Information Test: corroboration of previous findings and highlights on vulnerabilities

Transcription

1 The first independent study on the Complex Tril Protocol version of the P300-bsed Conceled Informtion Test: corrobortion of previous findings nd highlights on vulnerbilities Gáspár Lukács *, Bél Weiss b *, Ver Dlos, Tünde Kilencz, Szbin Tudj, Gábor Csifcsák,c Deprtment of Cognitive nd Neuropsychology, Institute of Psychology, Fculty of Arts, University of Szeged, Egyetem u. 2, 6722 Szeged, Hungry b Brin Imging Centre, Reserch Centre for Nturl Sciences, Hungrin Acdemy of Sciences, Mgyr Tudósok körútj 2, 1117 Budpest, Hungry c Deprtment of Psychology, University of Tromsø, Huginbkken 32, 9037 Tromsø, Norwy *The first two uthors contributed eqully to this work. Correspondence to: Gábor Csifcsák, Deprtment of Cognitive nd Neuropsychology, Institute of Psychology, Fculty of Arts, University of Szeged, Egyetem u. 2., 6722 Szeged, Hungry, Phone: , Fx: , e-mil: gborcsifcsk@yhoo.co.uk

2 Abstrct More thn dozen studies of the Complex Tril Protocol (CTP) version of the P300-bsed Conceled Informtion Test hve been published since its introduction (Rosenfeld et l., 2008), nd it hs been firly consistently proven to provide high ccurcy nd strong resistnce to countermesures (Rosenfeld et l., 2013). However, no independent uthors hve verified these findings until now. In the present, first independent study, we corroborte the ccurcy nd countermesure-resistnce of the CTP, when the probe item (criticl presented informtion, e.g., crime detil; P) vs. ll irrelevnt items (Ill) comprison is used for clssifying prticipnts s guilty or innocent, but we lso show tht the CTP is severely vulnerble to countermesures, when the P vs. the irrelevnt item with the lrgest P300 responses (Imx) comprison is used. This ltter mesure cn be defeted by creting oddbll items mong the irrelevnt items (through trgeting them with covert responses), nd thereby mking their P300 responses sttisticlly indistinguishble from those of the probe item. Prcticl implictions re discussed. Key words Memory Detection, Conceled Informtion Test, P300, Complex Tril Protocol, Countermesure, Verifiction 1

3 1. Introduction Undetected deception my hve high costs in certin scenrios, for exmple in connection with legl cses or counterterrorism however, met-nlyses show tht humns, without specil id, re rrely ble to relibly discriminte lies from the truth, most usully demonstrting judgment ccurcies similr to pure chnce (Bond nd DePulo, 2006; Hrtwig nd Bond, 2011; Krut, 1980). Moreover, no significnt individul differences cn be found; neither experience nor trining improves the ccurcy of judgments (Bond nd DePulo, 2008; Meissner nd Kssin, 2002). As technologicl id, the polygrph ws invented just bout century go, nd is tody widely used in mny countries ll over the world. While it provides higher thn chnce ccurcy, it suffers from vrious limittions, including severe vulnerbility to countermesures (Ntionl Reserch Council, 2003). 1.1 The P300 s tool for detecting conceled informtion A prominent lterntive under development is the P300-bsed deception detection tht is bsed on nlyzing neurl ctivity recorded by electroencephlogrphy (EEG). In n EEG exmintion, electrodes re plced on the sclp, through which electricl ctivity in the brin cn be detected. The P300 is n event-relted potentil with positive pek rising most prominently bove the prietl lobe, beginning usully round 300 ms fter stimulus presenttion (review: Polich, 2007). It is typiclly obtined through the oddbll prdigm: when presenting rndom sequence of stimuli, n infrequent stimulus will evoke the P300 wve if it is tsk-relevnt nd requires n overt or covert response tht is different from the rest of the stimuli (so-clled stndrds ). Importntly, the probbility with which stimulus occurs robustly influences the mgnitude of the P300: infrequent slient stimuli evoke lrger P300 wveforms n effect which 2

4 is considered to reflect the involvement of limited-cpcity cognitive processes in the genertion of the P300 (Polich, 2007). According to the influentil context-updting theory of the P300 (Donchin, 1981; Donchin nd Coles, 1988), this wveform represents the updting of stimulus representtions in working memory, process tht is highly context-dependent, i.e., is influenced by both immedite stimulus history nd tsk demnds (previous knowledge, expecttion, selective ttention, etc.). Alterntively, the P300 hs been linked to the formtion of decisions, reflecting the grdul ccumultion of evidence until decision boundry is reched (O Connell et l., 2012; Twomey et l., 2015). Finlly, more recent ccounts of the P300 emphsize the rectivtion of previously estblished stimulus-response ssocitions, process tht lso depends on stimulus frequency (Verleger et l., 2014; 2015). The sensitivity of the P300 to stimulus context nd tsk demnds cn be used in the Conceled Informtion Test (CIT), lso known s the Guilty Knowledge Test, deception detection method tht is bsed on the recognition of certin stimulus, for exmple crimerelevnt informtion, mong other, irrelevnt stimuli (Lykken, 1959; Verschuere et l., 2011; Verschuere nd Meijer, 2014). In n often used exmple to describe the CIT, vrious items re sequentilly presented to murder suspect, ny of which could be the murder wepon, for exmple: gun, knife, rope, etc. One of these items is probe item: the true murder wepon with which the ctul crime ws committed. All other items re conventionlly clled irrelevnt items. In EEG studies pertinent to our study, the number of different irrelevnt items typiclly rnges from four to eight, nd ech item (including the probe) is eqully repeted for exmple 40 or 50 times, presented in rndom sequence. It is ssumed tht the suspect will recognize the true murder wepon only if he/she hs prticipted in the murder. The recognition of the true 3

