# Reference signal when measuring EEG

When performing EEG, a number of electrodes are placed on the scalp to measure the electric potential difference between those locations and a reference. In the unipolar configuration, the reference is often the signal on another electrode (and is common to all measurement electrodes) or an average of the all measurement electrodes. In the bipolar configuration, the reference is specific to each measurement electrode (and is often another electrode located nearby). For examples, see here and here.

Questions:

1. For the unipolar configuration, why can't we just use Earth ground as our reference instead of having a reference electrode or performing an average of measurements on all the electrodes?
2. For the bipolar configuration, if the electrodes are too close to each other, wouldn't we measure very low voltages since much of the signal appearing on one electrode would also be picked up by the other?

• The amplifier might record the signal in such a way: Signal = (electrode - ground) - (reference - ground), which simplifies to Signal = electrode - ground - reference + ground .. and then you see that the ground signal disappears (the common mode is rejected): Signal = electrode - reference – S.A. Sep 13 '18 at 12:07