Contamination of the EEG by physiological and exogeneous artifacts requires that clinicians take extensive precautions, examine the raw EEG record, and remove contaminated epochs through artifacting. Impedance tests and behavioral tests help ensure the fidelity of EEG recording.
BCIA's Neurofeedback Essential Skills List requires applicants to identify and remove artifact sources in EEG recordings. Learn to recognize normal EEG patterns and identify and correct noncerebral origin signals like bridging artifacts. Once you understand the mechanics and appearance of common artifacts, Peper et al. (2008) recommend that you intentionally reproduce them to recognize and prevent them.
Brain map validity depends on the integrity of the raw EEG. You will need at least 45 seconds of reasonably clean data to construct a valid brain map from raw EEG recordings of 3-10 minutes. However, the ideal amount of clean data is approximately 2 minutes for adequate database comparison. Realize that longer recordings risk increased drowsiness artifacts and sleep. Therefore, speaking to the client occasionally to help them maintain alertness is helpful. Simply saying how much time is left every 1-2 minutes is usually enough.
Stage 1 sleep is a subtle, drowsy state that clients often do not recognize. This state change is seen as a decrease in alpha and an increase in theta amplitudes. Slow eye-rolling movements and decreased EMG and beta will be observed. Stage 1 sleep graphic © John S. Anderson. Note the increased theta amplitudes in the spectral displays for channels 1 and 2.
The less frequent the artifact, the shorter the required recording period. Disable low-pass and high-pass filters before editing to visualize electro-ocular and SEMG artifacts better.
Worst case, as with a hyperactive child, none of the EEG channels may contain usable data, and you will need to repeat the assessment. Where artifacts only contaminate a few channels, you may base the assessment on clean channels (Demos, 2019).
Strategies to Reduce EEG/QEEG Artifacts
Demos (2019) recommends several precautions to reduce artifacts in raw EEG recordings:
Demonstrate how to create artifacts for your clients using screen displays while they clench their teeth, move their eyes, blink, swallow, and fidget
Confirm the cap fits properly
Use reclining chairs with negligible neck cushioning that can force the head downward to minimize SEMG artifacts
Limit eyelid movement with cotton balls gently touching the closed eyelids, secured by a loose sleep mask, flexible band, or tape in the eyes-closed recording. There should be no pressure against the eyes
Ensure that impedance values or DC offset values are appropriate for your amplifier (values under 5 Kohms are expected for publishable research, but values of less than 20 Kohms are acceptable for general clinical sessions and do not require excessive skin abrasion)
Only record qEEG data when the raw waveforms appear clean
EEG artifacts, consisting of noncerebral electrical activity, can be divided into physiological and exogenous artifacts. Physiological artifacts include electromyographic, electro-ocular (eye blink and eye movement), cardiac (pulse), sweat (skin impedance), drowsiness, and evoked potential. Exogenous artifacts include movement, 60 Hz and field effect, and electrode (impedance, bridging, and electrode pop) artifacts.
Electromyographic (EMG) Artifacts
EMG artifacts interfere with EEG recording by volume-conducting signals from skeletal muscles. This artifact contains high-frequency activity that resembles a "buzz" of fast activity during a contraction. EMG is seen as fast beta activity in the qEEG. While some frequencies are between 10-70 Hz, most are 70 Hz or higher.
The graphic below shows how high-frequency filter (HFF) selection can affect contamination by this artifact. A high-frequency filter (low-pass filter) attenuates frequencies above a cutoff frequency. In the examples below, the cutoffs are 35 Hz and 15 Hz.
All the channels on the left side of the tracing show SEMG artifacts admitted by a 35-Hz high-frequency filter. The right tracing is free from SEMG artifacts since its 15-Hz high-frequency filter attenuates the higher frequencies that contain these artifacts.
The next graphic shows how gum chewing can generate SEMG artifacts by contracting the muscles of mastication. Graphic © eegatlas-online.com.
The frequency spectrum for SEMG artifacts ranges from 2-1,000 Hz. While strong muscular contraction can contaminate all frequency bands, including 10 Hz, the beta rhythm (at 70 Hz or higher) is most affected by this artifact. EMG artifact may create the appearance of greater beta activity than is present. Graphic © eegatlas-online.com.
Below is a BioGraph ® Infiniti EMG artifact display. Note how the amplitude of the EEG spectrum increases with each contraction.
Thompson and Thompson (2016) observed that EMG artifacts are readily detected because they affect one or two channels, particularly at T3 and T4 at the periphery and less often at O1, O2, Fp1, and Fp2.
You can identify EMG artifacts by visually inspecting the raw signal. The next graphic shows SEMG artifacts using a 70-Hz high-frequency filter.
Electro-Ocular Artifacts
Electro-ocular artifacts contaminate EEG recordings with potentials generated by eye blinks, eye flutter, and other eye movements. For example, an anxious patient's eyelid flutter may cause deflections at Fp1 and Fp2 (Klass, 2008).
These artifacts are due to the eye’s electrical field movement when it rotates and the contraction of the extraocular muscles. The eye creates an electropositive dipole at the front and an electronegative dipole at the back. Bell’s Phenomenon refers to the upward rotation of the eye when it closes, which causes an artifact seen as an apparent increase in EEG.
