The Clitoral Photoplethysmograph: A New Way of Assessing Genital Arousal in Women

  • Jeroen Gerritsen, MSc, (Emotional Brain BV—R&D FSD, Almere, the Netherlands)
  • Flip van der Made, MD, (Emotional Brain BV—R&D FSD, Almere, the Netherlands, Department of Gynecology and Obstetrics, Flevoziekenhuis, Almere, the Netherlands)
  • Jos Bloemers, MSc, (Emotional Brain BV—R&D FSD, Almere, the Netherlands)
  • Diana van Ham, MSc, (Emotional Brain BV—R&D FSD, Almere, the Netherlands)
  • Gunilla Kleiverda, MD, PhD, (Department of Gynecology and Obstetrics, Flevoziekenhuis, Almere, the Netherlands)
  • Walter Everaerd, PhD, (Department of Social Sciences, Clinical Psychology, University of Amsterdam, Amsterdam, the Netherlands)
  • Berend Olivier, PhD, (Department of Psychopharmacology, Utrecht Institute for Pharmaceutical Sciences, Utrecht, the Netherlands, Rudolf Magnus Institute of Neuroscience, Utrecht University, Utrecht, the Netherlands, Department of Psychiatry, Yale University School of Medicine, New Haven, CT, USA)
  • Roy Levin, PhD, (Sexual Physiology Laboratory, Porterbrook Clinic, Sheffield, Yorkshire, England)
  • Adriaan Tuiten, PhD (Emotional Brain BV—R&D FSD, Almere, the Netherlands, (Department of Gynecology and Obstetrics, Flevoziekenhuis, Almere, the Netherlands, Department of Psychopharmacology, Utrecht Institute for Pharmaceutical Sciences, Utrecht, the Netherlands)

DOI: 10.1111/j.1743-6109.2009.01228.x

Abstract

Introduction.

In the present study, we introduce clitoral photoplethysmography as an instrument to assess clitoral blood volume (CBV). In research on female sexual functioning, vaginal pulse amplitude (VPA), as measured using vaginal photoplethysmography, has been used extensively as a measure of vaginal vasocongestion. Measurement of clitoral blood flow has thus far been problematic, mainly because of methodological constraints.

Aim.

To demonstrate that CBV is a valuable, easy to use complementary measure for the female sexual response, offering additional information to the VPA.

Methods.

Thirty women with and without female sexual dysfunction (FSD) watched neutral and erotic film clips. At the end of the erotic clip, the session was interrupted to induce inhibition of the sexual response. Another neutral clip followed the interruption. VPA and CBV were measured simultaneously, as well as skin conductance levels (SCLs), to assess the amount of sympathetic activity.

Main Outcome Measures.

VPA, CBV, SCL.

Results.

For both FSD and non-FSD women, VPA and CBV increased when sexually explicit material was presented. Changes in skin conductance significantly predicted changes in CBV (b = -0.61, t1 = -3.88, P < 0.001), but not in VPA. A large increase in sympathetic activity was accompanied by a large decrease in CBV. Furthermore, a large increase in CBV at the end of the erotic film clip presentation, as compared with the neutral clip, was accompanied by a relatively small increase in VPA (b = -0.39, t2 = -2.25, P < 0.033).

Conclusion.

CBV is a valid and sensitive tool to measure the female genital response. In the present study, it was particularly useful in investigating sexual inhibition, when used in combination with SCL. Furthermore, high CBV appeared to inhibit VPA, suggesting that VPA reflects an automatic preparatory response rather than genital arousal per se.

Gerritsen J, van der Made F, Bloemers J, van Ham D, Kleiverda G, Everaerd W, Olivier B, Levin R, and Tuiten A. The clitoral photoplethysmograph: A new way of assessing genital arousal in women. J Sex Med 2009;6:1678–1687.

