Biomedical subjects

W Klimesch

Publications and source records attributed to W Klimesch.

At least 37 records · Page 2Linked to original sources

PubMed1999

EEG alpha and theta oscillations reflect cognitive and memory performance: a review and analysis.

Evidence is presented that EEG oscillations in the alpha and theta band reflect cognitive and memory performance in particular. Good performance is related to two types of EEG phenomena (i) a tonic increase in alpha but a decrease in theta power, and (ii) a large phasic (event-related) decrease in alpha but increase in theta, depending on the type of memory demands. Because alpha frequency shows large interindividual differences which are related to age and memory performance, this double dissociation between alpha vs. theta and tonic vs. phasic changes can be observed only if fixed frequency bands are abandoned. It is suggested to adjust the frequency windows of alpha and theta for each subject by using individual alpha frequency as an anchor point. Based on this procedure, a consistent interpretation of a variety of findings is made possible. As an example, in a similar way as brain volume does, upper alpha power increases (but theta power decreases) from early childhood to adulthood, whereas the opposite holds true for the late part of the lifespan. Alpha power is lowered and theta power enhanced in subjects with a variety of different neurological disorders. Furthermore, after sustained wakefulness and during the transition from waking to sleeping when the ability to respond to external stimuli ceases, upper alpha power decreases, whereas theta increases. Event-related changes indicate that the extent of upper alpha desynchronization is positively correlated with (semantic) long-term memory performance, whereas theta synchronization is positively correlated with the ability to encode new information. The reviewed findings are interpreted on the basis of brain oscillations. It is suggested that the encoding of new information is reflected by theta oscillations in hippocampo-cortical feedback loops, whereas search and retrieval processes in (semantic) long-term memory are reflected by upper alpha oscillations in thalamo-cortical feedback loops.

Alpha Rhythm↗

PubMed1999

'Paradoxical' alpha synchronization in a memory task.

The results of a specially designed memory search paradigm which maximizes episodic short-term memory (STM) and minimizes semantic long-term memory (LTM) demands show that the upper alpha band synchronizes selectively in those conditions and time intervals where episodic STM demands are maximal. This finding of a selective alpha synchronization occurring only in the upper alpha band and during highest task demands is surprising because it is well known that usually alpha desynchronizes during mental activity. Because experiments from our laboratory indicate that desynchronization in the upper alpha band is related to semantic LTM processes, the present finding suggests that a selective synchronization in this frequency band reflects inhibition of semantic LTM. It is assumed that once the capacity limits of STM are reached or exceeded, processing resources are no longer distributed and that potentially interfering, task irrelevant, brain areas or processing systems are inhibited.

Adult↗

PubMed1998

Theta synchronization in the human EEG and episodic retrieval.

Event-related desynchronization (ERD) and synchronization (ERS) was measured during episodic retrieval in a specially designed recognition task which forces subjects to avoid semantic search strategies. ERD represents the percentage of a decrease, ERS an increase in band power. The results show that only the theta band differentiates between good and bad episodic memory performers and that good performance is related to a large degree of theta synchronization. The delta and alpha bands did not yield significant effects. Topographical differences in theta ERS reveal that good performers use primarily their right hemisphere to retrieve episodic information. This finding agrees with respective results from PET studies.

Adult↗

PubMed1998

Induced alpha band power changes in the human EEG and attention.

Induced alpha power (in a lower, intermediate and upper band) which is deprived from evoked electroencephalograph (EEG) activity was analyzed in an oddball task in which a warning signal (WS) preceded a target or non-target. The lower band, reflecting phasic alertness, desynchronizes only in response to the WS and target. The intermediate band, reflecting expectancy, desynchronizes about 1 s before a target or non-target appears. Upper alpha desynchronizes only after a target is presented and, thus, reflects the performance of the task which was to count the targets. Thus, only slower alpha frequencies reflect attentional demands such as alertness and expectancy.

Adult↗

PubMed1998

Individual differences in brain dynamics: important implications for the calculation of event-related band power.

