Signal
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.
Read more about this topic: Vaginal Photoplethysmograph
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