How polygraph machines work

Back in the day polygraph machines resembled those devices we often see in movies: wires of which the one end was connected to the examinee (the person taking the test) and the other to a box. This box had a single sheet of scrolling paper, on which various needles were scribbling what seems to the untrained eye like a bunch of incoherent lines (but which are officially termed “tracings”).

While technology has changed and we’ve since moved from analogue polygraph machines fitted with paper (described above) to digital polygraph devices fitted with computer screens and various very complicated algorithms, the incoherent lines have remained. That’s because the polygraph, in essence, has remained the same machine. But what do those lines mean?

Herewith a brief look at the various recordings  and how they tie in with polygraph testing.

A quick definition of polygraph testing

Polygraph machines, or lie detectors, measure physiological responses when predetermined questions are asked during a polygraph examination. This is because a number of interesting things happen to our bodies when we try to conceal the truth: we tend to breathe faster, the heart rate goes up, and we start sweating.

These responses are captured and, when the test is complete, analysed by the polygraphist (the  examiner) to determine whether these recorded physiological responses indicate possible deception.

Each physiological response is measured by a specific component in the polygraph machine. Here we take a look at some of the most common components, and what they measure:

Pneumographs

These are rubber tubes filled with air, and usually come in twos – one strapped around the chest, and the other around the abdomen. They record the respiratory rate (breathing): expansion of the chest or abdomen during breathing causes air displacement within both of these tubes. The energy with which the air is displaced is converted into electronic signals by transducers.

These electronic signals are then passed through a number of algorithms before being displayed as a continuous graph on the polygraph machine’s monitor.

Tempting as it may be to attempt fooling the pneumograph reading by means of controlled breathing exercises, it’s worthwhile to keep in mind that they record even the most trivial fluctuations in breathing patterns; a faster or slower respiratory rate may be telling of such a strategy, and hence reveal a possible attempt at deception.

Cardio-sphygmograph

Heart rate and blood pressure are measured by the cardio-sphygmograph. This involves a blood pressure cuff strapped to the upper arm, which is then inflated. The movement of blood through the arm generates a sound which is transmitted through the air in the cuff. In older models a bellows was used to amplify the sound; nowadays transducers are used to convert the sound into digital signals.

Both the frequency and the magnitude of the sound travelling through the cuff are measured. Changes in frequency is used to measure heart rate, while the magnitude indicates blood pressure.

Conventional theory suggests that fear of being discovered when telling a lie automatically increases heart rate and therefore blood pressure. Modern algorithms, however, analyse and recognise change patterns in heart rate and blood pressure to help determine possible deception.

Galvanometers

Galvanometers measure electro-dermal activity, also known as galvanic skin resistance. A rise in respiratory rate, blood pressure, and heart rate brings a rather common physiological excretion to the fore: sweat.

Fingerplates are attached to two fingers – the finger containing a high concentration of sweat glands – to measure electrical conductivity. As the amount of sweat increases, the resistance of the electrical current decreases.

Much like the other variables mentioned above, perspiration may vary from one person to the next, which is why an important part of the polygraph examination is to establish a baseline for testing. As long as an examinee is eligible for polygraph testing, the equipment described above, the algorithms of the polygraph machine, and the experience of the examiner will contribute to the high success rate of polygraph testing.

I don’t know anything about lie detectors other than they scare the hell out of people! – Richard Nixon