Continuous estimation of parameters in skin electrical admittance from simultaneous measurements at two different frequencies

The impedance locus plot in the complex plane has been widely used to present data from measurements of the skin electrical properties. This classical method is, however, limited to electrodermal inactive sites, time-consuming when being performed at several frequencies and inconvenient when measuring evoked responses. To overcome these difficulties we have developed a new method to estimate skin electrical admittance parameters (conductance at extrapolated zero frequency G₀, phase angle (π/2)α and ion relaxation time τ) from simultaneous measurements at two different frequencies with a three-electrode lock-in amplifier measuring system. The new method can also be used to investigate the evoked responses of these parameters in psychophysiological research. The theoretical principles of the new technique are presented in the paper, and the instrumentation as well as measuring procedures are described. The results from experiments on frequency and time dependency of skin electrical admittance show that α is relatively constant or shows only moderate changes compared with G₀ and τ, which change in an opposite manner during evoked responses. Changing the DC offset in the excitation voltage from −1000 to 1000 mV had very little effect upon the estimated variables: G₀ and α (<4 per cent), whereas increase of the excitation alternating voltage amplitude gave a substantial increase in G₀ and α, and a corresponding decrease in τ.