As was pointed out in Chapter 1, sexual motivation is not activated by endogenous events but by an external stimulus. This is not the whole story, though. The external stimulus must initiate some processes within the central nervous system and these internal processes must sooner or later translate themselves into observable behavior, which means activity in skeletal muscles. This overt activity may or may not be associated with responses in the autonomous nervous system. In order to understand the mechanisms of motivation, it is essential to have some basic knowledge of the interactions between certain kinds of stimuli, central nervous activities and behavior. To that end, the incentive motivation theory elaborated by Dalbir Bindra (Bindra, 1969, 1974, 1976, 1978) is most useful. I will start this chapter with an introduction to that theory.
Environmental stimuli can be divided into three classes. Those that have no effect on an organism's behavior are neutral. The vast majority of stimuli belong to this class. Some stimuli may activate approach behaviors and once approached they may be consumed or interacted with in a variety of forms. An example is food, which is not only approached but also eaten if the organism is in an appropriate state. Another example could be a conspecific of the opposite sex. If the organism is in an appropriate state, it approaches a conspecific and, if that conspecific also is in an appropriate state, sexual interactions may be initiated. If not, social instead of sexual interactions may occur. Any stimulus inducing approach behavior in the organism is a positive incentive. Finally, some stimuli produce withdrawal. Examples are excessive heat, like that coming from a forest fire, or an intense smell of putrefaction, like that coming from a forgotten garbage bag, or some painful stimulus like electric shock. Stimuli producing withdrawal are negative incentives.
In addition to producing the behavioral responses of approach and withdrawal, incentive stimuli may activate visceral responses mediated by the autonomous nervous system or by the endocrine system. Examples of such actions of incentive stimuli may be the autonomous response of erection produced by an appropriate sexual incentive. Men and male primates and even male rats frequently respond with erection to stimuli emitted by a sexually receptive female. Similarly, negative incentives like an aggressive intruder may produce an endocrine response such as release of corticosteroids from the adrenal gland cortex as well as sympathetic responses such as release of catecholamines from the adrenal medulla and increases in heart rate and blood pressure. Thus, besides producing organized activity in skeletal muscles incentives may, or may not, produce visceral responses of many kinds.
One very important characteristic of incentives, positive or negative, is that the approach or avoidance behaviors they activate are arbitrary in the sense that there is no obligatory relationship between one incentive stimulus and a motor pattern. A steak may activate intense approach behaviors in a dog, but the specific motor patterns executed depend on whether the steak is made available in the dog's food bowl or on the dish of an honored guest. In the first case, the dog would run to the food bowl and ingest the steak with little decorum, while in the second case she would politely sit on the floor beside the guest, staring at him with begging eyes as he put a piece of steak to his mouth. If that failed to produce any result, the dog may start making discreet sounds or lightly scratch the guest's leg, or whatever the dog's experience tells her is the most efficient way to get hold of the alimentary incentive. In the same way, a male rat may press the lever in a Skinner box in order to reach a receptive female, or run over an electrified grid, or overcome whatever obstacle an experimenter may invent in order to humiliate his rodent subject. The point should be clear: there is no obligatory association between a particular incentive (like food or a female) and a particular motor pattern for approaching the incentive. The subject chooses the motor pattern most likely to lead to success according to the specific situation and his previous experience with this and similar situations.
A positive incentive will not always produce approach and a negative incentive will not always produce withdrawal. While an adult male rat will approach a sexually receptive female and eventually initiate sexual interaction with her, a prepubertal male will show much weaker approach behaviors and very little sexual interaction will occur. Likewise, a rat that has been deprived of food for 24 hours will rapidly approach and start to eat its normal food pellets when given the opportunity to do so, while a rat that just has finished a large meal of delicious chocolate pellets will pay no attention to her normal pellets. These petty examples show that a constant stimulus (a sexually receptive female or standard food pellets) may or may not have effects on the organism. Obviously, there is no necessary dichotomy effect - no effect. In most cases there are simply variations in the magnitude or intensity of response activated by the incentive stimulus. Nevertheless, it is clear that some internal mechanism that can account for the varying impact of constant incentives needs to be included in any theoretical model of motivational mechanisms. In incentive motivation theory this internal mechanism is called the central motive state.
