During the discussion of non-human sexual incentives, I mentioned an example of a visceral reaction provoked by an incentive, penile erection in the rat and monkey. We learned that rats enhance the frequency of spontaneous erection in response to odors from a distant female. However, this visceral reaction is not a constant response to an incentive stimulus, because we also learned that during actual cop-ulatory activity, erection in rats is not continuous but associated with mounting. The female odor, however, can be expected to be present with almost constant intensity throughout the copulatory interaction. Female odor cannot, therefore, be the stimulus controlling erection during copulatory encounters. Furthermore, the visceral response of erection does not occur in anticipation of a mount, for example during pursuit of the female, but during the initial stages of mounting when physical contact with the female has already been established. It appears, then, that erection in response to distant incentive stimuli emanating from an inaccessible female is controlled by stimuli different from those operating during copulatory interaction, at least in rats.
In the human, the situation may be similar. Particularly young men, and sometimes men not so young any more, get an erection in response to an incentive even in situations where sexual interaction is excluded from the outset. Reading a pornographic magazine could be such a situation. It is extremely unlikely, if not outright impossible, that the man reading it expects the lady on the photograph to materialize before him and propose sexual intercourse. He can have fantasies of that sort, but that is a question we will not address now. Just as the rat, he responds with erection to stimuli emitted by an inaccessible female. It should not be controversial to propose that men frequently display visceral reactions to sexual incentives long before actual sexual interaction takes place as well as in the absence of the possibility of such interactions. There are much data showing that this kind of response is not limited to men. In healthy women, enhanced genital blood flow is an extremely consistent reaction to sexual incentives in the form of pornographic movies or pictures with erotic content (see Rellini et al., 2005, for several references). It appears to be just as unfailing as erection in healthy men.
As pointed out in the preceding paragraph, the main visceral response to distant sexual incentives in the human is enhanced genital blood flow. This response is called sexual arousal, as mentioned long ago. The distinction between this visceral reaction and any mental state produced by sexual incentive stimuli should always be clearly maintained. Nevertheless, terms like 'subjective arousal' are sometimes used to refer to the conscious perception of genital arousal. It appears, though, that subjective arousal is only partially determined by genital blood flow. Studies in men and women have revealed modest correlations between questionnaire or other reports of arousal and objective measurements of genital response, viz. penile tumescence and vaginal blood flow (Laan et al., 1995). It is generally believed that the correlation is higher in men than in women (Heiman, 1977; Steinman et al., 1981). One probable reason for this is that erection is a more notorious event than vaginal lubrication. Sensory feedback from a fully or partially erect penis is probably more intense than that from a more or less lubricated vagina. In consequence, it is easier for men to be conscious of their degree of erection than it is for women to be conscious of their degree of vaginal lubrication. However, some recent data suggest that the coincidence between subjective and genital arousal in women may have been underestimated (Rellini et al., 2005).
I will now turn to an analysis of the stimulus control of the sexual arousal response in the human. The evidence for a role of olfaction in human sexual incentive processes is not overpowering. In fact, there are no data concerning the capacity of odorous stimuli to promote erection. Likewise, there are no data concerning the effects of odorous stimuli on vaginal blood flow. There is, however, a curious little experiment in which 28 young women either were exposed to a pornographic movie or were asked to produce sexual fantasies. While doing that, the women were exposed to one of three odors. A neutral odor (distilled water), a masculine odor (a commercial perfume belonging to the 'fresh fougere category', whatever that means) or a feminine odor (another commercial perfume belonging to the aldehydic floral category) were employed. Vaginal blood flow was estimated with a photoplethysmograph. Results showed that the odor failed to modify the response to the pornographic movie, while the 'masculine' odor enhanced vaginal blood flow during the production of sexual fantasies. The effect was seen in the follicular phase but not in the periovulatory phase of the menstrual cycle (Graham et al., 2000). This study suggests that synthetic odors may have some effect on the capacity of mental representations to stimulate vaginal blood flow. Combined with visual stimuli, though, the contribution of this kind of odor is marginal. A firm conclusion as to the role of olfactory incentive stimuli in the activation of sexual visceral responses is impossible in view of the limited data available.
