The Vaginal Pressure Inducer: A New Device to Test the (Un)pleasurableness and Tolerance of Vaginal Pressure and the Influence of Sexual Stimuli

ABSTRACT

To better understand the impact of sexual stimuli on genital pain, a new instrument was developed: the Vaginal Pressure Inducer (VPI). We administered gradually increasing vaginal pressure with the VPI to sexually functional women as they watched a neutral, erotic, or explicit sex film. Women had higher unpleasantness thresholds in a sexual context compared to a nonsexual context. Moreover, ratings of pleasurableness were higher in the sexual compared to neutral context and most so during the explicit sexual film. These results provide initial support for the suitability of the VPI to study determinants of pleasant and unpleasant appraisal of vaginal pressure.

Introduction

The influence of sexual arousal on genital pain (sensitivity) is unclear. Genital pain is most often elicited by tactile stimulation of the vulva-vaginal region and most frequently located at the vaginal introitus. This highly sensitive genital area is involved in both positive sexual states such as arousal and orgasm—leading to vasocongestion (Puppo, 2013) of the vestibular bulbs and lubrication (Levin, 2002)—and negative sensations such as genital pain. Impaired subjective sexual arousal, lack of lubrication, and increased pelvic-floor activity are assumed to be risk factors for genital pain (Farmer & Meston, 2007; Spano & Lamont, 1975; ter Kuile, Both, & van Lankveld, 2010). However, studies found contradictory results concerning sexual arousal in women with genital pain. Compared to sexual functioning women, women with genital pain showed impaired (Boyer, Pukall, & Chamberlain, 2013; Wouda et al., 1998) as well as unaffected (Brauer, Laan, & ter Kuile, 2006) levels of genital sexual arousal and impaired (Brauer et al., 2006) as well as unaffected (Boyer et al., 2013; Wouda et al., 1998) subjective levels of sexual arousal in response to sexual films. Activity of the pelvic-floor muscles has been shown to be activated by negative emotions, i.e., threat and fear (of pain) (Both, van Lunsen, Weijenborg, & Laan, 2012; van der Velde & Everaerd, 2001; van der Velde, Laan, & Everaerd, 2001), as well as positive emotions such as sexual arousal (Blok & Holstege, 1996; Blok, Sturms, & Holstege, 1997; Bohlen, Held, Sanderson, & Ahlgren, 1982; Both & Laan, 2007; Shafik, 2000). Taken together, these findings suggest that genital sensitivity, vaginal pressure, sexual arousal, and activity of the pelvic-floor muscles are interrelated and associated with emotional states.

Several theoretical models ascribe an important role to subjective as well as genital sexual arousal during vaginal penetration to prevent and overcome intercourse pain (Spano & Lamont, 1975; ter Kuile et al., 2010). Linking this with the aforementioned findings on the interrelation between sexual arousal, genital sensitivity/pressure, and pelvic-floor activity, it is surprising that so far only three studies have assessed the influence of sexual stimuli on genital (pain) sensitivity. One study determined vaginal sensitivity (Gruenwald, Lowenstein, Gartman, & Vardi, 2007), while the two other studies focused on sensitivity of the vulvar vestibule (Paterson, Amsel, & Binik, 2013; Payne et al., 2007). These studies show contradicting results on changes in genital sensitivity and vaginal pressure tolerance as a function of sexual arousal.

In the study of Gruenwald (Gruenwald et al., 2007), probes were inserted into the vagina to assess vaginal vibratory sensation (Vardi, Gruenwald, Sprecher, Gertman, & Yartnitsky, 2000) in response to sexual film clips. Results revealed that touch sensitivity of the vagina did not significantly change as a function of sexual arousal (Gruenwald et al., 2007). In two other studies, a vulvagesiometer (Pukall, Binik, & Khalife, 2004) was used to apply punctate pressure on the vulvar vestibule to measure pain thresholds (Paterson et al., 2013; Payne et al., 2007) and pleasurable sensitivity (Paterson et al., 2013). Sexual arousal was measured by self-report to almost having an orgasm by masturbation (Paterson et al., 2013), and by rating subjective levels of sexual arousal as well as by use of the labial thermistor clip to measure physiological sexual arousal in response to sexual film clips (Payne et al., 2007). Both studies found that pain sensitivity of the vulvar vestibule increased during sexual arousal (Paterson et al., 2013; Payne et al., 2007). Pleasurable sensitivity did not significantly change as a function of high subjective sexual arousal (Paterson et al., 2013). Both studies also measured touch thresholds of the vulvar vestibule as a function of sexual arousal using graded disposable filaments. Whereas sexual arousal induced by sexual films clips increased touch sensitivity (Payne et al., 2007), masturbation did not affect touch sensitivity of the vulvar vestibule (Paterson et al., 2013).

