Psychophysiology, 42 (5), 116 124. Blckwell Publishing Inc. Printed in the USA. Copyright r 5 Society for Psychophysiologicl Reserch DOI: 1.1111/j.1469-8986.5.259.x The reltionship between women s subjective nd physiologicl sexul rousl ALESSANDRA H. RELLINI, KATIE M. MCCALL, PATRICK K. RANDALL, AND CINDY M. MESTON Deprtment of Psychology, University of Texs t Austin, Austin, Texs, USA Abstrct Previous literture presents discordnt results on the reltionship between physiologicl nd subjective sexul rousl in women. In this study, the use of hierrchicl liner modeling (HLM) reveled significnt concordnce between continuous mesures of physiologicl nd subjective sexul rousl s ssessed during exposure to erotic stimuli in lbortory setting. We propose tht pst studies tht hve found little or no ssocition between the two mesures my hve been in prt limited by the methodology nd sttisticl nlyses employed. Descriptors: Femle sexulity, Physiologicl sexul rousl, Subjective sexul rousl, Vginl pulse mplitude, Photoplethysmogrphy Comprehensive ssessment of femle sexul rousl in lbortory setting involves mesurement of both physiologicl nd subjective responses. The reltionship between physiologicl nd subjective sexul responses in women is widely studied but poorly understood phenomenon. The definition of sexul response presented by the Dignostic nd Sttisticl Mnul of Mentl Disorders (DSM-IV-TR; Americn Psychitric Assocition, ) includes only physiologicl spects of sexul rousl: The rousl response consists of vsocongestion in the pelvis, vginl lubriction nd expnsion, nd swelling of the externl genitli (p. 543). The ssumption underlying this definition is tht physiologicl nd subjective sexul rousl re interdependent spects of the sme underlying ltent construct of sexul rousl (Bollen & Lennox, 1991). However, clinicl observtions nd lbortory studies hve often pointed to desynchrony in subjective nd physiologicl sexul rousl in women, but not in men (Ln & Evererd, 1995). In response to these results, Bsson et l. (1) hve proposed new clssifiction of femle sexul rousl dysfunction tht chllenges the concept of subjective nd physiologicl sexul rousl s inseprble events in women. This study ims to further explore the reltionship between physiologicl nd subjective sexul rousl by utilizing continuous mesures of the two vribles nd dt nlysis Ptrick K. Rndll is now t the Medicl University of South Crolin. This publiction ws mde possible by Grnt Number 5 RO1 AT224-2 from the Ntionl Center for Complementry nd Alterntive Medicine to Cindy Meston. Its contents re solely the responsibility of the uthors nd do not necessrily represent the officil views of the Ntionl Center for Complementry nd Alterntive Medicine. Address reprint requests to: Cindy M. Meston, Ph.D., University of Texs t Austin, Deprtment of Psychology, One University Sttion A8, Austin, TX 78712, USA. E-mil: meston@psy.utexs.edu. 116 strtegy tht is more sensitive to individul differences thn trditionl methods used in previous studies. To dte, the ssessment of physiologicl sexul rousl in women hs focused primrily on detecting chnges in vginl blood flow. Vginl engorgement occurs in unison with vginl lubriction nd together these provide the first mesurble physiologicl signs of sexul rousl in women. The three primry mens of ssessing vginl blood flow re vginl photoplethysmogrphy, indirect mesures of het dissiption, nd pulsed wve doppler ultrsonogrphy (for review, see Meston, ). The most frequently used of these is vginl photoplethysmogrphy, technique introduced by Sintchk nd Geer (1975). The vginl photoplethysmogrph is cler crylic, tmpon-shped device used to detect engorged nd unengorged tissue. The vginl probe ws designed to be esily inserted by the subject, nd positioning shield cn be plced on the probe s cble to stndrdize the depth of insertion between uses (Ln, Evererd, & Evers, 1995). The most relible nd sensitive (Ln, Evererd, & Evers) component of the signl is vginl pulse mplitude, which is believed to reflect phsic chnges in vginl engorgement with ech hert bet, mening tht higher mplitudes indicte greter engorgement (e.g., Geer, Morokoff, & Greenwood, 1974). The dependent vrible typiclly used is the mplitude of the pulse signl, which is mesured from the pek to the trough of the pulse wve. Anlyses of vginl pulse mplitude re usully conducted by verging cross specific stimulus presenttions, cross the highest 3 seconds of rousl, or cross selected time intervls. With regrd to the ssessment of subjective sexul rousl, mesurement usully involves sking the womn bout her subjective experience of rousl during prior sexul scenrio. In lbortory settings, the womn typiclly responds to questions bout whether (nd to wht degree) she felt sexully roused