5 murder wepon, s consequently slient item mong other items, will result in detectbly lrger verge P300 response. Numerous rticles on this subject hve been published since the first successful experiments strting from the lte 1980s (minly: Frwell nd Donchin, 1991; Rosenfeld et l., 1988). The gret mjority of these studies hve been conducted in the lbortory of J. P. Rosenfeld, where the Complex Tril Protocol (CTP) version of the P300-bsed CIT ws lso introduced, nd hs been used in more thn dozen studies by now (Rosenfeld et l., 2013, 2008). In most of these studies, the CTP hs been consistently found to chieve the gols of its conception: to improve generl ccurcy, nd more importntly, to resist countermesures tht were found to gretly reduce ccurcy in previously used methods (Mertens nd Allen, 2008; Rosenfeld et l., 2004). In relted review published in 2012, the necessity of n independent repliction of these otherwise successful series of studies ws lredy remrked (Ben-Shkhr, 2012), but, to the best of our knowledge, no such ttempts were reported to dte. Besides replicting some of the findings, the min purpose of our study ws to provide n outside view through reconsidertion of the previous studies with n emphsis on the findings relted to the resistnce to countermesures, which is considered to be key feture of the CTP method, distinguishing it from other deception detection methods (Rosenfeld, 2011; Rosenfeld et l., 2013) Uninvestigted effects of countermesures The most effective countermesures ginst the P300-bsed CIT were found to be conceled responses (e.g., smll physicl movements or reclling nme of person) tht re ssigned to specific irrelevnt items, nd executed when those items pper (Mertens nd Allen, 4

6 2008; Rosenfeld et l., 2004). It hs been resoned tht these covert responses mke the corresponding irrelevnt items relevnt during the tsk, nd thus, the probe item would not be the only relevnt item nymore (Rosenfeld et l., 2004). Consequently, the oddbll nture of the prdigm is wekened, resulting in reduced differences between the probe- nd the irrelevnt item-induced P300 responses thereby incresing the chnces for guilty prticipnt to be clssified s innocent. As severl studies seemed to prove the CTP highly countermesure-resistnt (Hu et l., 2012; Lbkovsky nd Rosenfeld, 2012; Rosenfeld et l., 2008; Rosenfeld nd Lbkovsky, 2010; Winogrd nd Rosenfeld, 2011), more recent reserch focused on other res (optimiztion of prmeters, etc., Hu et l., 2013; Meixner nd Rosenfeld, 2014; Rosenfeld et l., 2015, 2015b; Winogrd nd Rosenfeld, 2014). However, these studies hve been using two kinds of P300- bsed mesurements for the clssifiction of prticipnts s guilty or innocent, nmely, the P vs. Ill, nd the P vs. Imx mesures nd the countermesure effects on the P vs. Imx were not s thoroughly tested, s on the P vs. Ill mesure. The originl P vs. Ill mesure hs been regulrly used for clssifiction since the first P300-bsed CIT studies (Frwell nd Donchin, 1991; Wssermn nd Bockenholt, 1989), including ll CTP rticles (see Rosenfeld et l., 2013). The P vs. Imx mesure ws introduced long with the CTP s n lterntive nlysis method (Rosenfeld et l., 2008), nd hs been used nd reported in subsequent studies (Meixner et l., 2009; Meixner nd Rosenfeld, 2014, 2011; Rosenfeld nd Lbkovsky, 2010), but not in ll of them (see Rosenfeld et l., 2013). While the P vs. Ill mesure compres the P300 responses to the probe (P) with the P300 responses to ll irrelevnt items (Ill), the P vs. Imx mesure compres the P300 responses to the probe with the P300 responses to the one irrelevnt item tht hs evoked the lrgest verge 5

7 P300 mong ll the irrelevnt items (which is the Imx ). The dvntge of this mesure s it ws rgued by the uthors (Rosenfeld et l., 2008) is tht it my be ble to provide higher specificity (i.e., less flse positive clssifictions). In studies using both mesures, they proved to provide very similr ccurcies, lthough with the P vs. Imx mesure indeed hving, in generl, slightly higher specificity (Meixner et l., 2009; Meixner nd Rosenfeld, 2014, 2011; Rosenfeld et l., 2008; Rosenfeld nd Lbkovsky, 2010), which could indicte tht it is the preferble lterntive. Finding n item tht evokes the lrgest P300 response cn lso hve very importnt prcticl use in itself. When the relevnt detil (i.e., the probe item), is not exctly known, then group of items cn be shown to the suspect, out of which the one tht evokes the lrgest P300 responses would be selected s presumed probe. For exmple, terrorist ttck is bout to hppen, but it is not exctly known in which city, or on which dte the ttck will tke plce, lthough there re severl ssumed possibilities. In this cse, suspected conspirtor could be presented these ssumed possibilities to determine which of them evokes the lrgest P300, nd whether the P300 of this presumed probe is significntly lrger thn those of the other items. This is scenrio tht Meixner nd Rosenfeld (2011) tested in mock-terrorism experiment with very good results (Meixner nd Rosenfeld, 2011; nd more detils on the theory in Rosenfeld, 2011, p. 83). Since the P vs. Imx mesure tkes into ccount the lrgest irrelevnt P300 lone, it is considerbly more vulnerble to n outlier irrelevnt item tht evokes lrger P300 responses thn the rest of the irrelevnt items. In this regrd, the Probe vs. Imx pproch is not only more rigorous in clssifying exminees s guilty (Rosenfeld et l., 2008), but, to some extent, it could lso be sensitive to the use of countermesure technique, since n outlier cn lso be creted 6

8 voluntrily: beside the first oddbll, i.e. the probe item, one my crete secondry oddbll, through trgeting n irrelevnt item with unique covert response, while still keeping the mjority of irrelevnt items comprtively regulr. This scenrio would crete specil oddbll prdigm, with two slient items, i.e. the probe nd the trgeted irrelevnt item, ginst the mjority of the other items (Ktym nd Polich, 1999). Two rticles on the CTP hve been published tht hve exmined the effect of countermesures (covert responses) ginst less thn the hlf of the irrelevnt items, but neither of these reported P vs. Imx mesures (Hu et l., 2012; Lbkovsky nd Rosenfeld, 2012). However, one of these rticles (Hu et l., 2012) did report tht, in the cse of 2 countered irrelevnt items out of 8, the P300 responses to probe nd countered irrelevnt items were significntly lrger thn those of non-countered irrelevnt items, while, in the cses of 4 nd 6 countered irrelevnt items out of 8, only the probe but not the countered irrelevnt items evoked P300 responses significntly lrger thn non-countered irrelevnt items (Hu et l., 2012, p. 88). Despite this observtion, no further investigtion ws recounted in this direction. Our hypothesis pertinent to our study ws tht when only smll group of irrelevnt items re countered, t lest one of them will tend to evoke P300 tht pproximtes the P300 to the probe, nd consequently, the ccurcy of the P vs. Imx mesure will be significntly reduced. To provide cler proof of this vulnerbility, we used countermesures with few smll modifictions in order to enhnce them Restructuring nd simplifying countermesures Initil P300-bsed CIT methods included designted trget item mong the irrelevnt items, to which different behviorl response (key press) hd to be executed when it ppered. 7

