Three questions
- Does it have a field? Cerebral potentials fall off gradually across neighbouring electrodes. One electrode alone is an artefact.
- Does the field make anatomical sense? A "discharge" at Fp1 and O2 with nothing between does not.
- What was the patient doing? Video, the technologist's notes and the ECG channel answer most questions.
Eye movements
The eye is a dipole, cornea positive. A blink rolls the globe upward and makes Fp1 and Fp2 positive. Lateral gaze makes the electrode the eyes turn towards positive: looking left, F7 goes positive and F8 negative. 11
Muscle
EMG is faster and spikier than anything the cortex produces at the scalp, and it sits over muscles: temporalis, frontalis, the neck. Chewing gives rhythmic temporal bursts; a single motor unit can mimic a periodic spike.
Heart and pulse
ECG artefact is time-locked to the QRS. Pulse artefact is a slow wave at one electrode lying on a scalp artery, following each QRS by a constant delay.
Other physiological sources
- Sweat: very slow drifts, below 0.5 Hz, wandering from channel to channel.
- Glossokinetic: the tongue is a dipole too; speech and swallowing give frontal and temporal delta.
- Respiration and movement: slow swings in step with breathing, especially in electrodes the head rests on.
Electrodes and environment
An electrode pop is abrupt and involves one electrode. Mains interference is a perfectly regular 50 or 60 Hz. In intensive care the list grows: ventilator circuits with condensed water, chest physiotherapy, bed percussion, infusion pumps, dialysis and ECMO machines, pacemakers.
Rhythmic artefacts are the dangerous ones. Patting, tremor, chewing and ventilator artefact can all appear to evolve like a seizure. If a rhythmic pattern has no plausible field or stops when someone touches the patient, look at the video before treating.