ABSTRACT
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.
Subject(s)
Alpha Rhythm , Attention/physiology , Adult , Electroencephalography , Evoked Potentials , Female , Humans , Male , Reaction TimeABSTRACT
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.
Subject(s)
Cortical Synchronization , Evoked Potentials, Visual/physiology , Periodicity , Adult , Alpha Rhythm , Delta Rhythm , Discrimination Learning/physiology , Female , Humans , Male , Psychomotor Performance/physiology , Reaction Time/physiology , Theta RhythmABSTRACT
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.
Subject(s)
Alpha Rhythm/psychology , Cortical Synchronization/psychology , Language , Adult , Attention/physiology , Evoked Potentials/physiology , Female , Humans , Male , Memory/physiologyABSTRACT
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.
