The Vaginal photoplethysmograph (VPG) is a controversial type of plethysmograph. It consists of a clear acrylic, rod-shaped device that contains a light source, and a light detector. The light source illuminates the capillary bed of the vaginal wall and the blood circulating within it. The amount of light that is backscattered is thought to be directly related to the transparency of engorged and unengorged tissue and, hence, serves as an indirect measure of vasoengorgement, an indication of sexual arousal. However, there are many problems with the instrument including (1) lack of validation of its direct relationship with vasocongestion, (2) lack of an absolute scale precluding between-participant comparisons in research, and (3) its perceived invasiveness by some research participants.
The device was first introduced in 1967 by Palti and Berovici and refined in 1975 by Sintchak and Geer. It was the first practical and reliable device for the measurement of vaginal blood flow. The vaginal probe was designed to be easily inserted by the subject. Its introduction spurred great (research) interest in the area of female sexual arousal throughout the late 70s and early 80s. Research in this area has increased significantly recently, particularly since the introduction of Viagra for men.
The device remains the most frequently used method for monitoring vaginal blood flow.