Changes of power and coherence in the EEG during verb and noun processing

Khader, P., and Rösler, F.
Department of Experimental and Biological Psychology, University of Marburg, Germany

Power and coherence of 124 channels of the electroencephalogram (EEG) were analyzed while 16 subjects read either a verb or a noun which initiated a short meaningful phrase (e.g. “CUTS carpenter wood” or “WOOD carpenter cuts”). Thus, participants were enforced to activate the specific semantic and grammatical features of the respective word category. Short time FFT power and coherence spectra were derived from overlapping Hanning-windowed time epochs covering 1 s after word presentation. Theta-power decreased substantially relative to a prestimulus baseline (fixation period) at left anterior electrode sites for both nouns and verbs. This left frontal power attenuation was more pronounced for verbs compared to nouns. All other frequency bands showed less (alpha, beta) or no effects at all (gamma). Coherence of the theta-band revealed that left anterior electrodes became decoupled from left and right posterior sites, again more for verbs than for nouns. These spectral changes are interpreted as reflecting processing differences due to the grammatical status of the two word categories, i.e., a differential left frontal involvement of verbal working memory resources. In addition to FFT-based analysis spectral effects were also analyzed with wavelets in order to delineate possible similarities and dissimilarties of the two methodological approaches. Both methods provided comparable results.