What Else Does the Polygraph Measure?

I’ve done blogs that talk about the polygraph and sweat, the polygraph and breathing, the polygraph and heart rate, but I’m sure you’re all wondering, what else does the polygraph measure?

There are a couple more bits of kit that we use as examiners to monitor your physiology (body) of an examinee when we ask the questions.

The 2 bits of equipment that I will cover in the final blog of this ‘series’ are the photoelectric plethysmograph (PLE) and the movement sensor. Tongue twisters optional.

First, let us start with the photoelectric plethysmograph. I will shorten that to PLE in this blog, it just makes it easier for everyone.

So what else does the polygraph measure?

What is the PLE?

Definition of the Plethysmograph

Definition of the Plethysmograph

To give you the most generic definition possible I turn to Merriem Webster who define a ‘plethysmograph’ as:

“an instrument for determining and registering variations in the size of an organ, limb, or part resulting from changes in the amount of blood present or passing through it”

Simple enough so far right? It measures changes in your body.

In the case of polygraph, we do not measure the size of an organ or limb, we measure the amount of blood passing through your fingertips.

The photoelectric part refers to the type of plethysmograph. There are a few different types but this one uses an infrared light to pass through your fingertips.

This leads me nicely on to the term for the blood passing through your fingertips.

What Does It Measures

Definition of Vasomotor

Definition of Vasomotor

The PLE we use in the polygraph measures the peripheral vasomotor response.

In other words, the PLE is placed on your finger tip (periphery), and it measures the constriction or dilatation (opening or closing) of the blood vessels in that fingertip (vasomotor).

The How

A guy called Bruce Mehler on ResearchGate nicely explains how the photoelectric plethysmograph works.

Tailoring his explanation a little for the polygraph, we have a black clip that contains a light source that is combined with whats known as a photodetector.

This light source actually emits an infrared light. This light source passes through the finger and is absorbed by some parts of the finger and some passes through. Whatever light gets through is picked up by the photodetector.

The PLE tracing we see on the screen.

The PLE tracing we see on the screen.

The picture on the left is what we see on the computer screen: a constant pulse wave reflecting the pulsing of your heart and the blood pumping around your finger.

Going back to constriction and dilatation. If your blood vessels dilate it is called vasodilation and is an increase in blood flow which leads to a rise in that pulse rate we see on the screen.

If there is constriction (known as vasoconstriction), blood flow is reduced which in turn leads to a decrease in the pulse rate on the screen.

This is supported by another article on ResearchGate by Kirk Harry Shelley (scroll down to table 23.3 if you are interested).


If you find this interesting, please leave a comment and any feedback. I can only get better with your help.


Why Do We Measure This Vasomotor Response?

If you have read any of the other articles in this series you will see I mention the term ‘psycho-physiological’ and then talk about things like Orienting Response (OR), Defensive Response (DR) and Emotion.

To highlight this even more if you look at the picture of the definition of vasomotor you will note a small paragraph underneath. This specifically highlights how the constriction or dilation of the blood vessels is controlled in the brain. Psycho-physiological people.

Without going into too much complicated detail about those things, they are all psychological responses to a stimulus (e.g. a question or event) that cause a physiological reaction. One of those physiological reactions is as described above – the vasomotor response.

The Power of a Question

Lots of questions

Questions are very powerful and can elicit a response

The point of the question during the polygraph test is to ask about an issue that is usually quite significant to people; whether they are deceptive or truthful, which acts as the stimulus.

This question is thought to cause an OR or DR amongst other things because of the significance and novelty. You might also feel emotion because of any consequences, your desire to prove innocence, the desire to hide the truth and the social ramifications of being caught.

You might also be thinking hard about the situation, suppressing the truth etc. All of these cause a reaction.

These reactions are throughout the entire body and are thought to be based on evolution and their purpose for keeping us alive.


Do you have any questions for me? Please don’t hesitate to ask by contacting us.


Vasoconstriction vs Vasodilation

Certain psychological states can affect our blood vessels.

Certain psychological states can affect our blood vessels.

With an OR, there has been loads and loads of research (which is ever advancing) to show that if you react due to this OR, there will be a peripheral (your finger) vasoconstriction (less blood). Remember this leads to a reduction in that pulse rate we see on the screen.

The research I found is from this experiment with spiders, this research paper advancing the theory of OR, and looking at research from a guy called Sokolov.

A side point – I specifically say ‘peripheral’ because there is something known as ‘cephalic’ vasodilation. Cephalic relates to the head which we don’t measure.

How Does Evolution Help?

The theory is that an OR allows us to assess any situation and any stimulus but we don’t necessarily react yet. As a result, the blood flow to our extremities is reduced (due to peripheral vasoconstriction) in order to lessen blood loss from a potential attacker and any damage they may cause.

Moving to DR, there is peripheral vasodilation which leads to an increase in that pulse rate. This peripheral vasodilation means that more blood is being pumped to the extremities. This allows us to react more vigorously (fight or flight in some cases) as DR is often considered a bit more extreme as the stimulus has been assessed as more aversive or unpleasant.

All Very Good But Convince Me Again…

In the polygraph world we have a group of researchers who are conducting proper research to prove why the polygraph works.

One of those guys is Charles Honts, in a paper he did (Honts & Reavy 2015) he explored whether using a PLE and measuring vasomotor responses improved the accuracy of the detection of deception.

Basically, his research suggested that the accuracy of polygraph decision scores were increased “significantly” and that vasomotor changes are a “significant predictor” of deception.

The Movement Pad

I’ve left this component of the polygraph until last because we don’t measure it. It is used to ensure a person isn’t trying to sabotage the test.

Dog in a onesie

The use of a body suit (not unlike this dog onesie) to detect movement during questioning.

Moving around can and will effect every other thing we measure, so by having that pad we can check that. So don’t get cheeky.

There is research looking at the use of a full body suit and whether they are capable of detecting deception based on how much people move.

If you are interested, read this blog that I did about it.


If you found this blog interesting. Please share it with others. I aim to help people understand.

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