The next two graphics show eye movement artifacts due to rapid blinking. Graphics © eegatlas-online.com.
The next graphic shows eye blinks, sharp lateral eye movement, and slow lateral eye movement.
Below is a BioGraph ® Infiniti EEG display of eye movement artifact.
Below is a NeXus display of eye blink and EMG © John S. Anderson.
An upward eye movement will create a positive deflection at Fp1, while a downward eye movement may create a negative deflection. In a longitudinal sequential montage, the artifact is typically seen at frontal sites (Fp1- F3 and Fp2-F4). A left movement may produce a positive deflection at F7 and a negative deflection at F8 (Thompson & Thompson, 2015).
Rapid eye flutter may resemble seizure activity. Graphic redrawn by minaanandag on fiverr.com.
Cardiac and Pulse Artifacts
Cardiac artifacts occur when the ECG signal appears in the EEG (Jiang et al., 2019). This artifact may be produced when electrode impedance is imbalanced, too high, or when an ear electrode contacts the neck. The frequency range for ECG artifact is 0.05-80 Hz, contaminating the delta through beta bands. Since multiple electrodes detect this artifact, it can create the appearance of greater coherence than is present. Graphic © eegatlas-online.com.
You can detect cardiac artifacts by inspecting the raw EEG waveform's chart recorder, data acquisition, or oscilloscope displays. Cardiac artifacts appear as a wave that repeats about once per second (Thompson & Thompson, 2016). Below is a BioGraph ® Infiniti ECG artifact display.
ECG artifacts are easily recognized, especially with a separate ECG tracing, allowing a direct comparison between recorded ECG and artifact in the EEG. Sharp, regular, consistent artifacts at about 1 per second in the EEG are likely the result of the ECG. These artifacts are observed best in referential montages using earlobe electrodes A1 and A2 or mastoid electrodes M1 and M2. Any patterns in the EEG that are synchronous with the ECG tracing can be identified as such an artifact. ECG artifact graphic was produced by Garces et al. (2007).
Another cardiac-related artifact are the pulse artifacts that occur when an EEG electrode is placed directly over a blood vessel. The mechanical movement of the electrode as a vessel expands and contracts will produce a slow-wave pattern in the EEG that can be mistaken for delta activity and will affect topographic maps created from data containing such an artifact. This is sometimes called the cardioballistic artifact, though another similar-looking artifact is thought to be related to the subtle movement of the body as the heart beats. Graphic © eegatlas-online.com.
Sweat (Impedance) Artifacts
Sweat artifacts result from sweat on the skin changing the conductive properties under and near the electrode sites (i.e., bridging artifact). Sweating reduces electrode contact with the scalp and generates large-scale up-and-down EEG line movements in several frontal channels. Abrupt, unexpected stimuli often elicit this artifact and usually appear as isolated 1-2 Hz slow waves of 1-2-s duration at frontal and temporal sites (Thompson & Thompson, 2015). Graphic © eegatlas-online.com.
Bridging Artifacts
A short circuit produces bridging artifacts between adjacent electrodes due to excessive electrode paste application, a client sweating excessively, or arriving with a wet scalp. Bridging artifacts can cause adjacent electrodes to create a short circuit that produces identical referential EEG recordings or a flat line with a bipolar montage. The Fp1-F3 channel's reduced amplitude and frequency illustrate a bridging artifact.
Drowsiness Artifacts
Drowsiness artifacts involve the appearance of stage 1 or stage 2 sleep in the EEG. Stage 1 and stage 2 of sleep are most likely during eyes-closed recording. Sleep may occur during eye-closed awake recording. Graphic © eegatlas-online.com.
Two additional drowsiness artifact examples appear below. The first shows a brief episode of drowsiness lasting about 5 seconds with a dropout of the alpha rhythm (posterior dominant rhythm - PDR) and then a return of the PDR. The second example shows the end of a longer period of light sleep with a K-complex indicated in the F3-C3 derivation, followed by a return to a typical alpha rhythm.
Stage 1 sleep is a subtle, drowsy state that clients often do not recognize. Alpha amplitude (especially occipital) may decrease, and theta (especially frontal) may increase. Reductions in EMG and beta amplitude will accompany slow eye-rolling movements. Sleepiness may be accompanied by spike-like transients (vertex or V-waves). The graphic below © John S. Anderson shows increased theta during stage 1 sleep.
When you detect drowsiness artifacts during a training session, suspend recording and instruct your clients to move their hands and legs to increase wakefulness. To avoid this artifact, ask them to retire early and sleep for 9 hours if possible (Thompson & Thompson, 2003).
Evoked Potential Artifacts
Evoked potential artifacts (also called event-related potential artifacts) consist of somatosensory, auditory, and visual signal processing-related transients that may contaminate multiple channels of an EEG record. While evoked potentials increase recording variability and reduce reliability, they minimally affect averaged data (Thompson & Thompson, 2015).
Watch BPM Biosignals' YouTube video EEG: Visually evoked potentials (VEP).
