The effect of countermeasures against the reaction time based concealed information test on the P300 index of knowledge recognition: A combined RT and P300-based test

Behavioral reaction time (RT) to key (probe) stimuli in a concealed information test (CIT) is usually greater than RT to irrelevant stimuli, and this difference has been utilized as a sign of recognition of concealed information. The ability to voluntarily increase irrelevant RT would appear to be an obvious countermeasure to the RT-based CIT. This study examined the effect of such countermeasure use on the simultaneously recorded P300 event related potentials. There were two blocks of trials in the present study, based on the 3-stimulus protocol. On the first trial block, half the participants were tested on concealed recognition of their phone numbers without a countermeasure. In the second block, this subset of participants were tested on their birth dates, while they applied a countermeasure consisting of the mental statement of the phrase "yes sir" prior to the button press signaling irrelevant stimulus. The other half of the subjects received the reverse order of stimulus categories. Results were that probe RT exceeded irrelevant RT in the first block, but that this relationship was reversed on the second block. In contrast, although the probe P300 exceeded the irrelevant P300 in the first block, this difference significantly increased in the second (RT countermeasure) block, leading to more detections based on P300. Thus, there was a differential effect of this novel countermeasure (directed at countering RT) on RT and P300, which actually led to improved detection with P300, suggesting that both measures might be profitably used in field applications.