The topography of muscle activity in quantitative EEG.

Ten normal preadolescent subjects were studied on three occasions with quantitative EEG topography: two sessions recorded EEG that was free of artifact, but during the third the subjects were instructed to clench their teeth and tighten their faces to produce muscle artifact. The sessions were then compared for stability of various frequencies at standard scalp electrode sites. The posterior electrodes were stable among sessions for frequencies up to 24 Hz; the anterior electrodes were less stable, and above 24 Hz there were no stable electrode sites. Muscle artifact contaminates anterior electrode sites more than posterior sites, making the posterior scalp electrodes superior for studying beta activity in quantitative EEG. Frequencies above 24 Hz are contaminated at all sites and therefore cannot be assessed reliably in the presence of muscle artifact.