How does the lie detector

Webster’s Dictionary defines this apparatus as follows: “A tool for synchronous recording of traces of various pulses; in a broader sense, a lie detector. ”
Pulses are recorded on a moving sheet of paper. By “impulses” we mean changes in the activity of the vegetative nervous system (ANS), although this recording mechanism may reflect changes in any other activity. In Chapter 3 "Detection of deception by words, voice and plastic" I have already explained that changes in the activity of the autonomic nervous system (changes in pulse, blood pressure, skin temperature, sweating, etc.) are only indicators of emotional arousal. I also mentioned that some of these changes, such as increased respiration, sweating, pallor and blush, can be seen without a detector. The detector only registers them more carefully, reflecting even the smallest changes that cannot be seen with the naked eye, as well as recording all other manifestations of the ANS, such as, for example, the pulse, which is impossible to observe externally. This is achieved by amplifying signals from sensors attached to different parts of the body. Four sensors are usually attached to the subject: two pneumatic belts are tied around the chest and the stomach to record changes in the depth and speed of breathing; an apparatus for measuring pressure is wrapped around the forearm for a more accurate fixation of the activity of the heart; The fourth sensor, which marks changes in perspiration, is attached to the fingers with metal electrodes.
Webster's dictionary is right about the fact that this device is sometimes called a lie detector, but this in itself is wrong. Lying as such, the detector does not detect. Everything would be much simpler if there existed a certain unique feature that always and everywhere testifies to lies and never about anything else. But there is none. And although the attitude to the lie detector is very contradictory, everyone nevertheless agrees on one thing: he doesn’t detect a lie as such. The only thing he does is measure the intensity of manifestations of ANS arousal, that is, the physiological changes resulting from a person’s emotional excitement ^[132] .
The same thing happens with the behavioral signs of deception. Remember how I explained a little earlier that changes in facial expressions, gestures, and voice are not yet signs of lies as such. These are just signals of emotional arousal or evidence of thinking. And to conclude from this that a person lies, it is possible only on the basis of either the fact that his emotions do not correspond to his line of conduct, or that he is trying to hide them. And in this regard, the detector provides us with less accurate information about exactly what emotions are involved, than about behavioral signs. Microexpressions can detect anger, fear, guilt, and so on, and the detector will only show that a person has some emotions, but which ones will not determine.
In order to detect a lie, the detector operator compares the performance indicators of the SNS obtained during neutral questions (for example, “Today is Tuesday?” Or “Have you ever stolen anything?”) And during “decisive” questions (“You stole 750 dollars? "). A suspect is found guilty if the detector, with relevant questions, shows an increase in the activity of the ANS compared to neutral ones.
Detector tests, like reliance on behavioral signs, are highly vulnerable to what I call the Othello mistake. Recall how Othello failed to understand that Desdemona's fear was not the fear of a traitor who was caught, but the fear of an innocent victim who saw no way to prove his own innocence. Innocent people may experience very strong emotional arousal after learning that they are suspected of lying. Especially if they are suspected of committing a crime, asked about activities that jeopardize access to secrecy, so necessary for their work, or are accused of leaking classified information. And for some people, it is sometimes enough just the requirement to pass a detector test. The excitement can be especially strong if the suspect has reason to believe that the detector operator and the police have already conspired against him. In addition, fear is not the only emotion that a liar can experience: as I said in Chapter 2, “Why lies sometimes fail,” he can also feel guilt and delight in cheating. And any of these emotions causes an increase in the activity of the ANS and is marked by a detector. And also any emotion can be experienced not only by a liar, but also by an absolutely innocent person. In addition, what kind of emotion the suspect will experience depends, as I explained in Chapter 5, Basic Errors and Precautions, and from his personality, and from the existing relationship between him and the verifier, and from the expectations of both.