The role of hearing is equally unclear. An accomplished study evaluated the response to erotic/pornographic and romantic audiotapes in a sample of undergraduates (Heiman, 1977). Both questionnaire and physiological estimations of the intensity of response to the stories told on the tapes were performed. Physiological measurements included penile tumescence in men and vaginal blood volume in women. These genital responses were, not surprisingly, used as physiological indicators of sexual arousal. The pornographic stories stimulated erection and enhanced vaginal blood flow while the romantic stories were ineffective. Questionnaire responses coincided with the physiological indicators of sexual arousal. As always, the correlation between genital and questionnaire responses was higher in men than in women. This beautiful study seems to suggest that auditory stimulation enhances sexual arousal in both men and women. However, a problem with the interpretation of the data is that we cannot know whether the pornographic audiotapes evoked visual mental representations that were sexually arousing. It is possible to argue that the function of hearing was to evoke such mental imagery and that the imagery rather than the verbal content was the direct cause of arousal. The capacity of mental representations to induce sexual arousal has already been mentioned several times and the possibility that the verbal message was only triggering mental images needs to be seriously considered. In fact, sexually explicit visual imagery is an important part of physically arousing sexual fantasies (Leitenberg and Henning, 1995). Considering this it must be accepted that the Heiman (1977) study does not provide unequivocal support for a role of auditory stimulation in sexual arousal.
Another way to evaluate the importance of hearing for sexual arousal is to add auditory stimulation to visual stimuli. If sound plus image were more efficient than image alone, we might conclude that auditory stimulation indeed contributes to arousal. The experimenter-provided visual stimulation could be expected to neutralize any possible effects of mental imagery evoked by the sound. For example, a pornographic movie could be shown with and without the accompanying sound. The movie images would mask or at least substantially reduce the impact of any mental imagery occurring while watching the movie. One carefully controlled study employing a group of 133 young men, all of them undergraduate students, revealed that the sound accompanying the sexual activities depicted in pornographic movies indeed enhanced physiological arousal above that displayed after watching the same movies without sound. Physiological arousal was quantified by recording penile tumescence with a strain gauge. In addition, the students were asked to rate their level of arousal on a Likert scale. These ratings coincided with the physiological recordings in showing that movies presented with sound were more arousing than the same movies without sound (Gaither and Plaud, 1997). Pornographic movies illustrating different kinds of sexual activities were also compared in the same study. The activities included cunnilingus, fellatio, mutual oral sex, intercourse with the man superior, intercourse with the woman superior and intercourse with rear entry. In other words, many of the sexual activities making up any pornographic movie worth watching were shown. The results showed that fellatio was inferior to mutual oral sex and intercourse with female superior. This last finding does not appear to be particularly important in the present context, but it is a nice illustration of the precision of the analyses of the properties of a stimulus determining its impact on arousal that are carried out in some laboratories. Returning to the role of sound, it can be concluded that auditory stimulation contributes to sexual arousal even in a situation where visual imagery should be of reduced importance. Whether the sounds stimulated other kinds of imagery not brought to mind by the visual stimuli or not is unknown. There are other studies (e.g. Golde et al., 2000) confirming that auditory stimulation, although producing sexual arousal by itself, is inferior to the combination of visual and auditory stimulation.
It could also be argued that the possible confound produced by mental representations of sexually arousing stimuli is exaggerated. Although mental representation, fantasies, can be arousing to the point of orgasm, there are large interindividual differences both in the occurrence of fantasies and in their impact on sexual arousal. A small proportion of individuals does not seem prone to fantasizing at all, and others do it infrequently (Leitenberg and Henning, 1995). Several studies have determined that mental representation, even in situations where subjects are explicitly instructed to produce fantasies with sexual content, are inferior to external stimuli with sexual connotations. In one study, subjects were exposed to either a pornographic movie segment, pornographic slides, audiotaped descriptions of the activities displayed in the movie segments, written descriptions of them and, finally, the subjects were asked to produce mental representations of exactly the same activities as were exposed in the movie segment. It may also be important to note that the slides actually were frames taken from the pornographic movie segment, meaning that the factual content of all stimuli were identical. Data show that the pornographic movie enhances physiological arousal (evaluated as penile tumescence) more than all other stimuli. Furthermore, looking at slides, listening to an audiotape, or reading were equally effective, and superior to mental representation but inferior to the movie. Although mental representations did produce some arousal, their low effectiveness compared to the other stimuli suggests that they do not mediate the effects of these other stimuli on sexual arousal.