In sum, the evidence so far seems to indicate that sexual arousal increases genital pain sensitivity (Paterson et al., 2013; Payne et al., 2007) and does not affect pleasurable sensitivity (Paterson et al., 2013), whereas the results for genital touch sensitivity are contradictory with either no change (Gruenwald et al., 2007; Paterson et al., 2013) or increased sensitivity (Payne et al., 2007) as a function of sexual arousal. These findings contradict the important role of sexual arousal to prevent or overcome sexual pain/penetration pain (Bergeron & Lord, 2003; ter Kuile et al., 2010).

One way to explain these unexpected findings is that the punctate pressure for sensory testing might not be an accurate reflection of the pressure during masturbation and partner sex, which is a more pronounced and prolonged experience. The possibility of applying vaginal pressure in an—at least partially—pleasurable way would provide the opportunity to test if pleasurable genital touching together with subjective arousal can influence the subjective perception (pleasantness) of vaginal pressure along with an increase of the tolerance of vaginal pressure. To further assess the influence of sexual arousal on the subjective experience of vaginal pressure, a more prolonged, pronounced, and dispersed vaginal pressure in a sexual context is required.

To our knowledge, only one study induced a more prolonged and dispersed vaginal pressure in the vaginal introitus using an inflatable soft-rubber balloon to test painful sensation in women with and without genital pain (Bohm-Starke, Hilliges, Brodda-Jansen, Rylander, & Torebjork, 2001). Although this instrument seems well suited to induce vaginal pressure and assess prolonged and dispersed pressure in the vaginal introitus to determine vulvar (pain) sensitivity, a disadvantage is that a well-inflated balloon tends to slide out of the vagina. To induce (higher rates of) vaginal pressure, an instrument is needed to keep the inflated balloon in the vaginal introitus, which may also improve the standardization of the induced vaginal pressure. For this purpose, we have developed a new instrument: the Vaginal Pressure Inducer (VPI; see Figure 1). To assess the subjective experience of vaginal pressure, we explicitly asked participants to rate the subjective experience of pleasurable as well as painful pressure on the vagina in order to prevent priming the participants to appraise any physical sensation as painful rather than pleasurable. Also note that pleasurable sensitivity is typically a more important sexual motivator than (avoidance of) pain (Paterson et al., 2013).

Figure 1. The Vaginal Pressure Inducer (VPI).

The aim of the current study was to test the suitability of this new device, the VPI, to measure the subjective perception and tolerance of vaginal pressure in sexually functioning women. We expected that prolonged and gradually increasing vaginal pressure in a context of sexual arousal will increase the pleasurableness and the unpleasantness threshold of vaginal pressure. We also explored whether the level of sexual arousal matters as well, with high levels of sexual arousal leading to larger increases in pleasurable sensitivity/ decreases in pain sensitivity compared to low levels of sexual arousal. Additionally, we explored the influence of vaginal pressure on subjective sexual arousal during sexual films.

Method

Participants

Forty-two women were included in the study. The sample was recruited through advertisements at the university, in the local media, and by directly asking people. Inclusion criteria included absence of sexual problems, aged between 18 and 45 years, good command of the Dutch language, a steady heterosexual relationship for at least 3 months, and being sexually active including coitus. Women were excluded in case of pregnancy, breastfeeding, (post)menopause, major affective disorder, psychotic disorder, substance-related disorder, posttraumatic stress disorder resulting from abuse in the area of the pelvic floor and the genitals (e.g., sexual assault) according to DSM-IV-TR criteria (American Psychiatric Association, 2000), or if they were taking medication that was likely to interfere with sexuality. The study was approved by the Ethical Committee.

Materials

Stimulus materials

We selected six film excerpts of 5 minutes each. These six films consisted of one neutral acclimatization film, two high-arousal (HA) sexual films, one low-arousal (LA) sexual film, an HA nonsexual film, and a neutral film. Both HA sexual film excerpts depicted scenes involving manual and oral sex and intercourse. The LA sexual film excerpt depicted heterosexual seduction scenes with dressed actors showing petting and kissing. Fragments with bare body parts and genital touching were omitted from the LA sexual film. To control for effects of high arousal per se, we also presented an HA nonsexual film, comprising high emotion-eliciting excerpts from the films The Eighth Day, Life Is Beautiful, and Forrest Gump, ranked with highest scores on positive affect and (nonsexual) arousal (Schaefer, Frédéric, Sanchez, & Philippot, 2010).