9 However, Rosenfeld nd collegues (2008) hve resoned tht this tsk drins processing resources, diverting ttention from the recognition of the probe item, nd thus lso reducing the P300 response to it. Therefore the CTP ws devised so tht the probe nd irrelevnt items ll required the sme response, key press indicting merely tht the prticipnt sw the displyed item (Rosenfeld et l., 2008, pp. 906, 907). Additionlly, to hold ttention throughout the tsk, fter ech tril of displying probe or n irrelevnt item, simple secondry decision tsk ws presented, with rre trget item requiring button press different from the response to the nontrget items. In most of the following studies, this decision tsk involved strings of five identicl numbers, where the string of ws the trget, nd strings of four other numbers (22222, 33333, 44444, nd 55555) were non-trgets. Ech of ll these stimuli is presented for 300 ms, ppering within two seconds fter ech other, in the typicl tril structure. Thus, the CTP method my reduce cognitive lod during the probe-irrelevnt discrimintion tsk, but, overll, the combined tsk is firly demnding, nd especilly so if prticipnt tries to consistently execute number of predefined countermesures to vrious items. Our ssumption here is tht the CTP s resistnce to countermesures is t lest prtly due to this incresed worklod, nd therefore, reducing the difficulty of the execution of countermesures (i.e., simplifying them), will enhnce their effects. Seven different items were presented in our experiment, including one probe nd six irrelevnt items (following Hu et l., 2013). Out of the six irrelevnt items, we chose to hve two items for oddblls, insted of only one, in order to rise the possibility tht either one of them would evoke lrge enough P300 responses to defet the test when using the P vs. Imx mesure. In previous CTP studies on countermesures, prticipnts were instructed to execute different specific covert responses (typiclly: mentlly sy meningful personl nme, e.g., close 8

10 reltive s given nme) to ech irrelevnt tht ws trgeted for countermesures. Thus, to trget smll, two-item group of irrelevnt items, the originl countermesures could be used in the following wy: silently rticulting one specific nme whenever one of the two smll-group irrelevnt item ppers, nd nother specific nme whenever the other smll-group irrelevnt item ppers, while omitting covert responses whenever ny of the other items pper (exctly s in the following studies: Hu et l., 2012; Lbkovsky nd Rosenfeld, 2012; Rosenfeld nd Lbkovsky, 2010). In our version of countermesures, prticipnts were instructed to give conceled responses to ll items: one silent word when either of the two irrelevnt items tht belonged to the smll group ppered, nd nother silent word when ny of the other items ppered, including the probe. This cretes simple, but continully ctive second tsk tht my divert ttention from the probe, which becomes, to some extent, simply the prt of the lrger group of irrelevnt items. The reclling of the sme word for these items of the lrger group cn lso be described s giving them common ttribute, nd thus mking them overlpping. Consequently, the more the presented stimuli overlp with ech other in their ttributes, the smller the P300 mplitude differences will be (Azizin et l., 2006; Mrchnd et l., 2013). As the lest detectble countermesure, the silent, mentl rticultion of words ws introduced in the study of Rosenfeld nd Lbkovsky (2010), nd used in subsequent studies. These words were the first or lst nmes of the prticipnts, nd, in some occsions, the first or lst nmes of close reltives. Ltency mesures were not reported, but figures re provided on which it is consistently observble tht, on the group level, P300 responses to probes nd countered items peked during the sme time intervl (Hu et l., 2012, fig. 2; Lbkovsky nd Rosenfeld, 2012, fig. 5; Rosenfeld nd Lbkovsky, 2010, fig. 3; Winogrd nd Rosenfeld, 2011, 9

11 fig. 2). This shows tht the P300 ppers s rection to the item, nd not to the subsequently reclled silent word, nd it is therefore very likely tht the effect on the P300 is cused not by the meningfulness of the silent words, but by the meningfulness of the countered items, i.e., tht they re recognized s requiring specific nswer (Rosenfeld et l., 2013, p. 7, drew similr conclusions from some suggestive preliminry empiricl evidence, citing the bstrct of yet unpublished study: Winogrd nd Rosenfeld, 2012). Possible emotionl significnce of nmes my hve rousing effect, but higher rousl would simply led to generlly lrger P300 responses throughout the tsk, including those to the probe (Duncn et l., 2009; Polich, 2007). For the purpose of our bove described countermesure tsk, we sked our prticipnts to choose ny two simple, short (mximum two syllble) neutrl words, tht re esily distinguishble from ech other, nd which they would be comfortble to be repeting for minutes, i.e., for the durtion of the test Study outline Four groups were mesured: one innocent Control group, one simple guilty (SG) group with no instructions on countermesures, nd two other groups tht were instructed to use countermesures. One used our new countermesures (New-CM group), nd the other used the originl countermesures (Old-CM group) so tht we could replicte previous findings, while lso directly compring the two countermesure methods. In both groups, the countermesure use involved choosing smll, two-item group of irrelevnts; this set of items will be clled the I-2item, while the remining lrger, four-item group of irrelevnts will be clled the I-4item. While the New-CM group used the bove described countermesures, the Old-CM group used countermesures ginst the I-2item s described in previous studies, i.e., they were instructed to 10

