Abstract—The study of electroencephalography (EEG) for deception detection has long been regarded as an alternative to the standard polygraphy, whose main shortcoming is its unacceptably low level of reliability. Most of the EEG deception research has focused on the amplitude and topography of P300. However, the characteristics of the P300 component are tightly connected to the experimental design and hence countermeasures are easily available for P300-based deception detection. The goal of this study is to evaluate different epoching criteria for the extraction of EEG features that are most suitable for the discrimination between truthful and deceptive responses. In order to reach this aim, a modified version of the Guilty Knowledge Test was used where EEG recordings were obtained from four frontal electrodes and two midline electrodes. In none of the electrodes the P300 component differed between deceptive and truthful responses. Differences have instead been revealed through the extraction of response-locked epochs and analysis of area under the curve.

I. INTRODUCTION HE capability to reveal the information that is

intentionally concealed has long been sought for legal applications. Deception detection also bears clinical relevance; for instance, it would help in the recognition of subjects malingering cognitive impairments. Even though many shortcomings have long been known about the polygraphy, it is still the most widely used method in deception detection. The polygraph, in fact, measures physiological signals related to the autonomic nervous system. However, the same changes occurring in these signals during deception can occur as a consequence of different emotional states, i.e. anxiety, as well. Therefore, the interpretation of the outcomes is extremely difficult, subjective and error prone [1]. In order to overcome some of the downsides entailed by the polygraphy, other

Manuscript received April 2, 2007. This work has been sponsored in part by funds from the Defense Advanced Research Projects Agency (DARPA) Augmented Cognition Program, the Office of Naval Research (ONR) and Homeland Security, under agreement numbers N00014-02-1-0524, N00014-01-1-0986 and N00014-04-1-0119.

A.C. Merzagora is with the School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, PA 19104 USA (phone: 215-8951988; fax: 215-8954983; e-mail: a.merzagora@drexel.edu).

M. Izzetoglu is with the School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, PA 19104 USA (e-mail: meltem@cbis.ece.drexel.edu).

S. Bunce is with Department of Psychiatry, Drexel University College of Medicine, Philadelphia. PA 19104 USA. (e-mail: sbunce@drexelmed.edu).

B. Onaral is with the School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, PA 19104 USA (e-mail: banu.onaral@drexel.edu).

neurophysiological signals have been explored for their capability in deception detection, ranging from electroencephalogram (EEG) [2-8] to functional neuroimaging [9-18].

Research on event-related potentials (ERPs) has been especially intense. The main focus has been on the time domain analysis of the first one second post-stimulus interval data and principally the P300 topography and topology of the ERPs [4-7]. However, it is well-known that the P300 component is greatly dependent on the properties of the eliciting stimulus [19, 20]. Therefore, caution is needed in the generalization of the findings, since they are likely to be protocol dependent.

The aim of this study is to investigate EEG recordings obtained from different sites for time-domain differences between truthful and deceptive responses during a modified version of the Guilty Knowledge Test (GKT). The classical GKT has been modified in order to maximize the motivation to elude detection and to minimize subjects’ anxiety about being deceptive [21, 22]. Windows of different length were used for the extraction of EEG segments and they have been locked to either the stimulus or the response time. Features that were significantly different between deceptive and truthful conditions were provided by response-locked epochs, in particular by those recorded at frontal sites.

II. MATERIALS AND METHODS Participants: 16 adult healthy subjects were recruited for

this study. None of the participants reported previous history of neurological disorders, mental abnormalities or substance abuse. All subjects had signed an informed consent form approved by the Institutional Review Board at Drexel University.

Protocol: Participants were given a total of 5 cards, four of which (one from each suit) were face-up on the computer screen. Participants were then asked to choose between 3 sealed envelopes containing $50 and a fifth card (‘target’ card), known only to the subjects. The participant’s goal was to conceal the identity of this card. Subjects were told that, if able to successfully conceal the identity of the card, they would be able to keep the $50 in addition to their participation remuneration. The experimenter would attempt to discover the identity of the ‘target’ card by showing a card on the screen and asking questions such as “Do you have this card?”. Depending on the question card appearing on the screen, they were categorized as either ‘truth’ card, ‘lie’ card or ‘control’ card. Cards were presented for 3 s

Time-domain analysis of EEG during Guilty Knowledge Test: Investigation of epoch extraction criteria

Anna Caterina Merzagora, Meltem Izzetoglu, Scott Bunce, and Banu Onaral, Member, IEEE

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Proceedings of the 29th Annual InternationalConference of the IEEE EMBSCité Internationale, Lyon, FranceAugust 23-26, 2007.

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with an inter-stimulus interval of 12 s. Subjects’ responses and their timing to each card presented were also recorded for future analysis. The protocol consisted of 16 trials of each of the truth, lie and control cards. A thorough description of the protocol is in [8] .

EEG recordings and preprocessing: EEG signals were acquired using Ag/AgCl scalp electrodes. The recordings in 8 of the 16 subjects were obtained from frontal sites (F3, F4, F7, F8); in the remaining 8 subjects, the recordings were obtained from midline sites (Cz and Pz). All electrodes were placed according to the International 10-20 System. Vertical and horizontal electrooculograms (VEOG and HEOG) were also recorded from all subjects. Data acquisition was performed using a Neuroscan system (Neuroscan, Inc.) with an analog passband filter at 0.1-100 Hz and sampling frequency 500 Hz. Independent component analysis (ICA) was used for correction of the ocular artifact [23].

