Increased neuronal communication accompanying sentence comprehension.

The main purpose of this study was to examine large-scale oscillatory activity and frequency-related neuronal synchronization during the comprehension of English spoken sentences of different complexity. Therefore, EEG coherence during the processing of subject-subject (SS)- and more complex subject-object (SO)-relatives was computed using an adaptive fitting approach of bivariate auto-regressive moving average (ARMA) models which enabled the continuous calculation of coherence in the course of sentence processing with a high frequency resolution according to the dynamic changes of the EEG signals. Coherence differences between sentence types were observed in the theta (4-7 Hz), beta-1 (13-18 Hz) and gamma (30-34 Hz) frequency ranges, though emerging during the processing of different parts of these sentences: gamma differences were evident mainly during the relative clause while theta and beta-1 differed significantly following the end of the relative clause. These findings reveal no simple one to one map between EEG frequencies and cognitive operations necessary for sentence comprehension. Instead, they indicate a complex interplay and dynamic interaction between different EEG frequencies and verbal working memory, episodic memory, attention, morpho-syntactic and semantic-pragmatic analyses, which though distinct often co-occur.

Quantification of phase synchronization phenomena and their importance for verbal memory processes.

In the past years, interest in brain oscillations and their possible role in perceptual and cognitive processes has greatly increased. The two oscillations that have received the most attention are the theta and the gamma rhythm. In this study, the functioning and properties of phase synchronization parameters for these two frequency bands estimated by means of Gabor expansion were demonstrated with simulations for the phase-locking index (PLI) and the 1:1 as well as n:m phase synchronization indices. In order to demonstrate the importance of phase synchronization phenomena for memory performance, power, PLI and the 1:1 as well as n:m phase synchronization indices were calculated for EEG data on verbal memory encoding. These parameters showed various dissociations for recalled versus not-recalled nouns. In particular, the calculation of phase synchronization among different frequencies either at the same electrode or at different electrodes provided a completely new picture of dynamic neuronal interaction accompanying memory processing.

Cerebral information transfer during word processing: where and when does it occur and how fast is it?

Different regions of the brain have to interact to perform language processing. Such neural integration processes can be studied by measuring synchronization of oscillations. Coherence is the best known algorithm to study synchronization between two sites of the cortex with regard to selected oscillation frequencies. Phase coherence quantifies phase synchronization, in particular. In addition, cross phase (or simply phase) gives information about the direction and speed of the spread of oscillations. We use a new method to study short-time phases between different sites of the cortex in order to explore transient neural networks during word processing. Particularly, processing of abstract and concrete nouns was investigated by dynamic phase analysis of the alpha1 frequency band (8-10 Hz). Abstract and concrete nouns turned out to induce different dynamic networks of information transfer. Whereas processing of concrete nouns excites a widespread network for about 800 msec, the interhemispheric interactions during abstract noun processing are restricted to 300-500 msec after word presentation. Further, the direction of cerebral information transfer differs for the two word categories. Additionally, it could be shown that the propagation speed of information is slower for concrete nouns than for abstract nouns.