Electrodermal Activity — State-Of-The-Art Measurement And Techniques For Parapsychological Purposes - EDA

Journal of Parapsychology, The, June, 2000 by Stefan Schmidt, Harald Walach

EDA METHODOLOGY

The EDA measurement itself is not very difficult, if someone understands the underlying basic principles. These are the conditions and characteristics of electric activity of the skin, as well as the principles of recording and scoring electrical signals from the human body.

Physiological Principles

There are two phenomena regarding the electric activity of the skin. One is the skin potential. This stands for the skin's own electric activity without applying any external electric current (endosomatic measurement). The underlying principles of this phenomenon are not yet fully understood.

Exosomatic measurements refer to the phenomenon of changing electrical properties in the skin when an external voltage is applied. The application of a direct current (DC) constant voltage probe signal to the skin results in a current flow that changes over time. This measurement principle is called skin conductance. There are three more measurement principles (skin admittance, skin resistance, skin impedance) according to different probe signals (alternating current [AC] constant voltage, DC constant current, AC constant current). This change of electrical properties of the skin is strongly related to the activity of the eccrine sweat glands that are predominant on the palmar surfaces and feet. The production of sweat on these surfaces is ontogenetically related to grasping behavior, and it is quite clear that this moistening of the skin is responsible for the change in conductance. But moistening cannot be the only factor, as the skin's electrical properties often change faster than expected by this hypoth esis. For further details on the underlying principles, see Boucsein (1992) and Vossel (1990).

Measurement of Skin Conductance

If EDA is used as the dependent variable, then usually skin conductance is the appropriate method. Since parapsychologists always use EDA as an indicator (e.g., for psi) we will only describe the procedures for skin conductance in detail.

Nomenclature. Skin conductance is usually measured in "Siemens" units although sometimes "mho" is used. As the conductivity of the skin is very small, values are usually given in microsiemens ([micro]S) or micromho ([micro]mho) (1 [micro]S = 1 [micro]mho). In measuring skin conductance, one can distinguish two different principles: phasic and tonic ones. A tonic value stands for an activity that shows a certain amount of continuity over time. The tonic component of skin conductance is the skin conductance level (SCL). SCL is obtained by measuring the total amount of skin conductance and is related to a person's overall arousal. In contrast to tonic values, a phasic value stands for the change of the skin conductance within a short time period as a reaction toward a discrete stimulus. If a stimulus elicits an orienting response, the skin conductance rises for a certain time period and then returns to normal. This is called a skin conductance response (SCR). SCRs can be characterized by four different paramete rs (see Fig. 1): the amplitude (SCR amp.), latency of response onset (SCR lat.), the rise time up to the response peak (SCR ris t.), and the half time value of the recovery time (SCR rec 1/2).