12 counter one of the irrelevnt items by silently rticulting the nme of one of their prents, nd nother one with the nme of the other prent (see in Methods; 2.2. Procedure). To summrize our hypotheses: we expected successful repliction of the high ccurcy rtes with the P vs. Ill mesure in ll groups, but significnt drop in the detection rtes with the P vs. Imx mesure in the two CM groups (but not in the SG group) nd we expected this drop to be more pronounced with the enhnced countermesures in the New-CM group. 2. Methods 2.1. Prticipnts Sixty-six prticipnts were recruited through dvertisements proposing to try our EEG lie detection test, nd offering cfeteri voucher of Ft500 (pprox. 1.60) in cse they mnged to defet the test. Six prticipnts were excluded: four due to excessive mount of rtifcts in the EEG recording (over 50% of the trils hd to be rejected), nother due to extremely low ccurcy in the decision tsk (correct responses to the ctch trils of the item: 8.6%, correct responses in the cses of the four other strings of numbers: 91.8%), nd one due to n extremely low rte of correct responses in the min tsk (66.9% correct). The remining prticipnts consisted of 14 individuls in the Control group (ge = 24.5±3.98 yers, in the formt of MEAN±SD, s lso in the rest of this pper; 5 mles), 15 in the SG group (ge = 21.5±2.42 yers; 6 mles), 15 in the New-CM group (ge = 22.9±3.29 yers; 7 mles), nd 16 in the Old-CM group (ge = 23.0±3.48 yers; 6 mles). All prticipnts provided signed, informed consent, nd, t the end of the experiment, they ll received cfeteri voucher regrdless of the results of the exmintion. 11

13 2.2. Procedure In this CIT, we used prticipnts fmily nmes s probes except for the Control group, in which none of the presented items ws relevnt to the prticipnts. For this group, we refer to the Probe item s the irrelevnt item tht ws, unbeknownst to the prticipnt, rndomly ssigned with the sme EEG event mrker s the Probe (own fmily nme) of the other three experimentl groups. At the beginning of ech experiment, prticipnts were shown list of twenty Hungrin fmily nmes, nd were sked to indicte if ny of these nmes were prticulrly meningful (e.g., nme of close reltive or friend) or otherwise ppered to them mrkedly unique compred to the other nmes on the list. Irrelevnt items were selected from mong the fmily nmes tht were not indicted by the given prticipnt s slient. The Old-CM group prticipnts were instructed to use the given nmes of their prents to counter two out of the six irrelevnt items (lwys reclling their fther s nme when one specific item ppered, nd their mother s nme when nother specific item ppered). The New-CM group prticipnts were sked to choose ny two words tht were short (one or two syllble) nd esy to distinguish from ech other; for exmple up/down or dog/ct. One of these words ws reclled when either of two specific irrelevnt item ppered, nd the other ws reclled when ny of the other items (including the probe) ppered. We encourged prticipnts in this group to concentrte not on the item, but on which word-ctegory the item belongs to. In both CM groups, the words used for countermesure hd to be silently sid t the sme time or fter the response key ws pushed. None of the prticipnts hd ny informtion on the eventul irrelevnt items in the tsk until the tsk begn, nd countermesure using prticipnts hd to choose the smll group of two specific irrelevnt items during the beginning of the tsk. 12

14 The E-Prime softwre (Psychology Softwre Tools, Inc., Shrpsburg, USA) ws used to present stimuli nd record behviorl responses. Stimuli were presented in 100 cm distnce from the eye of the prticipnt, on 20 inch LCD screen. All presented chrcters were white on the blck bckground, with height subtending visul ngle of pproximtely.57. Ech tril begn with 100 ms bseline period for the recording of prestimulus brin ctivity. The probe or irrelevnt item (for the min tsk) ws then presented on the center of the screen for 300 ms. Following n inter-stimulus intervl tht rndomly vried between ms, one of the number strings (for the secondry tsk) ws presented for 300 ms. The next tril begn fter nother rndomly vrying intervl of ms. During ll intervls between stimuli, fixtion cross ws presented on the center of the screen. For schemtic depiction, see Figure 1. Figure 1. Exmple of tril in the CTP CIT tsk in this study. The probe nd irrelevnt stimuli were the given prticipnt s own fmily nme nd other, unfmilir fmily nmes. All these stimuli lwys required the sme response with the left hnd (pushing one rndomly chosen key out of five, with the corresponding finger). This ws followed by trget stimulus (11111) or non-trget stimulus (22222, 33333, 44444, nd 55555). In response to trget stimulus, key 13

15 hd to be pushed by the right middle finger, while in cse of non-trget stimulus, nother key hd to be pushed by the right index finger. In the full tsk, there were 350 trils in totl, consisting of the probe (prticipnt s fmily nme) nd six irrelevnt items (other fmily nmes), ech repeted 50 times, for totl of 50 probe nd 300 irrelevnt items presented in rndom order, followed by ny of the number strings with equl probbility (thus 10 times the 35 vritions of the piring of 7 nmes nd 5 number strings). Before the full tsk, countermesure using prticipnts were given prctice tsk tht rn exctly the sme wy s the full tsk (1 probe nd 6 irrelevnt items presented in rndom order), except tht prticipnts ssigned to the New-CM group were presented given nmes (with lwys the sme nmes - Ferenc for mles nd Ilon for femles - pointed out s probes 1 ), while those in the Old-CM group were presented month nmes (with Jnury s probe). Both CM groups were instructed to silently rticulte the very sme words (New-CM: two freely chosen neutrl words; Old-CM: prents given nmes) upon stimulus onset. This prctice tsk hd no time limit, prticipnts could finish whenever they felt redy; this usully took trils. As next step, prticipnts in ll four experimentl groups completed nother prctice tsk using fixed set of city nmes (1 probe nd 6 irrelevnt items with Budpest s probe in ll cses) tht lsted for only 35 trils (5 repetitions for ech stimulus). Here, prticipnts were sked to imgine themselves in rel lie detection sitution in which they would try to concel the recognition of the probes. However, prticipnts in the Control nd SG groups were not instructed to use ny specific countermesures. 1 These nmes were lwys different from the prticipnts own given nmes. 14

