EEG Artifact Identification 2 — Questions and Answers
Question 1: Which artifact appears as a regular, rhythmic waveform at the same frequency as the patient's heart rate?
- Eye blink artifact
- Pulse artifact (ECG artifact) (Correct answer)
- Electrode pop
- Glossokinetic artifact
Correct answer: Pulse artifact (ECG artifact)
Pulse artifact appears as rhythmic waveforms synchronous with the cardiac cycle, caused by electrode placement over a pulsating blood vessel or by ballistocardiographic effect.
Pulse artifact (also called ECG or ballistocardiographic artifact) appears when an electrode sits over or near a pulsating scalp artery. The resulting rhythmic waveform is time-locked to the QRS complex on the ECG channel. It is most common at temporal electrodes near the superficial temporal artery. Confirmation involves comparing the suspicious rhythm with the simultaneous ECG channel — if the waveforms are synchronous with the heartbeat, it is pulse artifact. Repositioning the electrode slightly away from the vessel usually resolves it. This artifact must be distinguished from rhythmic temporal theta, which has no ECG correlation.
Question 2: Eye blink artifact is primarily seen at which electrode positions?
- Occipital electrodes (O1, O2)
- Temporal electrodes (T3, T4)
- Frontal electrodes (Fp1, Fp2) (Correct answer)
- Central electrodes (C3, C4)
Correct answer: Frontal electrodes (Fp1, Fp2)
Eye blink artifact produces high-amplitude positive deflections most prominent at Fp1 and Fp2 because the cornea (positive pole of the eye dipole) moves upward toward these frontopolar electrodes during blinking.
The eye functions as an electrical dipole with the cornea positive and the retina negative. During blinking, Bell's phenomenon causes upward eye rotation, bringing the positive cornea closer to Fp1 and Fp2 electrodes. This creates high-amplitude (often >100 µV) positive deflections at frontal sites that diminish rapidly with distance posteriorly. Eye blink artifacts are synchronous bilaterally and have a characteristic morphology — sharp onset with slower decay. They are confirmed by EOG channels. Understanding the corneo-retinal dipole model is essential for distinguishing eye movement artifacts from frontal epileptiform activity.
Question 3: What is the most effective method for reducing 60 Hz electrical interference artifact?
- Increasing the high-frequency filter to 100 Hz
- Ensuring all electrode impedances are balanced and below 5 kOhm (Correct answer)
- Switching to a bipolar montage
- Increasing the recording sensitivity
Correct answer: Ensuring all electrode impedances are balanced and below 5 kOhm
Balanced, low impedances (below 5 kOhm) allow the differential amplifier to effectively reject common-mode 60 Hz noise. Impedance imbalance between electrodes is the primary cause of excessive 60 Hz artifact.
The 60 Hz (or 50 Hz in some countries) power line artifact is best controlled by maintaining low, balanced electrode impedances. EEG amplifiers use common-mode rejection (CMRR) to cancel signals that appear equally at both inputs. When impedances are mismatched, the 60 Hz signal is not equally attenuated, resulting in residual artifact. The ACNS recommends impedances below 5 kOhm and matched within 1 kOhm. While a 60 Hz notch filter can reduce the artifact, it also distorts spike morphology and should be a last resort. Proper skin preparation, adequate gel, and checking impedances before recording are the primary preventive measures.
Question 4: Glossokinetic artifact is caused by movement of which structure?
- The jaw muscles during chewing
- The tongue, which acts as an electrical dipole (Correct answer)
- The scalp muscles during frowning
- The diaphragm during breathing
Correct answer: The tongue, which acts as an electrical dipole
The tongue has a tip-negative, base-positive electrical dipole. Tongue movements generate slow potentials that are transmitted to scalp electrodes, particularly in frontal and temporal regions.
Glossokinetic artifact arises from the electrical dipole of the tongue (tip negative, base positive). During tongue movements — talking, swallowing, or even small involuntary movements — these potentials propagate to scalp electrodes, producing slow (delta range) rhythmic or semi-rhythmic activity most prominent in frontotemporal regions. It can mimic frontal intermittent rhythmic delta activity (FIRDA) or temporal slowing. Identification relies on correlating the activity with observed tongue or mouth movements and its characteristic morphology. Asking the patient to stop talking or hold still can help confirm the source.
Question 5: An electrode pop artifact on EEG is characterized by:
- Gradual onset sinusoidal waveform
- Abrupt, high-amplitude deflection confined to a single channel (Correct answer)
- Bilateral synchronous sharp waves
- Rhythmic activity at 14 Hz
Correct answer: Abrupt, high-amplitude deflection confined to a single channel
Electrode pop appears as a sudden, high-amplitude spike-like deflection isolated to one channel, caused by abrupt impedance change at a single electrode due to drying gel, poor contact, or mechanical disturbance.
Electrode pop artifact is caused by an abrupt change in impedance at a single electrode, producing a high-amplitude, sharp transient that appears in all channels containing that electrode. In a bipolar montage, it appears as a phase reversal at the affected electrode — deflecting upward in the channel where it is input 1 and downward where it is input 2. The key identifying feature is that it is confined to one electrode location (appears in channels sharing that electrode). It can mimic epileptiform spikes but lacks the physiological field distribution of true brain activity. Reapplying gel or replacing the electrode resolves the issue.
Question 6: Muscle artifact from the temporalis muscle can be distinguished from beta activity because muscle artifact typically:
- Has a frequency below 8 Hz
- Shows frequencies above 20 Hz with irregular sharp morphology maximal at temporal electrodes (Correct answer)
- Appears only during sleep
- Is enhanced by hyperventilation
Correct answer: Shows frequencies above 20 Hz with irregular sharp morphology maximal at temporal electrodes
Temporalis muscle (EMG) artifact produces high-frequency (typically >20 Hz) irregular, sharp activity maximal at temporal and frontal electrodes, which can obscure underlying brain activity in those regions.
Muscle artifact from the temporalis and frontalis muscles produces high-frequency activity typically above 20 Hz, often extending to 100+ Hz. It appears as irregular, sharp, spiky waveforms predominantly at F7/F8, T3/T4, and Fp1/Fp2 where these muscles overlay the skull. Unlike cerebral beta activity (13-30 Hz), which is relatively low-amplitude, smooth, and symmetrically distributed, muscle artifact is higher frequency, irregular in morphology, and amplitude varies with tension. Having the patient relax the jaw, slightly open the mouth, or use a bite block can reduce temporalis artifact. It is absent during sleep, which helps differentiate it from pathological fast activity.
EEG Artifact Identification 2
Which artifact appears as a regular, rhythmic waveform at the same frequency as the patient's heart rate?