OLD 3-STIMULUS, P300-BASED CIT (GKT)

J. Peter Rosenfeld, John Meixner, Michael Winograd, Elena

Labkovsky, Alex Sokolovsky, Xiaoxing Hu,Alex Haynes, Northwestern University

80% to 95% correct detection rates….but….

*Rosenfeld et al. (2004) and Mertens, Allen et al. (2008):These methods are vulnerable to Counter-measures (CMs)

via turning I’s into covert T’s.

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.

A CM is an attempt to defeat the test by converting irrelevants into covert targets

When you see a specific irrelevant, SECRETLY make some response, mental/physical.

After all, if you can make special response to TARGET on instruction from operator, you can secretly instruct yourself.

Irrelevant becomes secret target. It makes big P300. If P = I, no diagnosis.

Results from Rosenfeld et al. (2004): Farwell-Donchin paradigm(BAD and BCAD are 2 analysis methods.)Diagnoses of Guilty

Guilty Group Innocent Group CM Group

9/11(82%) 1/11(9%) 2/11(18%)

Amplitude Difference (BAD) method,p=.1

Cross-Correlation(BC-AD) Method, p=.1

6/11(54%) 0/11(0%) 6/11(54%)

Week BAD* BC-AD*

1: no CM 12/13(.92) 9/13(.69)

2: CM 6/12(.50) 3/12(.25)

3: no CM 7/12(.58) 3/12(.25)

*Note: BCD and BAD are 2 kinds of analytic bootstrap procedures.

…If somebody beat the test?

Would he pay the $100,000?

No worries about that if you are 100% confident that it can’t happen (‘cuz you rigged it!)

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.)

* “I saw it” response to S1. RT indexes CM use.

WEEK Hit Rate [Hit Rate] Week 1 (no CM): 11/12 (92%)

[12/12*( 100%)] Week 2 (CM): 10/11 (91%) [11/12*

(92%)] Week 3 (no CM): 11/12 (92%) [12/12*

(100%)]

Main Study: With false positive(FP) group.

  Confidence=.9 Confidence=.95   Test FPs Hits A’ FPs Hits

A’ Iall .08 .92 .95 0 .92

.98 Imax 0 .92 .98 0 .92

.98 

• 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

• 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.

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 4 Irrelevants: Say to

yourself your first name was the CM1, your last name as CM2. These are assigned prior to run.

*Why 2 irrels? Meixner &Rosenfeld(2010) showed countering all Irrels, not probe gives probe extra, special significance. They did a study with only 5 irrels, one of which was not countered. It had big P300. So doing CMs to all irrels is not a good strategy from perp’s perspective.

*Why mental CMs? They should be faster and a bigger challenge for our CTP.

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.

Elena Labkovsky & Peter Rosenfeld

SG

1CM

2CM

3CM

4CM

5CM

John Meixner & Peter Rosenfeld

How do you catch bad guys before crimes are committed, and before you know what was done, where,

when?

0

4

8

12

Guilty Innocent

Group

Am

plit

ud

e (µ

v)

Probe

Iall

A Mock Terrorism Application of the P300-based Concealed Information TestDepartment of Psychology, Northwestern University, Evanston, IL 60208-2700

Iall Imax Blind Imax

Guilty Innocent Guilty Innocent Guilty Innocent

1000 648 985 287 985 603

1000 610 999 416 998 602

955 598 889 476 892 649

996 611 898 430 893 605

994 150 946 17 943 689

909 475 698 284 761 547

945 600 677 365 702 536

997 555 959 250 961 569

999 586 908 217 907 565985 690 888 382 886 706

912 390 667 129 698 650

903 644 837 215 842 702

966 546 863 289 872 619

12/12 0/12 12/12 0/12 10/12 0/12

AUC = 1.0 AUC = 1.0 AUC = .979

Table 1. Individual bootstrap detection rates. Numbers indicate the average number of iterations (across all three blocks) of the bootstrap process in which probe was greater than Iall or Imax. Blind Imax numbers indicate the average number of iterations in which the largest single item (probe or irrelevant) was greater than the second largest single item. Mean values for each column are displayed in bold above detection rates.

CTP is a promising, powerful paradigm, against any number of CMs, mental and/or physical and RT reliably indicates CM use. The new “P900” might also.

jp-rosenfeld@northwestern.edu

Separated or split away from are called “splitting CMs”.

What happens if subjects are instructed to do CM and “I saw it” response at the same time? They lump these acts together. This is called “Lumping CMs.”

Xiaoxing Hu to the rescue! (with Dan Hegeman and Elizabeth Landry).

He simply increased irrelevants from 4 to 8, which should increase demand and RT…

Remember, Allen Hu gave the CMs to Ss in advance and let them rehearse.

And his subjects were geniuses, like you all…

So we are now working with 10 Irrelevant items… and 3,5,7 CMs.

… it is obvious that having to form—on the spot-- and hold 6 CMs for 6 of 8 Irrels in your head –as must happen in the field--is probably too hard for most bad guys to do.

Effects of feedback that focuses attention on probe-irrelevant dimension.

+First we tried 3SP to follow up Verschuere et al, (2009).+Probes were home towns. +Two groups: One got “You are lying” feedback (deception group).

The other (control group) received feedback about button pressing.

So in the next study, we use CTP with home town names (less salient), and feedback is about recognition in high awareness group (like previous “deception group.” In control

group, feedback is about irrelevancies: if they are holding still, not blinking, etc..

N200: The ROC analyses showed that N200 at Fz can effectively discriminate guilty from innocent participants above the chance level only in the high awareness condition (AUC=.72, p<.05).

P300: In the high awareness condition, the P300 could effectively differentiate guilty from innocent participants (AUC=.79, p<.01). However, the AUC in the low awareness condition was not significantly different from chance level (AUC=.59, p>.4).

Combining N200 and P300: The ROC analyses based on this combined measurement achieved the highest classification efficiency (AUC=.91, p<.001) compared with N200 or P300 indicator alone in the high awareness condition, but not in the low awareness condition.

In this study, a mock crime is committed: steal an exam from mailbox of Prof. Rosenfeld. So probe is incidentally acquired information during mock crime.

Subjects in 3 groups: 1.Guilty/Immediate, tested right after crime with BOTH CTP, IAT, 2.Guilty/Delay, tested a month later, 3.Innocent Group, tested right away.