Measures of event-related band power such as event-related desynchronization (ERD) are conventionally analyzed within fixed frequency bands, although it is known that EEG frequency varies as a function of a variety of factors. The question of how to determine these frequency bands for ERD analyses is discussed and a new method is proposed. The rationale of this new method is to adjust the frequency bands to the individual alpha frequency (IAF) for each subject and to determine the bandwidth for the alpha and theta bands as a percentage of IAF. As an example, if IAF equals 12 Hz, the widths of the alpha and theta bands are larger as compared to a subject with an IAF of, e.g., only 8 Hz. The results of an oddball paradigm show that the proposed method is superior to methods that are based on fixed frequencies and fixed bandwidths.

Adult↗

PubMed1998

A method for the calculation of induced band power: implications for the significance of brain oscillations.

A method for the calculation of significant changes in induced band power (IBP) is presented. In contrast to traditional measures of event-related band power (ERBP) which are composed of evoked and not evoked EEG components, the proposed measure for IBP is deprived from phase locked (or evoked) EEG activity. It is assumed that changes in IBP reflect the modulation of brain oscillations that are largely independent from ERPs. The results of a visual oddball task show that significant changes in IBP can be observed in response to the presentation of a warning signal (preceding a target or nontarget) and the imperative stimulus (i.e. a target or nontarget) in the alpha, theta and delta band. Only a few significant changes in IBP were obtained for the warning signal in the theta band although highly significant changes in ERBP were found. Our findings document that changes in IBP may be considered a phenomenon that is largely independent from the occurrence of ERPs. They underline the significance of oscillatory processes and suggest that induced rhythms are modulated by stimuli and/or events in a not phase locked way.

Adult↗

PubMed1998

The functional significance of absolute power with respect to event-related desynchronization.

The question is examined whether the extent of changes in relative band power as measured by event-related desynchronization (ERD) depends on absolute band power. The results for target stimuli of a simple oddball task indicate that the prestimulus (reference) level of absolute band power has indeed a strong influence on ERD. Whereas for the alpha band large band power in the reference interval is related to a strong degree of alpha suppression as measured by ERD, the opposite holds true for the theta band. Here, a low level of band power during the reference interval is related to a pronounced increase in band power during the processing of the target stimulus. In contrast to alpha and theta, ERD in the delta band is not influenced by the magnitude of band power in the reference interval.

Adolescent↗

PubMed1998

High-frequency components in the alpha band and memory performance.

We tested the hypothesis that alpha band power is correlated with memory performance. For a sample of 68 subjects, the ongoing electroencephalogram (EEG) was analyzed during three experimental conditions: eyes closed (EC), eyes open (EO), and memorizing words. The results show that the upper alpha band of approximately 10-12 Hz is related to memory performance. Only within this frequency range did we obtain significant positive correlations between memory performance and EEG power. A comparison of power spectra showed that within the range of the upper alpha band good performers had significantly greater normalized percent power than bad memory performers. In contrast to condition EC, the obtained relationship between upper alpha power and memory performance was particularly strong during EO and memorizing words.

Adult↗

PubMed1997

Brain oscillations and human memory: EEG correlates in the upper alpha and theta band.

The EEG was recorded while subjects judged whether sequentially presented feature-concept pairs are semantically congruent. Later and without prior warning they had to perform a semantic and episodic memory task. The results show that the upper alpha band is most sensitive to the encoding and processing of semantic information. It is only the upper alpha band that distinguishes between good and bad semantic memory performers and that shows significant correlations with semantic memory performance during that time period, semantic processing actually takes place. Even when the influence of episodic memory was removed by partial correlations, a reliable association between upper alpha desynchronization and semantic memory was observed.

Adult↗

PubMed1997

EEG-alpha rhythms and memory processes.

