The effects of secondary stimulus characteristics on men's sexual arousal.

The penile plethysmograph is believed by many researchers and clinicians to be the most valid and reliable device currently available for assessing male sexual arousal (Howes, 1995; Maletzky, 1995; Proulx, 1989; Zuckerman, 1971). Although the main use of the penile plethysmograph is in the detection and treatment of sexual deviations (e.g., Abel & Blanchard, 1976; Kelly. 1982) and sexual dysfunctions (e.g., LoPiccolo & Stock, 1986), it has also been employed to examine such phenomena as the classical conditioning (e.g., Plaud & Martini, in press; Rachman, 1966), operant conditioning (e.g., Rosen, Shapiro, & Schwartz, 1975), and habituation (e.g., O'Donohue & Plaud, 1991; Plaud, Gaither, Amato-Henderson, & Devitt, in press) of male sexual arousal.

A key element in sexual arousal research and plethysmographic assessments is the stimuli that are presented to elicit sexual arousal. The stimuli may vary within a study or assessment as well as between studies or assessments on a number of different characteristics (Howes, 1995), which can be divided into primary and secondary characteristics. Primary characteristics, for plethysmographic assessments, refer to those stimulus characteristics that are intentionally varied between categories (e.g., deviant versus normal) and are thought to be most salient in distinguishing between men according to their stated sexual preferences (e.g., age or sex of stimulus). For research studies, primary characteristics are those stimulus characteristics that are actively manipulated by the researchers, such as the verbal resistance of a sexual partner (Plaud, Bigwood, & Rosenkranz, 1996) or the description of condom use (Gaither, Rosenkranz, Amato-Henderson, Plaud, & Bigwood, 1996) in an audiotaped description of sexual activity. Again, the defining features of primary characteristics are that they are actively manipulated and thus thought to be the most salient characteristics in distinguishing different categories of stimuli. Therefore, depending upon the research question, some stimulus characteristics may be considered primary for one study (e.g., condom use when that is the characteristic under investigation) but secondary in another (e.g., condom use in some, but not all scripts when that is not the character under investigation).

Secondary characteristics may vary both within and between categories but are not actively manipulated and are not typically considered to be salient: They may even be considered idiosyncratic. The results of plethysmographic assessments are normally considered to be due to the manipulation of primary characteristics; yet, if I they are not controlled, some secondary characteristics may confound those results. The purpose of the current study was to determine whether the manipulation of two stimulus characteristics that are often not controlled in plethysmographic assessments would affect sexual arousal in a sample of male college students.

The mode of stimulus presentation is probably the secondary stimulus characteristic that has received the most consideration in published research. Researchers have repeatedly found that films/videos elicit greater physiological arousal than slides, still photographs, or audiotaped descriptions of sexual activity (e.g., Abel, Barlow, Blanchard, & Mavissakalian, 1975; Abel, Blanchard, & Barlow, 1981; McConaghy, 1974). Many of these studies, however, were, flawed in that the content of the stimuli was not matched between modes.

Julien and Over (1988), in the only study to date to match the content across modes, examined differences in male sexual arousal across five modes of stimulus presentation: film, slides, audiotaped descriptions (spoken text), written text, and fantasy. Julien and Over produced a film that was divided into eight two-minute segments. Each segment consisted of the same man and woman engaging in a different sexual activity. Photographs were taken simultaneously from the same angle as the movie camera for the slides. The spoken text an written text consisted of factual descriptions of the activities taking place in each segment. In the fantasy condition, the research participant was given a factual description of the activities he was to imagine, which also corresponded to each segment of the video. The researchers found that the film elicited significantly greater arousal than slides, spoken text, and written text, which did not differ significantly from one another. Fantasy elicited significantly less arousal than the other four modes.

