When the patient is relaxed with eyes closed, the background is usually characterized by the posteriorly dominant alpha rhythm, also known simply as the posterior dominant rhythm. Healthy adults typically manifest relatively low-amplitude, mixed-frequency background rhythms, also termed desynchronized. See Appendix 4 for representative common EEG artifacts seen during wakefulness. Another common artifact during the waking EEG is caused by swallowing and the related movement of the tongue, which similar to the eye is a dipole and causes a slow potential with superimposed muscle artifact. Since the positivity of the cornea rotates upward toward frontal electrode sites, a transient positivity, then negativity is recorded there. This occurs because the eye is a dipole, relatively positive at the corneal surface and negative at the retinal surface, and the eye moves characteristically upward during a blink according to Bell phenomenon, resulting in a moving charge and potential change. Extremely large-voltage, diphasic potentials in frontal regions result from blinks. Rapid eye movements (REMs), resulting from saccades and spontaneous changes of gaze, may be seen as small, rapid deflections in frontal regions. Most notable is the presence of low-amplitude, high-frequency activity arising from scalp muscles, often frontally dominant but seen throughout the tracing. Artifacts are common during the wakeful EEG, and one of the first hurdles of EEG interpretation is distinguishing these from cerebral signal.

Eeg normal vs epilepsy