To ensure accurate EEG measurements, it is crucial to minimize or eliminate line noise, commonly manifested as 50Hz or 60Hz interference. Despite the advancement in noise cancellation technologies, certain conditions might still lead to excessive line noise. Following the EEG User Guide meticulously ensures optimal preparation and execution of EEG measurements. Key practices include:
- Always incorporating the ground cable during EEG measurements.
- Maintaining consistency in the type of electrodes used for both the ground cable and EEG sensors.
- Thoroughly preparing the skin using recommended substances like Nuprep or alcohol. Prioritizing redoing the ground electrode if revisiting skin preparation.
- Monitoring the lifespan of EEG disc electrodes, with a recommendation of 10-20 uses for Ag/AgCl electrodes due to the gradual degradation of the layer, whereas sintered electrodes can sustain hundreds of uses.
- Verifying the expiry dates of all consumables such as conductive pastes and skin preparation gels.
- Ensuring the cleanliness of snap-on connectors to avoid signal quality degradation due to residue buildup.
- Applying an adequate amount of paste or gel on the electrode to cover it sufficiently while avoiding excessive amounts that might increase susceptibility to 50/60Hz interference.
Should the initial steps prove ineffective, consider:
- Positioning the system closer to the individual, such as on their lap.
- Detaching the laptop from its power supply.
- For NeXus-32 users, disconnecting the power adapter and opting for battery power.
If interference persists, further actions include:
- Sequentially turning off wireless and electronic devices to identify and eliminate the source of interference like lights, desk lamp with adapter/transformer, electrically motorized chair, air-conditioning. Check for possible power lines/cables.
- Encouraging the individual to move around the room with the NeXus to locate areas with lesser or greater interference, thereby pinpointing potential sources.
- Assessing the ground cable's functionality by removing it during a recording session and observing any increase in interference, signifying its operational effectiveness.
- Recording EEG and unplugging the ground cable after 30 seconds. Check if the interference increased after unplugging the ground cable. This means that the ground cable is working properly.
As a final measure, applying a notch filter through the BioTrace+ software can effectively filter out line noise. For guidance on setting up a notch filter, refer to the detailed documentation provided.
About 50/60 Hz Interference:
Electrical interference in EEG recordings can stem from various electronic devices and lighting, with typical household and office electrical systems operating at 110-230 Volts AC and alternating at a frequency of 50 or 60Hz. This interference is particularly prevalent where electrode contact is poor or in environments heavily populated with electrical devices and cables.
The BioTrace+ versions in English (EN) and German language contain 50Hz indicators and Notch filters.
The BioTrace+ version in English (US) contains 60Hz indicators and Notch filters.
In the BioTrace+ software 50 or 60Hz interference can be recognized in several ways:
Therapist screen (Frequency spectrum object indicating strong 50Hz interference)
Line graph object showing strong 50Hz interference
Signal Check screen
Access the Signal Check screen from any therapist screen, clicking this icon:
The line graph as well as the second traffic light indicates 50Hz (or 60Hz) interference