Self-reported nd physiologicl femle sexul rousl 117 or turned on during the preceding erotic film clip. A commonly used instrument is 7-point Likert film scle introduced by Heimn nd Rowlnd (1983) tht hs since been dpted nd expnded upon by mny reserchers (e.g., Meston & Gorzlk, 1995). These questionnires sk bout feelings of subjective sexul rousl, physicl sexul rousl (e.g., wrmth in genitls, genitl wetness or lubriction), positive ffect, negtive ffect, utonomic rousl (e.g., fster brething, fster hertbet), nd nxiety. Additionlly, other reserchers hve relied on nswers to women s rtings of strongest genitl senstions (e.g., Henson, Rubin, & Henson 1979), overll sexul rousl (e.g., Ln, Evererd, vn Bellen, & Hnewld, 1994), nd romntic feelings (e.g., Wilson & Lwson, 1976) to indicte subjective sexul rousl. In response to criticisms tht self-report mesures of subjective sexul rousl rely on retrospective recll of sexul feelings, some reserchers hve developed techniques for the continuous ssessment of sexul responding. Wincze, Hoon, nd Hoon (1977) were the first to employ this type of methodology by using lever mounted to tble, which could be moved by the prticipnt throughout the presenttion of nonsexul nd sexul films to indicte levels of rousl. Ln nd collegues hve lso employed continuous mesures of subjective sexul rousl in lbortory studies investigting femle sexul rousl (e.g., Ln, Evererd, vn Anhold, & Rebel, 1993; Ln, Evererd, vn der Velde, & Geer, 1995). The technique used in these studies involved mechnicl lever tht produced continuous signl connected to pnel of red light bulbs locted underneth the TV monitor. In Ln et l. s (1993) study, vginl pulse mplitude ws smpled every 13 s nd continuous subjective sexul rousl ws smpled every 1 s. Ln, Evererd, vn der Velde, et l. (1995) lso used the subjective lever, but instructed prticipnts to monitor their genitl senstions. Vginl pulse mplitude nd continuous mesures of genitl senstions were verged for 1-min film excerpts. Independently, both responses of subjective sexul rousl (e.g., Both, Evererd, & Ln, 3; Meston & Worcel, 2), nd physiologicl mesures of vginl pulse mplitude (e.g., Ln, Evererd, & Evers, 1995) hve shown to relibly increse with exposure to erotic stimuli. Correltions between subjective nd vginl pulse mplitude mesures hve rnged from low to moderte (e.g., Geer et l., 1974; Ln et l., 1994; Meston & Gorzlk, 1995). Using Likert scle to mesure subjective sexul rousl, correltions with vginl pulse mplitude hve rnged from.29 (Dekker & Evererd, 1988) to.76 (Henson, Rubin, & Henson, 1979) for sexully functionl women. Using continuous mesures of sexul rousl, Ln et l. (1993) reported between-subjects correltions of vginl pulse mplitude nd subjective sexul rousl tht were low overll nd sttisticlly nonsignificnt (r 5.26 to r 5.46). Given tht the reltionship between erectile response nd subjective sexul rousl yields high correltions in men (e.g., Ln & Evererd, 1995), the recurrent finding of low correltions between vginl pulse mplitude nd subjective sexul rousl in women hs led reserchers to question whether physiologicl chnges tht occur in the bsence of subjective sexul experience should even be considered sexul response. A number of explntions hve been proposed to ccount for the low concordnce noted between mesures of sexul rousl in women. Heimn (1976) hypothesized tht it my reflect n inbility for women to detect subtle chnges in vginl blood flow. Indeed, some studies hve noted tht higher levels of physiologicl sexul rousl result in improved response concordnce (e.g., Meston & Heimn, 1998). However, study found tht the degree of correspondence between subjective nd physiologicl sexul rousl mesures ws dependent upon detecting genitl chnge but independent of the strength of the genitl response (Ln, Evererd, vn der Velde, et l., 1995). Negtive ffect due to mn-mde erotic hs lso been tested s potentil explntion for the low concordnce between subjective nd physiologicl sexul rousl. Ln et l. (1994) reported tht women showed significntly higher subjective sexul responses to womn-mde films s compred to mn-mde films, but the reltionship between subjective nd physiologicl sexul rousl did not vry between types of film. Consistent with this finding, negtive nd positive mood induction chnged neither subjective nor physiologicl sexul responses (Ln, Evererd, vn Berlo, & Rijs, 1995). Demnd chrcteristics re nother fctor tht could fesibly ply role in the reported low concordnce between physiologicl nd subjective mesures of sexul rousl, mening tht there could be report bis in subjective sexul rousl for women. In study where demnd chrcteristics were mnipulted by sking group of women to become s sexully roused s possible, the correltions between subjective nd physiologicl sexul rousl were not significntly different in the demnd versus nondemnd conditions (Ln et l., 1993). Previous reserch indictes tht women my estimte their sexul rousl ccording to cues other thn chnges in genitl blood flow. Severl res of reserch hve provided evidence for the notion tht women my ttend to externl stimulus informtion s opposed to internl physiologicl sttes to determine their level of sexul rousl. Pennebker nd Roberts (1992) noted tht men re consistently more ccurte thn women t detecting physiologicl chnges (e.g., hert rte nd blood pressure). In fct, Korff nd Geer (1983) found tht the correltion between genitl nd subjective rousl could be improved by instructing women to specificlly ttend to genitl cues or overll bodily cues. Another vible explntion for the lck of concordnce often observed between mesures of women s sexul rousl reltes to the dt hndling used to ssess the ssocition between physiologicl nd subjective sexul rousl. Mny studies