Electroencephalography

Defining Electroencephalography

EEG is a noninvasive brain-imaging technique that measures electrical activity. Researchers place sensors across different locations on a person's scalp and record patterns of electrical activity. Electrical activity can be described by the place it occurs on the scalp, the time at which it occurs, its formation, and its voltage. Components of electrical activity described along these lines are referred to as event-related potentials (ERPs). For instance, the P300 component occurs 300 milliseconds (ms) after a stimulus appears with positive voltage. ERPs are associated with psychological processes, such as people's ability to pay attention to objects or remember presented stimuli. ERPs are a measurement of neurological activity that can be used to suggest which psychological processes occur in response to stimuli.

Use of the Procedure as Lie Detection

In the early 1990s, researchers began asking whether techniques of neuroscience, and EEG in particular, could be used to detect deception. Dr. Lawrence Farwell and his colleagues created one of the first paradigms that used neurological measurement to detect lies. First, they modeled their test after the oddball paradigm. In the oddball paradigm, participants recognize a target word when it appears on a computer screen among other irrelevant distracter words that are presented in quick succession. Farwell adapted the oddball paradigm to create a test that could detect whether people were aware of “guilty knowledge,” or information considered significant to a particular scenario or event.

In the original guilty knowledge test, participants read one of two hypothetical scenarios and memorize specific details about it. For instance, participants might remember the clothing worn by people in the scenario. Participants next perform an oddball paradigm task. They identify when a target word that is not related to details of the scenario appears in a sequential display of other distracting words. Importantly, among the distracters are words related to details of the hypothetical scenario previously memorized. For instance, participants may see the word coat—a word considered relevant to the memorization task.

Farwell suggested that EEG could measure ERP components evoked in response to targets and relevant and irrelevant distracters during the guilty knowledge test. A P300 component appears when participants see a target word or relevant distracters that are considered guilty knowledge. Importantly, the P300 is not evoked for irrelevant distracter words. The presence of P300 components indicates that both the target words and guilty knowledge words receive heightened attentional processing. From these data, Farwell suggested that the presence of a P300 component could identify whether a person had knowledge of an event, like a crime scene, that he or she might not choose to verbally share. For instance, if a suspect accused of a murder perpetrated by a knife is given the guilty knowledge test, the word knife may evoke the P300. Farwell suggested that the P300 could indicate whether a person witnessed and remembered details of a crime, regardless of his or her attempts to claim ignorance.

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