Key Words. Clitoral Photoplethysmography; Clitoral Blood Volume; Vaginal Pulse Amplitude; Sympathetic Activation

Introduction

Research on female sexual dysfunction (FSD) has featured several techniques for assessing physiological sexual responses. The most widely used among these is vaginal photoplethysmography 3, which can be used to measure either vaginal blood volume (VBV) or vaginal pulse amplitude (VPA). Of these two measures, VPA has proven to be the most valid 4.

In the current study, we introduce clitoral photoplethysmography as an instrument to assess clitoral blood volume (CBV), in a way very similar to VPA measurement. We aim to demonstrate that it is a user-friendly device, which yields an interesting complementary measure for the female sexual response. We have modified the vaginal photoplethysmograph to include an additional probe, which follows the shape of the vulva and which is placed between the labia minora. In this way, both VPA and CBV can be measured simultaneously. Sensitivity to changes in different aspects of the female genital response can thus be compared. This was investigated by presenting neutral and erotic film clips to 30 female subjects.

Besides vaginal vasocongestion, clitoral blood flow is of particular interest in studying the sexual response. Erectile tissue in the clitoris is in many ways analogous to that found in the male penis. In understanding male sexual (dys)function, research has focused primarily on erectile function. Doppler ultrasonography, for instance, has become an important measure to assess vascular changes in erectile tissue 56. It would be interesting to have a comparable measure for genital arousal in women. However, whereas VPA can be measured without the presence of an experimenter handling the device, this is not possible with clitoral Doppler ultrasonography 78, three-dimensional ultrasonography 9, magnetic resonance imaging (MRI) 10, or clitoral electromyography (EMG) 11. Such techniques will therefore be too intrusive, as the presence of the experimenter will most probably inhibit the sexual response and thus be a confounding factor.

Besides parasympathetic stimulation, a decrease in sympathetic activity is known to have a positive impact on the genital response [10–12]. Therefore, we decided to measure skin conductance levels (SCLs), along with genital blood flow. Yilmaz et al. [13] have shown that there is a strong relationship between sympathetic activity as measured from the palm of the hand and the clitoral bulb, the latter being measured using clitoral EMG. Sympathetic arousal levels were manipulated in the current experiment by interrupting the presentation of the erotic film clip. A loud noise via the intercom and the unexpected nature of the interruption— subjects believed they were alone when watching the explicit sexual videos—is assumed to yield an increase in sympathetic activity and inhibit the sexual response.

CBV is expected to be elevated during exposure to sexually explicit material in a way similar to VPA.

A decrease in sympathetic activity, which is supposedly necessary for blood volume to increase, would be reflected in lower SCLs during the presentation of the erotic film clips. During the interruption, an increase in sympathetic activity, as reflected by a higher SCL, is expected to be accompanied by a decrease in genital response. Extending the analogy to the penile erection in the male, it is expected that any inhibiting effect of the interruption will be more quickly visible and more pronounced in the CBV, as compared with the VPA.

As described further in more detail, we did indeed find an inverse correlation between SCL and CBV, and no such correlation between SCL and VPA. Furthermore, large VPA responses to sexual stimuli predicted relatively small CBV responses, and vice versa.

Method

Participants

Thirty-two heterosexual women were recruited for participation in this study (see Table 1). They were selected from our database of women who had participated in previous studies. The database contained relatively few healthy control subjects, all of which were contacted. Table 1 reveals no substantial differences between them and FSD patients in terms of age and testosterone levels. Twenty-two subjects were diagnosed with FSD (i.e. hypoactive sexual desire disorder [HSDD] and/or female sexual arousal disorder [FSAD], according to the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision [DSM-IV-TR] criteria). Subjects had been interviewed by an experienced psychologist and a gynecologist during the screening visit of the previous study (studies), and had been diagnosed using the DSM criteria. Additionally, they filled out the Sexual Functioning Questionnaire (SFQ). Women diagnosed as suffering from HSDD and FSAD for at least 6 months prior to the screening visit were included, provided they did not have any significant medical illness. During the previous studies, subjects received a single dose of an experimental treatment, which is assumed not to influence the current results. None of the subjects were using oral contraceptives. No experimental days were planned when subjects (FSD and non-FSD) were menstruating.

