Increasing the number of irrelevant stimuli increases ability to detect countermeasures to the P300-based Complex Trial Protocol for concealed information detection

We previously found that simultaneously executing a mental countermeasure and an explicit required response impairs reaction time (RT)‐based detection of countermeasure use in a P300‐ based concealed information test. To address this issue, we increased the numbers of irrelevant stimuli to eight, and manipulated the proportions of to‐be‐countered irrelevant stimuli from 25% to 50% to 75% in three groups. Results: Based on P300 data, 100% of the simple guilty (no countermeasure use) and 92% of the innocent subjects were correctly identified as having or not having concealed information. In the countermeasure groups, detection rates varied from 71% to 92% across the different groups. Notably, in the present study with eight irrelevant items, simultaneous countermeasure use was indicated by elevated RT in the 50% and 75% countermeasure proportion groups, which it was not, previously, with 50% (two) countermeasures and four irrelevants.