Assigned Versus Random Countermeasure-Like Responses in the P300-Based Complex Trial Protocol for Detection of Deception: Task Demand Effects
Applied Psychophysiology and Biofeedback, Vol. 34, pp. 209-220, 2009
13 Pages Posted: 6 Jun 2011
Date Written: June 20, 2009
The Concealed Information Test (CIT) is a credibility assessment protocol of an entirely different nature than the traditional lie detector test. Instead of attempting to detect actual lying (the goal of the commonly used Control Question Test), the goal of the CIT is to determine whether an individual possesses knowledge of specific details of a crime or event. For example, if a murder was committed at 800 Church Avenue using a .38 caliber revolver, the CIT seeks to determine whether a suspect recognizes the address and type of weapon.
The CIT presents subjects with various stimuli, one of which is a crime related item (the probe; such as the gun used to commit a murder). Other stimuli consist of control items that are of the same class (irrelevants; such as other potentially deadly weapons: a knife, a bat, etc.) such that an innocent person would be unable to discriminate them from the crime related item. If the subject’s physiological response is greater for the probe item than for irrelevant items, then knowledge of the crime or other event is inferred.
The CIT can be conducted through the measurement of a variety of physiological indicators, but one highly effective method is by measuring the electrical signals at the participant's scalp, termed Event Related Potentials (ERPs). One ERP component, termed the P300, is particularly large when an individual recognizes an item as meaningful, and is thus a good indicator of concealed information.
We recently introduced an accurate and countermeasure resistant P300-based deception detection test called the Complex Trial Protocol (Rosenfeld et al., 2008). When subjects use countermeasures to all irrelevant items in the test, the probe P300 is increased rather than reduced (as it was in previous P300-based deception protocols), allowing detection of countermeasure users. The current experiment examines the role of task demand on the Complex Trial Protocol by forcing the subject to make countermeasure-like response to stimuli. Subjects made either a simple random button response to both probe and irrelevant stimuli (experiment 1) or a more complex, assigned, button response to probe and irrelevant stimuli (experiment 2). We found that an increase in task demand reduced the effectiveness of the test. Using random responses we found a simple guilty hit rate of 11/12 with no false positives, but only a 4/11 hit rate for countermeasure-users. Using assigned responses we found a simple guilty hit rate of 8/15 with no false positives, and a 7/16 hit rate for countermeasure-users. We herein suggest that the high level of task demand associated with these countermeasure-like responses causes reduced hit rates.
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