16 We removed most of the keys from stndrd computer keybord so tht group of five keys remined on the left side (to be conveniently pushed by ech corresponding finger of the left hnd) nd two next to ech other remined on the right side (to be pushed by the right index nd right middle fingers). At ech ppernce of probe or irrelevnt item, key hd to be pushed with the left hnd, ech time rndomly chosen from the five keys; prticipnts were instructed not to follow ny specific sequence (see Rosenfeld nd Lbkovsky, 2010). During the recording, we monitored responses to verify tht prticipnts were indeed rndomly choosing keys, nd not pushing the sme key repetedly or following sequence (Rosenfeld nd Lbkovsky, 2010). In the secondry tsk, one of the keys on the right side hd to be pushed with the right hnd t the ppernce of number string; one with the middle finger for the string, nd the other one with the index finger when ny of the other number strings ppered ( 22222, 33333, or ). Prticipnts were told tht from time to time during the tsk there would be puse nd they would be sked to repet out loud the lst fmily nme tht ppered, nd more thn two incorrect nswers would men filing the test. They were ctully sked nine times during the experiment t rndom time points. None of the prticipnts hd more thn two incorrect nswers. All prticipnts in the three guilty groups were explicitly encourged to try to concel their nmes nd to defet the test. Prticipnts completed the whole experiment (including preprtions, prctice nd full tsks, nd debriefing) in minutes from their rrivl Electrophysiologicl recordings nd dt processing A BioSemi ActiveTwo Amplifier (BioSemi B. V., Amsterdm, Netherlnds) ws used with 32 Ag/AgCl electrodes recording EEG from stndrd sclp sites (Fp1, Fp2, AF3, AF4, F7, 15

17 F3, Fz, F4, F8, FC5, FC1, FC2, FC6, T7, C3, Cz, C4, T8, CP5, CP1, CP2, CP6, P7, P3, Pz, P4, P8, PO3, PO4, O1, Oz, O2) nd two dditionl electrodes plced bove the left nd right mstoids. The dt ws collected t smpling rte of 1024 Hz, without ny frequency filters. The recording reference nd the ground electrodes (Common Mode Sense nd Driven Right Leg electrodes in the ActiveTwo System; Metting vn Rijn et l., 1990) were plced in close proximity to the Cz position. Electrophysiologicl dt were processed with the EEGLAB toolbox (Delorme nd Mkeig, 2004) for Mtlb (MthWorks Inc., Ntick, USA). After chnging the smpling rte to 512 Hz (with Biosemi Decimtor 86), the dt ws high-pss nd low-pss filtered using Hmming-windowed sinc FIR filters with 0.3 Hz nd 30 Hz cutoff frequencies, respectively (Widmnn et l., 2015; Widmnn nd Schröger, 2012). Epochs strting t 100 ms before stimulus onset, nd ending t 1300 ms fter stimulus onset, were extrcted, with bseline correction bsed on the whole epoch length. The entire recording ws visully inspected for the removl of epochs with prominent rtifcts such s bseline fluctutions or musculr ctivity. Oculr rtifcts were removed with independent component nlysis (ICA, Hyvärinen nd Oj, 2000) implemented in EEGLAB. This method seprted independent subcomponents of the EEG, mong which those ssocited with eye movements were identified on the bsis of visul inspection of their single-tril ctivtions nd sclp topogrphy, nd rejected. After pplying new bseline correction (from -100 ms to 0 ms), the recording ws gin visully inspected to reject epochs with smller rtifcts. The men nd stndrd devition of the remining epochs for ech stimulus (i.e., for ech presented nme for ech prticipnt) ws 40.57±6.34. Finlly, the dt ws filtered gin by pplying Hmming-windowed sinc FIR low-pss filter with 6 Hz 16

18 cutoff frequency (Rosenfeld et l., 2008; Soskins et l., 2001), nd the EEG ws re-referenced to linked mstoids. For ll sttisticl nlyses, the P300 ws mesured t Pz only P300 mesure nd individul bootstrp nlysis For individul clssifiction using P300 wves, certin bootstrpping method hs been used in ll CTP studies, which compres the responses to the probe item with the responses to irrelevnt items (see lso: Frwell nd Donchin, 1991; Wssermn nd Bockenholt, 1989). This method uses pek-to-pek mesure (Rosenfeld, 2011; Rosenfeld et l., 2008; Soskins et l., 2001): in our cse, n lgorithm serched, on the verged epoch of certin stimulus type (s described below), for the mximum verge 100 ms segment between 500 nd 800 ms, nd then, between the midpoint of this segment nd 1300 ms, serched gin for minimum verge 100 ms segment. The choice of the serch window ws bsed on visully inspecting the grnd verge of ll prticipnts, verifying tht the P300 pek fell within the specified window (Keil et l., 2014; lso cited by Rosenfeld et l., 2015b). The resulting vlue is the mplitude vlue of the pek-to-pek P300, which will be referred to s P300pp in the rest of this pper. The procedure of the bootstrpping nlysis for the P vs. Ill mesure ws the following. First, single trils were chosen rndomly, with replcement, from ll probe single trils (i.e., trils in which the probe item hd been presented), nd verged into one epoch, from which P300pp ws clculted. The number of these chosen vlues ws equl to the number of vilble probe trils in cse of the given individul s results (i.e., the number of rtifct-free epochs out of the originl 50 recorded during the experiment). Second, sme number of single trils were gin chosen rndomly, with replcement, from ll irrelevnt single trils (i.e., trils in which one of the irrelevnt items hd been presented), nd verged into one epoch, from which 17

19 nother P300pp ws clculted. Third, the P300pp obtined from the probe trils ws compred to the P300pp obtined from irrelevnt trils, in order to determine whether the former is greter thn the ltter (with difference greter thn zero). These three steps were repeted 1000 times, with results possibly vrying ccording to the rndom choices with replcement. The end result of this procedure is number between , indicting the number of occsions in which the P300pp vlues of the probe trils were determined to be greter in comprison to those of the irrelevnt trils. The procedure for the P vs. Imx mesure is exctly the sme s the one for the P vs. Ill mesure, except tht the responses to the probe item were eventully compred to only one irrelevnt item, the one which hd evoked the lrgest P300pp, s mesured with the bootstrp nlysis. This individully vrying lrgest irrelevnt is clled the Imx. In order to select this Imx, n lgorithm seprtely compred ech of the six irrelevnt items to the probe, which gin resulted in number between , indicting the number of occsions in which the P300pp vlues of the probe were determined to be greter thn those of the given irrelevnt. The Imx ws then selected from mong ll these six irrelevnt items, to be the one in whose cse this number ws the smllest nd this smllest number is the result of the P vs. Imx mesure for the given individul Group level comprisons The distribution of behviorl dt (men item detection ccurcy nd rection times) ws entered into repeted-mesures nlyses of vrince (ANOVA) with Stimulus Type (min tsk: probe vs. Ill; probe vs. I-2item vs. I-4item; secondry tsk: trget vs. non-trget) s withinsubject fctor nd Group (I, SG, New-CM, Old-CM) s between-subject fctor. The comprison 18

