Active Shielded Cap

For instance, when you are doing EEG/ERP research "in the field" using a portable ERP system, you will not have your special shielded room with you. In these cases the ActiveShield will still give you good quality recordings. 

Example

The images on the right 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 ActiveShield 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.

Spectrum comparison and time-frequency plots of the signal at electrode CZ: using the Electro-cap (left), and the Shielded Cap (right).

Environmental noise is clearly reduced in case shielded electrodes are used!

How does it work?

The concept of the ActiveShield electrode can be compared to the working of the Faraday cage and is illustrated in the picture shown on the right. Each electrode cable ("core") is surrounded by a "shielding". The signal that is picked up by the electrode is actively fed back on the shielding resulting in a zero capacitance. This shields the signal in the core cable and prevents electrical fields from the outside to interfere with the signal in the core. The shielding is carried out for each electrode separately, resulting in the most optimal recording conditions. Active shielding of the electrodes works only when the EEG amplifier is able to provide the active feedback loop. The ANT Cognitrace amplifier the only amplifier available that can do this. To read more about this amplifier, please refer to the cognitrace ERP.

Why is it called active shielding?

Each single signal measured at the electrode is immediately (active) being fed back on the shielding. There is an active feed back loop for each electrode and the shielding is carried out for each electrode separately. 

When should you use shielded electrodes?

The shielded electrodes are recommended for experiments that are carried out in a room where you expect a lot of environmental noise, i.e 50 or 60 Hz noise. The special electrodes have proven to be very good in conditions where your subjects move around frequently, for instance, in an experiment inside a virtual reality room. In case your patient is restless and tense a recording with the ActiveShield electrodes also results in better and more reliable measurements.

Key benefits

• Easy to get to low impedances (get down to 5-10 kOhm immediately)
• Good signals, even when your impedances are high (around 150 kOhm)
• Movement of the electrode cables do not influence your EEG recording 
• Single electrodes can be replaced easily