The expression central motive state was probably introduced by Morgan in his famous 1942 textbook (Morgan, 1942). He ascribes four characteristics to it:
1 It is, at least partly, self-perpetuating. This merely means that it is not spontaneously fluctuating from one moment to another but shows a continuous and maintained functioning until some event reduces or terminates it. A food-deprived rat running in a maze for food reward at the end does not run in fits. It runs quite smoothly at each trial, but the intensity of approach behavior as well as of food consumption will be reduced when satiety is approached.
2 General activity increases with increasing intensity of the central motive state.
3 The central motive state activates specific behavior patterns. Supposedly, these behavior patterns should be related to the origin of the central motive state. If, again, food deprivation is the cause for the activity of the central motive state, behavior patterns likely to be activated are those related to search for food and eating.
4 The central motive state induces 'a set or potentiality for presenting various patterns of behavior when the appropriate stimulus conditions in the external environment are available. This is the priming property of the c.m.s.' (Morgan, 1942, p. 461). In other words, the central motive state primes the individual for reacting to certain stimuli, again supposedly, stimuli that have some relationship to the origin of the central motive state.
Points 1 and 2 were certainly most important in the kind of motivational discussions that were common in the early 1940s, but seem to be of slight interest in a contemporary analysis of motivation. Points 3 and 4, though, are rather central to modern incentive motivation theory. They imply, among other things, that there are many different central motive states, several of which may be active at the same time. The one that will translate into overt behavior is the one corresponding to the stimuli present in the environment. Any reader interested in motivation may find it intellectually rewarding to compare these statements with the views on response hierarchies, drive stimuli, external stimuli and incentive value presented in the writings of Clark Hull (Hull, 1943, 1951, 1952).
Bindra systematically avoids speculations concerning the intimate nature of the central motive state. This is probably a wise attitude, since it should be regarded and treated as an abstract concept without any necessary correspondence in reality. It is, nevertheless, a useful concept that allows us to explain exactly what it is intended to explain, namely the variations in reaction to a constant stimulus between different situations. Having said this, it may be appropriate to point out that we can make some speculations about the nervous bases of the central motive state associated with sexual behaviors. We even have data allowing us to go from speculations to specific hypotheses and occasionally from hypothesis to established fact. More about that will come further down and in other chapters.
Figure 2.1 illustrates what an incentive motivational model of sexual behaviors could look like.
FIGURE 2.1 A model for sexual incentive motivation. The text in italics represents the example of the male rat. (a) A reciprocal excitatory relationship functioning in such a way that the central motive state enhances the sensory system's sensibility to stimuli with sexual significance. When such stimuli are perceived, the sensory system excites the central motive state which, in turn, further sensibilizes the sensory system, i.e. the relationship is one of reciprocal positive feedback. (b) At a certain threshold level of activity, the central motive state engages a series of viscerosomatic activities preparing the subject for sexual interaction. (c) The appropriate environmental stimuli activate motor patterns that bring the subject in contact with the source of stimulation. During approach, additional incentive stimuli may be encountered. These will be centrally represented and enhance the central motive state through (a). (d) and (e) Provided that approach behaviors have been successful and that appropriate viscerosomatic reactions are being accomplished, the subject's behavior may change from unconditioned or conditioned instrumental responses to the execution of sexual reflexes. These are activated by tactile stimulation of the perineal or lower abdominal region. If the subject is sexually inexperienced, such stimulation is obtained accidentally. If the subject already has acquired sexual experience, then conditioned instrumental responses may facilitate the attainment of tactile stimulation necessary for activation of sexual reflexes. At the point of transition from approach to execution of copulatory reflexes, the behavioral sequence is aborted in the absence of tactile stimulation. In the case when sexual reflexes indeed are activated, sex behavior will normally continue until ejaculation. (f) The positive affect induced by ejaculation will feed back to the central motive state where a short-lasting inhibitory system is activated. (g) At the same time, the positive affect and associated processes of reinforcement will strengthen learning of associations between itself and environmental cues. These cues will acquire incentive properties in relation to the intensity of the positive affect that is experienced. MPOA, medial preoptic area; GnRH, gonadotropin-releasing hormone; T, testosterone. (Reprinted from Agmo, 1999. Copyright 1999, with permission from Elsevier.)
