Nowadays, people are becoming an expert to tell an untruth, they are skilled by lieing spontaneously. They are under the delusion that “telling an untruth is the only way to escape from the problem”. For such kinds of people, a lie detector would be a nightmare, and facing it would be a hell lotta challenge. We would have seen lie detectors being used for celebrities in reality shows. But we might not be familiar with their working principle. This blog will help you get more knowledge about the back work of lie detectors.

Lie Detection Test

For the so-called “lie detection test”, electrodermal activity is used to detect the deception. Electrodermal activity is abbreviated as EDA and it is also referred to as galvanic skin response. EDA is an autonomic(involuntary) response. Involuntary is such an action/response that cannot be controlled by ourselves. Electrodermal activity is the measurement of the eccrine gland’s activity.

Lie detector

Glands involved

There are two types of sweat glands, eccrine and apocrine. These eccrine glands are distributed all over the body and their activity is influenced by sympathetic nervous system activity resulting from external environmental stimuli. Types of external sensory stimuli that can elicit changes in EDA are listed below.

  • Touch
  • Odour
  • Taste
  • Light
  • Sound

Arousal Types

There are two types of arousals, namely physiological and psychological and these are based on the type of stimuli. When you are pricked by a needle, then you become physically aroused. This is an example of external sensory stimuli. When you are embarrassed, then there will be vasodilation of blood vessels(blushing) and you become psychologically aroused. These stimuli will initiate changes in arousal and thereby we sweat which increases skin’s electrical conductivity. This might seem a bit complex, let me tell it in a simpler manner.

Once we get aroused due to a stimulus, the sympathetic nervous system tends to increase the production of sweat. We all know that sweat comprises of water and electrolytes, so the more you sweat, the conductivity of the skin increases.


Skin electrical conductivity has two components namely skin conductance level(SCL) and skin conductance response (SCR). Skin conductance level is also known as tonic conductance level. Here conductance activity is measured in the absence of external environmental stimuli. Here patient’s tonic conductance level slowly changes according to his/her psychological state, hydration, skin dryness, etc. in a period of ten seconds to minutes.

SCL and SCR graph

Phasic conductance level

Whereas, skin conductance response which is also known as phasic conductance level is measured in the presence of external environmental stimuli. Here the patient’s phasic conductance level shows abrupt changes i.e abrupt increases called “peaks”. These peaks are referred to as skin conductance responses. The EDA is a combination of both tonic and phasic. It is the phasic waves that help in finding the lies. The units used for measuring EDA electrical conductivity are micro siemens. The normal skin conductance levels are in the range of 2 – 20 micro siemens. Electrodes are connected to the skin to measure the skin conductance levels.

Channels Involved

In a lie detector test, three channels are used to record and detect the changes. The first channel is known as the electrodermal channel which displays changes in EDA(Electrodermal Activity). The second channel is known as the “cardio channel” which is actually an arm plethysmograph that is used to record the changes in the blood volume of the upper arm associated with the cardiac cycle. This channel determines heart rate and indicates changes in pulse volume.

The third channel is placed around the patient’s chest to record respiration. The subjects are interrogated with 5 to 10 questions. When the patient is telling an untruth, then it will elicit an emotional response. The other changes we can observe are increases in the respiratory ratio of expiration to inspiration, an increase in blood pressure and arousal produced by critical questions that diminish with repetitions. These changes are noted through channels and are used to identify whether the subject is lying or not.


Even though this seems fascinating, this sometimes is affected by several factors varying from one person to another. This test is not so confirmative enough to declare a person guilty/innocent. For example, an innocent person’s emotional response might be increased due to the pressurized situation created while asking the question. This doesn’t mean that he is lying and it might ruin his career even though he is not guilty.

There are chances for a lot of false negatives and false positives to occur (false negative- the truth is positive but the prediction is negative, false positive- the truth is negative, prediction is positive). There is still a lot more to R&D so that it becomes a reliable one. Hope you enjoyed reading this and gained some knowledge about this.

Happy reading!

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