Countermeasure Mechanisms in a P300-Based Concealed Information Test
Psychophysiology, Vol. 47, Issue 1, 2010
9 Pages Posted: 6 Jun 2011 Last revised: 20 Nov 2012
Date Written: January 1, 2010
The detection of deception has been the focus of much research in the past 20 years. Though much controversy has surrounded one deception detection protocol, the “Control Question Test” (NRC 2003, Ben-Shakhar 2002), an alternative test, the Guilty Knowledge Test (GKT), developed by Lykken (1959, 1960), is based on scientific principles and has been well-received in the scientific community. The GKT presents subjects with various stimuli, one of which is a guilty knowledge item (termed the probe, such as the gun used to commit a crime). The other stimuli in the test consist of control items that are of the same class (termed 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 guilty knowledge item. If the subject’s physiological response is greater for the guilty knowledge item, then knowledge of the crime or other event is inferred. The Guilty Knowledge Test has been adapted for use through the measurement of Event Related Potentials. Specifically, the P300 component, which is large when an individual recognizes an item as meaningful, is the primary dependent variable.
We recently introduced an accurate, and countermeasure (CM)-resistant P300-based Guilty Knowledge Test (Rosenfeld et al. 2008). When subjects use CMs to all irrelevant items in the test, the probe P300 is increased rather than reduced, as in previous P300-based deception protocols, allowing detection of CM users. Evidence herein suggests this is partly due to an omit effect; the probe was the only uncountered item. 3 groups were tested: a guilty omit probe group performed an explicit response to each irrelevant item but not to the probe; an innocent omit irrelevant group saw only irrelevant items and omitted a response to one item; and a guilty no omit group had a guilty knowledge item as probe and performed an explicit response to each. We found a greater P300 amplitude to probes in the guilty omit probe condition as compared with the other two conditions, indicating a P300 enhancing effect of omitting a response to a single stimulus.
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