This study focused on stabilizing the sensations elicited by electrical stimulation applied to the skin through surface electrodes, a technique widely used in sensory feedback research. The relationship between absolute threshold (the minimum stimulus amplitude that causes sensation) and the parameters of an electrical equivalent circuit for skin impedance was examined with neurologically intact subjects. Electric current pulse stimulation at 100 Hz frequency and pulse widths between 0.08 and 0.5 ms was used to elicit a cutaneous sensation. The electrical equivalent circuit for skin impedance consisted of resistance Rp and capacitance Cp connected in parallel, plus resistance Rs connected in series to both.
First, we estimated the parameters of the equivalent circuit by the skin impedance measured with sinusoidal signals (Rps and Cps), and we examined their relevance to the absolute threshold. For Ag-AgCl electrodes used with electroconductive paste, variations over time of the frequency characteristics of Rps and Cps were found to be similar to the changes caused by repeatedly stripping the skin. Values of Cps and Rps at 10 Hz were found to relate to the absolute threshold with high correlation coefficients. For a solid-gel Ag-AgCl electrode, changes of absolute threshold, Rps and Cps caused by increasing size of the solid-gel electrode were similar to variations over time obtained using electrodes with electroconductive paste. Correlations between the threshold and values of Rps and Cps at 10 Hz were also observed.
Next, we estimated the parameters of the equivalent circuit by measuring stimulus pulse current and voltage waves of square-wave stimulus pulses using solid-gel electrodes (Rps and Cpr). We then examined their relevance to the absolute threshold. The absolute threshold increased with repeated electrical stimulation to the skin and decreased after adequate rest with no electrical stimulation. The value of Rpr decreased with repeated electrical stimulation and increased after rest, while the value of Cpr did not change. These variations became minor after preparation of the skin for removing the stratum corneum. A significant correlation between Rpr and the absolute threshold was observed. These results show that electrical stimulation to the stratum corneum affects both absolute threshold and skin impedance; they also indicate that real-time evaluation is important for stabilizing cutaneous sensation elicited by electrical stimulation.
The relevance of Rp to the threshold was shown in both methods that were used to estimate the parameters. Possible reasons for decreasing Rp were an increase of ionic conductivity due to osmosis of electroconductive paste into the stratum corneum or an increase of electric charge in the stratum corneum due to charge transfer by electrical stimulation through the skin. The increase of the threshold amplitude was considered to be the result of broadened current path due to increases of ionic or electric conductivities in the stratum corneum. It is necessary to clarify the mechanism of the relationship between the threshold and impedance parameters in order to develop a technique for stabilizing electrocutaneous sensation.
