How does a lie detector work? Vladimir Kodkin, Professor of the SUSU Department of Automated Electric Drive (who once took a polygraph test himself), answers.
The principles of operation of a lie detector are based on the fact that the human body reacts to attempts to deceive, causing changes in the functioning of internal organs.
If we consider a person as a complex structure, the polygraph partly intersects with the principles of diagnosing the human condition that I previously studied and conducted research on.
The simplest early devices were based on recording the electrocardiogram (ECG) and observing changes in the signal. You may have seen this in movies, especially old ones: the hero is asked to guess a number from 1 to 10, and when he says it, a sharp change in the electrocardiogram is recorded.
Most often, the choice fell on the heart, since ECG is a multifunctional signal comprising a large number of details and nuances that allow one to assess the physical condition. Changes most often concern the cardiac rhythm, although this is only one side of the issue. The heart is easy to measure and study, which makes it possible to build control schemes or diagnose a critical condition.
Determining the cause of extreme states of the body (for example, severe anxiety or deception) is not easy in itself. Probably, modern science has advanced further, and it has become possible to conduct more accurate measurements. Today's medicine is actively engaged in rhythmocardiography, a comprehensive study of changes in the work of the heart. Despite technological advances, it is quite difficult to establish for sure whether the recorded change is associated with an attempt to deceive. It is possible to record an extreme reaction, but it is wrongful to directly state that this is a sign of lying.
The methodology of asking questions plays a crucial role. I remember my own passing of the test: the questions were repeated many times, literally every fourth or fifth question returned to the same statement. This sequence created tension, forcing you to respond to the repeated questions again and again. The ability to remain calm and answer the same questions in an equally calm way was surreptitiously examined.
The procedure can take a long time: one hour and longer. Most likely, specialists are trying to catch some specific indicator confirming the presence of a problem or concealment of the truth. However, confidence in a person's guilt based on a polygraph examination alone is not enough.
Can a person consciously control their body and emotions to pass the test successfully? There are opinions that some people manage to do this. However, the development of methods to counteract these manipulations has led to the practice of repeating the same questions over and over again. I believe that only a specially trained agent or a high-level professional will be able to maintain complete control over a long period of time, and even resist such provocative questions. The majority of ordinary people have difficulty controlling their heart rate, breathing, and stress levels.
A polygraph test causes unusual sensations. At first, specialists conducting the test try to appear friendly and show interest in the interlocutor ("heart-to-heart talk"). Gradually, the tone of communication becomes harsher, the questions begin to sound rude and intrusive. The reaction of the person being tested does not particularly bother anyone: the main thing is to get a clear answer ("yes" or "no").
The process is accompanied by a peculiar process of switching off the moral side of the perception of the situation, turning into a mechanical testing of physical reactions. It would hardly occur to an ordinary person to ask such questions.
Thus, despite the existence of technologies that allow tracking physiological changes, the detector does not provide a 100% guarantee of accurate lie detection. The polygraph serves as an additional tool that helps investigators navigate difficult situations.