Brain oscillations in the EEG and their functional correlates: Measured by ERD and other methods

A variety of studies, particularly from our laboratory, have shown that event-related changes in band power (ERD) in different frequency bands reflect different cognitive processes. The interesting finding thereby is that (i) different types of cognitive processes are associated with band power changes in different but comparatively narrow frequency bands (of 2 Hz width) and that (ii) these findings are obtained only if the frequency bands are adjusted to the individual alpha frequency (IAF) of each subject. As an example, synchronization in the theta band (4-6 Hz below IAF) is associated with a successful encoding and retrieval of new (episodic) information whereas desynchronization in the upper alpha band (0-2 Hz above IAF) reflects (semantic) long-term memory processes. Because these findings are obtained only if the frequency bands were adjusted to IAF and because alpha reflects oscillatory processes in the brain, good arguments are at hand to assume that brain oscillations in general are related to cognitive processes. There are, however, possible objections against this assumption which will be tested in the proposed research project. One possible objection may arise from the fact that in almost all of our studies we used visually presented words. Because other research groups have found that (simple) acoustic stimuli induce strong alpha synchronization (instead of the expected desynchronization) the question arises whether or not our findings are restricted to cognitive processes within the visual domain. A series of experiments is suggested in order to test this and related questions about the general validity and possible extension of our results. Another important objection against the general validity of our findings refers to the question, whether (and if, to what extent) our findings are contaminated by the influence of event-related potentials (ERPs) or evoked EEG activity. In order to address this question, we use a newly developed method that is based on the "intertrial variance technique". This method allows (i) to calculated band power changes that are deprived from evoked EEG activity and (ii) to calculate (on a trial per trial basis) the percentage of that power of ERP components that are due to certain frequency bands.

The findings from project P13047 indicate that different EEG oscillations in the theta and alpha frequency range are associated with different types of cognitive and memory processes in particular. Whereas event-related changes in the theta band (of about 4-6 Hz) appear to be related to encoding and retrieval processes of a complex working memory system, the upper alpha frequency range (of about 10 - 12 Hz) responds selectively to sensory-semantic memory processes of a complex long-term memory system and the lower alpha band to attentional processes. It is important to emphasize that the functional specificity of these frequency bands can be observed only if frequency boundaries were adjusted to individual alpha frequency and if rather narrow bands were used Furthermore, we were able to demonstrate that theta and alpha oscillations are related to memory performance if a double dissociation between two types of power measurements (resting and event-related power) and two frequency bands (theta versus alpha) is taken into account. This double dissociation is given by two facts. First, with increasing task demands, event-related band power increases in the theta but decreases (desynchronizes) in the alpha frequency range. Second, the extent of an event-related EEG response depends at least in part on the extent of resting power but in opposite ways for theta and alpha. Whereas small resting theta power enhances theta synchronization, large resting alpha power enhances alpha desynchronization. These findings have led us to state a new hypothesis: If good cognitive performance is related to large resting but small task related upper alpha power (due to a large suppression of alpha), it should be possible to induce this type of EEG change by oscillatory stimulation, e.g., by repetitive transcranial magnetic stimulation (rTMS) at individual (upper) alpha frequency during a reference interval preceding task performance. We have tested this hypothesis in a recent study (in a collaboration with Prof. Gerloff from the University of Tübingen). The findings indicate that only rTMS delivered at the subjects individual alpha frequency (IAF) leads to a significant improvement in performance. Furthermore, the influence of rTMS at IAF on EEG parameters mimicked exactly that situation which we know is typical for good performance: increased reference power, decreased test power and, consequently, a large ERD. The interesting conclusion is that rTMS at IAF improves performance by way of those factors which are known to be of importance under normal conditions. Thus, the present findings suggest that the relationship between the dynamics of alpha desynchronization and cognitive performance is not correlative but causal in nature.