Some secondary stimulus characteristics are specific to certain modes of stimulus presentation. For example, the sex of the narrator as well as his or her tone of voice are characteristics that are specific to auditory stimulus presentation, whereas the attractiveness of the actors is specific to visual modes of presentation (i.e., slides and video/film). High, Rubin, and Henson (1979), in one of the few studies designed to examine the effects of mode-specific secondary stimulus characteristics, found that a video presented in color did not elicit greater physiological sexual arousal than the same film presented in black and white for eight men.

Another secondary stimulus characteristic inherent in videos/films is the presentation of the accompanying sound. Many studies reported in the literature, which involved the use of this mode of stimulus presentation, do not state whether the accompanying sound was presented (e.g., High et al., 1979; Julien & Over, 1988; McConaghy, 1974). Thus, it may not be proper to compare results between studies if this is not known, because the effects of this characteristic have not yet been investigated. Therefore, it seems vital that we delineate these effects to account for some variance between studies.

Some secondary stimulus characteristics are inherent in all modes of presentation, such as the type of sexual activities that are depicted. Some researchers who have use slides or videos used stimuli that present only a nude sexual pose (e.g., McConaghy, 1974). It has been demonstrated, however, that this type of stimulus does not elicit high levels of arousal. Thus, most researchers use stimuli that depict individuals engaging in some type of sexual activity. In describing their stimuli, however, many researchers state only that the stimuli depicted heterosexual activity. Thus, once again, it is not clear how much (if at all) this characteristic may affect the participants' arousal patterns.

In the Julien and Over (1988) study described previously, the participants showed similar patterns of arousal to the different sexual behaviors across modes. The stimuli depicting the couple undressing one another elicited the least arousal across all modes, whereas stimuli depicting fellatio, elicited the greatest arousal in three modes, and mutual oral sex (which includes fellatio) elicited the greatest arousal in the other two modes. It is entirely possible that this result was due to order effects (which were not analyzed), because the stimuli were presented in the same order within each mode across participants.

In the current study, we examined the effect of two stimulus characteristics--the presentation of video stimuli and the type of sexual activity depicted--on male sexual arousal using a penile plethysmograph and self-report. We hypothesized first that significantly greater arousal would be elicited when the accompanying sound was presented. Second, based on the findings reported by Julien and Over (1988), we hypothesized that video clips depicting fellatio and mutual oral sex would elicit the greatest levels of sexual arousal.

Method

Participants

One hundred thirty-three male undergraduate students at the University of North Dakota completed a packet of questionnaires in small groups as part of a larger study on volunteer bias in sexual arousal research (Plaud, Gaither, Hegstad, Rowan, & Devitt: 1997). At the end of the packet, students were given information regarding the current study and were asked to indicate whether they would like to participate. Nineteen students volunteered to serve as participants in this study (mean age = 23.68). This sample was predominantly White (n = 17, 89.5%) and single (n = 16, 84.2%). One participant dropped out of the study after completing one data-collection session. Therefore, data reported here are from the 18 participants who completed both data-collection sessions. Participants received four hours of research credit toward their psychology courses.

Apparatus

Physiological arousal (penile tumescence) was measured by a Type A mercury-in-rubber strain gauge and then recorded in centimeters by a penile plethysmograph manufactured by Parks Medical Electronics Inc. (model 240-A), Aloha, Oregon. The plethysmograph was connected to a Coulbourn Instruments transducer rack for transformation from analog to digital recordings. Data were then stored on a Gateway 2000 4DX2-66V with Intel processor computer via the Advanced CODAS data-acquisition system (Dataq Instruments Inc.).

A VCR placed in the control room was used to play the videos. The VCR was connected to a 19-inch color television set in the experimental chamber for display of the video segments. The VCR was also connected to a stereo system in the control room, which was used to present accompanying sound via a pair of headphones.

Stimuli

Twelve 60-second video segments, taken from commercial erotic videos, were presented. Each segment depicted one sexual behavior involving a man and a woman, which was preceded and followed by 15 seconds of a a screen. The behaviors included fellatio, cunnilingus, mutual oral sex, face-to-face intercourse with the male superior, face-to-face intercourse with the female superior, and intercourse with rear entry. There were 2 segments depicting each of the 6 behaviors for 12 segments. Each segment was recorded on a different videotape, thereby allowing for random ordering of segment presentation.

