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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →EEG noise comes from signals or electrical disturbances recorded alongside brain activity. Common causes include eye blinks, muscle tension, movement, sweat, poor electrode contact, loose leads, and 50 or 60 Hz power-line interference. Start by identifying the noise in the raw recording, then correct the physical or electrical cause where possible; filtering should be a selective last step because it can also remove or distort useful EEG information.
What causes noise in EEG recordings?
EEG artifacts are broadly physiological or equipment-related. Both can imitate brain activity or obscure it, and more than one cause can affect a recording at once.
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| Cause | What it can look like | First thing to check |
|---|---|---|
| Eye blinks and eye movements | Large deflections, often most visible in frontal channels | Whether the pattern coincides with blinking or gaze shifts |
| Muscle activity, talking, chewing, or movement | High-frequency activity, spikes, baseline shifts, or changing waveforms | Whether it tracks jaw, face, neck, or body movement |
| Cardiac or pulse activity | Rhythmic contamination | Whether it is pulse-synchronous; an electrode positioned over a blood vessel can contribute |
| Sweat or changing electrode contact | Slow baseline drift or generally unstable signal | Skin-electrode contact, sweat, and impedance changes |
| Loose electrodes, electrode pops, or moving cables | Sudden spikes, swings, intermittent disconnection, or an unstable baseline | Electrode seating and whether cable motion coincides with the disturbance |
| Power-line interference or ground loops | A persistent component near 50 Hz or 60 Hz, depending on the electrical system | Contact and impedance balance, grounding, shielding, lead layout, and nearby electrical sources |
These patterns are clues, not proof of a particular cause. Inspect multiple channels and relate the signal to what was happening during the recording before deciding whether to reject or filter it.
How to troubleshoot noisy EEG, step by step
- Inspect the raw recording first. View the unprocessed traces over time and, if available, their frequency spectrum. Note whether the disturbance is continuous or intermittent, rhythmic or broadband, limited to certain channels, or linked to movement. This helps distinguish likely line interference from contact problems and physiological artifacts.
- Check electrode contact and impedance. Confirm electrodes are secure and inspect impedance using the recording system’s procedure. The 2022 IFCN/ILAE routine clinical EEG standards suggest values below 5 kΩ and regard values below 10 kΩ as acceptable in that clinical context. These are not universal thresholds for every amplifier, electrode type, cap, or protocol. High or uneven impedances may make a setup more vulnerable to sweat, movement, electrode pops, and common-mode interference.
- Stabilize the participant and leads. Reduce unnecessary movement, adjust posture if needed, and secure cables so they do not tug or rub against the setup. If sweat or poor contact is suspected with a wet-electrode system, check the contact and reapply compatible conductive gel where appropriate, then recheck impedance. Follow the system maker’s compatibility guidance; gel does not address cable movement or grounding problems.
- Review the electrical setup. Check grounding, referencing, shielding, lead routing, and nearby sources of electrical interference. Consider whether a ground loop or poor contact is contributing to line-frequency noise. The AES/ILAE task-force report recommends correcting setup and grounding before relying on filtering; its report focuses on animal EEG acquisition, although the electrical principles are relevant more broadly.
- Process selectively and verify the result. A 50/60 Hz notch filter can reduce mains-frequency contamination when it is present. Use artifact rejection or component-based removal only when appropriate to the data and analysis. Compare the processed signal with the raw recording and document the processing so that removed information is not mistaken for an artifact-free original.
How to reduce 50/60 Hz interference
Power-line interference typically appears near 50 Hz or 60 Hz, depending on the electrical system. A narrow line-frequency component can be consistent with mains contamination, but a notch filter is not a substitute for fixing the acquisition setup.
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- Check electrode contact and whether impedances are markedly different across channels.
- Review grounding, reference connections, shielding, and cable layout; look for potential ground loops.
- Identify nearby electrical sources and determine whether changing their position or the recording arrangement reduces interference.
- Only after these checks, consider a notch filter at the relevant line frequency. Inspect the filtered trace against the raw one because filtering can distort or eliminate signal information.
The AES/ILAE task-force report states that proper grounding, referencing, and shielding are preferred to filtering for reducing line noise, because filtering can distort the signal and remove information.
Why high or imbalanced impedance matters
Impedance describes the electrical contact between an electrode and the skin. Poor or changing contact can destabilize a channel, while an imbalance among electrodes can undermine an amplifier’s ability to reject common-mode interference. High impedance can also leave recordings more susceptible to sweat, movement, and electrode-pop artifacts.
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Use the impedance guidance for your specific recording context rather than treating a single number as a rule for every EEG system. The below-5 kΩ suggestion and below-10 kΩ acceptability cited above come from 2022 IFCN/ILAE standards for routine clinical EEG; other equipment and protocols may have different requirements.
What filtering and artifact removal can—and cannot—do
Filtering can reduce selected frequency components, but it can also alter real signal in or near the frequencies being removed. Artifact rejection and component-based methods can help in some analyses, yet their suitability depends on the artifact, data, and method. An IEEE Access review notes there is no single standard method that optimally removes all EEG artifact types.
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- Keep an unprocessed copy of the recording.
- Choose a processing method for an identified artifact rather than applying filters indiscriminately.
- Compare processed data with the raw signal and check that relevant activity has not been removed or reshaped.
- Record the processing choices so results can be interpreted and reproduced.
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