What does the Lie Detector Test measure?

Aug 3, 2022 | News & Science, Polygraph Examiner

A lie detector test is used to determine if the subject is lying about the statements that he has made regarding the accounts of an event, particularly those involving crimes. During the examination, the subject is put under observation by the equipment of the polygraph which is attached to his body and along with that, he is under continuous observation of the examiner who is an expert in forensic Psychophysiology. It is based on the principle that when a subject is lying during stating facts regarding an event, it produces involuntary physiological changes which the lie detector can detect. The instrument measures and records the blood pressure, pulse, respiration and sweat glands of the subject. The belief is that deceptive answers during the polygraph test will produce responses in the above-mentioned aspects which are different from those of true answers.

The Lie Detector

This machine is connected to different medical components which measure and record four to six physiological responses of the subject. They all measure the physiological responses caused due to the sympathetic nervous system during questioning. There are transducers in the equipment which read the information received and turn them into digital signals that are stored on the computer and also visible on the monitor. The computer uses high level mathematical algorithms to analyse this information. The modern scientific methods have been created by using the knowledge of physiology, psychology, pharmacology, toxicology, etc. in determining whether the subject is being deceptive or not. The human body has a nervous control which has two parts – the central nervous system made up of the brain and the spinal cord; the autonomic nervous system made up of sympathetic and parasympathetic.
Our central nervous system primarily controls all the sensory functions which occur above a threshold and might be voluntary. However, the autonomic nervous system produces autonomic responses from the body. When a person is under duress or influence of fear, emotions or excitement, it is the sympathetic part of the autonomic nervous system which starts producing changes in the heart and pulse rate, blood pressure, respiration and perspiration, voice tracing etc. So when the investigation is going on with the polygraph attached to the subject’s body in front of the examiner, it is believed that the sympathetic nervous system starts acting and produces changes physiologically which the polygraph records. This is how the polygraph is able to measure and record the responses of the subject to various questions and determine the difference between truth and lie. The three medical instruments that the polygraph consists of are


The cardio-sphygmograph is a combination of a cardiograph and a sphygmograph. It doesn’t look much different from the regular blood pressure cuff that doctors use. The cuff is tied around the subject’s arms and it remains inflated during the investigation stage, which is usually about half an hour long. When the blood flows through the veins, it creates a sound which is transmitted through the air filled in the cuff which is used to amplify that sound. The blood pressure is measured by the amplitude of the sound and the heart rate is measured by the frequent changes occurring in the sound.


The Pneumograph is another instrument of the polygraph which is used to measure the subject’s respiratory rate. There are two air filled rubber tubes, one of which is tied across the subject’s chest and the other around his waist. As the subject breathes, even the most minute changes are caught by the air filled tubes and that data is saved on the polygraph.


The galvanograph is the instrument which records the perspiration that is produced. The skin on our fingertips consists of a high density of sweat glands, which makes it an ideal location to measure the amount of perspiration. The equipment has galvanometers, which are electrical sensors that are attached to the subject’s fingertips. The amount of sweat touching the galvanometers is indirectly proportional to the resistance of the electrical current passing through it. The changes happening in the sweat glands are recorded by the polygraph. Many forensic psychophysiologists believe that the pneumograph and the cardiosphygmograph are more informative and accurate than the data from galvanometer.

Once the lie detector test is over, the polygrams are produced which are composite records of the Cariosphymograph, pneumograph and the galvanograph readings recorded from the series of answers. The forensic psychophysiologist analyses the subject’s responses to all the questions that were asked during the test and gives scores for each of them. The total score is calculated and if it is large and positive enough, then the subject is believed to be true, but if it is largely negative, then the subject is believed to have been deceptive during the test. If the score is in some case close to zero, it means that the test was inconclusive.

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