Oral contraceptive use and female genital arousal: methodological considerations

Journal of Sex Research, August, 2005 by Brooke N. Seal, Lori A. Brotto, Boris B. Gorzalka

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).

Since the introduction of the VPP in 1975, a number of other psychophysiological instruments have been developed in attempts to quantify the female genital arousal response more accurately, and thus shed light on the mechanisms involved in female sexual arousal. Such instruments have included the labial thermistor (Cohen & Shapiro, 1970; Shapiro, Cohen, DiBianco, & Rosen, 1968), the heated electrode (Wagner & Levin, 1978), and magnetic resonance imaging (Maravilla et al., 2003). However, the VPP remains a popular instrument for the study of female psychophysiology because of practical advantages such as its ability to be inserted by the research subject, its ease of disinfection and handling, its relative durability with repeated use, the low cost of its laboratory hardware, and the fact that it is relatively unobtrusive once inserted.

The VPP has been employed in a wide array of research designs and in response to varied research questions pertaining to genital arousal. For example, it has been used to study gender differences (e.g., Heiman, 1977), effects of menopause (e.g., Brotto & Gorzalka, 2002; Laan & van Lunsen, 1997), female sexual dysfunction (e.g., Heiman, 1976; Meston & Gorzalka, 1996; Palace & Gorzalka, 1990; Wouda et al., 1998), sexual orientation (e.g., Chivers, Rieger, Latty, & Bailey, 2004), and in pharmacotherapy outcome trials (e.g., Laan, van Lunsen, & Everaerd, 2001; Meston & Heiman, 1998, 2002; Meston & Worcel, 2002; Rubio-Aurioles et al., 2002; Sipski, Rosen, Alexander, & Hamer, 2000a). Despite the widespread adoption of this methodology across international research sites, the current lack of established guidelines for using the VPP has led to methodological variation across studies, resulting in difficulties with cross-study comparisons and gaps in our understanding of the factors influencing female genital arousal.

The extent to which cross-study variability in design and analysis is responsible for discrepancies in the literature has received minimal attention, despite the potential for having significant consequences. For example, Palace and Gorzalka (1992) found dramatic differences in the strength of the vaginal blood volume response depending on the type of erotic stimulus employed, and whether it was drawn from educational or commercial sources. The method of data analysis has also been found to influence results, with within-subjects analyses being more likely than between-subjects analyses to reveal positive correlations of psychophysiological with subjective sexual arousal (Merritt, Graham, & Janssen, 2001). Hormonal status of research participants is a critical methodological factor to consider when interpreting results, in light of the finding that menstrual cycle stage can significantly influence vaginal responding (Graham, Janssen, & Sanders, 2000; Slob, Bax, Hop, Rowland, & van der Werff ten Bosch, 1996; Wincze, Hoon, & Hoon, 1976). However, hormonal status is rarely controlled for in VPP studies of premenopausal women.

Similarly, use of the oral contraceptive pill (OCP) is a methodological factor that is not consistently controlled for in female sexual arousal research using the VPE We arbitrarily chose to review articles published between 2000 and 2003 in hopes of examining this independent variable and identified 15 studies that employed the VPP in premenopausal women during this time. There was tremendous cross-study variability in terms of whether or not the OCP status of premenopausal participants was controlled for or even mentioned. Two studies excluded women using OCPs (Graham, et al., 2000; Meston & Heiman, 2002), two reported including women both using and not using OCPs (Polan et al., 2003; Tuiten et al., 2000), and 11 reports failed to mention the OCP status of the participants (Berman et al., 2001; Both, Everaerd, & Laan, 2003; Brotto & Gorzalka, 2002; Islam et al., 2001; Laan et al., 2002; Pras et al., 2003; Salerian et al., 2000; Sipski, Alexander, & Rosen, 2001; Sipski et al., 2000a, 2000b; Tuiten et al., 2002).