To exclude major medical and psychiatric illnesses, a general medical and gynecological anam- nesis was taken. Weight, height, blood pressure (supine and standing), heart rate, respiration rate, and body temperature were measured. A supine, 12-lead electrocardiogram was recorded and examined by a physician (and, if necessary, a cardiologist). A gynecological examination and a urine pregnancy test were performed to exclude pregnancy, vaginal infections, major surgery on the vagina and/or vulva, undetected major gynecological illnesses, or unexplained gynecological complaints. Cultures were taken to exclude Chlamydia or Gonococcus infections. Standard hematology tests were performed. Additionally, subjects were asked about childhood sexual abuse and other negative sexual experiences.
All subjects signed a written informed consent and received reimbursement for expenses for their participation. Two subjects (one FSD) were excluded for analysis because of technical problems during the experimental session, resulting in severe data loss. Additionally, SCL data for three subjects (one FSD) were missing, because of problems with the electrode setup. VPA and CBV data were thus analyzed for 30 subjects, and SCL data were ana- lyzed for 27 subjects. This study and the previous studies were carried out in agreement with Inter- national Conference on Harmonisation—Good Clinical Practice.

Apparatus and Stimuli

VPA was measured using a vaginal photoplethys- mograph, a tampon-shaped device containing a red light-emitting diode (LED, visible spectrum) and a photosensitive light detector (photodiode). The cabling is protected with silicon tubing. An addi- tional clitoral photoplethysmograph is attached to

the silicon tube, along with a silicon placement device (see Figure 1), holding it in the correct ana- tomical position. The shape of the clitoral probe follows the anatomical curves of the area surround- ing the urethral opening up to the clitoris, between the labia minora and just above the introitus. The distance between the placement device and the vaginal photodiode is 5 cm, and the vaginal probe is rotated 30° clockwise, as seen from behind the placement device. The LED and photodiode for the clitoral probe are located in an external connec- tion box. Signals are sent to and received from the probe through optical cables. In order to target the left clitoral bulb, the optical cables enter the probe at an angle of 45°, pointing to the right, as seen from behind the placement device. Positioning of the probes, as well as signal integrity, was tested during several pilot sessions. It was found that, like the common vaginal plethysmograph, the new device could be inserted by the participant and could yield a reliable and valid signal. Moreover, the CBV signal proved to be especially robust to movement artifacts. Like all subjects in the actual experiment, volunteers for the pilot study were asked afterward whether the probes fit comfortably and no prob- lems were noted.
SCL was recorded by means of two silver elec- trodes placed on the palm of the left hand. All signals were amplified and recorded using BioSemi ActiveTwo equipment (BioSemi, Amsterdam, The Netherlands) (electrodes, amplifier, and recording software).

For this study, neutral and erotic film clips were used. Neutral clips were a 10-minute clip containing concert footage (two versions) and two 6-minute clips of a nature documentary; erotic clips were 6-minute clips (two versions) showing cunnilingus and coitus, including visible penetra- tion. The erotic film footage was selected by female researchers and was edited in order to meet the following criteria: only heterosexual couples were shown, visible vaginal intercourse was shown within 30 seconds following onset of the clip, and foreplay that consisted of cunnilingus only. Strok- ing or kissing of the penis was not included, as these behaviors have been shown to be rated as less arousing by females [14]. The edited clips were judged by the other female investigators. All digitally sampled film clips were presented using Neurobehavioral Systems’ Presentation software (Neurobehavioral Systems, Albany, CA, USA).

The experiment took place in a closed, dimly lit, and soundproof experimental room containing the signal amplifier and a computer screen on which the film clips were presented. Subjects were seated in a comfortable chair and were provided with a blanket to cover their lap, in order to prevent external light from interfering with the measure- ment. An intercom was present to allow for a two-way communication (on demand) in case of possible problems or additional instructions.

