POLYGRAPH MACHINEBY- VIPUL CHAVAN11-509Biomedical Engineer

Polygraph machine

INTRODUCTION TO POLYGRAPH

A polygraph measures and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity.

Commonly referred to as “lie detector”. Invented in 1921 by John Augustus Larson. US federal government agencies such as the FBI and the CIA and

many police departments such as the LAPD use polygraph examinations to interrogate suspects and screen new employees.

Within the US federal government, a polygraph examination is also referred to as a psychophysiological detection of deception (PDD) examination

General theory behind polygraph

Arousal Increased ANS activity○Sweating○Respiration changes○Vasoconstriction○Pulse rate○Blood pressure○Specific patterns of arousal during questioning could indicate guilt or lying

TESTING PROCEDURE

Polygraph – Relevant/Irrelevant Test

Earliest method of polygraph testing Two kinds of questions

Relevant○ Deal with issue at hand

Irrelevant○ Deal with outside facts or details

Assumption:A liar or guilty person will be more aroused by relevant

questions than Irrelevant ones, while an innocent person will show no difference○ So, if arousal(relevant) > arousal(irrelevant) = lying

Polygraph – Comparison Question Test Most common method of polygraph interrogation

◦ Developed by John Reid Begins with extensive pre-test interview Three kinds of questions:

◦ 1. Irrelevant Is your name Orenthal James Simpson?

◦ 2. Comparison (aka probable lie) E.g. “Have you ever physically harmed

someone”◦ 3. Relevant

E.g. “Did you kill Nicole Brown Simpson”

ASSUMPTION

• A liar become more aroused by lying to the relevant questions than the comparison questions

• An innocent person will be more aroused by the comparison questions

• Arousal(relevant) > Arousal(comparison) = guilty