The PPG uses the reflection of a red light emitted into the skin to detect changes in the volume of blood in the upper layers of skin, typically recorded at the finger when using a polygraph. Physiological arousal is marked by a constriction in the pulse amplitude as blood is shunted from the extremity during activation of the sympathetic nervous system. See: Geddes (1974); Handler & Krapohl (2007); Kircher & Raskin (1988).