25-04-2010, 11:13 AM
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Psycho Physiological Credibility Assessment
INTRODUCTION
Lies are part of human life! People often tell lies to deceive others. many of us have heard about "lie detector ",some where seen in movies or read it in fiction stories. Does anything of this sort really exist? If yes, does it really detect lies? Yes it does. The first thing that comes to our mind is how an instrument with some electronic components can read out our mind. Though science and technology are growing at the speed of light, has it reached a stage where it can read mind? Not exactly! But it can approach to that stage by monitoring the physiological reaction by plotting graphs.
WHAT IT IS
Polygraph, a multigraph of physiological reaction. Lie detector or polygraph as it is technically called is an instrument that monitors the physiological reaction. It can detect only deceptive behavior of human mapped to lies. The underlying factor of this concept is a person is subjected to stress while trying to decieve.Biomedical technology is used to bring out this deception. Lying is a sort of defense mechanism used by human to avoid trouble but when it is a serious crime like murder, spying on a country, it is necessary to bring out the truth. in such a case polygraph provide a helping hand to the law.
PRINCIPLE APPLIED
When someone decieves, sometimes we are able to guess by his/her reactions whether they are deceiving or not. But most of the times we are unable to predict this. This principle of guessing is more widely used with the growing technologies to monitor the physiological reactions. It monitors the involuntary change that occurs in the body when the person is under stress. Biomedical instruments are used to monitor those physiological reactions. They are:
> HEART RATE
> RESPIRATORY RATE
> ELECTRO DERMAL ACTIVITY
> CHANGE IN BLOOD PRESSURE
TAKING THE TEST:
This test challenges the mind of the most stoical person. All the equipments are attached to the subject's body. A polygraph exam is along process that can be divided up in to several Stages.
1. PRETEST
2. INTEST
3. POST TEST
Forensic psychophysiologist, a trained examiner examines the subject (pretest) .he questions the subject (intest) and look for fluctuations in the graphs produced (post-test) to find the amount of deception.
PRETEST:
This consists of an interview between the examiner and the examinee, where the two individuals get to learn about each other. This may last about one hour. During this the examiner gets the examinee's side of the story concerning the events under investigation.while the subject is sitting there answering questions, the examiner also profiles the examinee. The examiner sees how the subject responds to questions and processes information. The examiner designs questions that are specific to the issue under investigation. The physiological reactions that are monitored are heart rate , heart sound,respiratory rate,galvanic skin resistance.
HEART RATE AS A FACTOR IN DETECTION OF DECEPTION:
Normally a heart beats 72 times per minute. But while decieving, the subject is under stress, hence the rate of heart beat increases. That is the frequency of heart rate increases. This involuntary response going in a person's body is detected by forensic psychophysiologist.hence by monitoring heart beat one can detect deception. In heart rate there are two parameters monitored.
1. Bioelectric signals produced by heart muscle cells.
2. Sound produced by the heart.
WHAT ARE BIOELECTRIC SIGNALS?
Certain types of cells inside the body, such as nerve and muscle cells are encased in semi permeable membrane that permits some substance to pass through the membrane and block the others. Surrounding the cells are body fluids. These are conductive solutions containing ions. The principle ions are sodium, potassium and chloride. The cell membrane readily permits potassium and chloride ions and block sodium ions. Since a balance is required between inside and outside of the cell, both according to concentration and electric charge, the inability of sodium to penetrate the membrane results in two condition.
1. The concentration of sodium ions inside the cell becomes much lower than the
fluid outside. Since sodium ions are positive, they would make the outside of the
cell more positive.
2. To balance electric charge, additional potassium ions which are also positive enter the cell, making the concentration higher inside than outside. Due to concentration imbalance of potassium ions, equilibrium is reached with a potential difference across the membrane, negative on the inside and positive on the outside. This membrane potential is called resting potential or bio electric potential. This potential is negative since it is measured from inside to the body fluids. A cell at resting potential is said to be polarized.
