Evoked coherence of EEG: 1. Mathematical foundation and methodological aspects

Journal/Book: Eeg-Emg-Z Elektroenz Elektrom. 1995; 26: P O Box 30 11 20, D-70451 Stuttgart, Germany. Georg Thieme Verlag. 215-221.

Abstract: Coherences of EEG, as sensitive indicators of the brain activity, find increasing interest in special literature. As a rule, however, these coherence values are calculated upon relatively large analysis intervals (from several seconds up to minutes) and reflect, this way, a general state of functioning rather than elementary operations of the brain. By contrast, the method proposed in this paper makes possible the recognition of short-lasting coherence phenomena within the context of CNS stimulus processing. For the purpose of distinction from ''steady-state'' coherences mentioned above, in analogy to evoked potentials, the terminus ''evoked coherence'' has been introduced. In the frame of this method, the ongoing EEG is decomposed by time-varying spectral analysis into components of variable frequency and amplitude, which are in good accordance with the classical frequency bands delta, theta, alpha and beta. Each of these frequency bands is fully represented by only one sinuidal oscillation. The component-related coherence values, then, are obtained by cross-correlation of corresponding components of different localization. Length and position of the post-stimulus analysis interval are chosen with regard to the components period - e.g. from 1/2 to 2 1/2 mean wave lengths after the stimulus onset. To eliminate random effects, these single-stimulus coherences will be averaged over a set of equal type stimuli. For getting a data basis, a simple odd-ball-paradigm has been used. Seven healthy, right-handed subjects were asked to count deviant tones within a tone series. The EEG was led from 16 positions of the head surface (reduced 10:20-system). Both for the target and standard tones, the results obtained show apparent enhancements of theta-coherences after stimuli between Pz and the temporal and the frontal regions of the cortex, compared with the resting condition. These effects were higher for the targets, which, additionally, caused higher intraparietal coherences in the delta-range. The results proved to be significant. Evoked coherences of EEG components seem to provide specific insights into the co-operation of different areas in specific stimulus processing of the brain.

Note: Article HJ Volke, Tech Univ Dresden, Inst Humanbiol & Biopsychol, Mommsenstr 13, D-01068 Dresden, Germany

Keyword(s): EEG; coherence; evoked coherence; time-variant spectral analysis; SCHIZOPHRENIC-PATIENTS; ELECTRICAL-ACTIVITY; BRAIN; PARAMETERS; MATURATION; ACTIVATION; THINKING; TASKS; POWER; AIDS

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