20 between probe, I-2item nd I-4item response ltencies ws necessry to show tht ny effects between probe vs. irrelevnt items in the New-CM nd/or Old-CM groups re due to the use of countermesures. Simple P300pp mplitudes i.e., P300pp clculted from ll single trils of the given stimulus type were nlyzed in three steps. First, repeted-mesures ANOVA ws used to ssess probe vs. Ill effects (Type s within-subject fctor) between experimentl groups. Then, probe vs. Imx vs. Iremining ANOVA ws used to investigte the efficcy of the Imx mesure in ll four groups. Finlly, with the probe vs. I-2item vs. I-4item sttisticl comprison between the two CM groups we imed to show tht the effect of countermesures ws more prominent in the New-CM group thn in prticipnts using the Old-CM technique. Results of the individul bootstrp nlysis (probe vs. Ill; probe vs. Imx) were used to clssify prticipnts s innocent or guilty. One my set cutoff rte, for exmple t 90% (Rosenfeld et l., 2013). In tht cse, when the P vs. Ill or P vs. Imx result for the individul is number lrger thn 900 (i.e., the P300pp vlues of the probe trils were determined to be greter in more thn 900 out of the 1000 clcultions), then the prticipnt is clssified s guilty. For illustrtion, we report clssifiction t severl possible cutoffs (t 90%, 70%, nd 50%), showing true negtive rtes (rtio of correctly identified innocent prticipnts) in the cse of the Control group nd true positive rtes (rtio of correctly identified guilty prticipnts) in the cses of the SG nd CM groups. However, for more comprehensive ssessment of clssifiction ccurcy, we clculted res under the receiver operting chrcteristic curve (AUROC curve, or simply AUC re under the curve; e.g., Ntionl Reserch Council, 2003, pp ). This method mesures true positive nd true negtive rtes t ll possible cut-off points nd gives n verged vlue tht cn rnge from 0 to 1, where 0.5 mens chnce level clssifiction, nd 1 19

21 mens flwless clssifiction (i.e. ll guilty nd innocent clssifictions cn be correctly mde t given cutoff point). The AUC ws first clculted for the P vs. Ill results for ech of the three guilty groups (SG, New-CM, Old-CM) pired with the P vs. Ill results of the Control group, nd the resulting AUCs were compred using z tests (Hnley & McNeil, 1982). Finlly, the sme clcultions nd comprisons were mde using the P vs. Imx results. We used n lph level of.05 for ll sttisticl tests except for the bootstrpping mesure. For ech ANOVA with significnt Group x Type interctions (p <.05), simple effects were tested using t-tests with Bonferroni correction. For violtions of sphericity, Greenhouse-Geisser corrected p vlues nd the relevnt epsilon (ε) correction re reported. In order to demonstrte the mgnitude of the observed effects, prtil et-squred (ηp 2 ) vlues re lso shown. 3. Results 3.1. Behviorl mesures Accurcies nd men rection times for the min nd secondry tsks for ll stimulus types nd ech experimentl group re shown in Tble 1. Tble 1 Mens (M) nd Stndrd Devitions (SD) for Accurcies nd Rection Times (RT) to Specific Types of Items, by Ech of the Four Groups Groups Control SG New-CM Old-CM M SD M SD M SD M SD Accurcies (%) Min tsk Probe

22 Ill I-2item I-4item Secondry tsk Trget Non-trget RT (ms) Min tsk Probe Ill I-2item I-4item Secondry tsk Trget Non-trget Note. Min tsk: Accurcies nd RTs during the min tsk with fmily nmes. Probe prticipnt s own nme; Ill ll nmes except the prticipnt s own; I-2item the two nmes tht belonged to the smller group of two items trgeted by rticulting the sme words in the New-CM group (prticipnts using the new countermesures), nd by rticulting two different words in the Old-CM group (prticipnts using the originl countermesures); I-4item the four nmes tht belonged to the lrger group of four items tht were trgeted by rticulting nother word in the New-CM group, nd simply omitted in the Old-CM group. There were no such groups of countered items (I-2item or I4-item) in the SG group (simple guilty prticipnts) or in the Control group (innocent prticipnts). Secondry tsk: Accurcies nd RTs during the secondry tsk with number strings. Trget the ctch trils of strings tht required response with the middle finger; Non-trget the rest of the number strings tht required response with the index finger. 21

23 In the min tsk, where ll stimuli required the sme response, mistkes nd omitted responses were very rre in ll conditions (see Tble 1), nd no sttisticlly significnt min effects or interctions were found (p >.2). In the secondry tsk, however, the prticipnts' ccurcies were significntly worse for trget stimuli thn for other strings (83±2.2% vs. 98±0.2%; F(1,56) = 51.7, p <.001, ηp 2 =.48). This effect ws not influenced by Group. In the min tsk, the prticipnts rection times were significntly slower for probe stimuli thn for ll the irrelevnts (Ill) (546 ±17 ms vs. 525±18 ms; F(1,56) = 21.6, p <.001, ηp 2 =.28), but this effect vried with Group significntly (F(3,56) = 5.6, p =.002, ηp 2 =.23). Bonferroni-corrected tests of simple effects reveled tht the probe vs. Ill comprison ws significnt for the SG (p <.001) nd Old-CM groups (p =.001) only. The min effect of Group ws not significnt. In the two CM groups, the I-2item nd I-4item stimuli were lso compred with ech other, nd the probe. The significnt Stimulus Type effect (F(2,58) = 12.1, ε =.754, p <.001, ηp2 =.29) indicted tht response times for probe (525±23 ms) nd I-2item (532±27 ms) stimuli were substntilly longer thn those obtined for I-4item stimuli (488±21 ms), with the Bonferroni post-hoc tests being significnt for both the P vs. I-4item (p <.001) nd I-2item vs. I- 4item (p =.002) comprisons. Agin, the min effect of Group nd its interction with Stimulus Type (P vs. I-2item vs. I-4item) were not significnt (p >.2). Regrding the secondry tsk, responses to trget stimuli were slower thn to other stimuli (609±12 ms vs. 539±15 ms; F(1,56) = 88.8, p <.001, ηp 2 =.61) nd this effect ws not influenced by Group. Interestingly, we hve lso found significnt Group min effect ( F(3,56) = 3.2, p =.035, ηp 2 =.15), but none of the post-hoc comprisons reched significnce level (p >.072). 22

