Desire is a term popular in the literature on human sexual behavior. Due to some undoubtedly unfounded prejudice against fashionable terms, I strongly dislike it. Its precise meaning is unclear and it is extremely difficult to use it in a comparative perspective. Proposing that a female rat desires to engage in copulatory activity is extremely repulsive. I have elsewhere argued, with convincing arguments, that the word 'desire' needs to be replaced with the word 'motivation' in any scientific text (Agmo et al., 2004). Motivation is a well established concept in the behavioral sciences and it can easily be integrated into a large body of theoretical constructions, where clear definitions are possible. The replacement of desire by motivation allows me, for example, to apply an incentive motivational analysis to the problems of approach to a mate and to the visceral reactions produced by incentive stimuli. Erection or vaginal lubrication can, for example, be analyzed and understood as visceral responses to sexual incentive stimuli.
Arousal or excitation are also imprecise terms. Some would use either of these terms as equivalent to motivation. The statement 'I am so aroused' could mean that the person making it feels an immediate urge to engage in sexual activity and/or that she perceives abundant vaginal lubrication. The same reasoning can be applied to the term 'excited'. Here I will not suggest a replacement term. Instead, I want to make clear that arousal, in the context of human sexual behavior, refers exclusively to enhanced genital blood flow. This is eloquently expressed in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM IV, American Psychiatric Association, 2000). The definition of female sexual arousal is: 'The arousal response consists of vasocongestion in the pelvis, vaginal lubrication and expansion, and swelling of the external genitalia' (p. 543). There is no explicit definition of arousal in men, but male erectile disorder appears as a subcategory of sexual arousal disorders. In men, erection is the main manifestation of enhanced genital blood flow. This means that the DSM IV equates sexual arousal with genital blood flow also in men. By now it should be clear that arousal, as employed by clinicians, has nothing to do with mental events, such as wanting sex, or anything of the kind. It refers strictly to an autonomous response, a visceral reaction.
The preceding clarification of terms can immediately be applied to a motivational analysis of the first phases of copulatory behavior, desire and excitation. I mentioned above that the presently most popular description of these phases divides the desire phase into endogenous and exogenous. The endogenous phase of desire, characterized by 'specific sensations which move the individual to seek out or become receptive to sexual experiences' (Kaplan, 1979, p. 10), sounds rather similar to what I have called sexual incentive motivation in its first phase, approach to a mate. The desire phase as defined above does not, actually, belong to what we have called copulatory behavior. We will return to that phase and its stimulus control in the chapter on human sexual incentives. For the moment it is sufficient to say that the addition of the desire phase, the phase where sexual motivation is activated, solves a fundamental problem that was not addressed at all in the Masters and Johnson model of human copulatory behavior. I think of the problem of the motivational background for enhanced genital blood flow during the arousal phase, the phase we now call excitation. By adding desire (motivation), activated by an external or internal incentive stimulus, we can explain how the enhancement of genital blood flow comes under the control of motivational mechanisms. Without a motivational mechanism, the origin of vaginal lubrication or erection would be unexplain-able. This would also be the case with variations in the intensity of lubrication or erection. Having introduced the motivational basis, we can now use degree of lubrication or erection as sensitive indicators of sexual motivation. Prior to these responses, however, sexual motivation must have been activated by external or internal (mental representations of sexual activities) incentive stimuli. The recognition that sexual motivation, or in incentive motivational terms, the activation of a central motive state by sexual incentives, needs to precede excitation and, furthermore, has to continue to be active during the entire excitation phase until orgasm allows for an elegant and comprehensible analysis of human sexual interactions.
Having such a fine model of human copulatory behavior, it must be regretted that no one has profited from the rather detailed descriptions of physiological arousal (excitation) to pursue an analysis of its control by motivation. In fact, the only study to have addressed the relationship between sexual motivation and intensity of the arousal response, which I know of, was not performed in humans but in male chimpanzees (Nadler and Bartlett, 1997). As a measure of arousal the authors employed the magnitude of erection during a sexual encounter with a receptive female. Erection was quantified simply by measuring the penis length at various moments. The momentaneous length measurements were then expressed as a proportion of length measured at maximum erection. This ingenious procedure made it possible to estimate the relative magnitude of erection throughout the sexual encounter. The relative magnitude of erection was then correlated with several measures of sexual performance, thought to depend on the degree of motivation. The results suggested that the relative magnitude of erection indeed is determined by motivational mechanisms similar or identical to those determining the intensity of other aspects of copulatory behavior. It is somewhat unfortunate that the extremely interesting conclusion is obscured by the use of a confusing theoretical framework. Nevertheless, this single study is all we have of experimental data concerning the relationship between the intensity of arousal (defined as intensity of genital blood flow) and motivation.
