J. Peter Rosenfeld, Elena Labkovsky, Michael Winograd,

Alex Haynes

Northwestern UniversityPsychology Department, Institute of Neuroscience

1 of 3 Stimuli on each trial: Probe (P), or Irrelevant(I), or Target (T). Subject presses either Target or Non-Target (NT) button. Both P and I can be Non-Targets. Special I is defined T.

This leads to 2 tasks for each stimulus: 1. implicit probe recognition vs. 2. explicit Target/Non-Target discrimination

Possible Result: Mutual Interference more task demand reduced P300 to P. CMs hurt Old test.

2 stimuli, separated by about 1 s, per trial,

S1; Either P or I…..then…..S2 ; either T or NT.

*There is no conflicting discrimination task when P is presented, so P300 to probe is expected to be as large as possible due to P’s salience, which should lead to good detection; 90-100 % in Rosenfeld et al.(2008) with autobiographical information. It is also CM resistant. (Delayed T/NT still holds attention.)

• Subjects were divided into three groups (n=12)• Simple Guilty (SG), Countermeasure (CM), and Innocent Control

(IC)

• All subjects first participated in a baseline reaction time (RT) test in which they chose a playing card and then completed the CTP using cards as stimuli.

• SG and CM subjects then committed a mock crime.• Subjects stole a ring out of an envelope in a professor’s mailbox.

Subjects were never told what the item would be, to ensure any knowledge would be incidentally acquired through the commission of the mock crime.

• All subjects were then tested for knowledge of the item that was stolen. There were 1 P (the ring) and 6 I( necklace,watch,etc).

• CM subjects executed covert assigned responses to irrelevant stimuli in an attempt to evoke P300s to these stimuli to try and beat the Probe vs. Irrelevant P300 comparison.

Condition Detections Percentage

SG 10/12 83

CM 12/12 100

IC 1/12 8

• A 3x2 (Condition x Stimulus Type) repeated measures ANOVA in the experimental block showed a main effect of condition, F(2,33) = 58.3, p < .001, stimulus type, F(1,33) = 16.4, p < .001, and a condition x stimulus type interaction, F(2,33) = 15.9, p < .001.

• An identical ANOVA in the baseline block yielded only an interaction of condition x stimulus type, F(2,33) = 22.0, p < .001.

• All CM subjects showed an increase in both Probe and Iall RT from the baseline to the experimental block, meaning CM use can be easily be detected.

• The average change in RT from the baseline to experimental

block was much significantly higher in the CM group.

• A 3x2 (Condition x Stimulus Type RT Change) repeated measures ANOVA yielded strong main effects of condition, F(2,33) = 45.5, p < .01, stim type RT change, F(1,33) = 23.7, p < .001, and an interaction, F(2,33) = 17.7, p < .001).

• As with autobiographical information, the CTP was found to be highly sensitive at detecting incidentally acquired concealed knowledge in a mock-crime scenario.

• Detection rates using the CTP compare favorably to similar polygraph CITs. The main advantage of the CTP over the old P300 or polygraph CIT is its resistance to CM use. The traditional covert-response CMs used to defeat past P300 CITs were found to be ineffective against the CTP, and actually led to larger Probe-Irrelevant amplitude differences and detection rates.

• CM use was also easily identified by a large increase in RT between the baseline and experimental blocks.

Since we saw in the previous experiment, and in Rosenfeld et al. (2008) that CMs actually lead to better detection, we thought we would incorporate explicit CM-like responses into the CTP protocol, in place of the “I saw it” S1 response.

So now we have a 5-button box for the left hand. The subject is instructed to press, at random*, one of the 5 buttons as the “I saw it” response to S1 on each trial with no repeats. T and NT (S2) stimuli and responses are as previously.

We also hoped that this would make CMs harder to do. It didn’t, but we caught the CM users anyway.

* We have done other studies with non-random, explicitly assigned responses also.

Autobiographical information (birthdates): One P and 4 I (other, non-meaningful dates).

3 Groups as before: SG,CM, IC.

NEW: mental CMs to only 2 of the Irrelevants: Say to yourself your first name the CM1, your last name as CM2. These are assigned prior to run.

Only one block per group (no baseline).

Group BT/Iall.9 BT/Imax.9

SG 13/13 (100%) 13/13 (100%)

IC 1/13 (7.6%) 1/13 (7.6%)

CM 12/12 (100%) 10/12 (83%)**These are screened via RT, which still nicely represents CM use

within a block.

CTP is a promising, powerful paradigm, against any number of CMs, mental and/or physical (we have studies where N=1,2,3,4 ), and RT reliably indicates CM use. The new “P900” might also.

jp-rosenfeld@northwestern.edu