Analog and computerized polygraphs

Polygraph testing equipment can be either analog or computerized (digital). Both systems allow the examiner to view the results while conducting the examination. The advantage of the computerized system is that the software can analyze the subject’s responses in real time, thus rendering the process less subjective. The examiner should use the latest, state-of-the-art lie detection testing equipment to conduct the polygraph exams. When you select an agency to provide polygraph tests, you are relying upon them for the accuracy and validity of the results. Regardless of which system is used, the Examiner is ultimately responsible for verifying the accuracy of the examination.

The three components of the polygraph instrument include the cardio-sphygmograph, the pneumograph, and the galvanograph. Blood pressure and heart rate are measured by the cardio-sphygmograph component of the polygraph, which consists of a blood pressure cuff that is wrapped around the subject’s arm. During the questioning the cuff remains inflated. The movement of blood through the subject’s veins generates a sound that is transmitted through the air in the cuff to a bellows that amplifies the sound. The magnitude of the sound relates to the blood pressure and the frequency of the changes in the sound relates to the heart rate. The pneumograph component of the polygraph records the subject’s respiratory rate. One tube is Splaced around the subject’s chest and a second is placed around his or her abdomen. These tubes are filled with air.

When the subject breaths, changes in the air pressure in the tubes are recorded on the polygraph. The galvanograph section records the amount of perspiration produced. It consists of electrical sensors called galvanometers that are attached to the subject’s fingertips. The skin of the fingertips contains a high density of sweat glands, making them a good location to measure perspiration. As the amount of sweat touching the galvanometers increases, the resistance of the electrical current measured decreases and these changes are recorded by the polygraph. Most forensic psychophysiologists (FPs) consider the cardiosphygomgraph and the pneumograph components more informative than the galvanograph.

A polygraph is a machine, commonly used in attempts to detect lies, that measures several of the physiological responses accompanying emotion. Some of the responses include perspiration, cardiovascular and breathing changes. They were mainly used in law enforcement and national security work. In the mid-1980’s, about two million American’s were reportedly being tested using polygraph machines because corporations were trying to screen applicants for the truth or to uncover employee theft (Holden,1986). In 1902, an earlier polygraph machine was invented by James Mackenzie. However, since it was unsuccessful, a new, more modern polygraph was invented in 1921 by John Larson.

Analog Polygraph

During the 1960’s and 1970’s, the polygraph machine was not as modernized as it is today. An analog polygraph is a kind of polygraph machine that can measure at least three different physiological responses. When being tested, a patient has complex rubber tubes attached to the chest and stomach area to record respiratory activity, a blood pressure cuff to record heart rate, and to record sweat gland activity, small metal plates are attached to the fingers. When the machine is turned on, the polygraph needles scratch lines onto a roll of paper. The lines represent the levels of stress the person is experiencing and that stress is believed to be caused by telling lies.

Since 1993, the computerized polygraph machine has replaced the analog machine. Just like the analog polygraph, the computerized polygraph measures a person’s heart rate, leg movements, sweating and other physiological responses. It is quite comprehensive and tells if a person is not telling the truth. The main difference between the analog and computerized polygraph is that the computerized makes its recording on a computers hard disk, which can be viewed on a screen or printed out.

A polygraph (popularly referred to as a lie detector) measures and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while the subject is asked and answers a series of questions. The belief underpinning the use of the polygraph is that deceptive answers will produce physiological responses that can be differentiated from those associated with non-deceptive answers; the polygraph is one of several devices used for lie detection

There are three parts of a polygraph machine. They are:

  • the cardio-sphygmograph consisting of a blood pressure cuff placed on the arm that remains inflated during the test – measures blood pressure and heart rate
  • the pneumograph consisting of two tubes filled with air, one placed around the chest and the other placed around the abdomen – measures respiratory rate
  • the galvanograph which consists of electrical sensors attached to the fingertips – records the amount of perspiration produced

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