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the benefits of active shielding

the benefits of active shielding

By using a shielded waveguard cap the user is able to record high-quality EEG in conditions where conventional electrode systems would not work. For instance, when doing EEG/ERP research using a portable ERP system without having access to shielded rooms. In these cases the active shielding of the electrode leads will continuously deliver you with good-quality data and protect it from the 50/60 Hz environmental noise. Moreover, active shielding protects data in situations when the subject is physically active. Active shielding works only when both the EEG amplifier and the cap are shielded. All eego systems are shielded come by deafult with a shielded waveguard cap.

Key benefits of active shielding:
•    Movement of electrode cables do not interfere with EEG recording
•    Good signals, even when your impedances are high
•    50/60 Hz environmental noise is shielded
•    No need for expensive and fragile pre-amplifiers at the cap

-> Example
The images below show a comparison between the power spectra of the EEG measured with a conventional cap (electro-cap, ECI) and that measured with the cap using the shielded electrodes. This recording was carried out in the USA in a normal EEG lab. No filtering was applied to the data. As you can see, the power spectrum of the conventional cap (shown on the left) shows a clear peak around the 60 Hz.

The picture below shows the clear reduction of 50Hz between unshielded vs. shielded recordings. With active shielding the feedback signal from the amplifier on the shield results in 'zero-capacitance' between core and shield. This makes the wires (thus whole cap) quite insusceptible to outside noise, e.g., caused by mains interference or movement of the cables.
Spectrum comparison and time-frequency plots of the signal at electrode CZ: using the Electro-cap (left), and the waveguard cap (right). Environmental noise is clearly reduced in case shielded electrodes are used!

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