1. Electrical Artifact
Electrical artifact is unwanted interference in EEG recordings caused by external electrical sources. The most common source of electrical artifact is interference from the power grid, which typically operates at 60 Hz in the USA (50 Hz in Europe and other parts of the world). This interference is generated by any electrical appliance, such as lights, computers, or even a cell phone charging nearby. Although modern EEG equipment is quite good at minimizing electrical artifacts, they may still occasionally appear in the signal.
Electrical artifacts appear as very fast and monotonous oscillations in the signal. They are characterized by their high frequency (60 Hz or 50 Hz), which is much faster than typical brainwave frequencies detected in EEG. Because brain activity in EEG is generally much slower (e.g., alpha waves range from 8 to 12 Hz), the fast electrical artifact is easy to identify and remove using a specific tool called a notch filter.
The notch filter selectively removes the specific frequency associated with the power grid interference (60 Hz or 50 Hz) without affecting the rest of the EEG signal. This is effective because no cerebral activity occurs at such a high frequency. Activating the notch filter ensures that the EEG data remains unaffected while eliminating the electrical artifact.
In the example shown, if the notch filter is not applied, the EEG signal from the Fp1 electrode might show significant interference due to the improper placement of the electrode. Electrical interference could cause sharp, fast oscillations in the signal, which would be removed by the notch filter.
2. Electrode Artifact
Electrode artifacts occur when the electrodes themselves cause interference in the signal. This can happen due to a variety of reasons, such as the electrodes becoming loose or being disturbed by the surrounding environment. One of the most common electrode-related artifacts is called an electrode pop, which happens when an electrode becomes loose or detaches slightly from the scalp.
An electrode pop is characterized by a sudden, sharp upward spike in the EEG signal, followed by a slower return to baseline. Unlike typical brainwave activity, the spike is not followed by any recognizable field pattern. This type of artifact is noticeable because the signal is disrupted by a sudden change, unlike the smoother transitions seen in normal brainwave activity.
In the example shown, the F7 electrode might be loose, causing a sudden spike in the EEG signal. If this artifact continues, it's important to check the electrode to ensure it's properly attached to the scalp. Regular maintenance and careful placement of electrodes can help minimize electrode-related artifacts and ensure accurate EEG recordings.
Conclusion
Both electrical and electrode artifacts can interfere with EEG recordings. Electrical artifacts are typically caused by external interference, such as power grid frequencies, while electrode artifacts arise from issues with the electrode placement or quality. Understanding these artifacts and knowing how to remove or correct them is crucial for accurate EEG data interpretation.