Penile plethysmography (PPG) or phallometry is measurement of blood flow to the penis, typically used as a proxy for measurement of sexual arousal. The most commonly reported methods of conducting penile plethysmography involve the measurement of the circumference of the penis with a mercury-in-rubber or electromechanical strain gauge, or the volume of the penis with an airtight cylinder and inflatable cuff at the base of the penis. Corpora cavernosa nerve penile plethysmographs measure changes in response to inter-operative electric stimulation during surgery. The volumetric procedure was invented by Kurt Freund and is considered to be particularly sensitive at low arousal levels. The easier to use circumferential measures are more widely used, however, and more common in studies using erotic film stimuli. A corresponding device in women is the vaginal photoplethysmograph.
For sexual offenders it is typically used to determine the level of sexual arousal as the subject is exposed to sexually suggestive content, such as pictures, movies or audio, although some have argued that phallometry is not always appropriate for the evaluation of sexual preferences or treatment effects. A 1998 large-scale meta-analytic review of the scientific reports demonstrated that phallometric response to stimuli depicting children, though only having a .32 correlation with future sex offending (accounting for approximately 10% of the variance), had the highest accuracy among methods of identifying which sexual offenders will go on to commit new sexual crimes. (None of the methods were strong predictors with most accounting for far less than 10% of the variance).
For prostatectomy nerve-sparing surgery, the surgeon applies a mild electrical stimulation near the cavernous nerves of penis to verify their locations and avoid operative trauma. Damage to these difficult-to-see nerves can cause erectile dysfunction outcomes. At the surgery's conclusion, the electrical stimulation penile plethysmograph result is a prognosis which helps to manage the erectile function outcomes earlier than the many months required for recovery.
Types
There are two types of penile plethysmograph: Volumetric air chamber When this is placed over the subject's penis, as tumescence increases, the air displaced is measured. Circumferential transducer This uses a mercury-in-rubber or indium/gallium-in-rubber ring or electromechanical strain gauge and is placed around the shaft of the subject's penis to measure changes in circumference.
The circumferential type is more common, but the volumetric method is believed to be more accurate at low arousal levels.
Significant suppliers of PPG machines include Behavioral Technology Inc. and Medical Monitoring Systems. The device is known to be used in Brazil, Britain, Canada, China, Czech Republic, Hong Kong, New Zealand, Norway, Slovak Republic, Spain, and the United States.
The surgical machine is supplied as CaverMap by Blue Torch Medical Technology, Inc.
A roughly equivalent procedure for women, vaginal photoplethysmography, measures blood through the walls of the vagina, which researchers claim increases during sexual arousal.
Methodology
Exact procedures can vary, but in one study a seventeen-year-old male sex offender was, at the start of each daily session, privately fitted with a Parks Medical Electronics mercury strain gauge attached to his penis and worn under trousers and underwear. The device had previously been calibrated with measurements from his penis in its flaccid state and at full erection as achieved through masturbation. This device was connected to a microprocessor, a battery pack, and a strain gauge amplifier, all of which were kept discreetly in a fanny pack. He was then driven daily to a college campus for multiple sessions to assess his sexual attraction to women and men, who were about 30–100 feet away from him. While remaining in a parked car, he was instructed to focus on either a specific woman or a specific man in the setting, imagine himself having sex with them, and allow himself to become sexually aroused if he was so inclined. The device then measured the percentage of full erection that he achieved. This was repeated over a period of 24 days, with the results showing a significantly greater level of attraction to women, especially when research staff were discreetly absent.