Movement Artifacts
Movement artifacts are caused by client movement or the movement of electrode wires by other individuals. Most of these artifacts are produced by brief changes in the electrode-skin surface connection. Cable movement is called cable sway.
Movement artifacts can produce high-frequency and high-amplitude voltages identical to EEG and EMG signals. While the delta rhythm is most affected by this artifact, it may also contaminate the theta band (Thompson & Thompson, 2016). Graphic © eegatlas-online.com.
The graphic below shows movement artifacts due to head movement (left), respiration (center), and tongue movement (right).
Below is a BioGraph ® Infiniti cable movement artifact display. Note the two voltage spikes at the beginning of the recording.
50/60 Hz and Field Artifacts
Both 50/60 Hz and field artifacts are external artifacts transmitted by nearby electrical sources. While 60-Hz artifact is a risk in North America, where AC voltage is transmitted at 60 Hz, 50-Hz artifact is a problem in other locations that generate power at 50 Hz. Their fundamental frequency is 50 or 60 Hz, with harmonics at 100/120 Hz, 150/180 Hz, and 200/240 Hz. Imbalanced electrode impedances increase an EEG amplifier's vulnerability to these artifacts. The 60-Hz artifact graphic below © John S. Anderson.
Below in red is a BioGraph ® Infiniti display of 60-Hz artifacts. Note the cyclical voltage fluctuations and 60-Hz peak in the power spectral display.
Radiofrequency Artifacts
Radiofrequency (RF) artifacts radiate outward like a cone from the front of televisions and computer monitors. Graphic © eegatlas-online.com.
Electrode Artifacts
There are several sources of electrode artifacts. Even with proper care, electrode surfaces can become corroded, and the leads and connectors can be damaged. Using sensors with mismatched electrode metals can cause polarization of amplifier input stages.
Impedance Artifacts
Unless skin-electrode impedance is low (under 5 KΩ for research and 20 KΩ for training) and balanced (under 1-3 KΩ ), diverse artifacts like 50/60 Hz and movement can contaminate the EEG signal, as seen in the P3 and Pz electrodes. Graphic © eegatlas-online.com.
Electrode Pop Artifacts
Mechanical disturbance can produce a unique artifact even when the impedance is low and balanced. Electrode pop artifact has a sudden large deflection in at least one channel when an electrode abruptly detaches from the scalp. This may also happen when there is a bubble or other defect in the gel or paste, and a charge builds up and subsequently "jumps" across the gap, resulting in a large electrical discharge. Graphic © John S. Anderson.
Glossary
50/60 Hz: external artifacts transmitted by nearby electrical sources.
balanced electrode impedance: skin-electrode impedances within 2-3 Kohms of each other.
bridging artifacts: a short circuit between adjacent electrodes due to excessive application of electrode paste or a client who is sweating excessively or who arrives with a wet scalp.
cable sway: the movement or displacement of electrode cables during the recording session. This movement can introduce artifacts or noise into the EEG signal, potentially interfering with the accurate interpretation of brain activity.
cardiac artifacts: the contamination of the EEG by the ECG signal.
drowsiness artifacts: in adults, 1-Hz (or slower) waveforms can be detected with greatest amplitude and reverse polarity at F7 and F8 may progress to 1-2 Hz slowing of the alpha rhythm.
EEG artifacts: noncerebral electrical activity in an EEG recording can be divided into physiological and exogenous artifacts.
electro-ocular artifacts: contamination of EEG recordings by potentials generated by eye blinks, eye flutter, and eye movements.
electrode pop artifacts: sudden large deflections in at least one channel when an electrode abruptly detaches from the scalp.
EMG artifacts: interference in EEG recordings by volume-conducted signals from skeletal muscles.
evoked potential artifacts (event-related potential artifacts): somatosensory, auditory, and visual signal processing-related transients that may contaminate multiple channels of an EEG record.
exogenous artifacts: noncerebral electrical activity generated by movement, 50/60 Hz and field effect, bridging, and electrode (electrode “pop" and impedance) artifacts.
field artifacts: external artifacts transmitted by nearby electrical sources.
high-frequency filter (HFF): a filter that attenuates frequencies above a cutoff frequency.
impedance artifacts: high and/or imbalanced impedance between the scalp electrodes and the skin can distort or disrupt the EEG signal, affecting the accuracy of the recorded brain activity.
movement artifacts: voltages caused by client movement or the movement of electrode wires by other individuals.
physiological artifacts: noncerebral electrical activity that includes electromyographic, electro-ocular (eye blink and eye movement), cardiac (pulse), sweat (skin impedance), drowsiness, and evoked potential.
pulse artifacts: noncerebral voltages due to the mechanical movement of an electrode in relation to the skin surface due to the pressure wave of each heartbeat
radiofrequency (RF) artifacts: interference caused by external electromagnetic signals, typically from nearby electronic devices or radio frequency sources. These signals can contaminate the EEG recording, leading to distortions or noise in the recorded brain activity.
sweat artifacts: changes in the EEG signal when sweat on the skin changes the conductive properties under and near the electrode sites (i.e., bridging artifact).
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