Biomedical subjects

M Doppelmayr

Publications and source records attributed to M Doppelmayr.

10 recordsLinked to original sources

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

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

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

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↗