The results of several experiments indicate that alpha frequency varies as a function of memory performance. It was found that in samples of age matched subjects alpha frequency of good memory performers is about 1 Hz-higher than those of bad performers. The difference in alpha frequency between good and bad performers reaches a maximum during the retrieval of information, is much smaller during encoding and is minimal--but still significant--during a resting period. These results suggest that alpha frequency may be a permanent and not only a functional parameter that determines the speed with which information can be retrieved from memory. The calculation of changes in band power indicate further that the upper alpha band is particularly sensitive to semantic memory demands. The lower alpha band, on the other hand, seems to reflect attentional processes. These findings are discussed on the basis of a hypothesis which assumes that EEG frequencies within the alpha band stem at least in part from the thalamus and that the activity of thalamo-cortical networks reflects processes that are related to searching, accessing and retrieving information from (scmantic) long-term memory.

Alpha Rhythm↗

PubMed1997

Event-related desynchronization in the alpha band and the processing of semantic information.

The hypothesis was tested whether event-related power shifts in the upper alpha band are specifically related to semantic memory processes. In Expt. 1 subjects had to judge whether pairs of sequentially presented words (W1-W2) were semantically congruent. In the following experiments subjects were presented the W1 words of Expt. 1 and were asked to perform a free association task in Expt. 2 and a cued recall task in Expt. 3. It is assumed that semantic memory demands dominate in Expt. 1, whereas working memory demands dominate in Expt. 3 and that Expt. 2 takes an intermediate position with respect to both types of task demands. A significant task-related power change that responds selectively to semantic processing demands was found for the upper alpha band and over the left side of the scalp. The lower alpha band, on the other hand, most likely reflects unspecific processing demands such as attention. A more general interpretation of these findings is that different cognitive processes such as semantic memory, perceptual encoding and attentional processes are reflected by band power changes in different and rather narrow frequency bands over localized regions in the brain.

Adult↗

PubMed1997

Theta synchronization and alpha desynchronization in a memory task.

In the present study, we examined the hypothesis that episodic encoding and retrieval processes are primarily reflected by a task-related increase in theta power. Individuals performed a recognition task with a total of 192 words. The electroencephalogram was recorded during the study and recognition phase. The results show that only those words that were later correctly recognized produced a significant increase in theta power during encoding. During the actual recognition processes too, a significant theta synchronization (increase in band power) was found for correctly remembered words only. In contrast to the theta band, remembered and not remembered words revealed a complex pattern of desynchronization in the lower and upper alpha band that was different during encoding and recognition.

Adult↗

PubMed1996

Theta band power in the human scalp EEG and the encoding of new information.

Task-related band power changes in the theta and alpha bands were examined during the encoding of new information in an implicit memory paradigm. The results showed significantly higher theta power during the encoding of those words which could be remembered in the later recall task, compared with words which could not be remembered later. In contrast to the theta band, alpha band power decreased during encoding. However, remembered words, compared with not remembered words did not show significant differences in the the alpha band. The increase in theta power during the successful encoding of new information is discussed with respect to a possible relationship with hippocampal theta, induced in the cortex via hippocampo-cortical feedback loops.

Adult↗

PubMed1996

Event-related desynchronization (ERD) and the Dm effect: does alpha desynchronization during encoding predict later recall performance?

Based on previous research which has shown that event-related desynchronization (ERD) in the lower and upper alpha band reflects attentional and semantic processing respectively, the present study examines the hypothesis whether event-related shifts in the two alpha bands are capable of predicting later recall performance. In an incidental memory paradigm, subjects first had to judge the category membership for a set of 96 words. Later, without prior warning, subjects were asked to recall the words. The results show that for good performers, the extent of ERD in the lower alpha band during the semantic encoding for words is significantly larger for remembered as compared to not remembered words, whereas for bad performers the ERD in the upper alpha band is significantly more pronounced. This type of Dm effect is particularly strong over parietal recording sites in both hemispheres. In referring to the proposed interpretation of the lower and upper alpha band, the present findings seem to indicate that in contrast to good performers, bad performers are less attentive or alert during encoding. Event-related potentials (ERPs) also yielded significant Dm effects at parietal recording sites.

Adult↗

PubMed1996

Memory processes, brain oscillations and EEG synchronization.