24 3.2. Electrophysiologicl mesures Event-relted potentils obtined for probe, Ill nd Imx stimuli for ll four experimentl groups re shown in Figure 2, wheres mens nd stndrd devitions for P300pp mplitudes evoked by probe, Ill, I-2item nd I-4item items re shown in Figure 3. Figure 2. Grnd verge event-relted brin potentil wveforms registered on the prietl electrode Pz, s evoked by the following stimuli: Probe (own fmily nme), Imx (the one irrelevnt fmily nme tht evoked the lrgest P300pp), nd Ill (ll irrelevnt fmily nmes); within ech of the four experimentl groups: Control (innocent), SG (simple guilty), New-CM (prticipnts using the new countermesures), Old-CM (prticipnts using the originl countermesures). Plese note tht on the group level, in the New-CM group, the P300pp evoked by the Imx item is even slightly lrger thn tht evoked by the probe. 23

25 Figure 3. Mens nd stndrd devitions of pek-to-pek P300 mplitudes registered on the prietl electrode Pz, for the following stimuli: Probe (own nme), Ill (ll irrelevnt nmes), I- 2item (the two nmes tht belonged to the smller group of two items trgeted by rticulting the sme words in the New-CM group, nd by rticulting two different words in the Old-CM group), nd I-4item (the four nmes tht belonged to the lrger group of four items tht were trgeted by rticulting nother word in the New-CM group, nd simply omitted in the Old-CM group); within ech of the four experimentl groups: Control (innocent), SG (simple guilty), New-CM (prticipnts using the new countermesures), Old-CM (prticipnts using the originl countermesures) P300pp mplitudes As expected, Probe stimuli evoked significntly lrger P300pp mplitudes thn irrelevnt (Ill) items (min effect of Stimulus Type: F(1,56) = 167.9, p <.001, ηp 2 =.75; Figure 3). 24

26 Furthermore, there ws significnt Stimulus Type x Group interction (F(3,56) = 20.5, p <.001, ηp 2 =.52). Bonferroni-corrected tests of simple effects indicted tht the P300 evoked by Probe items ws significntly smller in the Control group thn for SG prticipnts (p =.010), nd trend ws observed for the Control vs. Old-CM comprison (p =.053; nd p > 0.1 for the rest of the comprisons between ny two of the four groups). The P300 mesured for Ill items did not differ between groups (p >.999). When performing post-hoc Probe vs. Ill comprisons for ech group seprtely, highly significnt differences were found for the SG, New-CM nd Old-CM groups (p <.001), while mplitudes were comprble in the Control group (p =.728). The min effect of Group ws not significnt. When compring the P300pp mplitudes evoked by Probe items, Imx items (irrelevnt items with the lrgest P300pp), nd Iremining items (the mens of the other five irrelevnt items), the min effect of Stimulus Type ws significnt ( F(2,112) = 85.9, ε =.850, p <.001, ηp 2 =.61): the lrgest P300pp mens were for Probe, smller for Imx, nd smllest for Iremining (for ll comprisons, Bonferroni-corrected tests gve p <.002). The significnt Stimulus Type x Group interction (F(6,112) = 14.2, p <.001, ηp 2 =.43) indicted tht the P300pp mplitudes were significntly lrger for Probes thn for Imx items in the SG (p <.001) nd Old-CM (p <.001) groups, but were not found to be significntly different in the New-CM group (p >.999), nd were significntly lrger for Imx thn for Probe items in the Control group (p =.008) (Figure 2). Furthermore, the P300pp mplitudes were significntly lrger for Probes thn for Iremining stimuli in the SG, Old-CM nd New-CM groups, but not in the Control group (Bonferroni-corrected tests of simple effects for guilty groups: p <.001; for the Control group: p =.645). P300pp mplitudes were significntly lrger for Imx thn for Iremining in the Control, 25

27 New-CM nd Old-CM groups (p <.001), while only tendency ws observed in for SG prticipnts (p =.067). Agin, the min effect of Group ws not significnt. In order to test if incresed P300pp mplitudes in the CM groups were indeed cused by countermesure strtegies, i.e., due to incresed wveforms for the I-2item (the smller group of countered irrelevnt items), second repeted-mesures ANOVA ws performed with Probe, I- 2item, nd I-4item stimuli s levels of Stimulus Type, nd New-CM nd Old-CM groups s levels of Group. The significnt Stimulus Type x Group interction (F(2,58) = 3.5, p =.037, ηp 2 =.11) ws indictive of robust P vs. I-2item mplitude differences in the Old-CM group only (Bonferroni-corrected tests of simple effects: p <.001), while similr mplitude reductions between I-2item vs. I-4item stimuli were observed in both CM groups (New-CM: p =.033, Old- CM: p <.001; Figure 3). Finlly, to see whether the difference between I-2item nd I-4item differs in mgnitude between New-CM nd Old-CM groups, the ANOVA ws rerun with the Probe omitted. The Stimulus Type (I-2item, I-4item) x Group interction ws not significnt (F(1,29) = 1.5, p =.227, ηp 2 =.05) Individul clssifiction bsed on the bootstrp nlysis Prticipnts were clssified guilty or innocent bsed on the results of the P vs. Ill nd P vs. Imx mesures s described in Methods (Section 2.4. nd Section 2.5.). Correct detection rtes using cutoffs t 90%, 70%, nd 50% re shown in Tble 2, long with AUCs for ech group, for P vs. Ill nd for P vs. Imx, for which ROC curves re lso shown in Figure 4. In the cse of clssifiction using P vs. Ill mesures (SG: AUC =.976, CI:.930 1; New-CM: AUC =.943, CI:.858 1; Old-CM: AUC =.929, CI:.831 1), no significnt differences were found between the AUCs of ny two of the three guilty groups (p >.3). In the cse of P vs. Imx 26

