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.

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.

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.

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.

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.