DC and AC Exogenous Measurements

In both cases, this implies an exogenous, continuous DC current flow through the electrodes and the skin. Even if such a system is simple and practical, it is not particularly suitable for many purposes (1) DC current flow polarizes the electrodes and electrolyzes the skin, (2) the measurement of conductance is disturbed by possible varying emfs in the circuit, and (3) skin endosomatic DC potential cannot be registered simultaneously with DC conductance because of the DC current used. 

EDA is divided into tonic (level) and phasic (response) phenomena. Both tonic levels (skin conductance level, skin potential level) and responses (SCR, skin potential response) are of interest working with EDA. They are generated under autonomous nerve control of the active organs of the skin, in particular the sweat glands. 

As we have seen in the previous section, the DC method has been used for more than 100 years, in spite of the problems listed. The reason for this is (1) the simpler and more standardized curve forms obtained with DC and (2) the early start in the 1890s long before technology advances had made the AC method feasible. The simplest method of them all was of course the endogenous potential measurement, but the disadvantage of that method was a more difficult interpretation of the many waveforms that could come up. 

The AC method also has problems of its own. We have a new parameter, the measuring frequency, which must be chosen. But perhaps the largest problem is that impedance is a complex quantity related to a capacitor and a resistor coupled in series, and the inverse quantity admittance is related to a capacitor and resistor coupled in parallel. Resistance and conductance are inverse when using DC excitation, but with AC the resistance will not be the inverse of the conductance. In this case, it is obvious that resistance and conductance are no longer inverse, as discussed in Section 3.3, and conductance should be preferred to resistance since ionic conduction and polarization basically appear in parallel in biological tissue. 

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