Why low impedance is ensured during EEG experiments

Electroencephalography (EEG) is the recording of electrical activity along the scalp. It is often required to ensure a low impedance between the scalp and the electrodes. Here is why.

From the figure above,
Rin_amp is the input resistance of the amplifier the EEG electrode is connected to.
Vin is the true voltage of the EEG signal
Reeg is the impedance between the scalp and the electrode
The output voltage, Vout, is the voltage that appears at the input of the amplifier and is given by:

This output voltage is the one that is amplified by the op-amp.

As shown by the equation above, if Rin_amp is far greater than Reeg, Vout goes into the op-amp will be approximately equal to Vin and so there is much confidence that we are amplifying the the input voltage, Vin.

But if otherwise, Vout will be attenuated and will not reflect the input voltage, Vin.

Therefore the input impedance of the amplifier has to be far greater than the input impedance of the EEG signal. Also, it is necessary to keep the input impedance of the EEG signal as low as possible.