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Data Measured in Polygraph Tests For centuries, humans have looked for a reliable method to distinguish lies. In very old Hindu and Chinese communities, authorities “detected” lies by making the subject to chew a grain of rice and spit it out. A dry grain of rice would be considered a sign of the dry mouth of a liar. In India, if rice adhered to the mouth, it would mean guilt. While these methods were primeval and non-scientific, they nonetheless emphasized the elemental conjecture humans make in lie detection: lying can be detected using physiological indications. Each time a person lies or is asked a critical question, his heart can begin to race, elevating the body’s blood pressure. Also, the test subject may also hold his breath, inhale a large one, or perspire. These physiological irregularities are caught by the polygraph for the polygraph examiner’s interpretation. It is the discretion of the examiner to associate the sudden data changes with dishonesty. Cardiovascular Activity
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An air-filled cuff encircling the upper arm is used to collect blood pressure and heart rate data. When there are changes in blood pressure, the air pressure in the cuff changes as well. The polygraph machine records such changes and displays them on a computer screen, side by side with respiratory and perspiratory data.
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Respiration The respiration pattern of a subject is recorded by two pneumograph devices capturing thoracic volume change or movement as a person breathes. One pneumograph tube is placed around the chest and the other around the abdomen. Like the arm cuffs used to detect cardiovascular changes in a subject, the pneumograph tubes are also filled with air and connected to the machine. During inhalation and exhalation, the tubing air pressure changes, and each change will be reflected in the polygraph machine. Perspiration The measurement of sweat, scientifically called the measurement of galvanic skin resistance, is made possible by attaching a two-piece galvanometer to two of the subject’s fingertips. Through the galvanometer, a small electric current is sent into the skin from one fingerplate, and the amount of current that got through on the other fingerplate will be recorded. Dry skin conducts electricity poorly. However, during perspiration, water and salt from the sweat drives down skin resistance, allowing a bigger amount of electric current to flow on the surface of the skin. In other words, whatever amount of electric current is recorded by the galvanometer, indicates the amount of sweat that fingertips of the subject produced. Despite not being totally accurate, polygraph tests are constantly used by government authorities, especially law enforcement agencies, as an instructive tool. Through rapid technological advancements, humans will soon to strengthen the correlation between the psychological state of lying and its physiological indications.