The order in which the segments were presented for each participant was randomly determined with three restrictions: Each participant would see all 12 segments in both sessions, only 1 of the 2 segments depicting each behavior would be presented with accompanying sound in each session, and the 6 segments that were presented with sound in the first session would be presented without sound in the second session and vice versa.

A 3-minute video segment depicting animals moving about in a rain forest was presented as a sexually neutral stimulus. An 11-minute video consisting of a man and a woman engaging in a variety of different sexual activities (including the 6 experimental behaviors) was presented as a means to elicit a full erection.

Procedure

Two male researchers--a graduate student (the first author) and an undergraduate student--collected all data in this study at the beginning of the semester. The researchers called each potential participant (i.e., those who had volunteered to participate in the study) and set an appointment for an orientation meeting in the laboratory. During this meeting, the researcher showed the potential participant the plethysmograph and the other equipment that would be used in measuring his sexual arousal. He was also shown the experimental chamber, which contains a comfortable reclining chair, headphones, a television set, and a visual 10-point Likert scale hanging on the wall. The researcher explained the procedures that would be used, assure him of confidentiality, and answered any questions. This orientation meeting serve to aid the potential participant in making an informed decision regarding his participation in the study and adapt him to the laboratory environment.

The potential participant was given an informed consent statement to take home at the conclusion of the orientation meeting along with a research credit slip for one hour. He was then given at least 24 hours to deliberate on his decision to participate, at which time the researcher called him and, if he chose to participate, set appointments for both data-collection sessions. The appointments were always made for the same day and time one week apart.

When the participant arrived for the first session, the researcher led him into the experimental chamber and again explained the procedures to be used throughout the study. The participant was also informed that he may or may not hear the accompanying audio while he viewed the videos to ensure that he did not think that the headphones were broken the first time the accompanying audio was not presented. The researcher took the signed consent form, explained how to fit the strain gauge properly on the penis, answered any questions the participant had without divulging the hypotheses under investigation, and then left the room.

Communication was maintained between the participant and the researcher via a voice-activated intercom system. After the participant had placed the strain gauge on his penis and the headphones on his head, he notified the researcher that he was ready to begin. The researcher then instructed him to "get comfortable and relax."

After three minutes, the researcher presented the three-minute sexually neutral video segment an continuously corded the participant's physiological arousal with the plethysmograph. This served as a baseline measurement of the participant's penile circumference. At the end of the three minutes, the researcher stopped the tape and asked the participant to rate 10-point Likert scale (0 = not at all aroused, 9 = extremely aroused). This was then followed by a two-minute rest period.

The researcher next presented the 11-minute video described previously. At the end of the 11 minutes, the researcher again stopped the video and asked the participant to rate his sexual arousal using the Likert scale. This was then followed by a detumesence period, which lasted for two minutes, or until the participant's penile circumference had returned to its baseline measurement, whichever was longer.

After the detumesence period ended, the researcher played a one-minute video segment (the first experimental stimulus). At the end of the segment, the participant was asked to rate his sexual arousal. This was followed by a detumesence period, at the end of which the next video segment was presented. This procedure was repeated until all 12 segments had been presented.

Throughout the session, the researcher continuously monitored the participant's physiological sexual arousal on the computer screen in the control room. Continuous recordings were taken at a rate of 100 records per second while stimuli were being presented. During detumesence periods, however, data were not recorded.

After the last segment had been presented and the participant gave his rating of sexual arousal, the participant was told that the session was concluded. The participant was then instructed to take off the gauge, get dressed, and come into the control room where the researcher answered any questions he may have had, ensured that he was aware of his next appointment, thanked him for participating, and released him.

The same procedures were followed for the second data-collection session. At the end of this session, the researcher thoroughly debriefed the participant, asked him not to discuss it with anyone else, thanked him for his participation, and gave him a research credit slip for three hours.

