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Biomedical subjects

W Klimesch

Publications and source records attributed to W Klimesch.

52 records · Page 3Linked to original sources
PubMed1993

Alpha frequency, cognitive load and memory performance.

EEG-signals were recorded from subjects as they performed a modified version of Schneider's and Shiffrin's memory search paradigm. The hypothesis was tested whether individual (centre of gravity) alpha frequency, termed IAF, is related to memory performance and/or attentional demands. The results show that memory performance exerts the strongest effect on IAF. As compared to a resting period, the difference in IAF between age-matched good and bad memory performers reached a maximum when subjects were actually retrieving information from their memory. During retrieval, the IAF of good performers is 1.25 Hz higher than for bad performers. Attentional and task demands also tend to reduce IAF, but as compared to memory performance-to a much lesser degree. The results of amplitude analyses demonstrate further that during retrieval, alpha desynchronization is more pronounced for bad performers than for good performers. Taken together, the results indicate that a decrease in IAF is always related to a drop in performance.

Adult↗
PubMed1992

Functional topography during a visuoverbal judgment task studied with event-related desynchronization mapping.

Event-related desynchronization (ERD) of alpha components was studied in 10 subjects in a visuoverbal judgment task. EEG was recorded from 29 electrodes; ERD, measured by percentage of alpha power decrease, was calculated in 125-ms intervals and displayed in a time course over 7-s intervals and in the form of topographical ERD maps. ERD, interpreted as a measure of activated cortical areas, was studied in the upper and lower alpha bands. It was found that upper alpha band ERD was short-lasting (less than 1 s), localized to the posterior region and not found before stimulation; in contrast to this, lower alpha band ERD was longer-lasting (greater than 1 s), more widespread, with a left hemisphere dominance, and was already present before stimulation. ANOVA revealed significant interactions among alpha frequency bands, scalp location, and pre-/poststimulus intervals. The self-organizing topological Kohonen network was used to analyze the stability of consecutive ERD maps. There is evidence that topographical ERD patterns can be stable for about 300-400 ms.

Adult↗
PubMed1992

Semantic memory: complexity or connectivity?

The complexity hypothesis predicts that concept-feature verification times should be directly related to the number of features associated with a concept (the fan effect). Conversely, the connectivity hypothesis predicts that for interconnected semantic relationships, this verification time should be inversely related to the number of closely associated features (the reversed fan effect). In the present experiments, although the time required for episodic recognition memory decisions tended to be directly related to the number of features associated with a concept, semantic verification times were inversely related to the complexity of the concept. In addition, the concept's complexity was at least as good a predictor of performance in semantic tasks as was the strength of association between the concept and feature. Both of these results are interpreted in terms of the connectivity hypothesis.

Adult↗
PubMed1992

[P300 and dementia: diagnostic relevance II].

Recent research on the relationship between ERP-latencies and reduced information process capabilities with demented subjects has yielded divergent results. In the present study P300-latencies and RT are used in an attempt to differentiate between demented and nondemented subjects. Based on the "feature-integration-theory" of Treisman an experiment was designed in order to investigate more complex cognitive processes. The results showed that both P300-latency and RT may be used as additional diagnostic indices for cognitive capability.

Adult↗
PubMed1990

Topographical display and interpretation of event-related desynchronization during a visual-verbal task.

Multichannel EEG recordings were performed during a visual-verbal judgement task with a verbal response using an event-related paradigm. EEG trials of 7 sec were first digitally band pass filtered (10-12 Hz, 10-11 Hz, 11-12 Hz), and the amplitudes were then squared and averaged over all trials. This processing method results in a time course of alpha power and allows us to quantify the event-related desynchronization (ERD) in each EEG channel and to compute series of ERD maps in intervals of 125 msec. Analyzing the 10-11 Hz band, it was possible to study the time course and topographical pattern of cortical activation during visual encoding and cognition. Analysis of the 11-12 Hz band has enabled us, for the first time, to study the time course, spatial localization and extent of activation of speech, premotor and motor areas. We found that the speech centers were activated maximally 250 to 375 msec before speech onset and the SMA about 250 msec before speech onset. The results are preliminary, but demonstrate how much information can be extracted from the scalp EEG.

Acoustic Stimulation↗
PubMed1990

Event-related desynchronization, ERD-mapping and hemispheric differences for words and numbers.

Event-related desynchronization (ERD) is the amount of event-related decrease in alpha band power. In applying ERD as an index of cortical activation, the way in which attention and expectancy affect hemispheric differences for words and numbers was investigated. Subjects, 12 right-handed males, had to perform a semantic and a numerical classification task under two different counterbalanced expectancy conditions. Whereas under the high expectancy condition words and numbers were presented blockwise, they were presented randomly under the low expectancy condition. In the semantic task subjects had to indicate the category to which a word belonged; in the numerical task they had to judge whether a number was odd or even. Because 48 words and numbers were used in both expectancy conditions, each subject had to perform a total of 192 trials, practice trials not included. During each of the 192 trials, EEG-signals were recorded from 29 electrodes and analyzed in two frequency bands (6-10 Hz and 9-13 Hz). The data, which were also presented in the form of maps, were subjected to a 6-factorial ANOVA. The results reveal a complex pattern of interactions between the two frequency bands, expectancy conditions, stimulus types and the recording sites. The most important results concern the influence of expectancy. A consistent left hemispheric advantage could be observed under the high expectancy condition and in the lower alpha band only. This and other results seem to indicate that the lower alpha band is more sensitive to reflect expectancy and attentional processes.

