The EEG amplifier
The EEG amplifier performs a critical function in bringing data out of the brain and into the computer, and is the centrepiece of the EEG equipment. Electrical signals in the brain are very small and are measured in μV (microVolts). Despite this such signals can be detected reliably and amplified for use as biofeedback.
All EEG amplifiers are differential, which means that each channel takes a signal from a specific area of the scalp (active electrode) and another signal from a part of the body away from the brain or sometimes another position on the scalp depending on the montage (reference electrode). The second signal is then subtracted from the first leaving only a limited amount of data specific to the area surrounding the active electrode. In this way the brainwave signals can be isolated from the background electrical noise of the body, which is many times larger in amplitude and would otherwise drown out the signals.
Some portable amplifiers connect via a cable for both power and data, but most will use a wireless link in order to fully isolate the head from the computer. Should the computer hardware malfunction there is the potential for excess current to travel through the amplifier and into the head via the electrodes. Considering the tiny voltages being measured and altered by neurofeeback, galvanic isolation from the computer power supply is highly recommended as the voltage in the computer will be thousands to millions of times more powerful and nobody wants that inside their head for obvious reasons.
As mentioned above each channel has both an active and a reference electrode. In addition to this, the amplifier will have a single ground connection to complete the electrical circuit through the body for all channels, meaning an absolute minimum of 3 electrodes to record a channel of EEG data. Commonly for consumer brain training a maximum of 4 channels would be use at one time.
Electrodes can be made of different materials, the most common being pure silver. Other options include silver or gold plated copper, pure tin and silver chloride plated pure silver. The latter gives the best signal response for both EEG recordings and training applications.
It is common to use what are known as ‘linked references’, where two clips are attached to the earlobes and their signal combined with inside the amplifier, or with a jumper cable linking the two wires together before the connect to the amplifier. Cables are usually terminated at the equipment end with 1.5mm touchproof connectors, although other terminations are possible depending on the manufacturer.
In order to process EEG signals and provide feedback, a software platform is required. This will interface with the EEG hardware and allow the construction of protocols, often using object based programming environments with data flow being visualised using wiring and modules that perform specific functions.
Many EEG equipment manufacturers have their own proprietary software platforms, but the two most commonly used open platforms (i.e. non-hardware specific) are Cyberevolution’s BioExplorer and Proatech’s BioEra and both support a wide range of commercially available EEG amplifiers.
These are specific configurations which run within the EEG software and enable particular types of training being carried out. Common protocols include amplitude, frequency, connectivity, coherence and phase correlation. SMR up and Alpha up are examples of amplitude protocols while TAGSync is a phase coherence protocol.