Investigating the contamination of electroencephalograms by facial muscle electromyographic activity using matching pursuit

It has been widely recognized and previously reported that electrical field s from facial muscle electromyographic (EMG) activity can contaminate the e lectroencephalogram (EEG), even when closely spaced, bipolar electrode conf igurations are used (personal observations). We suspected that EEG signals evoked in response to pressure changes in the upper airway may include EMG contamination subsequent to muscle reflexes triggered by the stimuli. We ev aluated the potential contamination of the background EEG by voluntary acti vation of a facial muscle by obtaining simultaneous recordings in human sub jects of the EEG (from Cz-C4) and masseter muscle EMG (from a bipolar surfa ce electrode pair) before (quiet) and after voluntary tensing (VTen). Match ing pursuit analysis permitted identification of different time-frequency p atterns for each signal during the quiet period because the EMG signal has mostly atoms above 30 Hz compared to the EEG signal. However, the EEG showe d periods of low-frequency activity unmatched in the EMG TF pattern below 3 0 Hz. During the tensing, most of the atoms of both the EEG and EMG shifted to the higher frequency regions above 100 Hz, making the separation diffic ult. These results further suggest that the matching pursuit method may not separate the background EEG from phasic EMG signals, both of which an nons tationary in nature. (C) 1999 Academic Press.