This article tries to integrate results in memory research from divergent disciplines such as cognitive psychology, neuroanatomy, and neurophysiology. The integrating link is seen in more recent findings that provide strong arguments for the assumption that oscillations are a basic form of communication between cortical cell assemblies. It is assumed that synchronous oscillations of large cell assemblies--termed type 1 synchronization--reflect a resting state or possibly even a state of functional inhibition. On the other hand, during mental activity, when different neuronal networks may start to oscillate with different frequencies, each network may still oscillate synchronously (this is termed type 2 synchronization), but as a consequence, the large scale type 1 oscillation disappears. It is argued that these different types of synchronization can be observed in the scalp EEG by calculating event-related power changes within comparatively narrow but individually adjusted frequency bands. Experimental findings are discussed which support the hypothesis that short-term (episodic) memory demands lead to a synchronization (increase in band power) in the theta band, whereas long-term (semantic) memory demands lead to a task-specific desynchronization (decrease or suppression of power) in the upper alpha band. Based on these and other findings, a new memory model is proposed that is described on three levels: cognitive, anatomical and neurophysiological. It is suggested that short-term (episodic) memory processes are reflected by oscillations in an anterior limbic system, whereas long-term (semantic) memory processes are reflected by oscillations in a posterior-thalamic system. Oscillations in these frequency bands possibly provide the basis for encoding, accessing, and retrieving cortical codes that are stored in the form of widely distributed but intensely interconnected cell assemblies.

Alpha Rhythm↗

PubMed1996

Alpha frequency, reaction time, and the speed of processing information.

Several experiments indicate that alpha frequency is significantly correlated with the speed of information processing, as measured by reaction times. These data imply that alpha frequency is a timing mechanism for cognitive processes. However, this interpretation contradicts the well-accepted view that there is no single alpha rhythm, but instead an entire population of different alpha oscillations. We present arguments and report data in the attempt to demonstrate that this paradox can be resolved if different states of alpha oscillations are distinguished. The results of the present study show that during a state of desynchronization, task-related shifts in alpha frequency are not "real" because they are not related to reaction times. Instead, they most likely reflect power changes in different frequency bands.

Adult↗

PubMed1995

Clinical case report: opiate antagonist and event-related desynchronization in 2 autistic boys.

Event-related desynchronization and visual orientational behavior were examined in 2 autistic boys to determine if blockade of endogenous opioid activity facilitates cognitive processing at a cortical level. Before naltrexone, the boys showed no selective alpha blocking during exposure to either mother's pictures or white light. Unlike normals, they exhibited strong alpha band enhancement at temporocentral recording sites. Two hours after administering 0.5 mg/kg naltrexone, mother-as well as light-related alpha blocking appeared at occipital, occipitotemporal, and prefrontal sites. These effects were gone 24 h after dosing in one child, but persisted in the other. A parallel increase in visual pursuit in a social context was observed. These results affirm that autistic gaze aversion can be caused by excessive opioid activity interfering with corticothalamocortical processing of visual stimuli.

Alpha Rhythm↗

PubMed1994

Episodic and semantic memory: an analysis in the EEG theta and alpha band.

This study examines the hypothesis that in contrast to semantic memory processes that are assumed to be reflected primarily within the alpha band, episodic memory processes are related to activity within the theta band. EEG signals were recorded from subjects as they performed a semantic congruency and an episodic recognition task. In the semantic task, subjects had to judge whether or not sequentially presented concept-feature pairs (such as "eagle-claws" or "pea-huge") are semantically congruent. In the episodic task, which followed the semantic task without prior warning, the same word pairs were presented together with new distractors (generated by repairing known concept-feature pairs). Here, subjects judged whether or not a particular concept-feature pair was already presented during the semantic task. EEG data were analyzed using event-related desynchronization (ERD) as a measure for the amount of event-related changes in band power in the theta band and in the upper and lower alpha bands. The alpha band was determined individually, using the alpha peak frequency during the resting period as the cut-off point to separate the lower from the upper alpha band. The results, which are based on those identical word pairs that demanded a yes response in both tasks, showed that semantic memory processes are indeed primarily reflected in the upper alpha band whereas episodic memory processes are reflected in the theta band. The possible relationship between hippocampal theta activity and the encoding of episodic information is discussed.

Adult↗