Personally, I think it is premature to conclude that mental representations, fantasies, are unimportant for sexual arousal produced by external stimuli. The vividness of the mental representations may be heavily influenced by external stimuli, and that may be of some importance for the arousal response. At the same time, the position that external stimuli act by activating mental representations of a sexual nature and that these representations then activate some physiological processes leading to enhanced genital blood flow introduces an unnecessary complication. It is more parsimonious to pose that the external stimuli directly activate some physiological mechanisms leading to enhanced genital blood flow without the detour over representations. However, it is quite unlikely that visual stimuli in the form of printed or handwritten letters directly lead to sexual arousal. Likewise, an auditory stimulus consisting of someone reading a pornographic story is unlikely directly to produce sexual arousal in a listener. In both these cases, it is almost certain that the verbal message leads to mental representations, perhaps in the form of imagery, which then produces arousal. Otherwise we are forced to assume that the visual stimulus properties of a text with pornographic content differs, in some perceptible way, from the visual stimulus properties of, for example, a recipe for making a pumpkin pie.
Regarding an auditory stimulus, we would have to assume that the sonoric qualities of a pornographic story are perceptibly different from those of a travel story. This possibility appears extremely unlikely, forcing us to accept that sexually arousing verbal messages acquire their arousing properties via the activation of mental representations. A possible compromise between the notion that all stimuli producing sexual arousal need to activate mental representations with a sexual content in order to do so and the contrary notion of a direct connection between stimulus and arousal is to assume that some stimuli, like verbal accounts with arousing effects, act via mental representations while other stimuli, like a silent pornographic movie, directly stimulate sexual arousal.
The capacity of certain stimuli to produce sexual arousal in the absence of mental representations is probably most evident in the case of tactile stimuli. For example, mechanical stimulation of the clitoris is a most efficient stimulus for enhancing vaginal blood flow. Reflex connections between the sensory receptors in the clitoris and the clitoral/vaginal vascular system (Shafik, 1995) as well as between sensory receptors in the penis and its vascular supply have been described (see Giuliano and Rampin, 2004, for a review). Women with spinal cord transection at levels above the sacrum will respond to clitoral stimulation with tumescence of that structure and vaginal lubrication (Sipski et al., 2001). Men in a similar condition also experience erection in response to tactile stimulation of the penis (Riddoch, 1917; Kuhn, 1950; Courtois et al., 1993). Women and men with spinal transection do not consciously register the stimulation, meaning that they do not feel it. This makes it extremely unlikely that the mechanical stimulus activates mental representations with a sexual content. Indeed, it is just as unlikely that this is the case as it is unlikely that the visual stimulus characteristics of a pornographic text directly modify clitoral/vaginal blood flow. The inevitable conclusion must be that the compromise hypothesis outlined above is correct or at least far more likely than any of the alternative hypotheses.
A summary of current knowledge of the stimulus control of human sexual arousal is now in place. Humans, women and men, respond to certain kinds of distant stimuli with enhanced genital blood flow. These stimuli may be visual and/or auditory. Olfactory stimuli seem to be of slight importance. The exact characteristics of the visual stimuli that carry their sexually arousing properties have not been determined in any detail, but it is clear that color is of little importance. Black and white movies are efficient, but no direct comparison between such and color movies has been done. Nevertheless, I allow myself to conclude that color is not essential. We also know that moving pictures are more efficient than slides. The reason for the superiority of motion is not known, but it may be speculated that moving pictures look more realistic than slides. It is also indisputable that the theme illustrated in the pictures and movies must be sexual, preferably depicting an explicit copulatory activity such as vaginal intercourse in varying positions, fellatio, cunnilingus or anal intercourse. Geometric figures or movies depicting landscapes or flower gardens are inefficient. Concerning verbal messages, either read or listened to, the content needs to be sexual if arousal is to be produced. This is probably a triviality and I will not insist upon the point. Non-verbal auditory stimuli are probably also efficient. Humans produce a lot of sounds in many contexts, and at least some humans vocalize intensely while displaying copulatory activities. During copulation, there are also sounds coming from other parts of the body than the mouth or nose. I do not think of vulgarities such as farting, but the movement of the genitals in and out of body orifices also produce sounds of varying intensity and quality. Paraphernalia, such as the bed or dining table, may also make rhythmic noises during copulatory activity, to much amusement for the neighbors and to the dismay of the sound producers. All these non-verbal sounds associated with sexual activity might have arousing properties. This possibility has not been unequivocally evaluated, but the fact that the addition of non-verbal sounds to a pornographic movie enhances its arousing effect suggests that these kinds of sounds indeed are arousing.
