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Vasomotor
Some polygraphs are equipped with a device that is designed to detect changes in the size of capillaries at the fingertip. Called a photoplethysmograph, it is sensitive to physiological reactions in the form of capillary constriction. The photoplethysmograph has two important components: a red light and a light sensor. The red light is shone into the finger and the light sensor detects any light that is reflected back. The red light that enters the skin is either absorbed, or when it encounters blood, it is reflected back to the sensor. On the polygraph, the signal shows up as a pulse wave. When a person lies, the body typically activates systems that move the blood away from the extremities, including the fingers, and shunts it toward the major muscles. The movement of the blood out of the fingers means that less of the red light is reflected back to the light sensor. This causes a narrowing of the signal that shows on the polygraph. See the diagram below for how this is normally displayed.
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