EEG is defined as “the difference in voltage recorded b/w two electrodes and plotted over time, when at least one of them is placed over the scalp (see definition of EEG).” One pair of electrodes usually make up a channel. Each horizontal tracing corresponds to an electrode pair placed on a particular area of the patient's scalp.
In usual EEG recordings, 16 channels are recorded in parallel - appearing as 16 traces (as shown in figure). However number can go up to 40 depending on the indication.
Each channel of EEG recording is produced by the output of one differential amplifier. This type of amplifier (also known as balanced amplifier), is constructed to amplify only the difference in the potential b/w its two inputs, identical voltages appearing at the two inputs are not amplified and create a flat line output.
This is one way of reducing contamination of the physiological signal by electrical noise, (for example, 60 Hz noise from line voltage devices) because this noise tends to be the same at all electrode positions and cancels out when a difference in potential is formed. On the other hand, physiologic signals are usually different at different electrode positions.
References:
- Electricity and Electronics in Clinical Neurophysiology : Clinical Neurophysiology by Jasper R. Daube (F. A. DAVIS COMPANY)
- Technological basis of EEG Recording : Electroencephalography - Basic Principles, Clinical Applications and Related Fields by Ernst Niedermeyer (WILLIAMS AND WILKINS)
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