ABSTRACT
The heart of neurofeedback is the measurement of the electroencephalography (EEG) signal from the scalp. All EEG systems require the basic elements, which consist of the placement of active sensors somewhere on the head, as well as reference and ground connections. A variety of sensor materials are used for EEG applications. EEG is the most direct and objective means of measuring the electrical activity of the brain without actually going inside the head with invasive sensors. A differential amplifier is important because the subject's body (and head) is awash in electrical noise, both from within and without the body. In a practical EEG device, the common mode rejection ratio must be 100 decibels or more, meaning that the differential gain must be 100,000 or more times larger than the common-mode gain. This allows the amplifier to tolerate noise many times larger than the actual signal, while still rejecting it.
