The Electrodermal Response
|SCL||Skin Conductance Level|
|SCR||Skin Conductance Response|
|SRL||Skin Resistance Level|
|SRR||Skin Resistance Response|
|SPL||Skin Potential Level|
|SPR||Skin Potential Response|
Older terminology no longer in use, such as the galvanic skin response, has not been included in the table. The resistance and conductance measurements are reciprocals, of course; however, one or the other might turn out to be linearly related to the stimuli under study and be somewhat more useful as a result.
27.4 MEASUREMENT SITES AND CHARACTERISTIC SIGNALS
As discussed above, EDA is best measured at palmar sites. Suggested locations for electrode placement are given in Figure 27.2. In general, the electrodes used are of the Ag/AgCl type which are recessed from the skin and require the use of a suitable electrode paste. Since this is a reversible type of electrode, polarization and bias potentials are minimized. This is obviously of importance since such contributions introduce artifact in the SP and SC determinations. There is also a half-cell potential under each electrode, but if these are similar and overlie identical chloride concentrations their effects are equal and cancel. For this reason an electrode paste with NaCl at the concentration of sweat (approximately 0.3% NaCl) is to be preferred.
As described in Figure 27.2, the reference site should be abraded, a procedure that may possibly remove the corneum and introduce much reduced contact resistance. The site itself, on the forearm, is selected to be a neutral (nonactive) location so that only good contact is required. Although the removal of the corneum at the active site would interfere with the examination of the system there, no such requirement needs to be imposed at the reference site, since it should be nonactive.
Fig. 27.2 Suggested electrode sites on the palm for the measurement of skin resistance and skin potentials. (Redrawn from Venables and Christie, 1980.)
Shown in Figure 27.3 are signals characteristic of SCR and SPR waveforms. Those identified as having slow recovery, shown in Figure 27.3A, have a duration of around 40 s, with phasic amplitudes of around 2 µS for conductance and 10-20 mV for potential. Since the amplitude values depend on electrode area in a nonlinear way, these values cannot be readily normalized and, consequently, are difficult to compare with others. Data collected by Venables and Christie (1980) give a mean SCL of 0.3 µS and SCR of 0.52 µS in a study of a particular population (N = 500-600). Rapid-recovery SCRs and SPRs are shown in Figure 27.3B.
The electronics associated with measurement of EDR is fairly simple. For exosomatic conditions either a constant current or a constant voltage source is used. As illustrated by Venables and Christie (1980), the circuit in either case consists of a battery with voltage EB connected to the skin through a series resistance RA; the circuit is completed by the skin resistance Rs. Constant current conditions can be implemented by letting RA be very large. (In the example given, EB = 100 V; RA = 10 MΩ; and, even for high values of skin resistance (i.e., , corresponding to 4 µS), the current differs from a nominal 10.0 µA by under 2.5%.) For constant-voltage conditions RA is small compared to Rs, so the voltage across Rs is the fixed battery voltage. In the constant-current case, the skin voltage Vs(t) is measured and
For constant-voltage conditions the voltage VA is measured across the series resistance. Then
Present-day practice utilizes a battery voltage Eb of 0.5 V, whereas constant current and constant voltage are better obtained electronically.
For endosomatic measurements the skin potential is desired, and the optimum condition is where the input resistance of the amplifier is very high compared to the skin resistance. The use of an operational amplifier is called for. Additional requirements are evident from the sample waveforms in Figure 27.3: in general, an input voltage in the range of +10 to -70 mV at a bandwidth of from DC to a few Hz. Geddes and Baker (1989) suggest 0-5 Hz for tonic measurements, with 0.03-5 Hz being adequate for phasic measurements. Recommendations for electrodermal measurements were drawn up by a committee selected by the editor of Psychophysiology and published by that journal (Fowles et al., 1981). The paper by MacPherson, MacNeil, and Marble (1976) on measurement devices may also be useful.
Fig. 27.3 (A) Upper trace is a slow-recovery SCR, whereas middle and lower are monophasic negative SPRs.
(B) The upper trace is a rapid-recovery SCR, whereas the middle and lower traces are positive monophasic SPRs. (Redrawn from Fowles, 1974.)
Fig. 27.4 A simplified equivalent circuit describing the electrodermal system. Components are identified in the text. (From Fowles, 1986.)
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Fowles DC, Christie MJ, Edelberg R, Grings WW, Lykken DT, Venables PH (1981): Committee report: Publication recommendations for electrodermal measurements. Psychophysiol. 18: 232-9.
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Geddes LA, Baker LE (1989): Principles of Applied Biomedical Instrumentation, 3rd ed., John Wiley, New York, N.Y.
MacPherson RD, MacNeil G, Marble AE (1976): Integrated circuit measurement of skin conductance. Behav. Res. Methods Instrum. 8: 361-4.
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Venables PH, Christie MJ (1980): Electrodermal activity. In Techniques in Psychophysiology, ed. I Martin, PH Venables, pp. 2-67, John Wiley, New York.