Recording and display conventions
Recording and display conventions[edit | edit source]
The EEG machine is a differential amplifier. It takes the electrical information from one signal source or electrode and compares it to another source, rejecting any common features (common mode rejection). The resulting signal reflects a measurement of the net difference in voltage between the two electrodes (here designated as gridpoints G1 and G2) over time. In this way the EEG displays a continuous tracing of the electrical activity from a particular region of the cortex in relationship to another cortical region or in relationship to a non-brain reference.
By convention, the tracing for each channel of the EEG reflects the electrical activity of the first electrode or grid point (G1) in reference to the second (G2). Again by convention, each derivation or channel displays a waveform with an upward deflection when the net difference of the voltage between the two electrodes is negative (G1-G2<0). If G1 is positive or less negative than G2 (G1-G2>0), then the waveform has a downward deflection. Thus, there are two possible interpretations for the waveform produced by each derivation pair. For example, if G1 is more negative than G2 or G2 is more positive than G1, then the waveform points upwards. It depends on which electrode is the active electrode and in what direction.
There is a rational basis for the convention of displaying electrically negative events as an upward deflection on the EEG. Epileptic discharges result in a paroxysmal negative charge at the surface of the cortex, and conventional EEG tracings produce a rapid upward deflection often described as a sharp wave or spike. This is why a transient negative potential at any given electrode is described as a surface negative event. Vice versa, a transient positive potential is described as a surface positive event.