hve smpled severl dt points for vginl pulse mplitude nd then computed n verge of these points. This computed verge hs then been used in correltionl nlyses with single Likert-scle question intended to ssess subjective experience of sexul rousl (e.g., Brody, Ln, & vn Lunsen, 3; Geer et l., 1974; Meuwissen & Over, 1991; Morokoff & Heimn, 198) or men composite of severl Likert-scle questions of subjective sexul rousl (e.g., Meston & Gorzlk, 1995). This methodology significntly reduces the richness of the dt nd does not llow for n ssessment of how chnges in one mesure my be ssocited to chnges in the other mesure. There re severl possible sttisticl resons why mny previous studies hve filed to find significnt reltionship between physiologicl nd subjective sexul rousl. On verge, the smple size of such studies hs been pproximtely prticipnts, with some studies reporting on smples s smll s 6 women (Wincze et l., 1977). Although smll smple sizes re often pproprite when looking t physiologicl mesures with low error vrince, introducing Likert dt reduces the power of the nlyses, thus incresing the potentil for Type II errors. In ddition, the mjority of previous studies investigting the reltionship between subjective nd physiologicl sexul rousl in
118 A.H. Rellini et l. women hve computed correltions bsed on within-prticipnts repeted mesures (e.g., correltions nd regressions run on ech individul prticipnt). These methods provide informtion on the experiences of individul prticipnts, but no inferences cn be mde to the overll group or to between-group differences (e.g., women with nd without sexul dysfunction). Other studies hve tried to compenste for the lck of between-subjects nlyses by conducting group correltions or regressions bsed on one dt point for subjective sexul rousl nd one dt point for vginl pulse mplitude for ech individul prticipnt. The wekness of this method is tht chnges in vginl pulse mplitude nd subjective sexul rousl re stripped to one verge point, which provides limited informtion when the nlysis is investigting reltionship over n extended period of time. To tke into considertion individul differences, repetedmesures ANOVAs hve often been used to nlyze multiple vginl pulse mplitude nd subjective dt points. However, severl of the ssumptions tht re required for repeted-mesures ANOVA do not llow for the nlysis of the reltionship between vginl pulse mplitude nd continuous mesures of subjective sexul rousl. One ssumption of repeted-mesures ANOVA is tht observtions re collected t equl intervls for ll prticipnts. If the resercher wnts to reduce the dt to intervls equl to or smller thn 1 s, the frequent movement rtifcts present in vginl pulse mplitude violte this ssumption. Moreover, the longer the intervls used to reduce dt, the more informtion is lost on the reltionship between subjective nd physiologicl sexul rousl. The lrge between-subjects vrince tht chrcterizes vginl pulse mplitude dt (Jnssen, 2) violtes the repeted-mesures ANOVA ssumption of equl vrince nd covrince of the dt t different points in time. Between-subjects vrince in vginl pulse mplitude cn be due to probe plcement nd ntomicl nd physiologicl chrcteristics of the prticipnt (e.g., resting levels of vginl musculr tone) s well s n impednce effect on the mplitude of the signl (Jnssen, 2). For these resons, repeted observtions re often collected on group of individuls, but these between-groups observtions re not necessrily comprble. One wy to nlyze such nested dt is to use structurl eqution modeling. However, the lrge smples nd equl number nd spcing between observtions required for this technique render it problemtic for ssessing vginl pulse mplitude signls. We propose tht more pproprite wy to nlyze the reltionship between vginl pulse mplitude nd subjective sexul rousl is to continuously nd simultneously mesure the two vribles throughout exposure to sexul nd nonsexul films nd to utilize hierrchicl liner modeling (HLM) for the sttisticl nlysis. A mjor dvntge of HLM is tht it conducts within subjects nlysis of the subjective/physiologicl sexul rousl reltionship nd uses the coefficients tht describe this reltionship (i.e., slope nd intercept) s outcome vribles to test differences between prticipnts. With this methodology, individul differences in vginl pulse mplitude levels do not pose problem with interprettion becuse the nlysis is conducted within subjects nd the only between-subjects comprison is bsed on the strength of the within-subjects reltionship between vginl pulse mplitude nd continuous subjective sexul rousl. HLM is usully dopted to nlyze repeted mesures dt (Level 1 dt) nested within subjects (Level 2 dt; Bryk, Rudenbush, & Congdon, 1996). There re severl dvntges to this type of nlysis. HLM does not require the ssumption of independence of observtions, improves the estimte of effects within individul units, simultneously estimtes vrince nd covrince components for within-subjects nd between-subjects levels of nlysis, nd hs lower Type I error rtes (Rudenbush & Bryk, 2). Furthermore, HLM cn ssess individul differences s predictors of the degree of the reltionship. Given tht the literture hs reported brod rnge in levels of concordnce between subjective nd physiologicl sexul rousl