Sensitization, a facilitation of behavior produced by strong or noxious irrelevant stimuli, or by repeated presentations of certain kinds of stimuli, has been described with regard to sexual behavior in rats. Electric shock to the skin or tail pinch have repeatedly been found to facilitate some aspects of male copulatory behavior or to induce copulatory behavior in sexually inactive males (Barfield and Sachs, 1968; Wang and Hull, 1980; Leyton and Stewart, 1996). Again, I know of no data from female rats. Nevertheless, there is no doubt that irrelevant stimuli may enhance the intensity of sexual behavior in male rats. There may well be similar studies in other non-human species, but I have not invested the effort needed for finding them in any search. With regard to the mechanisms behind sensitization, it is generally assumed that the irrelevant stimulus increases general arousal, making the animal more reactive to any stimulus in the environment. Since the salient stimulus in all experimental procedures for studying male sexual behavior is the receptive female, and since a likely response to this stimulus normally is copulation, the enhanced arousal takes the expression of facilitated copulatory behavior.
Regarding sensitization in the human, we are in a most unusual situation. The only experimental studies I know of were performed in women! The purpose was not at all to study sensitization of sexual responses, but instead to determine the effects of activation of the sympathetic nervous system or the influence of anxiety on sexual arousal. The stimuli or actions employed for generating sympathetic activation and/or anxiety were completely irrelevant as far as sex is concerned. Yet they were found to enhance responses to sexually relevant stimuli. This coincides perfectly with the definition of sensitization given earlier in this chapter. Thus, although the scientists performing the studies I soon will mention did not interpret their results in terms of learning, I will use their data as evidence in favor of sensi-tization of sexual responses.
The oldest paper containing observations relevant to the issue of sensitization that I have been able to localize reports a study in which women viewed either an anxiety-producing film or a neutral travel film shortly before viewing a pornographic movie. The anxiety sequence 'depicted in vivid detail the aftermath of several tragic automobile accidents, including occupants' death cries' and the neutral sequence depicted a travelogue of Nova Scotia. Changes in vaginal blood volume in response to the pornographic movie were, not unexpectedly, taken as indicators of sexual arousal. The women having viewed the anxiety-producing movie showed a larger vaginal response to the pornographic movie than women having watched the neutral movie (Hoon et al., 1977). Similar results were obtained in another study (Palace and Gorzalka, 1990). Again, an anxiety-evoking film fragment preceding exposure to a pornographic film segment enhanced the vaginal response. Some kind of semimystical account in terms of emotion effects could certainly be used for explaining these data, but a more parsimonious explanation would be to suggest that the anxiety-inducing movie simply had sensitized the women to the sexual stimuli in the ensuing pornographic movie. By saying that, we limit ourselves to a pure description of behavior (enhanced vaginal response to sexually arousing stimuli following exposure to an irrelevant stimulus) without any speculation as to cause. We just employ the label corresponding to the behavioral effects observed. If we would like to go a step further and speculate about the physiological mechanisms behind the behavioral effect, we could, for example, propose that activation of the sympathetic nervous system during exposure to the anxiety-inducing movie was the cause for the heightened response. This possibility was actually mentioned by Palace and Gorzalka (1990).
Another study offers some information relevant to the hypothesis that sympathetic activation sensitizes the organism to sexual incentives. Thirty-eight women answered a questionnaire evaluating 'trait and state anxiety' after having seen a pornographic movie. 'Trait anxiety' is something like a stable individual characteristic reflecting a dispositional tendency to experience anxiety, while 'state anxiety' is an acute emotional response consisting of feelings of apprehension and enhanced sympathetic nervous system activity. The vaginal response to the movie was assessed by photoplethysmography. It was found that women scoring moderately high on the 'state anxiety' scale showed a larger increase in vaginal blood flow than women having either low or high scores. These data were interpreted in terms of sympathetic influences on sexual arousal in response to a sexual stimulus (Bradford and Meston, 2006).
As it turns out, the role of sympathetic activation in sexual arousal in women has been evaluated more directly in a series of well-designed studies. In order to achieve a rather intense sympathetic activation, women were asked to exhaust themselves doing physical exercise, always in the form of cycling. The demanded speed and adjusted workload was such that the poor women exercised at a constant 70% of their maximum oxygen uptake. Fifteen minutes after a 20-minute exercise a pornographic movie was shown and the vaginal response was recorded with photo-plethysmography. On another occasion, the women saw a pornographic movie without preceding exercise. It was found that the vaginal response to the pornographic movie was significantly larger after exercise than after rest. Curiously, exercise had no effect on the women's subjective rating of the intensity of sexual arousal. The authors concluded that the results provide indirect support for the notion of a facilitatory influence of sympathetic activation on physiological sexual arousal (Meston and Gorzalka, 1995, 1996; Meston, 2000). I would add that they also show that a sexual response may be sensitized.
Further evidence for sympathetic activation as the cause underlying at least some forms of sensitization of sexual arousal by irrelevant stimuli comes from pharmacological studies. The adrenergic agonist ephedrine or its vehicle was administered to women in a double-blind repeated measures design. About 40 minutes after taking the drug or vehicle, the women watched first a short neutral film and then a pornographic movie while the vaginal response was recorded with photoplethys-mography. Ephedrine had no effect on vaginal blood flow during the neutral film. However, the response to the pornographic movie was larger after ephedrine than after the vehicle (Meston and Heiman, 1998). For those not familiar with basic physiology, I just mention that the sympathetic nervous system releases noradrenaline, which acts at adrenergic receptors throughout the body. Ephedrine should, then, simulate some of the effects of sympathetic activation. The results obtained could, therefore, be considered as supporting the idea that enhanced sympathetic activity stimulates the response to sexual stimuli. The latter is what we have called sensitiza-tion and we can therefore conclude that sensitization of sexual responses is mediated by the sympathetic nervous system.
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