Physiological Processes Measured by the Polygraph
This appendix summarizes scientific knowledge about the three main physiological processes that are measured by the polygraph: cardiovascular, electrodermal, and respiratory.
Cardiovascular activity is governed by the sympathetic and parasympathetic nervous systems, with the former acting through the postganglionic neurotransmitter norepinephrine to speed the heart and increase blood pressure and the latter acting through the postganglionic acetylcholine to slow the heart and lower blood pressure. The baroreceptorheart rate reflex serves to maintain blood pressure: baroreceptors (pressure-sensitive receptors) mostly within the carotid sinus increase firing in afferents to the nucleus of the tractus solitarius in response to an increase in blood pressure, which in turn inhibits sympathetic motor neurons in the intermediolateral cell column of the cord and excites the parasympathetic source nuclei in the nucleus ambiguus and dorsal motor nucleus of the vagus. The resulting decrease in sympathetic activation further slows heart rate and reduces ventricular contractility and reciprocal increase in parasympathetic activation slows the beat of the heart and reduces cardiac output. Together with reductions in adrenergic vasoconstrictor tone, the baroreceptor actions compensate for the disturbance and restore blood pressure. The opposite pattern of autonomic control (i.e., sympathetic activation and reciprocal parasympathetic withdrawal) is triggered by a
sudden lowering of blood pressure (e.g., during assumption of an upright posture) (Berntson, Cacioppo, and Quigley, 1991).
The baroreflex displays the essential characteristics of a feedback-regulated, homeostatic servomechanism that responds to perturbations and acts to restore basal blood pressure. But blood pressure regulation is far more complex. Indeed, blood pressure changes can be seen in anticipation of a perturbation, before any change in baroreceptor afference. Examples include the increased blood pressure just prior to assumption of an upright posture or in anticipation of threat or danger. To some extent, these likely reflect simple Pavlovian conditioning, in which stimuli (environmental or cognitive) that predict an impending perturbation can serve as conditioned stimuli for an anticipatory, compensatory adjustment (Dworkin, 2000).
Not only can sympathetic and parasympathetic activation within the autonomic nervous system be reciprocal—as implied by arousal theories—but it can also be uncoupled, coactivated or coinhibited (e.g., Berntson, Cacioppo, and Quigley, 1991, 1993). Reciprocal activation fosters a rapid and dramatic change in effector status (e.g., heart rate); uncoupled activation affords more fine tuning (e.g., vagal withdrawal in response to mild exercise); and coactivation or coinhibition can regulate or mute the functional consequences of underlying neural adjustments.
Importantly for the interpretation of polygraph data, individual differences in the mode of autonomic activation to acute psychological stressors have been identified, with some individuals showing primarily sympathetic increases, others primarily vagal withdrawal, and others showing reciprocal sympathetic activation (Berntson et al., 1994; Cacioppo et al., 1994). In addition, Obrist, Light, and colleagues demonstrated that active coping tasks (those with which one copes by doing something, e.g., mental arithmetic) tend to elicit beta-adrenergic (e.g., cardiac) activation and increased blood pressure, whereas passive coping tasks (those with which one copes by enduring; e.g., cold pressor) tend to elicit alpha-adrenergic (e.g., vasomotor) activation (e.g., Light, Girdler, and Hinderliter, in press) and increased blood pressure. Individual differences have been found in these cardiovascular patterns as well, with some individuals showing greater cardiac reactivity and others greater vasomotor reactivity (Light et al., 1993; Kasprowicz et al., 1990; Sherwood, Dolan, and Light, 1990).
In sum, cardiovascular responses to stimuli that may be considered arousing are multiply determined, and there are individual differences in terms of the direction and extent of cardiovascular reactivity that is observed. These findings call into question assumptions about cardiovascular signals of arousal that are consistent across individuals.
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).
Respiration can be modified by the central and the autonomic nervous systems. The respiratory centers in the medulla and pons contain neurons that fire spontaneously to initiate inspiration. In addition, respiration is modified by autonomic reflexes arising from the lungs, heart, blood vessels, and upper airways. For instance, central chemoreceptors (e.g., in the brainstem) are responsive to carbon dioxide concentrations, peripheral chemoreceptors near the large vessels of the heart are sensitive to oxygen concentrations in the blood, and stretch receptors in the lungs are sensitive to the extent of lung inflation.
Respiration is easily brought under voluntary control, and variations in respiration can produce changes in heart rate and electrodermal activity. Therefore, respiration needs to be monitored to determine whether responses to relevant and control questions are artifacts. For instance, a sharp sniff can reliably produce an electrodermal response. If an examinee were to sniff sharply following control but not relevant questions, it might appear that the individual’s responses to the relevant questions were relatively small and, therefore, reflected general stress, arousal, or anxiety rather than deception. In polygraph testing, the rate and depth of respiration are measured by strain gauges positioned around the chest and abdomen because breathing can produce fluctuations in the girth of either or both. The strain gauge provides a measure of relative amplitude; if the strain gauge moves during the session, amplitudes recorded prior to this movement could not be compared to those recorded following the movement.
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Cacioppo, J.T., G.G. Berntson, P.F. Binkley, K.S. Quigley, B.N. Uchino, and A. Fieldstone 1994 Autonomic cardiac control. II. Noninvasive indices and baseline response as revealed by autonomic blockades. Psychophysiology 31:586-598.
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Light, K.C., R.J. Turner, A.L. Hinderliter, and A. Sherwood 1993 Race and gender comparisons: I. Hemodynamic responses to a series of stressors. Health Psychology 12:354-365.
Orne, M.T., R.I. Thackray, and D.A. Paskewitz 1972 On the detection of deception. A model for the study of physiological effects of psychological stimuli. Pp. 743-786 in Handbook of Psychophysiology, N.S. Greenfield and R.A. Sternbach, eds. New York: Holt, Rinehart, and Winston.
Sherwood, A., C.A. Dolan, and K.C. Light 1990 Hemodynamics of blood pressure responses during active and passive coping. Psychophysiology 27:656-668.
Stern, R., W.J. Ray, and K.S. Quigley 2001 Psychological Recording, 2nd ed. New York: Oxford University Press.