General information about the PPG

Penile plethysmography (PPG) is measurement of blood flow to the penis, typically used as a proxy for measurement of sexual arousal. It involves the measurement of the circumference of the penis with a mercury-in-rubber strain gauge. This is also called measurement of “tumescence”. The strain gauge is connected to a computer, which records electrical impedance that occurs as the penis expands. A software program transmits 27 erotic audio/video narratives to a monitor and headphones. The goal is to administer stimuli that will elicit erectile responses sufficient to discriminate between deviant and nondeviant interests. THERE ARE NO NUDE IMAGES OF CHILDREN WHATSOEVER! However, there are a handful of very mild nude images of adults.  

For a man accused or convicted of a sexual offense the PPG is typically used to determine the level of sexual arousal as he is exposed to sexually suggestive audio scenarios. A corresponding device in women is the vaginal photoplethysmography, but Arizona Forensic Services does not offer this service. 

The two most common commercial brands of the PPG are the Limestone System and the Monarch System.  Both systems incorporate standardized, non-pornographic images of a variety of subjects across gender, age, and sexual activity.  Neither systems uses nude images of children, but they do incorporate a handful of very mild nude images of adults.  Both systems include audio presentations of erotic scenarios that involve the depicted subject.  The audio material is designed to “cue in” the evaluatee to the visual stimuli.  An advantage of the Limestone system is it incorporates a Real Child Voices Stimulus (RCV) set.  The RCV features both male and female voices ranging from child to adult and professionally produced sound effects.  The audio recordings of children were produced in a “blind” method that meets ethical standards. Recent research is showing increased accuracy with the RCV set.  Arizona Forensic Services utilizes the Limestone System.  

Who are appropriate subjects for the PPG?

The PPG is the most advanced means of measuring sexual arousal patterns for 1) a man accused of a sexual offense or, 2) a probationer who has been convicted of a sex crime and is now back in the community. The PPG can be a critical component in identifying what is and is not sexually arousing for a man (age, gender, coercive vs. consensual).  This in turn can better inform pretrial negotiations, post-trial sentencing, or assess community safety for a probationer.  

About the PPG Lab

The lab facility maximizes privacy and minimizes distraction. The evaluatee is placed in a separate room from the lab clinician. 

  • He places a mercury strain gauge on his penis in private outside of the view of the technician. 
  • He sits on a comfortable chair that is covered by a disposable, soft medical drape sheet to ensure safety and good hygiene. 
  • He is provided a hand towel to place across his lap to avoid exposure during test administration.
  • The testing incorporates countermeasure technology that includes a breathing monitor placed across his stomach and a “pressure cushion” that measures movement and tightening of gluteal muscles.  These gauges are used to monitor arousal, movement, and attempts to fake responses. The PPG technician pays close attention to potential cues suggesting deception such as furtive hand and arm movement, breathing rhythm changes, and unusual erectile tracing patterns displayed on the computer monitor. 
  • The visual stimuli used in 27 erotic narratives are displayed using a full-size television monitor that is sufficiently large to dominate the subject’s visual field. There are no nude images of children and only a handful of mild nude images of adults. 
  • Headphones for used for hearing the audio narratives.
  • A keypad is used to track the subject’s attentiveness, provide self-report of erectile response, and encourage compliance with testing procedures. 
  • The technician is able to communicate with the subject during testing. 
  • The testing lasts for approximately 1.5 hours.