Description of the Vaginal Pressure Inducer

The VPI consists of an inflatable vaginal balloon that is partially inserted into a synthetic handle (Figure 1). The handle ends in a flange, which can be placed against the opening of the vagina. The flange prevents too deep insertion, to ensure introital pressure. In the middle of the flange a balloon is fixed by pumping just enough water (initial value), so that the balloon extends 2 cm out of the flange. The balloon is gradually filled with body temperature water by a pump until a length of 4 to 6 cm. When the balloon is filled, an outward omnidirectional pressure is given to the surrounding tissues. The tube of the balloon is connected to a pump to fill the balloon gradually and controllably with water (0.83 ml per second). The maximal volume of pumped water after 120 seconds is 100 ml, by which the inflatable balloon can reach a maximal circumference of 16 cm outside the body (Figure 2).

Figure 2. VPI with max filled balloon.

To insert and fixate the VPI, an extension piece is placed around the handle with four openings to attach Velcro straps, which are subsequently attached to a waistband to create a kind of pants. By wearing these pants and tailoring the straps, the flange is naturally placed against the vulva inserting the balloon into the introitus of the vagina (Figure 3). Figure 4 shows the inflated balloon after 20 seconds; for the picture the Velcro straps are removed. The VPI can be put on by the participant herself and can be remotely controlled to respect privacy of the participant. Because the level of pressure on the vaginal wall can be influenced by physical variances and fluctuations in the activity of the pelvic-floor muscles, a pressure gauge is used to allow an objective measure of induced pressure on the vaginal wall. To control temperature, one thermometer is fixed at the underside of the balloon inside the handle. By use of isolated tubes and a hot water tray with a thermometer, the water is kept at circa 37˚C. A new sterilized balloon is used for each woman. The VPI is sterilized with alcohol after each experimental session.

Figure 3. VPI with straps.

Figure 4. VPI inserted balloon straps removed.

Procedure

The experiment consisted of a single session that took place in a sound-attenuated laboratory room at the university. After we obtained informed consent, the participant was led to the test room where she received detailed information about the setup of the experiment. The participant was instructed to stop the vaginal pressure as soon as the pressure felt unpleasant by use of a button. Then the participant was left alone to prepare herself using audio support from the research assistant if needed. When the participant was ready to start, the computer program started. To acclimatize the participant, the experiment started with the presentation of a neutral acclimatization film with pressure induction. Because the data of the acclimatization trial are not critical for the aims of the study, these data are not included in the present article. The experiment continued with one of the HA sexual films without vaginal pressure, followed by four randomized conditions with pressure: an HA sexual film, an LA sexual film, an HA nonsexual film, and a neutral film. Which of the two HA sexual films was paired with the VPI was counterbalanced between participants (see Figure 5).

Figure 5. Procedure experiment. HA = High Arousal, LA = Low Arousal.

Each condition started with short written information about that condition and a reminder to check the fastening of the instrument (VPI). In all conditions with pressure induction, two minutes after the start of the film the vaginal balloon was gradually filled until the participant pressed a button in order to terminate the pressure. By pressing the button, the movie stopped and the VPI balloon was immediately deflated to its initial level by draining the water. If the participant did not press the button, the movie ended after 5 minutes (i.e., 3 min. of vaginal pressure). To prevent carry-over effects, in between films women completed a letter-span working memory task of 5 minutes. Then the next film excerpt was presented with the same procedure, until all conditions were administered.

In order to control the instruments, the level of vaginal pressure and temperature ratings were shown by a feedback bar on the monitor of the researcher. At the end of the experiment, a short exit interview took place to probe women's reactions to the experimental procedure followed by a questionnaire regarding the film clips. The total duration of the experimental session was about 1.5 hours.

Dependent measures

Threshold of unpleasant vaginal pressure in seconds

The time taken to reach the threshold of unpleasant vaginal pressure (TvP) from start pressure to stop pressure is reported in seconds (TvPs). When the button was not pressed, the TvP was coded as 180 seconds (maximal duration of the film with pressure).