When there is an active central motive state and an appropriate stimulus, behavior is usually activated. As always, behavior is activity in skeletal muscles. This means that the incentive stimulus impinging on the active central motive state in the end must activate motor neurons. The process could be conceived as a kind of automatism where efferent stimulation excites the central motive state which, in turn, excites motor neurons, in the same way as a sensory neuron excites a motor neuron in a reflex arch. In motivated behavior, large amounts of sensory neurons, perhaps from several modalities, vastly distributed in the brain, may connect to large numbers of motor neurons, vastly distributed in the brain. These connections might involve several short interneurons and perhaps also longer projection neurons. Both kinds of neurons may be modulated by the central motive state, making the connection between sensory input and organized motor output either impossible or possible with varying probability, in the same way as other polysynaptic reflexes are modulated by interneurons of several kinds. The point here is that motivated behavior, like sex, can be regarded as a sequence of automatic reactions to environmental stimuli. This allows us to understand and explain the behavior without any reference to goal or purpose. An automatic response occurs as a consequence of a specific pattern of stimulation, whether we want it or not, and without any purpose. An erection in response to a beautiful student during a lecture has certainly no purpose, except for reminding us of the poena reciproca, the humiliation of not having control over our organism with our will, as so eloquently exposed by Saint Augustine.
The preceding discussion is applicable to both human and non-human sexual incentive motivation. However, as repeatedly pointed out in Chapter 1, the human may replace an external incentive stimulus with the mental representation of such a stimulus. The requirement that an incentive be present in the environment does not, therefore, apply to the human. As we will see, this difference has some important consequences. For example, while male primates almost never masturbate in the absence of sexually relevant external stimuli, men frequently do so. They may intentionally produce external sexual incentive stimuli, by opening a pornographic magazine or turning on one of the many TV channels offering pornographic movies, but masturbation may also be activated solely by mental representations of sexual incentives. Female primates rarely masturbate and they never do so in the absence of adequate sexual incentive stimuli. Most women masturbate, and they usually do it in the absence of external sexual incentives. Like men, they may produce such incentives on purpose, but again pure mental representation may be enough for activating the behavior. Thus, while non-human sexual behaviors are never or almost never performed in the absence of sexual incentives, humans may engage in such behaviors at any moment.
If we conceive the scheme incentive stimulus (real or in the form of a mental representation) - central motive state - motor output as an automatism, we gain a lot in conceptual power and scientific rigor. Not only do we get rid of purpose, hence teleology, but we can also propose mechanistic cause-effect explanations. This can be done at the purely behavioral level. If we maintain the central motive state constant, we can determine the relationships between incentive stimuli and ensuing responses in the way behaviorists do, without any consideration of processes within the nervous system. If, on the contrary, we maintain the incentive stimuli constant, we can manipulate the nervous system and determine changes in motor output. This would eventually allow us to understand the cellular processes underlying the abstract concept of central motive state. Both approaches are currently used, and have been used for a long time, but they have not been integrated into a coherent theoretical framework. The next few chapters should illustrate the fruit-fulness of the conceptual framework offered by incentive motivational theory in the form it has been outlined here.
Once approach behaviors have been successful, copulation may be initiated. In contrast to the arbitrary approach behaviors, copulation in non-human mammals consists of a series of stereotyped motor patterns. These motor patterns are essentially somatic (thrusting, contraction of penile muscles, ejaculation in the male or lordosis in the female of many species) or autonomous (erection, seminal emission) reflexes or fixed action patterns, to employ a term popular in classical ethology. Some scientists are of the opinion that the motivational mechanisms determining the likelihood of appearance of the sexual reflexes or fixed action patterns are best described in terms others than those employed in incentive motivational theory. The motivational model proposed by Konrad Lorenz many years ago (Lorenz, 1950) is a viable, or perhaps even excellent, option. Likewise, the motivational framework in Clark Hull's learning theory is perfectly adequate. In fact, Hull used some of Frank Beach's work on female rat lordosis as basis for his theoretical analysis (Hull, 1943). Both the Lorenz and Hull models posit that there is an inverse relationship between the intensity of the stimulus required to activate a behavior pattern and the intensity of motivation to display the pattern. Thus, a weak stimulus will activate behavior only if motivation is strong, whereas a weakly motivated animal will respond only to a powerful stimulus. Interestingly enough, both models concentrate on the probability of response activation and not on the intensity of response. The reason is certainly that the intensity of a reflex response varies little, while the probability of occurrence may vary from 0 to 1. I have earlier suggested that the incentive motivational framework is essential for an understanding of the mechanisms controlling the arbitrary approach behaviors, while the Hullian or Lorenzian frameworks may be better for understanding the occurrence or non-occurrence of copulatory reflexes (Pfaff and Ágmo, 2002). My opinion has not changed.
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