Oral Contraceptive Use and Female Genital Arousal: Methodological Considerations‪.‬

The use of the vaginal photoplethysmograph (VPP) over the past three decades has enriched our understanding of the factors that influence and are implicated in female sexual arousal. Its development allowed for many of the observations made by Masters and Johnson (1966) with respect to physiological sexual excitement to be quantified objectively. The VPP consists of a clear, acrylic, tampon-shaped probe that is inserted vaginally. A light-emitting diode projects a beam of infrared light that illuminates the capillary beds of the engorging vaginal tissue. Embedded in the probe is a photosensitive light detector, which detects the amount of backscattered light reflected from the vaginal wall (Sintchak & Geer, 1975). The relative degree of back-scattered light detection from the engorged to the unengorged state is interpreted as an indirect measure of vasocongestion within the area (Laan & Everaerd, 1998). Because of the absence of an absolute metric in the VPP, within-subjects protocols are necessary wherein the subject is exposed to neutral and subsequently erotic stimuli, and responses to the two conditions are compared. Two signals are derived from the VPP: one that reflects a general pooling of blood in the vaginal tissue (vaginal blood volume) and one that reflects moment-to-moment changes in blood flow that occur with each heart beat, with larger amplitudes reflecting higher levels of vasocongestion (vaginal pulse amplitude; VPA). Because of its sensitivity and specificity, VPA is the preferred measure when using the VPP (Laan, Everaerd, & Evers, 1995).