in women, HLM cn be useful tool to ssess the modertors in this ssocition. Method Prticipnts Twenty-five women were recruited from the community using newspper dvertisements. Exclusion criteri ssessed during stndrdized phone interview included history of dibetes, thyroid disorder, crdiovsculr disese, neurologicl disese, stroke, psychitric or psychologicl dignosis, nd current use of psychoctive mediction. Prticipnts sexul functioning ws ssessed by phone interview, nd only women who reported no concerns regrding sexul rousl, desire, orgsm, or sexul pin were scheduled for n ppointment. Of the originl 25 prticipnts, dt from 1 womn were excluded from nlysis becuse of technicl difficulties tht my hve distorted her results. Also, dt from 2 women were excluded becuse they did not show significnt increse in vginl pulse mplitude in response to the erotic film, mening tht they showed slope of in their regression of vginl pulse mplitude over time. Although it is not unusul for 3% of prticipnts to show no increse in vginl pulse mplitude in response to erotic videos, the dt from these prticipnts were not included becuse the empiricl question posed in the present study focused on how vginl pulse mplitude impcts subjective sexul rousl nd vice vers. Thus, lck of chnge in vginl pulse mplitude would not hve provided ny meningful informtion bout chnges in subjective sexul rousl. Anlyses were conducted on the remining 22 prticipnts. All prticipnts included in the study reported heterosexul orienttion nd current involvement in sexul reltionship. For prticipnts chrcteristics, see Tble 1. Tble 1. Prticipnt Chrcteristics Men (SD) Rnge N (%) Age 27.5 (8.12) 19 53 Rce Africn-Americn 3 (13.6) Cucsin 14 (63.6) Hispnic 4 (18.2) Eduction Some high school or less 1 (4.5) High school grdute 4 (18.2) Some college 5 (22.7) 2 yer degree 9 (.9) 4 yer degree 3 (13.7) Mritl sttus Single, never mrried 17 (77.3) Mrried 2 (9.1) Divorced 2 (9.1) Seprted 1 (4.5)
Self-reported nd physiologicl femle sexul rousl 119 Further exclusion criteri included () under the ge of 18; (b) currently pregnnt; (c) self-report of n ctive psychologicl disorder including orgnic mentl syndromes nd disorders, schizophreni, delusionl disorder or psychotic disorders not clssified elsewhere, bipolr disorder, eting disorders (such s norexi nervos nd bulimi), nd pnic disorder; nd (d) currently receiving ny medictions known to ffect vsculr or sexul functioning (including ntidepressnts nd ntihypertensives). Prticipnts were told tht the purpose of the investigtion ws to exmine their physiologicl nd emotionl responses to brief visul stimuli, which would include erotic content. Mesures Physiologicl sexul rousl. The vginl photoplethysmogrph ws used to ssess vginl response to the erotic films. A dt cquisition unit Model MP1WS (BIOPAC System, Inc.) nd softwre progrm, AcqKnowledge version 3.7.3 (BIOPAC Systems, Inc., Snt Brbr, CA), were used for the trnsformtion of nlog/digitl dt. The vginl pulse mplitude signl ws smpled 8 times per second nd the mplitude of ech pulse wve ws recorded in millivolts. Continuous subjective sexul rousl. To mesure subjective sexul rousl continuously nd simultneously during exposure to the erotic film, the Femle Sexul Psychophysiology Lbortory t the University of Texs t Austin developed device we termed the rousometer. The rousometer consists of computer opticl mouse (Intellimouse by Microsoft s ) mounted on wooden trck divided into 1 eqully spced intervls, from 2 to 7. Given tht vginl pulse mplitude is mesured only in positive chnges from bseline, only the positive vlues ( 7) were used in the nlysis. Prticipnts were instructed tht 2 nd 1 reflect feelings of being sexully turned off, isneutrl, nd 1 to 7 reflect incresingly higher levels of feeling sexully turned on. Prticipnts were told tht while the vginl photoplethysmogrph detects the wy your body responds to the erotic stimuli, through the rousometer you will indicte your subjective experience of how turned on or sexully roused you feel. The mjority of women who prticipte in studies in our lbortory re fmilir with use of computer mouse, thus incresing the reltive ese in using the rousometer over prior continuous methodologies. At ech numeric intervl tht the computer mouse psses, prticipnts feel slight resistnce on the mouse indicting tht they re chnging from one numeric vlue to nother. The resistnce provided by the rousometer llows prticipnts to monitor the level of rousl tht they re indicting without hving to focus their ttention wy from the television screen. We believe this technique would be less likely to distrct the womn from processing erotic cues thn prior techniques tht provided visul rther thn tctile feedbck on reported rousl levels. The rousometer is connected to pointer on computer in the djcent experimenter s room. A softwre progrm written in MtLb detects the position of the pointer with respect to the y- xis of the computer s monitor every.5 s. A score of 7 on the rousometer corresponds to the highest point on the screen, nd corresponds to the middle of the screen. Vlues from to 7 were the rnge chosen for the rousometer to mtch the vlues of the Likert Film Scle (Heimn & Rowlnd, 1983); however, units of movement in rousometer