When a cell membrane is excited by the flow of ionic current or by some form of externally applied energy, the membrane changes its characteristics and begins to allow some of the sodium ions to enter. These movements of sodium ions into the cell constitute an ionic current flow. This process of disturbing the cell from its polarized state by the rushing in of sodium ions is called depolarization.
Once the rushing in of sodium ions is stopped, the in lying sodium ions are rushed out by an active process called sodium pump. Thus the cell again becomes polarized. This process of converting the cell from depolarized state back to polarized state is called repolarization.
PHYSIOLOGICAL NATURE OF BIOELECTRIC SIGNAL
Q
I R
Na+ Na+
k+
Na+ Na+
TYPES OF WAVES ORIGIN AMPLITUDE mV DURATION second
P-wave atrial depolarization or contraction 0.25 0.12-0.22 (PR-interval)
R-wave (QRS
complex) repolarization of atria and depolarization of ventricles 1.6 0.07-0.1
T-wave ventricular repolarization 0.1-0.5 0.05-0.15 (ST-interval)
U-wave repolarization of inter ventricular system <0.1 0.2
(TU-interval)
SOUNDS OF HEART
Listening to sound produced by organs and vessels is called auscultation. A normal heart beat produces two distinct sounds often described as "lub-dub".
Lub:-it is a long and a dull sound caused by the closure of atrio-Ventricular valves, which permits flow of blood from the atria into the ventricles but prevent in the reverse direction.
Dub:-it is a short and sharp sound caused by the closure of semi lunar Valves, which release blood into pulmonary and systemic circulatory systems.
There is sometimes a third heart sound especially in young adults which occurs from 0.1-0.2 second after the second heart sound. This is due to the rush of blood from atria into the ventricles.
CORRELATION OF HEART SOUND WITH BIOELECTRIC SIGNALS:
The first heart sound occurs at the time of QRS-complex. The second heart sound occurs at the time of end of t-wave.
HEART RATE MONITORING.
The bioelectric signals generated in the heart muscle cells are converted to electric signals. Also the sound signals of the heart are converted to electric signals and the graph of those signals is monitored.
output
TRANSDUCTION OF BIOELECTRIC SIGNALS.
SURFACE ELECTRODES:
It is employed to pick up electrical signals. It is a type of biopotential electrode. They use metal-electrolyte interface. The interface of metallic ions in the solution with their associated metals results in an electric potential which is called electrode or half-cell potential. This potential acts as voltage source. This potential is the result of the difference in the diffusion rate of ion in and out of the metal. Equilibrium is produced by the formation of the layer of charge at the interface. This charge is a double layer, with the layer nearest to the metal being on one polarity and the layer next of opposite polarity.
This double layer acts as capacitor. There is some leakage potential. This acts as a resistor which is in parallel with the capacitor. There is some skin resistance to the electrode. This acts as series resistor to the parallel circuit. These parameters form the electrical equivalent of the electrode interface circuit.
V=E1-E2 3*
R]
R2
CI
R3
R4
C2
El
E2
-vw
R5
The electrode measures the ionic potential at one point of the body. The bioelectric potential is actually the ionic potential difference between two points of the body. Hence to obtain bioelectric potential two electrodes are required. This combination of two electrodes acts as a transducer. The two electrodes should be of same type to ensure that we obtain the true bioelectric potential of the body.
ISOLATION AMPLIFIER
It increases the input impedence.Low pass filter with carrier frequency 10 kHz to reduce the
Interference due to surgery. High and over volt protection to withstand volt during refibrillation. Notch filter to eliminate interference due to line frequency.
CONDENSER MICROPHONE:
The conversion of heart sound to electric signal can be done using a transducer called condensor microphone.there is two main categories.
1. air coupled microphone
2. contact microphone
In air coupled microphone, the movement of chest is transferred in air cushion and presents low mechanical impedance to the chest. But contact microphone is directly coupled to the chest wall and presents higher impedance, high sensitivity, low noise and light weight. Contact microphone is mostly preferred.