Subjective ratings

After each condition the participant was asked to rate the subjective experience of pleasurable and painful vaginal pressure sensations (“Watching the film to which level did you experience the following feeling? (“pleasurable/ painful pressure on the vagina”) and the subjective sexual arousal (“At this moment, to which level do you feel sexually aroused?” on three 7-point Likert scales with the extremes on a 7-point scale not at all to very strong (range:1–7). Higher scores indicates higher levels of pleasurable/ painful pressure sensations and subjective sexual arousal.

Statistical analyses

To calculate the influence of the four conditions (HA sexual, LA sexual, HA nonsexual, neutral) on the output measures (subjective sexual arousal, unpleasantness thresholds, and the ratings of pleasurable and painful vaginal pressure), we conducted repeated measure ANOVA with condition as a within-subjects factor. If the Mauchly's test of sphericity indicated that the assumption of sphericity had been violated for the four conditions, we corrected degrees of freedom using Greenhouse-Geisser estimates of sphericity (ϵ =.33). Contrasts were used to compare the conditions to each other.

Results

Feasibility and tolerability

The VPI was well tolerated by the women. None of the women showed negative reactions or interrupted the experiment. The women valued the experiment on a 10-point Likert scale ranging from unpleasant to pleasant, with a mean score of 7.2 (range: 1–10). Exit interviews revealed mainly positive reactions of women concerning the experiment varying from “a bit weird,” to “stimulating” and “interesting”; with respect to the vaginal pressure, reactions ranged from “not very pleasant” to “doable.”

Descriptive data

Fifty-five women were interested in participation in the current study. Eleven women were excluded due to no relationship (eight), use of antidepressants (two), and no response (one). After inclusion, two women dropped out due to illness, leading to a sample of 42 women. One experimental session was terminated prematurely due to sound problems. Due to technical problems, output data of the unpleasantness thresholds of two women were not saved. Consequently, subjective ratings were obtained from 41 women, and complete data sets (including the thresholds and intravaginal pressure) were obtained from 39 women.

The mean age of the women was 24 years (range: 17–35, SD = 6 years), and the women were in general well educated (54% higher education). Thirty-eight women used hormonal contraception. The mean score of the Female Sexual Functioning Index (Rosen et al., 2000; ter Kuile, Brauer, & Laan, 2006) was 30.4 (range: 19–35; SD = 3.4), indicating that sexual functioning was within a normal range (Rosen et al., 2000).

Mean mmhg (millimeter of mercury) values across conditions varied between 111.29 mmhg at start of the film clip (initial value vaginal pressure) to 279.35 mmhg when the pressure was terminated, with a maximum measured value of 473.35 mmhg. Mean, minimum, and maximum values of the unpleasantness thresholds (in seconds), the intravaginal pressure at threshold time (in mmhg), and the subjective ratings of pleasurable and painful pressure and sexual arousal across conditions are shown in Table 1.

Table 1. Unpleasantness thresholds, intravaginal pressure and subjective ratings by condition.

	HA Sex		LA Sex		HA Non-Sex		Neutral +		HA Sex
	+ Pressure		+ Pressure		+ Pressure		Pressure		No Pressure
	Mean	(SD)	Mean	(SD)	Mean	(SD)	Mean	(SD)	Mean	(SD)
Unpleasantness threshold (in seconds)	77.3	(62.1)	79.3	(64.6)	70.1	(59.5)	67.3	(56.5)	—	—
Min–Max	5.73–180		5.88–180		2.25–180		4.57–180		—	—
Intravaginal Pressure (in mmhg) at threshold time	270.9	(54.9)	279.4	(53.9)	265.1	(42.9)	263.2	(46.6)	120.9	(24.6)
Min–Max	200.5–435.6		193.0–398.6		204.2–364.7		197.9–391.1		40.8–164.4
Pleasurable pressure (1–7)	4.24	(1.66)	3.44	(1.6)	2.33	(1.62)	2.81	(1.6)	3.02	(1.72)
Painful Pressure (1–7)	2.49	(1.49)	2.61	(1.6)	2.81	(1.6)	2.81	(1.54)	1.15	(0.42)
Subjective Sexual Arousal (1–7)	4.15	(1.51)	2.73	(1.4)	1.81	(1.1)	1.98	(1.35)	3.49	(1.44)

Note. HA Sex + Pressure= High Arousal Sex film with Pressure; LA Sex + Pressure = Low arousal Sex film with Pressure; HA Non-Sex + Pressure = High Arousal Non- Sexual film with Pressure; HA Sex No Pressure = High Arousal Sex film without Pressure; Mean = Mean rates from start pressure to stop film (pressure). SD = Standard Deviation Min = Minimum; Max = Maximum.

