with respect to a reference interval. Tonic power is measured in terms of (log) band power during a reference interval. RESULTS: Large group differences in tonic and phasic lower theta were found for occipital sites where dyslexics show a complete lack of pseudoword processing. For words, only controls show a highly selective left hemispheric processing advantage. CONCLUSIONS: Dyslexics have a lack to encode pseudowords in visual working memory with a concomitant lack of frontal processing selectivity. The upper theta band shows a different pattern of results which can be best interpreted to reflect the effort during the encoding process.

Child↗

PubMed2000

Simultaneous desynchronization and synchronization of different alpha responses in the human electroencephalograph: a neglected paradox?

This study aims to resolve a paradox. Experiments measuring alpha band power report an event related decrease (desynchronization) in alpha activity, whereas those measuring evoked alpha report synchronization. During a recognition memory task with human subjects, we measured the evoked (phase locked) and induced (not phase locked) alpha response. The findings reveal that evoked alpha is due to a transient phase locking (at about 100-200 ms poststimulus) of three alpha sub-bands which can be observed only at parieto-occipital sites. In contrast, induced alpha shows a widespread pattern of desynchronization at most recording sites. Thus, opposite alpha responses occur within similar time windows. Evoked alpha synchronization may reflect cortical inhibition which serves to increase the signal to noise ratio for activation processes following immediately later.

Adolescent↗

PubMed2000

The time locked theta response reflects interindividual differences in human memory performance.

Recent research indicates that an increase in theta band power is related to episodic memory performance. In this study with human subjects, the evoked (time locked) and induced (not time locked) theta response is analyzed in a recognition task. The results show a strong evoked theta response during an early retrieval period of up to 400 ms. Only for good memory performers theta is strictly time locked, indicating that theta peaks appear in preferred time windows after a target is presented. This effect--which coincides with a large P3--suggests that good performance requires a strict timing of different processing stages that correspond to cycles of theta activity.

Adult↗

PubMed2000

Theta oscillations and the ERP old/new effect: independent phenomena?

OBJECTIVES: The hypothesis is examined whether a memory-related change in induced band power (oscillatory old/new effect) is functionally related to a memory-related increase in ERP positivity (ERP old/new effect). METHODS: In order to avoid a confounding on the measurement level, induced band power (IBP) was used as a measure that is devoid of the influence of evoked components. The EEG was recorded during a recognition memory task. RESULTS: The results show that compared to correctly rejected words, targets (remembered words) elicit a significantly larger P300. An oscillatory old/new effect was found for the delta and theta but not for the alpha band. It is manifested by an increase in delta and theta IBP which is significantly larger for targets than for correctly rejected words. It can be observed during the same time interval and shows the same topographic distribution as the ERP old/new effect. Most importantly, however, the ERP old/new effect (as well as the P300 itself) is generated by very slow frequencies which lie below the delta band. CONCLUSIONS: These findings demonstrate that the two types of old/new effects are functionally related. Possible physiological mechanisms underlying this relationship are discussed in terms of a threshold change in the cortex (generating the P300) that occurs during an increase in hippocampal theta activity (generating an increase in induced theta power).

Adult↗