Results

Physiological Data Reduction

Physiological data were collected via the CODAS software and stored individually for each session, totaling two sessions per participant. Maximum and minimum circumference for each of the 14 stimuli (1 baseline, 1 "maximum," and 12 experimental) of each session was calculated via the CODAS software. Percentage of full erection was then calculated for each stimulus period.

The Effect of Stimulus Characteristics

To determine whether sexual arousal was affected by stimulus characteristics (sound and the behavior depicted), separate 2 x 6 x 2 (sound x behavior x session) repeated measures analyses of variance (ANOVAs) were performed on the physiological data and the self-report data. The physiological data (i.e., percentage of full erection) and self-report data were found to be highly correlated (r.78).

For the physiological data, the only significant effects found were the main effect for sound, F(1, 15) = 17.73, p [is less than] .001, with greater arousal elicited during the trials in which sound was presented (56.3% vs. 48.6%), and the behavior main effect, F(5, 75) = 4.55, p [is less than] .005. Figure 1 presents the mean percentage of full erection collapsed across the four stimulus periods depicting each behavior (e.g., two fellatio with sound, two fellatio, without sound). Follow-up Tukey's tests revealed that stimuli depicting fellatio (43.4%) elicited significantly less arousal than stimuli depicting mutual oral sex (55.6%) and stimuli depicting intercourse with female superior (58.9%). No other significant differences were found.

[Figure 1 ILLUSTRATION OMITTED]

For the self-report data, there were significant main effects for behavior, F(5, 70) = 3.38, p [is less than].01, and sound, F(1, 14) = 18.12, p [is less than] .001, as well as a significant behavior x sound interaction effect, F(5,70) = 3.14, p [is less than] .025. The session main effect was not significant, nor were any other interactions.

Figure 2 displays the mean self-report of arousal for each of the 12 behavior x sound combinations. Although participants reported being more aroused when sound was present than absent for all six behaviors, follow-up Tukey's tests revealed that the difference was significant at the .05 level only for stimuli depicting intercourse with male superior (5.10 vs. 3.57). Comparing the self-report of arousal across behaviors when sound was absent (i.e., comparing across the dark bars in Figure 2), the only significant difference was between stimuli depicting fellatio and intercourse with female superior (3.20 vs. 4.33). Finally, comparing across behaviors when the sound was presented (i.e., comparing across the patterned bars in Figure 2), stimuli depicting fellatio (3.53) elicited significantly less arousal than stimuli depicting intercourse with rear entry (4.47), intercourse with male superior (5. 10), mutual oral sex (4.43), and intercourse with female superior (4.57).

[Figure 2 ILLUSTRATION OMITTED]

Discussion

Currently there is a debate over whether stimuli used in plethysmographic assessments should be standardized (see Howes, 1995; Maletzky, 1995). The researchers and clinicians who are opposed to standardization cite the need to tailor stimuli to clients' specific fantasies or behaviors to maximize the discriminability of the assessment (i.e., increase the possibility of detecting truly deviant sexual arousal patterns, Maletzky, 1995). Supporters, on the other hand, believe that a standardized set of stimuli should be developed to cover the gamut of sexual deviations. Thus, because stimuli would be constant across all assessments, the results could be truly compared across individuals and/ or research studies and national norms could even be developed (Maletzky, 1995).

Howes (1995), in a study of plethysmographic assessment centers throughout the United States, found great variance between and even within centers for the types of stimuli used in assessments. The stimuli vary on at least two separate dimensions. The first dimension is the mode of presentation. Although audiotaped descriptions of "normal" (i.e., heterosexual behavior involving consenting adults) and paraphilic behaviors are used most commonly, slides, videos, or structured fantasies are used as well.

The second dimension on which stimuli differ is the content. This can very easily differ from one assessment to the next, depending upon the particular sexual deviation for which the client is being assessed. In assessing pedophilia, for instance, slides may depict children of different ages who are clothed, partially clothed, or nude and either simply posing or engaging in sexual activities. The type of sexual activity depicted is a further variant that is rarely even considered, or at least rarely discussed in the literature.