were clculted in percentges (% 5 nd 1% 5 7). This continuous mesure improves upon previously dopted questionnire mesures becuse it llows monitoring of prticipnts levels of sexul rousl every.5 s rther thn once per video. Likert scle subjective response to the erotic film post exposure. To compre subjective responses to previous studies of this nture, n dpted version of the Film Scle by Heimn nd Rowlnd (1983) ws lso dministered in this study. Seventeen of the originl 41 questions were retined in this version nd four fctors were derived from this questionnire: subjective experience of physiologicl sexul rousl (six items), subjective experience of mentl sexul rousl (two items), negtive ffect (five items), nd positive ffect (four items). Items were rted on 7-point Likert scle indicting not t ll to intensely. Sexul functioning. To ensure tht prticipnts were in the norml rnge of sexul functioning, they completed the Femle Sexul Function Index (Rosen et l., ). The sexul functioning questionnire is composed of 19 items divided into fctor-nlytic derived subscles: desire (two items), rousl (four items), lubriction (four items), orgsm (three items), stisfction (three items), nd pin (three items). Ech subscle hs been tested to reflect internl consistency within n cceptble rnge (Cronbch s 5.89.97). Interitem relibility is within the cceptble rnge for sexully functionl women (Cronbch s 5.82.92). Test retest relibilities ssessed using 4-week intervl rnged between Person s r 5.79 nd.86. Divergent vlidity hs been estblished using the Locke Wllce Mritl Adjustment Scle (Person s r 5.53 for women with femle sexul rousl dysfunction, Person s r 5.22 for sexully functionl women). Stimulus MterilsFFilms Film stimuli consisted of 14-min udiovisul film which included () 1-min disply of the word relx, (b) 3 min of trvel film (neutrl stimuli), nd (c) 1 min of n erotic film. The erotic stimuli depicted heterosexul couple engging in foreply nd sexul intercourse nd hve previously been shown to induce sexul rousl in women in our lbortory (e.g., Meston & Worcel, 2). Procedure Visits were scheduled between dys 14 nd 28 of the menstrul cycle to control for potentil differences in sexul response ssocited with hormone levels. Upon rrivl, the prticipnt signed consent form nd completed the sexul functioning questionnire (Rosen et l., ) nd questionnires ssessing demogrphic vribles. In privte internlly locked room, the prticipnt inserted the vginl photoplethysmogrph nd relxed on reclining chir for 1 min (hbitution time) before exposure to nonerotic/erotic film sequence. Throughout the exposure, continuous levels of physiologicl nd subjective sexul rousl were recorded. Prticipnts were instructed to move the rousometer throughout the erotic film exposure to indicte their levels of subjective sexul rousl ( turned on ). The films were shown on 42-in. Sony plsm TV mounted on wll pproximtely 6 feet from the chir in which the prticipnt rested. The experimenter controlled the VCR nd the dt collection from n djcent room. Following the film sequence, the prticipnt completed the Film Scle, ws debriefed, nd compensted $5.
1 A.H. Rellini et l. Dt Reduction Vginl pulse mplitude ws smpled 8 times per second cross the neutrl nd erotic film segments. An experienced resercher visully inspected the dt for outliers nd deleted movement rtifcts, which were defined s sudden nd drstic chnges in vginl pulse mplitude. Using the AcqKnowledge softwre progrm, the resercher then recorded the pek-to-trough vlue in millivolts for ech pulse during the entire experimentl session. Dt from the rousometer nd the photoplethysmogrph were then verged cross 1-s intervls, resulting in 18 dt points during the neutrl nd dt points during the erotic film. A pulse tht fell cross two 1-s intervls ws ssigned to the intervl in which the pek ended. Dt Anlysis HLM cn be ctegorized s multilevel process (for discussion of HLM, see Bryk et l., 1996). The intercepts nd slopes of liner regressions of subjective sexul rousl predicted by vginl pulse mplitude were computed for ech prticipnt (Level 1). The slopes nd intercepts of Level 1 becme the outcome vribles for Level 2 in liner model to test whether overll women showed significnt reltionship between continuous subjective sexul rousl nd vginl pulse mplitude. The results described in this study re bsed on the following models: Level 1 nlyses evluted both vginl pulse mplitude prediction of continuous subjective sexul rousl nd the reverse (continuous subjective sexul rousl prediction of vginl pulse mplitude). The following is n exmple of the model wherein subjective sexul rousl is predicted by vginl pulse mplitude (VPA): Subjective sexul rousl ¼ b j þ b 1j ðvpaþþr ij ; ðlevel 1Þ where b is the intercept or the expected subjective sexul rousl of prticipnt whose vginl pulse mplitude is the prticipnt s verge; b 1 is the slope or the expected chnge in subjective sexul rousl ssocited with unit increse in vginl pulse mplitude; nd r is the error term, or the unique effect ssocited with ech observtion. Additionlly, four Level 2 models (Bonferroni 5.5/ 4 5.1) were run on the strength with which ech of the Film Scle fctors (mentl sexul rousl, reported physiologicl sexul rousl, positive ffect, negtive