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Data Measured in Polygraph Tests For centuries, humans have looked for a reliable method to distinguish lies. In early Hindu and Chinese cultures, authorities “detected” lies by instructing the subject to chew a grain of rice and spew it out. A dry grain of rice would be associated with the dry mouth of a liar. In India, if rice stuck to the mouth, it would be a sign of 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. Every time a person lies or is asked a delicate question, his heart may start to race, increasing the body’s blood pressure. Also, the test subject may also hold his breath, inhale a large one, or perspire. Such physiological irregularities are recognized by the polygraph to be interpreted by the polygraph examiner. It is the judgment of the examiner to equate the significant data changes with dishonesty. Cardiovascular Activity
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To record blood pressure and heart rate, an encircling, air-filled cuff will be placed on the upper arm. When there are changes in blood pressure, the air pressure in the cuff changes as well. The polygraph machine takes note of such changes, displaying them on a computer monitor simultaneously with respiratory and perspiratory data.
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Respiration Two pneumograph devices, which capture changes in volume or movements in the thoracic cavity, record the subject’s respiration pattern while he breathes. One pneumograph tube goes around the chest while the other is strapped around the abdomen. Like the arm cuffs that detect cardiovascular changes in a subject, the pneumograph tubes are also air-filled 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. The galvanometer functions by sending a small electric current into the skin from one fingerplate and recording the amount of current that was able to reach the other fingerplate. Dry skin is a bad conductor of electricity. 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. While not a hundred percent accurate, polygraph tests are commonly used as an instructive tool by law enforcement agencies and many government authorities. Through rapid technological advancements, humans will soon to strengthen the correlation between the psychological state of lying and its physiological indications.