P300 is a brain wave derived from the electroencephalogram (EEG), which has recently been used as a novel information channel in the detection of deception. The traditional channels are recorded from the autonomic nervous system and include physiological activity such as respiration pattern, blood pressure, and skin conductance. In contrast, the EEG is a record of sequential, spontaneously changing voltages as a function of time, recorded from the scalp surface in humans. It reflects the spontaneous activity from the underlying cerebral cortex. If as these changing voltages occur, a discrete stimulus event (such as a light flash) occurs, the EEG breaks into a series of somewhat larger peaks and troughs, called components. This series of waves is called an event-related potential (ERP).
These early peaks and troughs represent sensory activity (exogenous ERP components), and the later (endogenous) components may represent the psychological reaction to the sensory events. P300 is the name of one heavily researched ERP. It is elicited by stimulus events that are rare and meaningful to subjects. For example, if a stimulus series consists of a set of randomly occurring first names, each presented singly on a display screen about every 3 s, and the subject’s own first name is one of the stimuli presented about 15% of the time, with the remaining 85% of the presentations being of other, unfamiliar names, the P300 will be elicited by the rare, meaningful (subject’s own) name. P300 is named in respect of its positive (P) polarity and its occurrence at about 300 to 800 ms after the stimulus onset. Simple stimuli such as brief sounds elicit early P300 peaks (300—400 ms), whereas more complex stimuli such as words elicit later peaks (500-800 ms).
It occurred to Dr. J. Peter Rosenfeld and colleagues in the early 1980s that P300 might be used in deception detection situations to index recognition of the presentation of crime scene details known only to perpetrators (and the authorities) and not to innocent suspects. The protocol would involve presentation (usually on a display screen) of items of information, such as possible murder weapons (e.g., pistol, rifle, knife, axe). The guilty party, but not the innocent subject, would react with a P300 to the actual murder weapon (e.g., the pistol), called the probe stimulus. Neither guilty nor innocent subjects would react to the other, irrelevant items from the weapons category, which were not actually used in the crime, as the guilty party would know. Thus, the difference in P300 amplitude between the probe-evoked ERP and the irrelevant-evoked ERP indicates guilt. This protocol was closely related to the Guilty Knowledge Test (GKT) invented by David Lykken in 1959, which used autonomic nervous system responses to stimuli. One difference was that in the P300 protocol, there was usually a third stimulus type used, also rarely presented, called the target. This was typically one other irrelevant item but one to which the subject is told to respond by pressing a unique button. In one version of the protocol, the subject is told to press a “No” button (for “No, I don’t recognize this”) in response to both probes and irrelevant items and “Yes” (“I do recognize this”) in response to targets. Of course, in saying “No” to the probe, the guilty subject lies, but it is hoped that his P300 ERP reveals his guilty recognition all the same. The target stimulus is used to force attention onto the display screen, since the three stimulus types are presented unpredictably in random sequence, and if the subject neglects to respond to the target stimulus as instructed, the operator knows that the subject is not paying attention and will report this to the authorities. But if the subject is always paying attention, he or she cannot avoid seeing the probe stimuli, which evoke P300s in guilty subjects.
Early reports in the 1990s (by J. Peter Rosenfeld and colleagues and by John J. B. Allen and Emanuel Donchin and their respective coworkers) with this protocol reported high overall accuracy (80-95% correct classification of guilty and innocent subjects), and they were received with considerable enthusiasm; it was naively believed that because the P300s occurred with such short reaction times (fractions of a second post-stimulus) relative to the slow autonomic reaction times in the GKT, the P300-based protocols would resist countermeasures (CMs), intentional covert responses subjects can learn to make that can defeat the GKT. Unfortunately, J. Peter Rosenfeld and colleagues showed in 2004 that P300-based GKTs were also vulnerable to CMs. The guilty subjects were simply trained to covertly respond (e.g., with secret toe wiggles) systematically to irrelevant stimuli, thus turning them into P300-evoking targets. It then became impossible to distinguish between probe and irrelevant P300s, whose typical difference without CMs indexed guilt. Reports from John J. B. Allen’s lab showed similar results.
However, in 2006, J. Peter Rosenfeld and colleagues reported that a second-generation P300-based deception test using a wholly novel protocol was accurate and highly resistant to CMs. More than 100 subjects have been studied to date, and the accuracy rates have been 90% to 100% in many experiments, dropping by only 0% to 10% with CM use. Moreover, a new feature built into this new protocol alerts operators about CM use. In the new protocol—called the Complex Trial Protocol (CTP), two stimuli are presented on each trial, and there are four possible trial types. The first stimulus is either a probe or an irrelevant, and the subject responds with one simple behavioral acknowledgment that the stimulus has been seen. About 1 to 1.5 s later, a second stimulus is presented, which is either a redefined target or not one. The subject here signals target or nontarget. The subject’s absolute behavioral reaction time to the first stimulus is significantly increased if a CM is being used, and the reaction time to irrelevants, which without a CM is less than or the same as that to probes, is usually increased to much greater than probe reaction time if a CM accompanies the irrelevant. Thus, occasionally successful CM use or attempted but unsuccessful CM use has always been detected. The probe P300 amplitude actually increases during CM use (unlike what is seen with the older protocol based on three trial types—probe, irrelevant, or target). Such an increase means that the CTP is still likely to see a probe-irrelevant difference even if the irrelevant P300 increases, as expected, during CM use. It appears that this new protocol is powerful because its multiple demands made on the subject force attention on the key stimuli, thus enhancing P300 responses to them.
Other brain-activity-based dependent indices of deception have been suggested and researched in preliminary ways. These approaches have different theoretical foundations. Dr. J. Peter Rosenfeld and colleagues have also examined the P300 amplitude distribution (not simple amplitude) across the scalp (a kind of “brain map”) as a promising new index of deception. The motivation for pursuing this new approach is, again, the possibility of removing CMs. It was simple to develop CMs for the earlier P300-amplitude-based protocols because the antecedents of P300 amplitude—rareness and meaningfulness—are relatively well-known. If one knows the antecedents of P300, then one knows how to manipulate it. On the other hand, very little is known about how to manipulate the amplitude distribution across the scalp, thus facilitating the creation of a CM method.
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