ffect) predicted growth in continuous subjective sexul rousl during exposure to the erotic film (Level 1: subjective sexul rousl predicted by time). The following is n exmple of the model wherein the overll Film Scle is the predictor nd subjective sexul rousl is the outcome vrible: Subjective sexul rousl ¼ b j þ b 1j ðtimeþþr ij ðlevel 1Þ b j ¼ g þ g 1 ðfilmscleþ j þ u j b 1j ¼ g 1 þ g 11 ðfilmscleþ j þ u 1j ; ðlevel 2Þ where g is the men subjective sexul rousl for ll prticipnts; g 1 is the chnge in the subjective sexul rousl ssocited with one unit increse in Film Scle; g 1 is the verge subjectivetime slope for ll prticipnts; g 11 is the chnge in the subjectivetime slope ssocited with one unit increse in Film Scle; u j is the unique effect of ech prticipnt on subjective sexul rousl holding Film Scle constnt; nd u 1j is the unique effect of ech prticipnt on the subjective-time slope holding Film Scle constnt. Tble 2. Mens (SD) of Prticipnt Sexul Functioning Sexul functioning domins (rnge) Prticipnts (N 5 22) Helthy controls reported by Rosen et l. () Desire (2 1) 7.86 (1.39) 6.9 (1.89) Arousl (4 ) 17.81 (1.92) 16.8 (3.62) Lubriction (4 ) 19.5 (1.) 18.6 (3.17) Orgsm (3 15) 13.38 (1.77) 12.7 (3.16) Stisfction (3 15) 12.5 (3.61) 12.8 (3.3) Pin (3 15) 13.6 (1.73) 13.9 (2.79) These four Level 2 models were computed to explore the vlidity of the continuous subjective sexul rousl. A moderte to high reltionship between the subscle of mentl sexul rousl nd the continuous subjective sexul rousl ws expected to confirm the vlidity of the continuous subjective sexul rousl method. Results Sexul Functioning Prticipnts were evluted for current sexul functioning using the sexul functioning questionnire (Rosen et l., ). All women included in the finl smple (n 5 22) scored within one stndrd devition from the men of women who re sexully functionl s reported by Rosen et l. (; see Tble 2). Correltions between Vginl Pulse Amplitude nd Likert Film Scle Averge vginl pulse mplitude vlues were clculted for the entire nonerotic nd erotic film sections nd the percentge of increse from neutrl to erotic ws clculted for ech prticipnt (1 dt point per prticipnt). This dt reduction procedure hs been commonly used in studies of this nture (e.g., Brody et l., 3). Person s correltions showed tht vginl pulse mplitude ws not significntly ssocited with the Likert Film Scle nd, specificlly, no significnt reltionship ws found between vginl pulse mplitude nd subjective physiologicl sexul rousl, r() 5.134, p 5.56, subjective mentl rousl, r() 5.82, p 5.73, negtive ffect, r() 5.67, p 5.77, nd positive ffect, r() 5.121, p 5.. Reltionship between Vginl Pulse Amplitude nd Continuous Subjective Sexul Arousl A Person s r correltion clculted on continuous subjective sexul rousl nd vginl pulse mplitude for ech prticipnt (within-prticipnt correltion) rnged between r 5.8 nd.79. Sixteen of the 22 women showed significnt correltion (r4.37). Additionlly, individul regressions indicted tht vginl pulse mplitude significntly predicted continuous subjective sexul rousl in 16 of the 22 prticipnts. When using HLM, the nlysis showed tht for the overll smple, vginl pulse mplitude significntly predicted the levels of continuous subjective sexul rousl, b 5 46.81, t 5 3.63, po.1. Figure 1 illustrtes the slopes for ll the prticipnts nd indictes the verge slope for this popultion. The comprison of the unconditionl model (model with no predictors) with the model using vginl pulse mplitude s predictor showed tht residuls re smller in the model with vginl pulse mplitude, w 2 (4) 5 341., po.1, suggesting tht vginl pulse mplitude contributes significntly to the prediction of continuous subjective sexul rousl.
Self-reported nd physiologicl femle sexul rousl 121 Stndrdized Subjective Sexul Arousl 1 5 2 4 6 8 1 12 Stndrdized VPA Figure 1. Individul regressions for vginl pulse mplitude (VPA) predicting subjective sexul rousl. Note. Regression line tht best describes the popultion of slopes. 2.8 2.6 2.4 2.2 2. 1.8 1.6 1.4 1.2 5 3 1 55 5 45 35 3 25 1 8 Figure 2 illustrtes the chnges in vginl pulse mplitude nd continuous subjective sexul rousl for 4 prticipnts for which the reltionship cn be clerly seen. After grphing the reltionship for ech prticipnt, one of the prticipnts showed reltionship between vginl pulse mplitude nd continuous subjective sexul rousl distinctly superior to tht of ll other prticipnts. The prticipnt lso showed vginl pulse mplitude bseline tht ws lmost 1 times lrger thn the ones shown by the other prticipnts. To test if the significnce previously found ws due to this outlier, second HLM ws run on the entire smple without this prticipnt s dt. The reltionship between vginl pulse mplitude nd continuous subjective sexul rousl remined significnt without the outlier, b 5 36.52, t 5 3.8, po.1. To ensure tht excluding the outlier from the nlyses would not crete problems, ll nlyses were first conducted on the entire smple nd then on the smple without the outlier. The significnce levels remined the sme in both nlyses; 1 9 8 7 6 5 4 5 3 1 18 16 14 12 8 5 3 1 Figure 2. Exmples of vginl pulse mplitude (VPA) nd continuous mesures of subjective sexul rousl during the erotic film regressed by time for four prticipnts.
