Countermeasure mechanisms in a P300-based concealed information test

We recently introduced an accurate and countermeasure (CM)-resistant P300-based deception detection test ( J.P. 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. Three 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 concealed information 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.