Analysis of Variance↗
PubMed1990

[P300 and dementia: diagnostic relevance. I].

Visual evoked potentials, including the P300 response, and reaction time were recorded from 20 patients with multi-infarct dementia, 20 nondemented patients with brain-infarct diseases, and 20 age-matched control subjects. Additionally the patients were assessed using the WAIS-index. Prolonged P300 latency and reaction time occurred in both of the patient-groups, correlated significantly with the WAIS-index. However, effects regarding to P300 latency and reaction time were not strong enough to allow individual classifications.

Adult↗
PubMed1990

[Event-related desynchronization and the selection of an alpha-frequency band for quantifying cortical pre- and poststimulus activation].

The amount of event-related decrease of increase in alpha-band power is termed "Event-Related Desynchronization" or ERD. In this study, ERD is used as an index of cortical activation, which not only can be applied to monitor poststimulus but prestimulus processes as well. Usually, ERD is analyzed within a fixed range of alpha-frequency, typically within a range of 8-12 Hz. alpha-frequency, however, shows a considerable between subject variation, even in a sample of subjects with comparable age. The purpose of this study, thus is to show as to how the range of alpha-frequency affects the calculation of ERD. Two different methodological approaches were compared. First, ERD was calculated within a fixed range of 8-12 Hz. Second, ERD was calculated within an individual range, which was adjusted to the individually determined alpha-frequency of each subject. A sample of ten right handed subjects participated in an episodic memory experiment, in which target words, shown in a earlier session of the experiment, were to be identified. A time span of 375 ms preceding and following the presentation of a word was defined as the "pre-" and "poststimulus" interval. EEG-signals were recorded unipolarly from 29 leads. The results show a rather localized increase of ERD in the poststimulus interval. In the prestimulus interval on the other hand, the average magnitude of ERD is much lower and the topographical distribution less localized. Most importantly, however, when comparing the two methods of analyzing ERD, those ERD-values calculated within an individually determined alpha-band, are superior in order to differentiate between the pre- and poststimulus interval.

Adult↗
PubMed1987

[Visual evoked potentials and hemispheric differences: on the functional significance of homologous regions].

A total of 18 right handed subjects participated in an experiment, in which visually evoked potentials were recorded from four scalp electrodes (01, 02 and P3, P4) when words or flashes of diffuse light were presented as stimuli. The evoked potentials were subdivided into three components, an early component (Komp1, 0-150 ms) and two late components (Komp2, 150-300 and Komp3, 300-450 ms). For each of these components and for each subject, correlations were computed between different recording sites. The results show that homologous correlations (01:02, P3:P4) yield values, which are significantly higher than the values of the intra- (01:P3, 02:P4) and diagonal interhemispheric (01:P4, 02:P3) correlations. This result, which holds true for all of the three components in a similar way, is independent from the type of presented stimuli. In contrast to homologous correlations, intra- and diagonal interhemispheric correlations do vary as a function of stimulus type and different components as well. Here, the results show that the degree of correlation between different recording sites is positively related to the degree of cognitive load: with increasing cognitive load, the degree of synchronous neural activity increases. Homologous correlations are independent from the type of stimuli (words and pictures) and from the type of VEP-components (Komp1, 2, and 3) as well. Thus, it can be concluded that homologous correlations reflect attentional rather then cognitive processes.

Adult↗
PubMed1986

[Dynamic EEG mapping--an imaging procedure for studying perceptive, motor and cognitive brain performance].

The spatial pattern and time course of cortical activation can be studied when the event-related desynchronization in 30 EEG recordings is being quantified, averaged over about 60 events and displayed in form of maps with integration times of 125 ms (8 maps/s). This method was named dynamic mapping and used for investigating cortical activation pattern during the following tasks: Planning and execution of voluntary self-paced thumb movements, tactile stimulation of the foot sole and recognition of words, displayed shortly on a computer terminal. All these experiments resulted in localized activation pattern.

Cognition↗
PubMed1981

[Problem of classifying the commotio cerebri into severe and slight injuries (author's transl)].

Our material of medical legal investigations backs up the theory that the time of amnesia as part of the commotio cerebri seems to be prolonged under the influence of alcohol, and that retrograde amnesia is more frequent in drunken patients. A purely contusional damage of the skull of overtired and/or drunken persons more often leads to concussion of the brain. The case histories never contained detailed information about the exact duration of unconsciousness. Therefore, the common classification of the commotio cerebri into severe or slight injuries does not seem to be justified. It should better be defined as a slight injury.

Alcoholic Intoxication↗
PubMedDate not supplied

The value of P300 in the diagnosis of cognitive impairment in stroke.

The latency of the P300 and the simple reaction time (RT) generated by visual stimuli were investigated in a group of 43 healthy normals and in 24 stroke patients. The stroke patients were in a chronic phase of their disease. Twelve patients were without cognitive deficit, while 12 showed the symptoms of dementia. In the normals, P300 and RT showed a positive correlation with age. In comparing 12 age-matched normals with stroke patients with and without dementia, a similar latency of the P300 was found in the normals (347 msec) and in the stroke patients without dementia (349 msec). The patients with cognitive impairment showed a significant increase in their latency (434 msec). The RT was significantly different in the normals (421 msec) and stroke patients without dementia (533 msec) as well as in the stroke patients with dementia (663 msec). A significant correlation between the P300 and the RT was found.

Journal Article↗