Detection of Deception: Event-Related Potentials

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 irrelevantevoked 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.

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