Procedure

Upon entering the laboratory, SCL electrodes were applied to the subject’s hand and were con- nected to the amplifier. The signal was then checked, after which the subject was left alone in the experimental room and was instructed to insert the probes. When the subject notified the experimenter through the intercom that the probes were in place, signals were checked and the experiment was started.

The session started with a 10-minute neutral film clip (music) to allow VPA and CBV to reach baseline levels, following insertion of the probes. The actual experiment then started with a 6-minute neutral film clip (nature), followed by a 6-minute erotic clip. At the end of the erotic clip, the subject was called through the intercom to inform her of a fabricated computer problem and was told that the session would resume shortly. The final neutral film clip (6 minutes, nature) was then presented. At the end of the session, the par- ticipant was allowed to remove the probes and leave the experimental room.

Participants were randomly assigned one of two versions of the experimental protocol, each con- taining different concert and erotic film clips and a different presentation order of the nature docu- mentary clips, in order to minimize effects caused by either the order of presentation or the contents of any specific erotic or musical film clip. Procedures for both versions were identical.

Data Reduction and Analysis

VPA

The VPA reflects phasic changes in vaginal engorgement corresponding with each heartbeat. VPA was defined as the peak-to-trough amplitude of the pulse wave and was calculated by acquiring the means of all peaks and troughs and by subtract- ing those. Data from the photoplethysmograph were sampled at 256 Hz and were filtered offline (high-pass 1 Hz, 48 dB/oct; low-pass 1.5 Hz, 48dB/oct), in order to isolate the alternating current (AC) component from the direct current (DC) coupled amplifier, and reduce respiration artifacts and high-frequency oscillations. Large movement artifacts (more than 100% increase for a small number of isolated periodic cycles) were manually removed following visual inspection of the data. The data were then divided into 30-second segments for each 6-minute film clip, thus yielding 12 discrete values reflecting VPA during different stages of the film clip. Finally, in order to eliminate interpersonal differences and obtain meaningful data, VPA scores during the erotic clip and the neutral clip following the inter- ruption were related to activity during the first 6-minute neutral clip, using the following formula:

VPArel = ([VPAx − VPAneu ] VPAneu ) × 100,

with VPArel being the relative increase (%) in VPA related to the first neutral clip, VPAx being any 30-second segment value during the erotic and the final neutral clips, and VPAneu being the average VPA score during the first neutral clip.

CBV

CBV was assessed by analyzing the DC component of the signal from the clitoral photoplethysmo- graph. The AC signal proved to be relatively small and insensitive to increasing sexual arousal, when compared with the DC signal. (Because we used a DC coupled amplifier in the experimental setup, both AC and DC components were available for analysis. See Figure 2 for an illustration of the analysis of the DC signal). CBV data were sampled at 256Hz and were filtered offline (low-pass 0.03 Hz, 24 dB/oct). Again, data were divided into 30-second segments. The data were baseline cor- rected for each subject by subtracting the minimum value during the session from the actual values for each segment. The CBV signal proved to be resis- tant to subject movement, especially when com- pared with VPA; therefore, no additional artifact rejection was needed. The square root of each value was calculated in order to further eliminate inter- personal differences in the CBV signal. These values were then used to produce relative scores in a way similar to the VPA procedure, i.e., based on the first 6-minute neutral clip.

CBVcorr_x = √(CBVx −CBVmin ), CBVrel =([CBVcorr_x −CBVcorr_neu]

CBVcorr_neu )×100,

with CBV {check pdf} root of the baseline corrected score, CBVx being any 30-second segment value during the erotic and the final neutral clips prior to correction, and CBVmin being the minimum value of all the segments following the return to baseline clip. CBVrel is the relative increase (%) in corrected CBV related to the first neutral clip, as expressed by CBVcorr_neu, the average corrected CBV score during the first neutral clip.

