The most sensitive measure in laboratory studies of the detection of deception has been electrodermal activity (e.g., Orne, Thackray, and Paskewitz, 1972). Electrodermal activity varies as a function of the eccrine glands, which are innervated by the sympathetic branch of the autonomic nervous system, but the postganglionic neurotransmitter is acetylcholine rather than norepinephrine (the postganglionic sympathetic neurotransmitter for most visceral effectors). This means that circulating catecholamines (epinephrine, norepinephrine), which can have an excitatory effect on autonomic effectors, have no effect on eccrine gland or electrodermal activity.
Electrodermal activity is measured by passing a small current through the skin to measure skin resistance or its reciprocal, skin conductance. Deviations from basal levels (e.g., responses to relevant and control questions) are called electrodermal responses (EDRs). Whether the electrodermal activity is measured and depicted in terms of skin resistance or skin conductance is not arbitrary. For instance, whether the EDR is interpreted as larger to a relevant question or a control question can vary depending on type of measurement and basal electrodermal activity levels (Dawson, 2000).
Eccrine glands can be thought of as tiny tubes with openings at the surface of the skin (Stern, Ray, and Quigley, 2001). The more activation of a given eccrine gland, the greater the secretion into the gland or onto the surface of the skin and, consequently, the lower the resistance to current flow across this area of the skin. Because eccrine glands are concentrated in the palms of the hands and soles of the feet, the set of eccrine glands between two electrodes on the fingers or palms can be conceived as variable resistors wired in parallel. The total electrodermal activity (or output of eccrine glands) at any given moment, therefore, can be measured by summing the values of all the active resistors wired in parallel. Because the sum of resistors in parallel equals the sum of the conductances, changes in skin conductance need not be corrected for basal levels to measure the effect of a given stimulus.
In polygraphy, this means that the deflections associated with relevant or control questions can be used to gauge an individual’s response to the question only if the readout is in terms of skin conductance. Even when measuring skin conductance, however, stimuli that elicit the responses are so numerous as to make it difficult to isolate its specific psychological antecedent (e.g., Landis, 1930).