It consists of a diaphragm which acts as rotor and a back plate acts as stator of a variable capacitor. The two electrodes are placed very close to each other. When a regulated dc voltage is supplied through a high a resistance across the electrodes, a constant charge is maintainted.it is given by c=q/v.The vibrations produced by the chest wall changes the position of diaphragm. This results in change in voltage.
PHONOAMPLIFIER:
The voltage produced in the condenser microphone is of the order of millivolts.the preamplifier is used to amplify in the order of volts.phonoamplifier has two stages. First stage has an amplification of about 20, second stage about 50.hence the total gain is 1000.Continuous variation of gain can be achieved through 22 kilo-ohms potentiometer.
FILTERS:
High pass filter of gradual slope of attenuation is used. Band pass filters produce transients and masks the splitting of heart sounds. In abnormal hearts additional heart sounds called murmurs. In the case of murmurs where greater selectivity is required, high pass filters with sharper slopes are required.
RESPIRATORY RATE:
Pneumographs, rubber tubes filled with air are placed around the test subject's chest and abdomen. When the chest or abdominal muscles expand, the air inside the tubes is displaced. In an analog polygraphs, the displayed air acts on a bellows, an accordion like device that contracts when the tube expands. A digital polygraph also uses the pneumographs, but employs transducers to convert the energy of the displayed air in to electronic signals.
ELECTRODERMAL ACTIVITY:
This is also called as galvanic skin resistance. This is basically a measure of sweat on finger tips as they are one of the most porous areas of the body. We sweat more when we are under stress. Finger plates called galvanometers are attached to the fingers of the subject. These plates measures skins ability to conduct electricity. When skin is hydrated, it conducts electricity more easily than when it is dry.
INTEST:
This is the actual test. Here the examiner asks certain number of questions out of which only a few will be relevant to the issue or crime being investigated. The others are "control questions". A control question is a very general question, such as "have you ever stolen anything in your life?".. .a type of question that is so broad that almost no one can honestly respond with a"no".if the person answers no, the examiner gets an idea of the reaction that the examinee demonstrates when being deceptive. While asking questions he simultaneously monitors the physiological reactions which are plotted as graphs in the monitor. POSTTEST:
The examiner analyses the data of physiological responses and makes a determination regarding whether the person has been deceptive or not. If there are significant fluctuations that show up in the results, this may signal that the subject has been deceptive. Especially when a person displays different responses to a question that was asked repeatedly.
MISINTERPRETATION OF RESULTS:
There are times when a polygraph examiner misinterprets a person's reaction to a particular question. The human factor of a polygraph exam and the subjective nature of the test are two reasons why polygraph exam results are seldom admissible in court. Here are two ways that a response can be misinterpreted:
¢ FALSE POSITIVE:
The response of a truthful person is found to be deceptive.
¢ FALSE NEGATIVE:
The response of a deceptive person is found to be truthful.
If we look at laboratory-based studies, false positive errors occur more than false-negative errors.
Critics of polygraph exams say that false-positive errors occur in real world scenarios, which biases the system against the truthful person. These errors are likely to occur if the examiner has not prepared the examinee properly or if the examiner misreads the data following an exam.
APPLICATIONS:
> NATIONAL SECURITY
> CRIMINAL INVESTIGATION
> FEDERAL BUREAU OF INVESTIGATIONS
CONCLUSION:
American polygraph association has certified that a polygraph is 90% to 98% truthful. The outputs from the post-test are fed in to the computer, which uses an algorithm developed by applied physics lab of Johannes Hopkins University. It's all up to the experience of the examiners to line up the questions and interpreting the results. The American government now realizes their mistake of overseeing the test and not subjecting Robert Hansenn (who was spying on America for Russia) to this assessment. After this incident the American government is keen in using this in various fields. They want this to get upgraded to be 100% accurate. With such a progress in this field why can't an instrument read a person's mind! Yes, it will. In a very short span of time this field will spread widely all over the world.and one day, very soon our country's dream of "satyameva iavate"will be fulfilled through this.
