Module 5 - Polygraph Testing & Forensic Document Analysis

Lesson 1 - Stress Indicators & The Polygraph Instrument

Page 4

Polygraph Data

Collection of Physiological Data by the Polygraph Instrument

The polygraph instrument does not, as the name "lie detector" suggests, detect lies. Rather, the polygraph instrument collects and displays from a subject during an interrogation four specific types of physiological data associated with being deceptive. Each type of data is collected and displayed in a form that a polygraph examiner is able to interpret.

Heart Rate and Blood Pressure Data

A circular blood pressure cuff (or sphygomometer) is placed on one arm and is connected to the polygraph instrument through tubes. Air is pumped into the cuff causing it to constrict a major artery in the arm called the brachial artery. When blood flow in the brachial artery is stopped, the air in the cuff is slowly released causing the brachial artery to open again. A sound is created when blood begins to pump through the brachial artery again. This results in a change in pressure within the tubes connected to the polygraph. This change in pressure causes the sensitive needles of the polygraph to move, creating a cardiospymograph. In the digital polygraph, the pressure change in the tubes is converted into electrical signals, interpreted, and then displayed on a computer monitor.

Breathing Rate Data

Two pneumographs (rubber tubes filled with air) record volume changes during breathing or respiration. One pneumograph is placed around the chest and the other pneumograph is placed around the abdomen. Each pneumograph has an air-filled rubber tube connected to the polygraph instrument. When the chest or abdominal muscles of a subject expand during breathing, the air inside the tubes attached to the polygraph is displaced. This displacement of air inside the polygraph tubing causes the sensitive needles of the analog polygraph to move. In the digital polygraph, the pressure change is converted to electrical signals, interpreted, and then displayed on a computer monitor.

A blood pressure cuff and two pneumographs from a polygraph instrument. Click to enlarge.

Photo courtesy of Sergeant A. Kowalyk, 2005.

Perspiration Data

A galvanometer consisting of two small metal electrodes is attached to two fingertips on the same hand. The galvanometer records skin conductivity by sending a small electric current into the skin from one of the electrodes and determining how much of the electric current was allowed to pass to the other electrode. Dry skin is a poor conductor of electricity, but the water and salt in perspiration (sweat) allows a larger amount of electricity to travel along the skin surface. Therefore, the amount of electricity recorded by the galvanometer reflects the amount of sweat being produced by the subject.

Pathological Lying

The pathological liar may become a prisoner of his lies, and the desired personality of the pathological liar may overwhelm the actual one."

- Dr. Charles Dike: Forensic Psychiatrist & Clinical Instructor of Psychiatry at Yale University School of Medicine

The only type of person that could lie repeatedly and likely not exhibit the sympathetic nervous symptoms related to deception is the pathological liar. To obtain any conclusive results from a pathological liar during a polygraph examination is difficult. The pathological liar tells repeated lies over a number of years for no clear reason. Often when a pathological liar tells the same lie repeatedly, he or she begins to believe that it is a true statement. When this occurs, the person will likely be able to pass a polygraph exam because he or she thinks he or she is telling the truth. The lies from a pathological liar are unplanned, impulsive, and often woven into complex stories. Pathological liars may admit that what they have said is false when confronted, but then they will often proceed to change their stories.

Whether pathological liars have control over their lies is still unclear. Currently, many medical experts consider pathological lying to be a symptom of a pre-existing psychiatric disorder and not a distinct mental disorder. However, new scientific evidence has emerged that may change this assumption.

A study from the University of Southern California (USC) has shown that the brains of some pathological liars are different. This suggests that pathological lying could be an unconscious behavior in some people and that it is a distinct mental disorder.

In the British Journal of Psychiatry (October 2005), the USC researchers reported that they found pathological liars had 14% fewer neuron cells in their brains and greater than 20% more white matter in the prefrontal cortex of their cerebrums compared to normal people. The prefrontal cortex is the top layer of the cerebrum and controls cognitive functions such as critical thinking and learning; white matter contains cells that connect brain cells to one another. The conclusions from this study were drawn using the results from a series of psychological tests, interviews, and magnetic resonance images (MRI) of 49 subjects. Medical experts believe the excess of white matter may give pathological liars the ability to be more effectively deceitful.

Charles Dike, a forensic psychiatrist and clinical instructor in psychiatry at Yale University School of Medicine, has said that a clear understanding of pathological lying is important so that the legal system can decide whether pathological liars should be held responsible for their behavior. This could help determine how to deal with pathological liars who provide false testimony while under oath in criminal court cases.

Did you know?

Children are not good at lying; however, this changes at about age 10 at which time a rapid growth of white matter in the cerebrum occurs. After this point, children can become proficient at lying.

Source: Swaminathan, Nikhil. Pathological Liars' Brains Physically Different. (www.seedmagazine.com) September 30, 2005.