Vaginal photoplethysmography (VPG) is measurement of vaginal vasocongestion. A vaginal photoplethysmograph is a clear acrylic, menstrual tampon-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. As the amount of blood in the vaginal tissue increases, more light is reflected into the photosensitive cell of the device. VPG is the most common way to assess vaginal blood flow and is widely used to measure genital sexual arousal in women. However, debate has arisen as to how reliable the technique is.
VPG was first introduced in 1967 by Palti and Berovici, who affixed a light source and photosensitive cell onto a gynecological speculum and recorded vaginal pulse waves. Sintchak and Geer improved on the device in 1975 by using a vaginal probe. The vaginal photoplethysmograph was the first practical and reliable device for the measurement of vaginal blood flow.
The output of the VPG can be filtered into two types of signals, which have different properties. The direct current signal, is a measure of vaginal blood volume (VBV) and reflects the total blood volume in the vaginal tissues. The alternating current signal is a measure vaginal pulse amplitude (VPA) and reflects the pressure change within the blood vessels of the vaginal wall associated with each heartbeat . While changes in VBV occur in response to sexual and anxiety-inducing stimuli, changes in VPA only occur in response to sexual stimuli. Since VPA is a more sensitive and specific measure of sexual arousal, many researchers use it instead of VBV.
VPA is defined as the peak-to-trough amplitude of the vaginal pulse wave. It is calculated by subtracting the means of all troughs from the means of all peaks experienced during stimulus presentation.VPA lacks an absolute scale of measurement; each unit of change (mV) does not correspond directly with a physiological change (cf. penile plethysmography). Since VPA does not have a standard unit of measurement it is difficult for researchers to make between-participant comparisons. Being said that, in 2009, a group of engineers at Rensselaer Polytechnic Institute developed a real-time wavelet-based algorithm that can create a relatively accurate data analysis system using VPA signals. They claimed that their method has made it possible to perform between-participant comparisons.
There is an overall poor correlation (r = 0.26) between women's self-reported levels of desire and their VPG readings suggesting that vaginal blood flow is not a reliable indicator of female sexual arousal and a better method is needed. Men using the penile plethysmograph have a far greater correlation between reported arousal and blood flow. An improved technique with better correlation is Laser Doppler imaging of genital blood flow.
VPG was designed to be easily inserted by research participants; however, some participants perceive insertion of the VPG to be invasive. Past research suggests that studies using VPG result in small and unrepresentative samples. However, more recent assessments of women’s willingness to participate in sexual psychophysiological research using VPG found no evidence of sampling bias influencing results.
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Vaginal photoplethysmograph. Image: Courtesy of The Kinsey Institute for Research in Sex, Gender, and Reproduction
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