EEG artifacts

EKG artifacts are charcterized by regular, monophasic, low-amplitude, spiky waveforms that are time-locked with the QRS complexes in the EKG leads.

Myogenic artifacts are characterized by high frequency, ‘spiky’ appearing activity in the anterior (frontal) leads (due to frontalis muscle contraction), or temporal leads (due to temporalis mucle contraction), along with an absence of a physiological field denoting non-cerebral source.

Eye blinks are characterized by relatively higher amplitude positive/downward deflections that are symmetrically seen in the frontal (anterior) leads maximally (Fp1 and Fp2). The globe of the eye intrinsically has a dipole where the cornea is positive and the retina negative. The conjugate upward movement of the eye globes associated with blinking induces a relatively positive potential at Fp1 and Fp2 electrodes thus generating a positive/downward deflection on EEG, known as Bell's phenomenon.

Eye fluttering is chracterized by rhythmic 3-4 Hz activity in the frontal leads (restricted to Fp1 and Fp2 electrodes) that appear similar to eye blinks, albeit in trains.

Lateral eye movements are charcterized by out-of-phase deflections in the anterior leads (Fp1-F7/F7-T3 and Fp2-F8/ F8-T4 electrodes). Due to the intrinsic dipole of the eye globe where the cornea is positive and the retina negative, a right-sided eye movement will cause F8 to be positively charged and F7 to be simultaneously negatively charged, whereas a left-sided eye movement will cause F7 to be positively charged and F8 to be simultaneously negatively charged. The same movements can also be seen as undulating waveforms in the LOC-ROC channel where electrodes are placed on the left outer canthus (LOC) and right outer canthus (ROC).

An electrode pop is characterized by a high-voltage waveform confined to a single electrode, which can occur due to poor contact of the electrode with skin, poor connection of electrode to the wire, or a fractured lead wire. It tends to be higher amplitude than those of the surrounding electrodes and presents with a rapid rise followed by a slower fall. There is often recurrence within a short time if the affected electrode is not fixed and may have pseudoevolution.

50/60 Hz artifact is characterized by diffuse, high-frequency, activity in the 50 or 60 Hz range which is an environmental, electrical artifact produced by AC line frequencies (60 Hz in United States, 50 Hz in Europe). The notch filter can be used to eliminate this artifact.

A lateral rectus spike is characterized by a vert short, relatively low voltage spiky waveform at the beginning of a fast, lateral eye movement. These are frequently recorded in F7 and F8 electrodes immediately prior to saccadic horizontal or oblique eye movements. This artifactual waveform, which represents an isolated motor unit potential in the lateral rectus muscle, is most noticeable during rapid eye movement (REM) sleep but is also often seen during wakefulness.