Time-domain analysis: For each ‘truth’ and ‘lie’ card presentation, epochs were extracted from the EEG data in three different ways. 1) Stimulus-locked short epochs were extracted with a 0.2

s pre-stimulus baseline and a 1 s post-stimulus window. 2) Stimulus-locked long epochs were extracted with a 0.2 s

pre-stimulus baseline and a 10 s post-stimulus window. 3) Response-locked short epochs were extracted with a 0.2

s pre-response baseline and a 1 s post-response window. Baseline correction was performed and grand averages of

all trials and subjects were obtained for the two conditions

(‘truth’ and ‘lie’ cards) for each channel. Statistical analysis: Differences were evaluated using

repeated measure ANOVAs with Geisser-Greenhouse correction on each of the investigated features; Tukey-Kramer test and test for multiple comparisons were performed as post-hoc analyses. The p-value was set at 0.05.

III. RESULTS Stimulus-locked short epochs: No difference between

truthful and deceptive responses was visible in the grand averages obtained from frontal sites (fig. 1). Differences in the amplitude of the P300 were not significant for any of the considered electrodes (fig. 2 and 3). However, statistical analysis of the area under the curve between 0.4 to 1 s revealed differences between ‘lie’ and ‘truth’ cards in Cz and Pz. The area was smaller for ‘truth’ cards than for ‘lie’ cards in both channels (fig. 1 and 2).

Stimulus-locked long epochs: None of the considered channels showed any difference in the time-domain analysis

Fig. 1. Grand averages of stimulus-locked short epochs at each of the 6 considered channels. The solid red lines represent the grand average for “lie” cards and the dashed blue lines represent the grand average for the “truth” cards. Significantly different features are depicted onto the single channel graphs (AUC: area under the curve).

SLS P300 (microV) SLS auc (microV*s) RLS auc (microV*s)

Chn: CzLie Truth

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5

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15

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Chn: PzLie Truth

Fig. 2. Statistical features extracted from recordings at the midline sites for the different epoching criteria. Stars denote statistical significance (p<0.05). (SLS: stimulus-locked short epochs; RLS: response-locked short epochs; auc: area under the curve.)

SLS P300 (microV) RLS peak (microV) RLS auc (microV*s)

Chn: F7Lie Truth

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-5

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5

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Chn: F8Lie Truth

Chn: F3Lie Truth

Chn: F4Lie Truth

Fig. 3. Statistical features extracted from recordings at the frontal sites for the different epoching criteria. Stars denote statistical significance (p<0.05). (SLS: stimulus-locked short epochs; RLS: response-locked short epochs; auc: area under the curve.)

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when comparing responses to ‘lie’ and ‘truth’ cards in long

epochs. Grand averages for each of the channels are plotted in fig. 4.

Response-locked short epochs: Fig. 5 shows for each channel the averages of the recordings following deceptive and truthful responses. In the midline electrodes, analysis of the area under the curve revealed a significant difference in the time range going from 0.1 to 0.4 s after the response. In particular, both in Cz and Pz the area under the curve for ‘lie’ cards is smaller than for ‘truth’ cards (fig. 2 and 5), whereas in stimulus-locked epochs the area under the curve was bigger in deceptive responses. Similar analyses were performed in the frontal channels. This time the area under the curve in the time range from 0.1 to 0.2 is found to be statistically different for deceptive and truthful responses in sites F3 and F4. Moreover, a peak was identified between 100 and 200 ms post-response in the frontal channels; the peak was found statistically different in F7 and F3 (fig. 3 and 5).

IV. DISCUSSION In this study, new frameworks for the study of deception

detection are provided through time-domain features such as area under the curve and alternative epoching criteria, i.e. response-locked epoching. Analysis of the area under the curve in specific time ranges provided some significant differences, suggesting that it can serve as another feature in discriminating truthful and deceptive responses. Furthermore, extraction of response-locked EEG epochs has never been investigated before for deception detection, yet revealed promising differences in this study suggesting that more valuable results could be obtained by the re-alignment of the epochs based on the response time of the subjects. Differences are more evident in the frontal sites. In particular, a peak occurring about 150 ms after the response shows significant differences between ‘truth’ and ‘lie’ cards in the left frontal channels (F7 and F3). The frontal channels placed more towards the midline (F3 and F4), instead, reveal differences in the area under the curve right after the response (0.1-0.2 s).

The analysis of longer epochs failed to provide any visible difference in the time-domain other than those present in the shorter stimulus-locked epochs. Moreover, none of the considered frontal and midline channels showed any significant difference in the P300 amplitude between truthful and deceptive responses, though it has been widely reported in the literature. This lack of difference was probably due to the specific protocol used, the modified Guilty Knowledge Test. However, even if differences were found in the P300 amplitude, it might still not be considered as a reliable feature in deception detection; since it has already been proven that easy countermeasures exist to invalidate it [24]. Hence, the standard analysis of the P300 peak is found to be not suitable for deception detection in the modified GKT protocol.

The statistically different time-domain features found in

Fig. 4. Grand averages of stimulus-locked long epochs at each of the 6 considered channels. The solid red lines represent the grand average for “lie” cards and the dashed blue lines represent the grand average for the “truth” cards.

Fig. 5. Grand averages of response-locked short epochs at each of the 6 considered channels. The solid red lines represent the grand average for “lie” cards and the dashed blue lines represent the grand average for the “truth” cards. Significantly different features are depicted onto the single channel graphs (AUC: area under the curve).

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this work will be further explored in future studies for the automatic detection of deception. The classification will be performed together with other possible discriminating features that can be extracted through the analysis in other domains. In particular wavelet analysis will be considered in the future since it has already been proven to be able to provide statistically different features [8].

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