Peter Rappelsberger

Long-range EEG synchronization during word encoding correlates with successful memory performance

Distinct cortical activity during memory encoding of words, which were either recalled or not, was reported by a number of studies. This activity was mainly found at frontal and temporal/parietal brain regions. However, it was not clear if these regions interact with each other or work independently. In order to get a functional measure of the degree of neuronal large-scale cooperation, we calculated EEG coherence, which provides a statistical measure of synchronization between two EEG signals per frequency band. Therefore, coherence enables us to assess the functional interaction between cell assemblies of distant brain regions. The purpose of our study was to investigate if successfully recalled words show enhanced cortical synchronization compared with not recalled ones. Additionally, the influence of stimulus modality and the way different EEG frequencies participate in this process was examined. The EEG of 25 participants was recorded during memory encoding of concrete German nouns, either presented auditorily or visually and stimuli were separated according to the participants memory performance. Recalled nouns exhibited overall enhanced synchronization but showed typical patterns, especially between anterior and posterior brain regions in all frequency bands except the alpha-1 band (8-10 Hz). Recalling nouns was accompanied by increased synchronization between more distant electrodes in relation to an increase of synchronization between adjacent electrodes. Moreover, the degree of intrahemispheric synchronization was higher for recalled nouns. The pattern of EEG coherence and amplitude changes during verbal memory encoding allowed us to assess the probability that nouns would be recalled or not.

An E-Health Solution for Automatic Sleep Classification according to Rechtschaffen and Kales: Validation Study of the Somnolyzer 24 × 7 Utilizing the Siesta Database

To date, the only standard for the classification of sleep-EEG recordings that has found worldwide acceptance are the rules published in 1968 by Rechtschaffen and Kales. Even though several attempts have been made to automate the classification process, so far no method has been published that has proven its validity in a study including a sufficiently large number of controls and patients of all adult age ranges. The present paper describes the development and optimization of an automatic classification system that is based on one central EEG channel, two EOG channels and one chin EMG channel. It adheres to the decision rules for visual scoring as closely as possible and includes a structured quality control procedure by a human expert. The final system (Somnolyzer 24 × 7™) consists of a raw data quality check, a feature extraction algorithm (density and intensity of sleep/wake-related patterns such as sleep spindles, delta waves, SEMs and REMs), a feature matrix plausibility check, a classifier designed as an expert system, a rule-based smoothing procedure for the start and the end of stages REM, and finally a statistical comparison to age- and sex-matched normal healthy controls (Siesta Spot Report™). The expert system considers different prior probabilities of stage changes depending on the preceding sleep stage, the occurrence of a movement arousal and the position of the epoch within the NREM/REM sleep cycles. Moreover, results obtained with and without using the chin EMG signal are combined. The Siesta polysomnographic database (590 recordings in both normal healthy subjects aged 20–95 years and patients suffering from organic or nonorganic sleep disorders) was split into two halves, which were randomly assigned to a training and a validation set, respectively. The final validation revealed an overall epoch-by-epoch agreement of 80% (Cohen’s kappa: 0.72) between the Somnolyzer 24 × 7 and the human expert scoring, as compared with an inter-rater reliability of 77% (Cohen’s kappa: 0.68) between two human experts scoring the same dataset. Two Somnolyzer 24 × 7 analyses (including a structured quality control by two human experts) revealed an inter-rater reliability close to 1 (Cohen’s kappa: 0.991), which confirmed that the variability induced by the quality control procedure, whereby approximately 1% of the epochs (in 9.5% of the recordings) are changed, can definitely be neglected. Thus, the validation study proved the high reliability and validity of the Somnolyzer 24 × 7 and demonstrated its applicability in clinical routine and sleep studies.

Low-resolution brain electromagnetic tomography revealed simultaneously active frontal and parietal sleep spindle sources in the human cortex.

Analyses of scalp-recorded sleep spindles have demonstrated topographically distinct slow and fast spindle waves. In the present paper, the electrical activity in the brain corresponding to different types of sleep spindles was estimated by means of low-resolution electromagnetic tomography. In its new implementation, this method is based on realistic head geometry and solution space is restricted to the cortical gray matter and hippocampus. In multichannel all-night electroencephalographic recordings, 10-20 artifact-free 1.25-s epochs with frontally, parietally and approximately equally distributed spindles were marked visually in 10 normal healthy subjects aged 20-35years. As a control condition, artifact-free non-spindle epochs 1-3s before or after the corresponding spindle episodes were marked. Low-resolution electromagnetic tomography demonstrated, independent of the scalp distribution, a distributed spindle source in the prefrontal cortex (Brodmann areas 9 and 10), oscillating with a frequency below 13Hz, and in the precuneus (Brodmann area 7), oscillating with a frequency above 13Hz. In extremely rare cases only the prefrontal or the parietal source was active. Brodmann areas 9 and 10 have principal connections to the dorsomedial thalamic nucleus; Brodmann area 7 is connected to the lateroposterior, laterodorsal and rostral intralaminar centrolateral thalamic nuclei. Thus, the localized cortical brain regions are directly connected with adjacent parts of the dorsal thalamus, where sleep spindles are generated. The results demonstrated simultaneously active cortical spindle sources which differed in frequency by approximately 2Hz and were located in brain regions known to be critically involved in the processing of sensory input, which is in line with the assumed functional role of sleep spindles.