The results of the current study may provide modest support for stimulus standardization, although further research is warranted. The mode of stimulus presentation and the type of sexual behavior depicted were both shown to affect physiological and subjective measures of sexual arousal, which were highly correlated. For both measures, videotaped stimuli elicited significantly greater levels of sexual arousal when the accompanying audio was presented than when it was not. The particular behavior that elicited the greatest level of arousal differed between the two measures, although stimuli depicting fellatio elicited the least arousal for both measures.

These results are in contrast to Julien and Over's (1988) findings in which stimuli depicting fellatio elicited the greatest level of arousal in three out of five modes of stimulus presentation and stimuli depicting mutual oral sex elicited the greatest arousal in the other two modes. This may, in part, be attributed to procedural differences between the two studies. In the current study, stimuli were presented in a random order for each participant with a two-minute detumesence period between presentations. Julien and Over presented their stimuli in the same order across modes to all participants (undressing, mutual fondling, mutual masturbation, cunnilingus, mutual oral sex, fellatio, intercourse in several positions, intercourse terminating in ejaculation) with only a 30-second interval between stimuli. Figure 1 (Julien & Over, 1988, p. 137) shows that participants' sexual arousal increased until the fifth or sixth presentation, after which it began to decrease. It is possible that this decrease was due to physiological fatigue, or habituation. Thus, in other words, the difference in arousal between behaviors may have been caused by order effects and not by the actual behavior being depicted.

The significance of the results from the current study lies in the fact that sexual arousal was affected by the type of sexual behavior depicted. Two separate video clips of each sexual behavior were presented to maximize the possibility that differences would be based upon the specific behavior rather than some other idiosyncratic characteristic of a particular video (e.g., the attractiveness of the woman). A future study might use several different video clips of the same man and woman engaging in different sexual behaviors to reduce the variance associated with these other characteristics.

A limit to the current study is a possible confound between the sexual behavior depicted and stimulation by the male versus female anatomy. In other words, video segments depicting cunnilingus, for instance, tend to focus on the woman's vulva, whereas segments depicting fellatio tend to focus on the man's penis. Thus, it is possible that the participants in the current study were aroused by the visual presentation of a woman's vulva and not by the act of cunnilingus, whereas their arousal may have been inhibited by the visual presentation of another man's penis. If this is the case, however, researchers and clinicians using visual stimuli depicting people engaging in sexual activities need to consider this point in choosing their stimuli. Future researchers may consider using audiotaped depictions of the different sexual behaviors instead of video clips to reduce this possible confound.

The results of the current study should serve as a caution to researchers to consider secondary stimulus characteristics when designing their studies. For instance, researchers should be careful to match their stimuli for mode of presentation (including whether accompanying audio is to be used) and type of sexual activity depicted. More research is needed to determine what other types of stimulus characteristics affect arousal patterns.

Finally, the generalizability of this study may be somewhat limited because of self-selection of participants or a "volunteer bias." Only 19 of 133 male college students agreed to participate in this study when given the opportunity. Although this issue is discussed much more thoroughly by Plaud et al. (1997), in general, we found that those students who agreed to participate tended to be more sexually liberal (i.e., lower sexual guilt, more homosexual partners, more total sexual experiences) than students who did not participate. These differences are in line with the findings of other researchers (e.g., Strassberg & Lowe, 1995; Wolchik, Braver, & Jensen, 1985).

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Manuscript accepted September 20, 1996

This article is based on the master's thesis of George A. Gaither with the direction of Joseph J. Plaud. This research was supported by a Student Research Grant from The Society for the Scientific Study of Sexuality. Special thanks to Michael Franklin for his assistance in data collection.

Address correspondence to Joseph J. Plaud, Ph.D., Department of Psychology, University of North Dakota, P.O. Box 8380, Grand Forks, ND 58202-8380. Telephone: [phone]. Fax: [phone]. E-mail: [e-mail].