28 mesures (SG: AUC =.981, CI:.943-1; New-CM: AUC =.776, CI: for; Old-CM: AUC =.911, CI:.811-1), the AUC of the SG group ws significntly lrger thn the AUC of the New-CM group (z = 2.21, p =.027) mening tht the results of SG group s guilty prticipnts, compred with the results of New-CM group s guilty prticipnts, were significntly more distinct from the results of the Control group s innocent prticipnts. No significnt differences were found between the AUCs of the SG nd the Old-CM groups (z = 1.29, p =.199) or between the AUCs of the New-CM nd the Old-CM groups (z = -1.29, p =.196). 2 Figure 4. ROC curves showing the true positives rtes of the three guilty (SG, Old-CM, New- CM) groups in function of the flse positive rtes of the Control group (for the results of the P vs. Ill on the left, nd for the results of the P vs. Imx on the right). 2 In order to demonstrte the overll effect of countermesures, we lso performed dditionl AUC clcultions for the CM groups merged into one CM group tht cn be defined simply s prticipnts instructed to use countermesures (resulting in AUC =.935, CI: using P vs. Ill; nd AUC =.846, CI: using P vs. Imx). In the cse of the P vs. Imx mesure, the AUC of the SG group ws significntly lrger thn the AUC of this merged CM group (z = 2.23, p =.026), i.e., the P vs. Imx results of this merged CM group were more distinct from those of the Control group. Agin, no significnt differences were found in the cse of the P vs. Ill mesure (p >.3). 27

29 Tble 2 P vs. Ill nd P vs. Imx Bootstrp Results for Ech Prticipnt P vs. Ill P vs. Imx Subject Control SG NewCM OldCM Control SG NewCM OldCM Men TR TR TR AUC Note. Group verges of the bootstrpping results re given below ech corresponding column, in boldfce (nd rounded to whole numbers). TR: true negtive rtes (in the cse of the Control group) nd true positive rtes (in the cses of the SG nd CM groups) of correct individul clssifictions (number of correctly clssified prticipnts / number of prticipnts) bsed on the P vs. Ill or P vs. Imx mesures, with possible cutoffs, for illustrtion, t 900 (TR-9), 700 (TR- 7), nd 500 (TR-5) where numbers below/bove the given cutoff men innocent/guilty clssifictions, respectively, for the given prticipnt. AUC: res under the curve for the three 28

30 guilty (SG, Old-CM, New-CM) groups for the two mesures (P vs. Ill nd P vs. Imx), where ech AUC uses the Control group s results of the sme mesure to clculte clssifiction efficiency. One of the two countermesure-trget irrelevnt items ws the Imx. 4. Discussion 4.1. Effects of smll group countermesures on the P vs. Imx mesure The min purpose of our study ws to show tht the CTP version of the P300-bsed CIT, which hs repetedly been climed to be highly resistnt (or even immune) to countermesures (Rosenfeld, 2011; Rosenfeld et l., 2013, 2008), cn in fct be severely vulnerble to certin countermesures, when using the P vs. Imx mesure, i.e., when the probe (the criticl informtion, e.g., crime detil) is compred to the Imx (the one irrelevnt informtion which hs evoked the lrgest P300pp responses). An effective countermesure cn be ccomplished by covert responses to smll group of irrelevnt items, nd different response, or no response t ll, to ll other items. This mkes the items of the smll group subjectively unique compred to the others, thereby evoking prominent P300pp wves, which cn pproximte or even overcome those evoked by the probe, reducing detection rtes when using the P vs. Imx mesure (Figure 2). To show this, for one, we hve instructed the prticipnts in the Old-CM group to use countermesures tht were used in previous studies, but whose effect, when used only on smller group of irrelevnt items, hs not been tested on the P vs. Imx mesure, until now. For nother, we hve lso introduced slightly modified new version of these countermesures, which ws used by the prticipnts in the New-CM group. In this ltter cse, ll items were divided into smller nd lrger group, nd ll items required covert response ccording to 29

31 group membership. The smller group included two irrelevnt items, trgeted to be oddblls, nd the lrger group included ll the five other items (the probe nd the four other irrelevnt items). Furthermore, silent rticultion of simple, esily distinguishble words (insted of personl nmes) were used for covert responses, in order to simplify the countermesure tsk. The effects of this new countermesure did prove to be somewht more effective thn the originl one, nd consequently helped us provide more convincing proof for the vulnerbility of the P vs. Imx mesure. Our min results indeed show tht prticipnts in both CM groups hve used the countermesures ginst the P vs. Imx mesure with success, generlly chieving to be more difficult to distinguish from innocent prticipnts, thn those guilty prticipnts who were not instructed to use countermesures (AUC =.85 for CM prticipnts vs. AUC =.98 for simple guilty prticipnts) though this difference ws especilly pronounced for those who used the new countermesures (AUC =.78 for New-CM prticipnts). Importntly, we lso found, sme s previous studies (Hu et l., 2012; Lbkovsky nd Rosenfeld, 2012; Rosenfeld et l., 2008; Rosenfeld nd Lbkovsky, 2010; Winogrd nd Rosenfeld, 2011), tht the P vs. Ill mesure (probe compred to ll irrelevnt items) provided high detection rtes not only in the SG group (AUC =.98), but lso in both CM groups (AUC =.93 using the originl, nd AUC =.94 using the new countermesures), with no significnt differences between the three groups. This lso mkes our study the first to show tht the results of the P vs. Ill nd the P vs. Imx mesures, which hve been shown to provide very similr ccurcies in ll previous experiments (Meixner et l., 2009; Meixner nd Rosenfeld, 2014, 2011; Rosenfeld et l., 2008; Rosenfeld nd Lbkovsky, 2010), cn in fct differ gretly. 30