We know a lot more of the stimulus control of the arousal response than we know about its control by motivational mechanisms. There are diverse reasons for this. The most important is that lack of enhanced genital blood flow in some situations is a rather common clinical problem. In men, it leads to erectile dysfunction, a condition called impotence in ordinary language. Impotence has been of much concern to medical practitioners for many years and much basic research concerning the physiological mechanisms of erection has been prompted by this interest. Concepts like motivation do not form part of a physiologist's conceptual framework and is rarely, if ever, mentioned in their analyses of erectile mechanisms. Although motivation is of no concern to the physiologist, she/he needs to have some idea of the stimuli that activate the physiological response under scrutiny. The clinicians, for their part, need some reliable diagnostic procedure for determining the severity and/or the characteristics of the erectile dysfunction in their patients. A standard procedure is to expose the patients to some kind of erotic stimulus, like a pornographic movie, and measure the arousal response to that stimulus. This is easily done by measuring penile circumference with a strain gauge. Clinicians, like physiologists, are not particularly interested in psychological concepts like motivation and they use non-motivational concepts when trying to understand and treat impotence. This is even the case with impotence treatments centered on manipulations of motivational mechanisms. The famous sensate focus training described by Masters and Johnson (1970) is essentially a procedure leading to enhanced sexual motivation by the generous employment of several sexual incentive stimuli, including mechanical stimulation of the genitalia. However, the lack of a coherent theoretical framework adequate for sexual functions impeded the recognition of this fundamental fact and the term motivation is not used at all in the discussion of treatment of impotence. In fact, the only time the word motivation appears in Human Sexual Inadequacy is when describing the patient's reasons for seeking treatment. These reasons were called motives, and the single use of the term motivation was awkward indeed: '. . . intense motivation for reversal of symptomatology . . .' (Masters and Johnson, 1970, p. 357). Perhaps this little example illustrates the fact that clinicians have their worries far away from motivational mechanisms as understood in motivation theory.
Our discussion of the very basic question of the relationship between motivation (desire) and the following phases of human copulatory behavior is far shorter than its theoretical and clinical importance merit. The reason for this brevity is that very little is known. Factual knowledge is frequently supplanted with more or less founded speculations and I will finish this section with some speculative proposals.
First, by applying the incentive motivational framework outlined earlier, we can establish that sexual incentive stimuli have the dual effect of activating motivation, behaviorally manifested as approach to the sexual incentive, and visceral reactions, notably enhanced genital blood flow. The sexual approach is a paramount feature of van de Velde's (1965) first phase of human sexual interactions, the prelude, and it can be assumed to coincide with the desire phase in more recent models of human sexual behavior. An extremely important question here is whether the two aspects of sexual motivation, approach and enhanced genital blood flow, are always activated simultaneously or not. The answer to that question becomes crucial to our understanding of deficient sexual arousal, called arousal disorder in women and erectile dysfunctions in men. We will return to this important problem in the chapters dedicated to these disorders. Before that, we have to acquire some knowledge about sexual incentives, which will be the subject of the two following chapters. Here I only want to make the reader cognizant of the existence of the question.
Second, some sexual incentive stimuli, be they proximal or distant, may not be able to activate sexual motivation in a particular situation and/or at a particular moment. Other stimuli may, however, be effective in that situation. Caressing the face or lower arms or kissing may be inefficient for enhancing sexual motivation in a woman at a particular moment, but mechanical stimulation of the clitoris might be highly effective. The question is whether the woman would allow this stimulation to occur if the previous stimuli had failed to activate sexual motivation. This is another issue that will become of some importance when we arrive at problems with hypoactive sexual desire disorders. Are these disorders caused by insufficient exposure to efficient sexual incentive stimuli, or are they caused by hypoactivity of the central motive state, making the individual unresponsive to such stimuli? For the moment I will not consider this fundamental topic. Nevertheless, a general principle, maintaining that the intensity of stimulation determines the probability of activation of motivation, and consequently also the degree of arousal, can be reiterated. This principle is not new and surprising in any way. As mentioned in the first section of this chapter it was established, in general terms, more than 50 years ago, and it has had a wide application in ethology and related fields (Lorenz, 1950). It also coincides with the characteristics of the concept of drive stimulus as employed in Hullian theory (Hull, 1943).
Third, 'spontaneous' desire, or 'endogenous' desire, meaning the sexual motivation that is activated without any known reason, is certainly not spontaneous. Some external stimuli may activate sexual desire and there is no reason to assume that all such stimuli must be available for the conscience. Any stimulus may have become a conditioned sexual incentive and it is quite possible that the conditioning process never formed part of our conscious experience. When and if stimuli of this kind activate sexual incentive motivation, we would call it a case of spontaneous or endogenous desire activation since we are not conscious of the connection between the stimulus/stimuli and the central motive state.
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