122 A.H. Rellini et l. Positive Affect Negtive Affect Low+Affect Hi+Affect Low-Affect Hi-Affect Subjective Sexul Arousl 1 8 Foreply Vginl Sex Foreply Vginl Sex 1 8 Erotic Video Mentl Sexul Arousl Physiologicl Sexul Arousl Low Mentl Hi Mentl Low Physio Hi Physio Subjective Sexul Arousl 1 8 Foreply b Vginl Sex 1 8 Erotic Video Foreply Vginl Sex Figure 3. The impct of the four scles mesured with the Film Scle questionnire (positive ffect, negtive ffect, reported physicl sexul rousl, nd reported subjective sexul rousl) on the chnges in continuous subjective sexul rousl during exposure to the erotic videos. The regression line corresponding to low indictes the increse in continuous subjective sexul rousl for n individul with score of 1 on the Film Subscle, wheres the regression line corresponding to high indictes the increse in continuous subjective sexul rousl for n individul with score of 4 on the subscle. Different lower cse letters (, b) indicte sttisticl difference between slopes. No significnt sttisticl difference between intercepts within grphs. therefore, only results of nlyses on the smple minus the outlier re reported. In the results presented here, the b coefficient mens tht for 1 mv of increse in vginl pulse mplitude, continuous subjective sexul rousl showed 36.52 unit increse. Becuse the verge SD in continuous subjective sexul rousl during the exposure to the erotic film is 22., the increse of 36.52 units in subjective sexul rousl corresponds to 1.6 SD, which is considered lrge effect size. A test of the between- nd within-subjects vrince (vrince components of Level 1 nd Level 2) showed tht 42% of the vrince in continuous subjective sexul rousl ws due to within-subjects vrince, indicting tht there is 58% of vrince in the model tht is ttributble to between-subjects vrince. This ws confirmed by test of the significnce of between-subjects differences in vginl pulse mplitude, w 2 () 5 1525.1, po.1, indicting the presence of substntil between-subjects difference on the subjective/vginl pulse mplitude reltionship. These results confirm the importnce of using method to nlyze the dt tht is sensitive to between-subjects vrince. Vrince t Level 2 ws 1557.69, w 2 (, N 5 21) 5 15,1.93, p o. 1. In other words, lthough on verge continuous subjective sexul rousl ws strongly ssocited with vginl pulse mplitude, some individuls showed weker or even nonsignificnt reltionship. In ddition to vginl pulse mplitude predicting continuous subjective sexul rousl, continuous subjective sexul rousl lso predicted vginl pulse mplitude, b 1j 5.12, t 5 2.1, po.1. Individul vribility in vginl pulse mplitude ws on verge 58.56 mv, w 2 (, N 5 21) 5 11,464.57, po.1, which corresponds to 95% of the vrince explined by the model, indicting tht vginl pulse mplitude vried more between subjects s compred to the vrince during exposure to the nonerotic/erotic video sequence. Reltionship between Likert Film Scle nd Continuous Subjective Sexul Arousl Levels of mentl sexul rousl s mesured by the Likert Film Scle predicted the increse of continuous subjective sexul rousl during the erotic video, g 11 5.1, t 5 5.88, po.6. This cn be interpreted s n increse of 1 units on the continuous subjective sexul rousl scle (rnge 1) for ech increse of 1 point in the mentl sexul rousl of the Likert Film Scle (rnge 1 7). This result cn be lso understood s n
Self-reported nd physiologicl femle sexul rousl 123 increse of 1% on the continuous subjective sexul rousl scle for ech 14.3% increse on the mentl sexul rousl fctor of the Likert Film Scle. The nlysis of vrince showed significnt difference between prticipnts in this reltionship tht remins unexplined by the model, w 2 (, N 5 21) 5 1399.8, po.1. Negtive ffect, g 11 5.1, t 5 1.66, p 5.1, positive ffect, g 11 5.1, t 5.49, p 5.62, nd perceived physicl sexul rousl, g 11 5.1, t 5 1.3, p 5.3, did not predict continuous subjective sexul rousl (see Figure 3). Discussion The present study ws the first to nlyze the reltionship between continuous subjective sexul rousl nd vginl pulse mplitude using HLM, sttisticl technique designed to nlyze between- nd within-prticipnts differences simultneously without losing sensitivity towrd individul differences. The results of this study suggest tht vginl pulse mplitude nd subjective sexul rousl re indeed significntly relted in women during lbortory studies, nd tht the limittions of pst studies in finding n ssocition between the two mesures my hve, in prt, been the result of the methodology nd sttisticl nlyses employed. This study lso confirmed the high between-prticipnt vrince in vginl pulse mplitude lso shown in pst reserch (for review, see Jnssen, 2). Thus, we believe it is importnt tht sttisticl nlyses used to exmine this type of dt tke into considertion individul differences. Pst studies hve often found significnt correltions between vginl pulse mplitude nd subjective sexul rousl when exmining dt within n individul prticipnt (e.g., Korff & Geer, 1983; Wincze et l., 1977). However, none of