Manipulation check sexual stimuli

There was a significant main effect of condition on sexual arousal, F(2.5, 100.07) = 56.11, p <.001, r =.36. Contrasts revealed that ratings of sexual arousal of the HA sexual film, F(1, 40) = 84.18, p <.001, r =.68, and the LA sexual film, F(1, 40) = 16.29, p <.001, r =.29, were significantly higher than the ratings of the neutral film; the ratings of sexual arousal of the HA sexual film were significantly higher than the ratings of the LA sexual film, F(1, 40) = 41.05, p <.001, r =.5.

Unpleasantness threshold

There was a significant main effect of condition on the threshold of unpleasant vaginal pressure (TvPs), F(2.38, 90.51) = 4.76, p <.01, r =.05. Contrasts revealed that compared to the neutral film, both the HA sex film, F(1, 38) = 4.95, p <.05, r =.12, and the LA sex film, F(1, 38) = 7.91, p <.01, r =.17, increased the threshold of unpleasant vaginal pressure. When compared to the HA nonsex film, the threshold of unpleasant vaginal pressure was also increased by both sex films: the HA sex film, F(1, 38) = 5.07, p <.05, r =.12, and the LA sex film, F(1, 38) = 7.17, p =.01, r =.16. Although the tolerance of the vaginal pressure increased in the context of sexual arousal, no cross-conditional differences were found in the measured level of intravaginal pressure.

Pleasurable and painful pressure

There was a significant main effect of stimulus type on pleasurable vaginal pressure, F(2.06, 82.44) = 30.4, p <.001, r =.27. Contrasts revealed that the ratings of pleasurable vaginal pressure of the HA sexual film, F(1, 40) = 30.85, p <.001, r =.44, and the LA sexual film, F(1, 40) = 15.89, p <.001, r =.28, were significantly higher than the ratings of the neutral film; the ratings of the neutral film, F(1, 40) = 9.24, p <.01, r =.19, were significantly higher than the ratings of the HA nonsexual film; the ratings of the HA sexual film were significantly higher than the ratings of the LA sexual film, F(1, 40) = 14.27, p ≤.001, r =.26. There was no significant main effect of condition on painful vaginal pressure.

Vaginal pressure and subjective sexual arousal

Additionally, we explored the influence of vaginal pressure on sexual arousal. A paired-samples t test revealed that the HA sexual film with pressure evoked significantly higher levels of subjective sexual arousal, t(1, 40) = 3.66, p =.001, d = 0.57, than the HA sexual film without pressure.

Discussion

The current study aimed at testing a new device, the Vaginal Pressure Inducer (VPI), which was developed to gradually induce increasing vaginal pressure in a standardized and controlled manner in the vaginal introitus. Because this is a first exploratory study aiming to validate the instrument, we focused specifically on the influence of sexual stimuli on the threshold of unpleasant vaginal pressure and the level of pleasurable and painful vaginal pressure in a sexually functional sample of women. The results confirmed that unpleasantness thresholds as well as ratings of pleasurable vaginal pressure increased in the context of sexual stimuli when compared to the neutral condition. Comparing HA sexual stimuli to LA sexual stimuli, the results confirmed that HA sexual stimuli are associated with higher ratings of pleasurable vaginal pressure, but HA sexual stimuli were not associated with heightened unpleasantness thresholds of vaginal pressure. Only HA sexual stimuli had a positive effect on the experience of vaginal pressure, whereas HA nonsexual stimuli negatively affected ratings of pleasurable vaginal pressure, indicating the specificity of our effects. Furthermore, the results suggested that vaginal pressure in the context of an HA sexual film increased subjective sexual arousal compared to an HA sexual film without pressure. The results showed that gradually increasing vaginal pressure, as applied by the VPI, was well tolerated by the women. The results provide initial support for the VPI as a device to measure the threshold and pleasurable appraisal of vaginal pressure in the context of sexual stimuli.

