Chapter 2: Artifacts

Characteristics of artifact

• ▪ If the activity is limited to a single channel or electrode, it should be assumed to be an artifact until proven otherwise.

• ▪ Repetitive, irregular, or rhythmic waves that occur simultaneously in unrelated head regions.

Physiological artifacts

Eye movements

• ▪ Eyeball acts as a dipole with positive polarity of 100 mV at the cornea with respect to the retina.

• ▪ Bell's phenomenon (upward eye deviation during eye closure) causes positive electrical activity at Fp1 and Fp2.

• ▪ Lateral eye movement causes positivity to the side of which gaze is directed to (either F7 or F8).

• ▪ Oblique eye movement is more difficult and can be misinterpreted as a focal abnormality.

• ▪ Courses of asymmetric eye movements include:

  • ▸ Decreased movement of one eye or eyelid

  • ▸ Absence of an eye or retina

  • ▸ Asymmetric electrode placement

  • ▸ Frontal skull defect

Rapid Eye Movement (REM)

(Figure 2-1)

• ▪ With lateral eye movements, the eyes are moving to the side where positivity is noted. This is caused by the positivity of the cornea (100 mV positive compared to retina) coming closer to the F7 or F8 electrodes making positive the one toward which the eyes are moving.

• ▪ Asymmetric waves with steeper rise than fall.

• ▪ REM stage in healthy children does not occur within the first cycle but after one complete cycle (stage 1 to 4 and then back from 4 to 1), usually 90 minutes after sleep onset. If REM sleep appears near the onset of sleep (early REM), narcolepsy must be considered. However, early REM can be seen in individuals withdrawing from CNS depressants such as barbiturates or alcohol.