Some proximate stimuli, here meaning touch stimuli, are sexually arousing. There are innumerable descriptions of the use of tactile stimuli for producing sexual arousal in the popular literature and many in more serious works. The arousing properties of tactile clitoral or penile stimulation have probably been known since the dawn of mankind. In the 1920s, this old knowledge entered into some marriage manuals, which recommended sexually inexperienced young men to stimulate the clitoris of their equally inexperienced wife before penetrating her vagina. The purpose was to enhance her enjoyment of the marital duties. Curiously enough, it was even recommended that the clitoral stimulation should be performed with the mouth and tongue. In some countries, the application of this advice would have been a criminal offense. More socially acceptable forms of arousing tactile stimulation are non-genital mouth-to-body contact and mouth-to-mouth or tongue-to-tongue contacts. All these behaviors frequently go under the label 'kissing'. I have never seen any laboratory study of the genital response to kissing, but anecdotal evidence suggests that this activity in fact may be arousing. I will not analyze non-genital tactile stimulation, but it is probably safest to conclude that it may be sexually arousing, at least in some cultures.
If we were to try to establish the order of arousing potency of different kinds of sexual incentive stimuli, we would probably find that genital tactile stimulation is the most efficient. The order of arousing potency of other kinds of tactile stimulation and of distant stimuli is unclear.
We have now briefly summarized current knowledge concerning the stimulus control of sexual arousal in the human. I have retained the old term 'arousal' throughout this chapter, although we learned in Chapter 2 that it has been replaced with the term 'excitation', at least when talking about the sequence of human sexual behavior. I have done so for a very simple reason: in the clinical literature, the term 'arousal' is consistently employed when talking about enhanced genital blood flow in women. Lack of genital response to sexual stimuli is actually named 'female sexual arousal disorder'. In men, the equivalent condition used to be called impotence, but the name erectile disorder has become increasingly popular. I do not know why the term 'arousal disorder' is not employed when talking about men, although the problem referred to is a problem of arousal. Furthermore, the cause of impotence as well as of lack of clitoral/vaginal response to sexual stimuli is lack of enhanced genital blood flow both in women and men. Thus, it might seem reasonable to use the same term for the same disorder in both sexes. Why this is not the case could perhaps be explained by sociologists or feminists. I have an additional reason for having preserved the term 'arousal'. The phase of the human sexual response cycle now called 'excitation' encompasses events immediately preceding actual copulation and copulation. In the Masters and Johnson (1966) model, the 'arousal' phase seems to precede copulation. In the preceding pages, we have mentioned many studies of 'arousal' where copulation never took place. Listening to pornographic stories, or watching an adult movie (using a very stupid American euphemism) was never followed by copulation in any of the studies mentioned. 'Arousal' was studied as an isolated phenomenon, not as part of a sequence of events. 'Excitation', as it is understood in the discourse on human sexual behavior would, therefore, have been inadequate.
We will now turn to an analysis of the stimulus control of events preceding arousal. The arousal response was described as a visceral response to sexual incentive stimuli. Such stimuli also activate somatic processes. Salient among those are the display of approach behaviors, leading to establishment of initial contact with a potential mate, as mentioned many times already. Very little is known about these behaviors and their control by external stimuli or mental representations. Whereas our knowledge of sexual arousal is sufficient for presenting some informed speculations and a few firm conclusions, we can do nothing of the sort with regard to approach to a potential mate. The following section is, consequently, essentially a work of fiction. I am deeply embarrassed over this sad fact and I decided to put that section after the discussion of sexual arousal with the hope that most readers will be exhausted by now and prefer to immediately jump to the next chapter, which is far more exciting.
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