SCL

Skin conductance data were sampled at 256 Hz and were filtered offline (low-pass 10.73 Hz, 12 dB/oct, and 50-Hz notch filter), after which the data were again divided into 30-second intervals. These values were log-transformed to compensate for interpersonal differences in baseline SCL.

Results

All VPA and CBV scores mentioned in this section are relative changes as described previously. All analyses below are performed for all subjects, because a one-way analysis of variance revealed no significant group differences (FSD vs. non-FSD).

Neutral vs. Erotic Stimuli

As expected, both VPArel and CBVrel showed a significant increase during the erotic film clip presentation, as compared with the first 6-minute neutral fragment. One-sample t-tests revealed that the relative change in VPA deviated significantly from 0 (M = 0.77, t[29] = 6.24, P < .001), as did the relative change in CBV (M = 0.36, t[29] = 3.50, P < 0.002). The SCL, however, did not show such an effect, i.e., it remained unchanged. Note that the relative change is calculated by comparing the erotic clip minus the final 30 seconds, during which the interruption occurred, to the entire neutral clip.

Inhibition

To investigate the effects of the interruption of the session during the viewing of the erotic film clip, relative activity during the final 30 seconds of the erotic clip was subtracted from the relative activity during the first 30 seconds of the consecutive neutral film clip for all measures. Regression analysis was then performed including all subjects, which showed that change in SCL significantly predicted the change in CBV (b = -0.61, t[27] = -3.88, P < .001), with a higher SCL change being accompanied by a greater decrease in CBV (see Figure 3). No such relationship was found between SCL change and VPA change. (Figure 4 shows the relationship between SCL and CBV/ VPA. Subjects are divided on the basis of their SCL scores, see legend).

VPA vs. CBV

Like VPA, CBV responded to the presentation of erotic film clips. Further analysis revealed different responding patterns for both measures. When looking at the supposed peak of sexual arousal during the film clip presentation, the penultimate 30-second segment, just before the interruption, the relative VPA score significantly predicted the relative CBV score (b = -0.39, t[29] = -2.25, P < .033, see Figure 5). R2 values increased when moving toward the end of the trial, and significance was only reached at the very end.

Discussion

In this study, we introduced clitoral photoplethysmography as a new way of measuring the sexual response in the female, which has the advantage that it is suitable for unsupervised use, contrary to other techniques of assessing clitoral blood flow [5–9]. It is therefore possible to measure a broader range of sexual response in a more private and naturalistic environment. The results suggest that CBV is a valuable additional tool, providing data that the vaginal photoplethysmograph cannot: it proved sensitive to inhibition of the sexual response, in contrast to the VPA. Second, the inverse relationship between VPA and CBV at moments of high sexual arousal suggests that VPA may be a more automatic, preparatory response rather than a measure of genital arousal per se. FSD groups in this study did not show significant differences on the measures of genital response, and therefore their effectiveness was assessed by analyzing all subjects.