The finding that sexual stimuli increased the unpleasantness threshold and pleasurableness of vaginal pressure opposes the pattern of earlier studies showing that sexual arousal increased genital (pain) sensitivity (Paterson et al., 2013; Payne et al., 2007), while vaginal vibratory sensation (Gruenwald et al., 2007) and pleasurable ratings remained unchanged (Paterson et al., 2013). These contradictory findings may partly be attributed to methodological differences across studies. The VPI is the first instrument by which genital pressure can be applied in private by means of a remotely controlled device. Studies thus far required the presence of a research assistant applying the sensory tests. This may have moderated the outcome unintentionally, for example, by inducing heightened self-awareness, distraction from sexual stimuli, or negative affect (e.g., shame). Research has shown that female sexual arousal is context-dependent and thus highly influenced by external laboratory cues (Basson, 2002; Heiman, 1980; van Lankveld et al., 2014). These methodological differences may preclude a direct comparison between the current data of the VPI and extant measurements of genital sensitivity.

Note that the current study is the first to demonstrate elevated unpleasantness thresholds and ratings of pleasurable vaginal pressure as a function of subjective sexual arousal in a sexually functional sample. This might be a clinically relevant finding, since genuine vaginal pressure apparently is appraised as pain in genital pain patients (Farmer et al., 2013). It might be interesting to further explore this option in a future study by comparing women with and without genital pain. This may provide targets for intervention and help to develop evidence-based personalized interventions of genital pain. The finding that an increase of subjective sexual arousal is associated with increased ratings of pleasurable vaginal pressure in the context of HA sexual stimuli, compared to LA sexual stimuli, confirms the positive influence of higher levels of subjective sexual arousal on the appraisal of genital pressure sensations. Additionally, indications were found that vaginal pressure may heighten subjective sexual arousal during HA sexual films, which may implicate that vaginal pressure can potentiate sexual arousal in sexually functional women.

The findings illustrate the sensitivity of the VPI to differentiate the level of pleasurableness of vaginal pressure as a function of different arousal levels. This suggests that the VPI seems well suited to assess the influence of several vulnerability factors on the appraisal of vaginal pressure and the relationship with genital pain. The VPI may also be useful for diagnostic and treatment purposes, for example, learning to associate high levels of subjective sexual arousal to gradually increasing levels of vaginal pressure (counterconditioning). The prolonged and gradual increase of vaginal pressure, together with the remote control of the VPI, provide a suitable instrument to assess vaginal pressure tolerance in a sexual context.

Limitations

One limitation of the current study is the experimental design. Considering that sexual arousal in women is highly context-dependent, we expected that the experience of vaginal pressure is probably highly related to intrapersonal and relationship factors, which are not included in the current study. To receive more information about the ecological validity of the experienced vaginal pressure, we will ask participants to what degree their experience with the VPI was consistent with their experience of vaginal pressure during intercourse outside of the laboratory. To increase the external validity of the VPI, the influence of partners' presence in the lab will be assessed in future studies.

In order to assess the suitability of the VPI and given the invasive nature of the instrument, we decided to start the experiment with an acclimatization trial followed by an HA sexual film without pressure. Concerning the possibility of a priming effect, the data on the influence of vaginal pressure on (sexual) affect should be interpreted with caution. Future studies should confirm this finding using a fully randomized order of presentation. Additionally, the test–retest reliability of the VPI has to be further investigated.

The current study focused on the affective dimension of pain using a within-subject design by assessing the influence of subjective sexual arousal on the appraisal of vaginal pressure. To prevent eliciting a hypervigilance response toward possible pain sensations in participants, we measured the unpleasantness thresholds and not pain sensitivity by gradually increasing vaginal pressure. To provide more insight in the relationship between the (un)pleasurableness of vaginal pressure and genital pain sensitivity, the VPI could also be used to obtain objective data for pain sensitivity (the amount of pumped water, ml, along with the measured intravaginal pressure, mmhg) in future studies.

A restriction of the VPI is that the instrument is hard to combine with physical indices of genital arousal. Future research needs to explore other types of physical arousal measures that may be compatible with the VPI, such as oxygenation-temperature method, lubrication, and the vaginal pulse amplitude (VPA), to get a more complete picture of the role of sexual arousal in relation to vaginal pressure.

Conclusions

The current findings indicate the importance of sexual stimuli for the heightening of unpleasantness thresholds as well as the increase of pleasurableness of vaginal pressure. Explicit sexual stimuli fortified the pleasurableness of vaginal pressure paralleled by an increase of subjective sexual arousal. The results provide initial support of the VPI as a device to investigate various determinants of the tolerance and pleasurable appraisal of vaginal pressure in a context of sexual arousal.