Abstract
This study examined the effects of a mental countermeasure on the Guilty Knowledge Test (GKT) by P3. Twenty-eight subjects enacted a mock crime in which they stole one of five jewels in the pouch. The subjects were randomly divided into two groups : a countermeasure (CM) group and a non-countermeasure (NCM) group. In the GKT, each picture of five jewels was presented on a CRT display and the subjects were required to press a button as soon as they recognized the stimulus. In addition, only the CM group was instructed to count backwards in si-lent by sevens from 200 during the experiment. The results showed that P3 amplitudes to the relevant stimuli from Cz and Pz were smaller in the CM group than in the NCM group, although P3 amplitudes to the relevant stimuli were consistently larger than to the irrelevant stimuli. The correct detection rate was 50% in the CM and 79% in the NCM group. These findings indicate that GKT by P3 is immune from the effects of the countermeasure.