SCL significantly predicted the CBV response, following the interruption of the experimental session. Subjects showing the largest increase in sympathetic activity showed the greatest decrease in CBV. Interestingly, no such effect was found on VPA. A sufficient decrease in sympathetic tone is necessary for the erectile bodies to fill with blood [10]. Conversely, a sudden increase in sympathetic activity (e.g., orientation or even startle response) appears to cause a contraction of smooth muscle tissue in the clitoral erectile tissue and results in a net outflow of blood. It has previously been shown that sympathetic activation by directly stimulating the paravertebral sympathetic chain in the lumbar area can have antierectile effects in male rats [15]. It is conceivable that a similar pathway exists in the human female, and that it is triggered by environmental/social factors as well. In this case, being disturbed when viewing sexually explicit material will have a startling effect, as will the contact with the experimenter when they were expecting to be alone in the room when watching the erotic film. The distraction or confusion will most likely inhibit the sexual response. Changes in VPA were not found, which indicates that the sympathetic vasoconstrictive influence on the arterial plexus in the vaginal wall is different from its influence on the clitoral erectile tissue. Vasoactive intestinal peptide (VIP) and nitric oxide (NO) are the two major neurotransmitters involved in clito- ral vasocongestion. In the vaginal wall, nitric oxide synthase is only sparsely distributed [16]. It is conceivable that sympathetic stimulation has a greater modulating effect on NO, when compared with VIP, which would explain the lack of vaginal response to sympathetic arousal. A combination of clitoral photoplethysmography and the measurement of electrodermal activity might thus prove to be an indicator for inhibition of the sexual response. These findings are in contrast with Meston and Gorzalka [17], reporting an increase in VPA and VBV responses to erotic film clips following an increase in sympathetic activity. There, the increase in sympathetic activity was achieved by having subjects engage in physical exercise 10 minutes prior to the film clip presentation. It is conceivable that physical exercise and the subsequent prolonged sympathetic activation (i.e., increased blood flow throughout the body) facilitate the sexual response. However, in the context of sexual stimulation, an acute increase in sympathetic activity causes CBV to decrease because of the constriction of smooth muscle tissue in the erectile tissues. Note that in the current study, VPA was not noticeably affected.

A closer look at the VPA and CBV during the presentation of sexually explicit stimuli revealed a negative correlation between the two, most notably (and significantly) at the end of the film clip. A large relative increase in VPA, compared with the neutral clip, was indicative of a small relative increase in CBV and vice versa. We suggest that a feedback mechanism is responsible for reducing blood flow toward the genitalia, when the cli- toral bodies are filling with blood. This effect would be most pronounced in VPA, the pulselike character of which indicates a strong relationship with arterial blood flow toward the genital areas. This idea is supported by the finding that the effect is only found at the very end of the film clip, prior to the interruption, where sexual arousal is presumed to be at its maximum during the trial. The negative feedback would be strongest during exactly that period. This would imply that an increase in VPA actually precedes clitoral engorgement (even though there will be a significant overlap, this being a gradual process). The effect of interrupting the erotic film clip indicates that CBV is not just slower to respond to changes in sexual arousal, as it decreased sharply with increased sympathetic activity, whereas the VPA did not. We suggest that the two measures reflect different processes. It has been shown before that VPA in women reflects a nonspecific sexual response, which does not correlate with subjective reports [18]. In contrast, men did show a genital response more in accordance with their sexual preferences. It should be noted that the male genital response was defined as an increase in penile circumference, which would be comparable to the CBV as measured in the current study, rather than VPA.

A number of things should be taken into account when interpreting these data. First of all, it would be desirable to directly compare both vaginal and clitoral photoplethysmography with a different technique of assessing the genital response, like labial temperature [19], which can also be used unsupervised. Other methods of assessing clitoral hemodynamics, as described previously, have been used as an index of the genital response. Clitoral MRI revealed a clear change in clitoral blood flow following presentation of sexual stimuli, but like the current study, did not clearly discriminate between FSD patients and healthy controls [8]. Doppler ultrasonography did reveal an increase in blood flow after subjects watched an erotic film clip, when compared with a neutral condition, but failed to discriminate between erotic and other positive (humorous) clips [5]. This, combined with the problems inherent to the experimental setup, would make these techniques less useful. Measurements should also be performed over several experimental days to investigate test–retest reliability. Furthermore, a correlation between physiological and subjective measures of sexual arousal would help elucidate some complex issues surrounding sexual dysfunction. It would also be interesting to attempt to replicate the results in research on male sexual function mentioned before. This does, however, introduce a problem, because online monitoring (i.e., during the presentation of the stimuli, like the “arousometer,” described by Rellini et al. [20]) would be the most informative, but focusing attention explicitly on sexual arousal most likely directly influences the sexual response. Finally, it should be noted that in this study, the subject sample was heterogeneous with both FSD patients and healthy controls included in the study. Because of time constraints, we were unable to plan experimental days only during specific periods in the menstrual cycle (although no measurements were performed when they were menstruating). Testosterone levels are found to be fluctuating during the menstrual cycle, peaking around the moment of ovulation [21]. Moreover, both clitoral blood flow and clitoral volume have been found to fluctuate during the menstrual cycle in women who were not sexually aroused [22].

