ABSTRACT
This contribution deals with EEG coherence and phase analyses using two different techniques. First, Fourier transform was applied to compute cross-spectra between pairs of electrode signals of EEG trials of constant length. The cross-spectra of a number of trials were averaged, yielding coherence and phase spectra representing the mean properties within the length of the trials, usually 1 or 2 seconds. Second, an adaptive autoregressive-moving-average (ARMA) model was used to compute instantaneous coherence and phase values with a time resolution in the millisecond range. In the first experiment EEG was recorded during auditory presentation of concrete and abstract nouns. 19 right-handed female native German speakers participated. The main coherence results using the Fourier approach relate to the Alpha-1 band (8-10 Hz) and the Beta-1 band (13-18 Hz). In the Alpha-1 band both word classes revealed about the same changes during word processing, suggesting that this band reflects processes common to both word classes. In contrast, in the Beta-1 band, clear differences were found. These differences concern mainly the involvement of visual and frontal association areas, probably due to visual images evoked by the concrete nouns. The second experiment was conducted with 25 right-handed females. Concrete and abstract nouns were presented auditorily but also visually. The main coherence results using the Fourier approach revealed that the Alpha-1 band is sensitive to the modality of stimulus presentation, but does not distinguish between the memorization of abstract and concrete nouns. In contrast, coherence differences independent of stimulus modality were found in the Delta, Theta and Beta-1 bands. Measurement of time relations, i.e. studies of the direction of information transfer, was made using both the Fourier approach for time intervals and the ARMA approach at time instants during the memorisation of nouns. Due to the highly dynamic process, with changing directions of information transfer, the Fourier approach, based on 1 s trials after stimulus onset, yielded only very coarse estimations of the time relations during word processing. The short-lasting properties during word processing
were smeared and to a great extent extinguished. In single subject studies, the ARMA approach clearly demonstrated an occipital-frontal information transfer in the Beta-1 band for the visual stimulus presentation. During auditory stimulus presentation temporal sites tended to lead occipital sites but there was also a trend for occipital sites to lead central and left frontal sites. KEYWORDS: EEG coherence; Phase relations; Fourier spectra; Adaptive ARMA model; Word processing; Concrete and abstract nouns; Visual and auditory word presentation
1. INTRODUCTION
The EEG has been used for decades as a valuable tool for diagnostic purposes in neurology and psychiatry. However, it contains much more information than can be extracted by pure visual examination. Many efforts have been made, and highly sophisticated mathematical procedures and computer techniques have been applied, to extract that information.