The SIESTA project polygraphic and clinical database

The SIESTA project had two major goals: developing new tools for analyzing computer-based sleep recordings and creating a reference database for sleep-related features. Basically, both goals have been reached, although validation and fine tuning of the sleep analyzer is still on-going. Investigations on the Web interface will be finished soon and a documentation of the database (including a CD-ROM with all test forms and all clinical, psychometric and actigraphic data as well as all R&K-scorings) will be published. Besides its scientific impact, the SIESTA project also emphasizes two other important aspects: the need of national and international cooperation between different experts and disciplines and the importance of standardized methods in scientific and clinical research.

Artifact processing in computerized analysis of sleep EEG - a review.

Quantitative analysis of sleep EEG data can provide valuable additional information in sleep research. However, analysis of data contaminated by artifacts can lead to spurious results. Thus, the first step in realizing an automatic sleep analysis system is the implementation of a reliable and valid artifact processing strategy. This strategy should include: (1) high-quality recording techniques in order to minimize the occurrence of avoidable artifacts (e.g. technical artifacts); (2) artifact minimization procedures in order to minimize the loss of data by estimating the contribution of different artifacts in the EEG recordings, thus allowing the calculation of the ‘corrected’ EEG (e.g. ocular and ECG interference), and finally (3) artifact identification procedures in order to define epochs contaminated by remaining artifacts (e.g. movement and muscle artifacts). Therefore, after a short description of the types of artifacts in the sleep EEG and some typical examples obtained in different sleep stages, artifact minimization and identification procedures will be reviewed.

EEG coherence within the 13–18 Hz band as a correlate of a distinct lexical organisation of concrete and abstract nouns in humans

Coherence analysis was applied to the EEG of 19 female participants who had to memorize auditorily presented abstract and concrete nouns. The EEG was recorded from 19 scalp electrodes (10/20 system). Significant differences between both word classes were found only in the beta 1-band (13-18 Hz) whereas the alpha 1-band (8-10 Hz) revealed coherence patterns which were identical for both word classes. These results indicate that the alpha 1-band reflects cognitive processes that were common to both word classes, whereas the beta 1-band seems to be closely related to associative processes and more complex cognitive functions.

Theta synchronization predicts efficient memory encoding of concrete and abstract nouns.

Functional and topographical differences between processing of spoken nouns which were remembered or which were forgotten were shown by means of EEG coherence analysis. Later recalled nouns were related with increased neuronal synchronization (= cooperation) between anterior and posterior brain regions regardless of presented word category (either concrete or abstract nouns). However, theta coherence exhibited topographical differences during encoding of concrete and abstract nouns whereby former were related with higher short-range (mainly intrahemispheric), later with higher long-range (mainly interhemispheric) coherence. Thus, theta synchronization possibly is a general phenomenon always occurring if task demand increases and more efficient information processing is required. Measurement of EEG coherence yields new information about the neuronal interaction of involved brain regions during memory encoding of different word classes.

Current source density analysis: Methods and application to simultaneously recorded field potentials of the rabbit's visual cortex

This paper deals with the application of current source density (CSD) analysis to simultaneously recorded intracortical field potentials of the rabbits visual cortex. Recordings were made with multielectrodes with either 8 contacts at distances of 300 μm, or 16 contacts at distances of 150 μm on one carrier needle. For synchronized activities, a spatial resolution of 150 μm turned out to be sufficient to record all depth-varying details of the field potentials; for seizure potentials even a spacing of 300 μm was adequate in most cases.For practical application, an appropriate spacing of the measuring points has to be chosen for a satisfactory estimation of the first and second derivatives of the field potentials. For this reason an interpolation procedure is applied to reduce the spacing from 300 μm or 150 μm electrode contact distances, respectively, and to obtain intermediate values at 75 μm distances. With this spacing satisfactory estimations of the second derivative are obtained.Theoretically, CSD analysis has to be made three-dimensionally, but under certain conditions which are discussed, a one-dimensional analysis can be applied. An unknown quantity is σz, the vertical conductivity. It turned out that average values obtained from different experiments are not representative and that the vertical conductivity has to be measured in every experiment. This is caused by the great individual differences of the cortices even if the same stereotactic coordinates are chosen. Therefore, in every experiment relative conductivity measurements are performed. The influence of different conductivity values within the various layers and the influence of a conductivity gradient is discussed and demonstrated by examples.

The reference problem and mapping of coherence: A simulation study

SummaryFour techniques are applied to record EEG signals: bipolar recording, referential recording, common average reference recording and source derivation. For the interpretation of EEG parameter maps knowledge of the properties of the applied recording technique is essential. Bipolar recordings are not discussed in this paper. The application of reference and common average reference recording has the disadvantage of an unknown reference potential. This disadvantage is much larger with the use of source derivation because every electrode signal has its particular reference signal. This must be taken into consideration when coherence estimations are made. With actual EEG records the influence of the reference cannot be determined unambiguously. However, simulation studies enable some essential conclusions. In this paper by means of autoregressive processes EEG signals with given power and coherence properties were simulated and different recording situations using the same data set were reconstructed. The essential result is that computation and mapping of coherences yield the most reliable results when reference recording is used. However, measures to ensure a low reference signal must be taken.

Calculation of event-related coherence—A new method to study short-lasting coupling between brain areas

SummaryThis article deals with the estimation of event-related coherence (ERCoh) and its application to the planning and execution of self-paced index finger movement. ERCoh estimation complements the event-related desynchronization (ERD) measurements of rhythms within the alpha band. ERCoh yields information of the functional relationships between different brain areas as a function of time. The time resolution is 125 msec. Before movement onset a contralateral ERCoh increase was found between premotor and motor areas. This coherence increase was accompanied by an ERCoh decrease in parallel to the ERD over the contralateral centro-temporal areas. During movement, the ERD became bilaterally symmetrical. Simultaneously, interhemispheric coherence between contralateral and ipsilateral sensori-motor areas increased.