Conclusion

We conclude that CBV appears to be a more informative and more sensitive measure for assessing

the female genital response than VPA. In the present study, it was particularly useful in investigating sexual inhibition, when used in combination with SCL. Furthermore, high CBV appeared to inhibit VPA, suggesting that VPA reflects an automatic preparatory response, rather than genital arousal per se.

Future research using clitoral photoplethysmography may open up the possibility for a more complete understanding of female sexual functioning.

Acknowledgment

  • The authors wish to thank Gert Camfferman (Emotional Brain and Department of Psychopharmacology, Utrecht University) for his valuable efforts in developing and implementing the clitoral photoplethysmograph.
  • Corresponding Author: Adriaan Tuiten, PhD, Emotional Brain, Inc., Louis Armstrongweg 78, 1311 RL, Almere, The Netherlands. Tel: +3154646; Fax: +31- (0)36-5497186; E-mail: [email protected]

Conflict of Interest: Jeroen Gerritsen is psychophysiologist at Emational Brain Inc (EB) and PhD candidate. Flip van der Made is resident Ob/Gyn at the Flevoziekenhuis Almere, The Netherlands. Jos Bloemers is researcher/psychologist at EB and PhD candidate. Diana van Ham is researcher/psychologist at EB and PhD candidate. Gunilla Kleiverda is gynecologist at the Flevoziekenhuis. Walter Everaerd is advisor of EB and supervisor of the PhD-thesis of Flip van der Made, Jos Bloemers and Diana van Ham. Berend Olivier is super- visors of the PhD-thesis of Flip van der Made, Jos Bloemers, and Diana van Ham. Adriaan Tuiten is CEO/ psychologist of/at EB. This study was supported by Emotional Brain Inc.

Statement of Authorship

Category 1

(a) Conception and Design

Adriaan Tuiten; Flip van der Made; Jeroen Gerritsen

(b) Acquisition of Data

Diana van Ham; Jeroen Gerritsen; Flip van der

Made

(c) Analysis and Interpretation of Data

Adriaan Tuiten; Jeroen Gerritsen; Flip van der Made; Jos Bloemers; Walter Everaerd; Roy Levin; Berend Olivier; Gunilla Kleiverda

Category 2

(a) Drafting the Article

Jeroen Gerritsen; Flip van der Made; Diana van Ham

(b) Revising It for Intellectual Content

Roy Levin; Walter Everaerd; Berend Olivier; Jos Bloemers; Adriaan Tuiten; Gunilla Kleiverda

Category 3

(a) Final Approval of the Completed Article

Jeroen Gerritsen; Flip van der Made; Jos Bloemers; Diana van Ham; Gunilla Kleiverda; Walter Ever- aerd; Berend Olivier; Roy Levin; Adriaan Tuiten

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Footnotes

  1. 27
  2. 29
  3. Geer JH, Morokoff P, Greenwood P. Sexual arousal in women: The development of a measurement device for vaginal blood volume. Arch Sex Behav 1974;3:559–64.
  4. Laan E, Everaerd W, Evers A. Assessment of female sexual arousal: Response specificity and construct validity. Psychophysiology 1995;32:476–85.
  5. Golijanin D, Singer E, Davis R, Bhatt S, Seftel A, Dogra V. Doppler evaluation of erectile dysfunction—part 1. Int J Impot Res 2007;19:37–42.
  6. Golijanin D, Singer E, Davis R, Bhatt S, Seftel A, Dogra V. Doppler evaluation of erectile dysfunction—part 2. Int J Impot Res 2007;19:42–8.
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