Many researchers have asked us this question. What we have found is that impedance matching pastes and gels are not direct shorts like wires. We have measured the conductance of impedance matching pastes and gels and found that they are typically about the conductance of the scalp. Therefore, a millimeter separation of the tripolar electrode elements provides enough resistance in the paste that currents flowing through it cause a potential difference (Ohm’s law: V=IR). These potentials cannot be measured with conventional EEG amplifiers, so we designed our own preamplifier, the t-Interface 20, to go between the t-Leads and the conventional EEG amplifier.
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