抄録
This study examined effects of physical and mental countermeasures on the P300-based guilty knowledge test (GKT). Twenty participants were required to choose on out of five cards, to record it number, and put the card in an envelope to be handed to an experimenter. The number on the all cards was 6 for the all participant. In the GKT, participants were presented one target, one probe, and four irrelevant stimuli in the random order. A probe stimulus is the chosen card that only the participants would know, while irrelevant and target stimuli are not the chosen cards. Participants were required to push a right button as quickly and accuracy as possible when the target stimulus was presented on the computer display, and to push a left button as quickly and accuracy as possible when probe and irrelevant stimulus were presented on the computer display. In addition, all participants were instructed to conduct one of 2 types of countermeasures. In a physical countermeasure condition, participants were instructed to up the ankle when stimuli are presented. In a mental countermeasure, participants were instructed to count backward in silent by sevens from 200 during the experiment. The order of the two types of countermeasures was counterbalanced for each participant. In both the conditions, amplitudes of P300 were larger for the target stimuli than the probe and irrelevant stimuli but were not different between the probe and irrelevant stimuli. Peak amplitudes of P300 for the target stimuli did not differ between in the physical countermeasure condition and in the mental countermeasure condition. The physical and the mental countermeasures reduced the accuracy of the P300-based GKT. Thus both the countermeasures were equally effective. These results clearly demonstrated that the P300-based GKT has no special immunity to the countermeasures.
