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

It has been widely recognized and previously reported that electrical fields from facial muscle electromyographic (EMG) activity can contaminate the electroencephalogram (EEG), even when closely spaced, bipolar electrode configurations 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 evaluated the potential contamination of the background EEG by voluntary activation of a facial muscle by obtaining simultaneous recordings in human subjects of the EEG (from Cz-C4) and masseter muscle EMG (from a bipolar surface electrode pair) before (quiet) and after voluntary tensing (VTen). Matching pursuit analysis permitted identification of different time-frequency patterns 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 showed periods of low-frequency activity unmatched in the EMG TF pattern below 30 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 difficult. These results further suggest that the matching pursuit method may not separate the background EEG from phasic EMG signals, both of which an nonstationary in nature. (C) 1999 Academic Press.