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L A Farwell

Publications and source records attributed to L A Farwell.

6 recordsLinked to original sources
PubMed2001

Using brain MERMER testing to detect knowledge despite efforts to conceal.

This experiment examined the accuracy and reliability of the memory and encoding related multifaceted electroencephalographic response (MERMER) technique for detecting information related to events subjects have experienced, despite subjects' efforts to conceal that knowledge. Information obtained through interviews was used to develop stimulus sets consisting of words and phrases presented to subjects visually by computer. Sets were composed of three types of stimuli: life experience-related (Probes), stimuli the subject was asked to memorize and respond to (Targets), and irrelevant information (Irrelevants). Each set of stimuli was tested on two individuals: (1) one individual who had participated in the event in question--and thus had the relevant information stored in his/her brain, and (2) one who had not. Six subjects were tested. Electrical brain responses to the stimuli were recorded non-invasively from the scalp and analyzed. MERMERs, (memory and encoding related multifaceted electroencephalographic responses), of which the P300 is a sub-component, were used to determine whether the subject had the relevant information stored in his brain (information present) or not (information absent), thus indicating whether or not each subject had participated in the real-life event in question. Bootstrapping was used to analyze and compare the responses to the three types of stimuli. As predicted, MERMERs were elicited by Probe stimuli only in the subjects who had participated in the investigated event, by Target stimuli in all subjects, and in no case by Irrelevant stimuli. For each of the six subjects, brain MERMER testing correctly determined whether the subject had participated in and consequently knew about the event in question (information present) or had not participated (information absent). The statistical confidence for this determination was 99.9% in five cases and 90.0% in one case. The article concludes with a discussion of areas of future research and the potential for using this new technology as an investigative tool in criminal cases.

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PubMed1993

Optimal digital filters for long-latency components of the event-related brain potential.

A fundamentally important problem for cognitive psychophysiologists is selection of the appropriate off-line digital filter to extract signal from noise in the event-related brain potential (ERP) recorded at the scalp. Investigators in the field typically use a type of finite impulse response (FIR) filter known as moving average or boxcar filter to achieve this end. However, this type of filter can produce significant amplitude diminution and distortion of the shape of the ERP waveform. Thus, there is a need to identify more appropriate filters. In this paper, we compare the performance of another type of FIR filter that, unlike the boxcar filter, is designed with an optimizing algorithm that reduces signal distortion and maximizes signal extraction (referred to here as an optimal FIR filter). We applied several different filters of both types to ERP data containing the P300 component. This comparison revealed that boxcar filters reduced the contribution of high-frequency noise to the ERP but in so doing produced a substantial attenuation of P300 amplitude and, in some cases, substantial distortions of the shape of the waveform, resulting in significant errors in latency estimation. In contrast, the optimal FIR filters preserved P300 amplitude, morphology, and latency and also eliminated high-frequency noise more effectively than did the boxcar filters. The implications of these results for data acquisition and analysis are discussed.

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PubMed1991

The truth will out: interrogative polygraphy ("lie detection") with event-related brain potentials.

The feasibility of using Event Related Brain Potentials (ERPs) in Interrogative Polygraphy ("Lie Detection") was tested by examining the effectiveness of the Guilty Knowledge Test designed by Farwell and Donchin (1986, 1988). The subject is assigned an arbitrary task requiring discrimination between experimenter-designated targets and other, irrelevant stimuli. A group of diagnostic items ("probes"), which to the unwitting are indistinguishable from the irrelevant items, are embedded among the irrelevant. For subjects who possess "guilty knowledge" these probes are distinct from the irrelevants and are likely to elicit a P300, thus revealing their possessing the special knowledge that allows them to differentiate the probes from the irrelevants. We report two experiments in which this paradigm was tested. In Experiment 1, 20 subjects participated in one of two mock espionage scenarios and were tested for their knowledge of both scenarios. All stimuli consisted of short phrases presented for 300 ms each at an interstimulus interval of 1550 ms. A set of items were designated as "targets" and appeared on 17% of the trials. Probes related to the scenarios also appeared on 17% of the trials. The rest of the items were irrelevants. Subjects responded by pressing one switch following targets, and the other following irrelevants (and, of course, probes). ERPs were recorded from FZ, CZ, and PZ. As predicted, targets elicited large P300s in all subjects. Probes associated with a given scenario elicited a P300 in subjects who participated in that scenario. A bootstrapping method was used to assess the quality of the decision for each subject. The algorithm declared the decision indeterminate in 12.5% of the cases. In all other cases a decision was made. There were no false positives and no false negatives: whenever a determination was made it was accurate. The second experiment was virtually identical to the first, with identical results, except that this time 4 subjects were tested, each of which had a minor brush with the law. Subjects were tested to determine whether they possessed information on their own "crimes." The results were as expected; the Guilty Knowledge Test determined correctly which subject possessed which information. The implications of these data both for the practice of Interrogative Polygraphy and the interpretation of the P300 are discussed.

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PubMed1988

Talking off the top of your head: toward a mental prosthesis utilizing event-related brain potentials.

This paper describes the development and testing of a system whereby one can communicate through a computer by using the P300 component of the event-related brain potential (ERP). Such a system may be used as a communication aid by individuals who cannot use any motor system for communication (e.g., 'locked-in' patients). The 26 letters of the alphabet, together with several other symbols and commands, are displayed on a computer screen which serves as the keyboard or prosthetic device. The subject focuses attention successively on the characters he wishes to communicate. The computer detects the chosen character on-line and in real time. This detection is achieved by repeatedly flashing rows and columns of the matrix. When the elements containing the chosen character are flashed, a P300 is elicited, and it is this P300 that is detected by the computer. We report an analysis of the operating characteristics of the system when used with normal volunteers, who took part in 2 experimental sessions. In the first session (the pilot study/training session) subjects attempted to spell a word and convey it to a voice synthesizer for production. In the second session (the analysis of the operating characteristics of the system) subjects were required simply to attend to individual letters of a word for a specific number of trials while data were recorded for off-line analysis. The analyses suggest that this communication channel can be operated accurately at the rate of 0.20 bits/sec. In other words, under the conditions we used, subjects can communicate 12.0 bits, or 2.3 characters, per min.

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