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What is the Polygraph Examination?

The modern Polygraph instrument can monitor and record up to five measures:

1) Respiration,

2)  Electro  dermal  activity,  

3) Cardio changes,  

4) Relative  changes  in  the  total  blood  volume

using  the  (Photo  Plethysmograph) and  

5)  Movement  to  detect  certain  physical


The  Polygraph  instrument  merely  records  momentary  changes  in  a  person’s  level  of

physiological  arousal.    There  is no  lie  response per  se.    That  is,  there  is no  unique  response

pattern which occurs every time a person lies, and never under any other circumstance.  Many

things  cause  the  body  to  respond;  noise,  sights,  being  touched,  telling  a  lie,  even  random

thoughts. The examiner must structure the testing situation to eliminate or control extraneous

sources of reactions. For example, repeating the test questions several times controls responses

to  random  thoughts.  The  examiner’s  diagnosis  of  deception  is  an inference  based  upon  the

elimination of alternative causes of responses.

1) Respiration.

2) Electro dermal activity.

3) Cardio changes.

4) Photoelectric Plethysmograph.

The  Photo  Plethysmograph  adds  new  physiological  data  to  existing  polygraph  information by

measuring  relative  changes  in  the  total  blood  volume  in  a  body  segment  and  by  rapidly

detecting any changes in pulse blood volume. The Photo Plethysmograph uses a photo-sensitive

cell to measure infrared light reflected or passed though human tissue. The light is scattered by

red blood cells so that the amount of light reaching the photo sensor after passing through the

tissue  indicates  the  amount  of  blood  that  it  had  to  pass  through.  Empirical  research  on  the

effectiveness  of  the  Photo  Plethysmograph  has  revealed  that  this  technique  provides  an

independent  index  of  sympathetic  arousal  and  that  the underlying  physiological  principles  of

the Photo Plethysmograph are scientifically based and reliable.

(1) Relevant question: Pertain directly to the matter under investigation.

For example:“Did you commit this crime?

(2) Irrelevant question: Totally unconnected to the matter under investigation.  

For example:“Is today Wednesday?

(3) Comparison  question: Only  indirectly  related  to  the  matter  being  investigated.    The

Question is designed to either provoke a lie, or to create concern about whether or not the

examinee is concealing information from the examiner.

For  example: “Before  the  age  of  20,  have  you  ever  deliberately  told  a  serious  lie,  to

someone that trusted you?

Question Types?

Innocent Examinee

The  innocent  examinee  is  answering  the  relevant  questions  truthfully. He  /  she  is  lying  to  or

doubtful about the comparison questions, which now have greater  signal  value. He / she  thus

react more or greater to the comparison questions than relevant questions.

Guilty Examinee

The  guilty  examinee  is  lying  to  both  the comparison  questions  and  relevant  questions,  but  is

more concerned about him or her lie to the relevant questions because of their signal value and

fear  of  detection. He  or  she  thus  reacts  more or  greater  to  relevant  questions  than  to  the

comparison questions.

PSYCHOLOGICAL SET: Signal value - Fear of detection


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