these studies reported the verge strength of this reltionship nd, more importntly, they filed to determine whether the reltionship between vginl pulse mplitude nd subjective sexul rousl is significnt overll in women. The study of the reltionship between vginl pulse mplitude nd subjective sexul rousl often uses dt with nested structure tht hs chrcteristics incomptible with the ssumptions of repeted-mesures ANOVA, correltion, nd liner regression. To use these sttisticl methods, dt for ech prticipnt need to be reduced to one point which corresponds to the men for vginl pulse mplitude nd the men for subjective sexul rousl. This method does not tke into considertion the richness of the repeted dt nd, more importntly, ssumes tht vginl pulse mplitude represents the sme physiologicl experience for ll prticipnts, n ssumption tht is violted by wht is known bout vginl pulse mplitude. HLM, on the other hnd, ws developed to nlyze nested dt nd is highly sensitive to individul differences. In ddition, HLM cn simultneously clculte within- nd between subjects vrince, provides more efficient estimtion of effect, nd hs lower Type I error rtes (Rudenbush & Bryk, 2). Significnt individul differences in vginl pulse mplitude nd in prediction of continuous subjective sexul rousl were noted in this study. Correltions were not significnt between the subfctors of the Likert Film Scle nd percentge of vginl pulse mplitude increse during the nonerotic/erotic video sequence (s per nlyses conducted in most prior studies). However, significnt ssocitions between vginl pulse mplitude nd continuous subjective sexul rousl were observed when using HLM to nlyze the sme dt. Interestingly, significnt portion of the vrince in the subjective/physiologicl reltionship ws ssocited with between-subjects differences. This provides support for the hypothesis tht the lck of significnt reltionship between subjective nd physiologicl sexul rousl noted in prior correltionl studies my hve prtilly been ttributble to the type of nlyses used to nlyze dt tht is chrcterized by substntil between-subjects vrince. In the present study, we found tht, on verge, n increse of 1 mv in vginl pulse mplitude corresponded to n increse of pproximtely 37 units (rnge 1) in continuous subjective sexul rousl, which corresponds to lrge effect size (SD 5 1.6) in continuous subjective sexul rousl. It is interesting to note tht the literture on the reltionship between self-reported emotionl sttes nd nonsexul physiologicl senstions re low (Pennebker, Gonder-Frederick, Cox & Hoover, 1985), suggesting tht internl physiologicl sttes re not necessrily ssocited with n emotionl response. The significnt reltionship between physiologicl nd subjective sexul rousl noted here my, thus, be prticulrly meningful if considered within the lrger context of subjective responses. Given tht the rousometer is new instrument, n lterntive explntion of these results could be tht this device mesures something seprte from womn s subjective experience of sexul rousl. To test the vlidity of the rousometer, series of HLM nlyses were used to exmine the reltionship between questionnire mesures of mentl nd physiologicl sexul rousl nd dt from the rousometer. Likert scle levels of the mentl sexul rousl but not physiologicl sexul rousl significntly predicted continuous subjective sexul rousl, suggesting tht the mentl sexul rousl fctor of the Likert Film Scle nd the rousometer mesure closely relted constructs. Also, mesures of positive nd negtive ffect were not ssocited with levels of continuous subjective sexul rousl, thereby providing further evidence tht the rousometer ws detecting chnges specific to mentl sexul rousl rther thn ffect or physiologicl sexul rousl. In conclusion, vginl pulse mplitude ws found to significntly predict levels of subjective sexul rousl in sexully helthy women nd the inverse reltionship ws lso true. We suggest tht future studies tht exmine the reltionship between subjective nd physiologicl sexul rousl in women smple dt throughout the film stimuli for both subjective nd physiologicl indices nd employ sttisticl nlyses tht re designed to nlyze nested dt. The nturl question tht follows from this study will involve investigting wht modertes the strength of the reltionship between subjective nd physiologicl sexul rousl. Given tht some women seem to hve much stronger ssocition between physiologicl nd subjective sexul rousl thn others, it is importnt to understnd the individul differences tht my explin this vrince. For exmple, orgsm consistency during intercourse ws observed to correlte with the reltionship between subjective sexul rousl (mesured using Likert scle) nd vginl pulse mplitude in 26 menopusl women (Brody et l., 3). We propose tht future studies should dopt continuous mesures of subjective nd physiologicl sexul rousl in combintion with HLM in the investigtion of other potentil modertors or meditors such s ge, sexul functioning, nd sexul self-efficcy.
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