How Sexual Desire Works- The Enigmatic Urge

by Frederick Toates

  The action of these two levels of control is evident in the wanting phase of sexuality. Incentives can act through the low-level system to engage appetitive behaviour, whereas aversive events can act to cause withdrawal from a situation or immobility. The low-level system would take a considerable weight of the control of sexual behaviour in a case where an individual pursues an instant turn-on, exemplified by the male being turned on by the sex worker.

  The high-level system calls up memories, anticipates the future and takes into account long-term considerations and it takes up a large amount of processing capacity. This system might add to the effect of the low-level system, for example a person is happy to be captured by a sexual stimulus and carefully pursues it. I would suggest that the high-level system dominates the control of behaviour in those cases just used to exemplify where desire was not automatically triggered by physical properties. Rather, it took time to emerge, dependent upon memories, emotional connectedness and interpretation of the behaviour of the partner.

  A common incentive system

  Basics

  Obviously we know whether we are feeling lust or hunger! There are distinct systems in the brain underlying particular wants. However, evidence also suggests a common process that contributes to any want (Panksepp, 1982); an ‘incentive approach system’ serves the range of motivations, for example sex, food and drugs, described as a ‘common currency’ (Berger and Shiv, 2011). Any such incentive can engage the common system, which brings the person into closer contact with the incentive.

  A consequence of the common mechanism is that there can be ‘spill-over’ between different incentives. That is to say, excitement by incentive 1 (e.g. the sight of a drug to someone using drugs) might also be seen in a stronger approach to incentive 2 (e.g. sexual incentive). Incentives 1 and 2 might be simultaneously present and add their effects together or the effect of incentive 1, presented on its own, might be seen in increased wanting towards incentive 2 that is presented immediately afterwards. The common approach (‘wanting’) system would seem to form the basis of the ubiquitous observation that advertisers exploit sex to create or amplify other wants. There is no better demonstration of this than motor shows, where the latest fashionable model usually has at least one scantily clad female draped over its bonnet.

  Future discounting

  The phenomenon termed ‘future discounting’ or ‘future myopia’ (Wilson and Daly, 2004; van den Bergh et al., 2008) also points to common features of different desires. Numerous studies have shown that people are impatient in that they prefer to obtain a reward today rather than tomorrow and prefer a smaller reward (e.g. £10) over a bigger reward (e.g. £15) in one month.

  Sexual arousal causes discounting of the value of future monetary gain, as does heroin or nicotine craving. Sexual cues make males impatient to get soda pop and candy bars! Wilson and Daly exposed men to images of women whom they found attractive and this increased the men’s tendency to discount the future in terms of monetary reward. This was observed not only for images of women but also by viewing and touching a woman’s bra, as compared to a T-shirt. This triggered impatience for a candy bar (van den Bergh et al., 2008). Women exposed to images of attractive men did not show an increased tendency to discount the future3 and neither did men exposed to images of women that they did not find attractive.

  Differences between individuals in their wanting

  Some bases of desire remain idiosyncratic. However, certain differences between individuals can be understood in terms of identifiable processes.

  Personality

  Motivation can arise from either the pursuit of attractive goals (the approach system, exemplified by seeking sexual pleasure) or the avoidance of aversion (a so-called avoidance system, exemplified by having sex to avoid relationship conflict or social rejection). Individuals differ in how much relative weight is placed upon these two systems (Cooper et al., 2008). If someone seeks help for loss of desire, it could be useful to look carefully at the goals set within any relationship. Setting approach goals (e.g. to plan to take a holiday together) might spill over into increased sexual desire. This is in contrast to setting avoidance goals (Impett et al., 2008).

  Sometimes women feel bad after engaging in consensual but unwanted sex (Meston and Buss, 2009). Whether they feel good or bad depends largely upon whether the sex was engaged in as part of approach or avoidance motivation. If the intended goal was a desired one, such as to please a partner, women tend to feel good afterwards. If it was a goal of avoiding an undesirable effect, such as losing the partner, the consequences are more likely to be negative, involving such feelings as shame. Rather than being analogous to resisting addictive drugs, the situation would be rather like forcing oneself to take a bad-tasting medicine.

  Individual differences in the strength of wanting can be linked to differences in the relative sensitivity of the approach system (van den Bergh et al., 2008). Extraverts are characterized by a large weight upon approach motivation, high sensitivity to social rewards and relatively high pleasure levels derived thereby (Larsen and Augustine, 2008). This is a feature of Jeffrey Gray’s ‘reinforcement sensitivity theory’ (see Corr, 2008). Individuals showing a high level of ‘reward sensitivity’, i.e. ‘wanting sensitivity’, were found to show a relatively high level of discounting of the future (van den Bergh et al., 2008). A predictor of promiscuous sexual activity is the personality characteristic of extraversion and impulsive sensation-seeking (Schmitt and Shackelford, 2008).

  Risky sexual behaviour can also be based upon avoidance motivation (Cooper et al., 2008). Escape from aversion can dominate behavioural control and negative moods can exacerbate the strength of incentive pull, associated with short-term relief from aversion but with long-term risks. Neuroticism is associated with strong avoidance tendencies (Cooper et al., 2008).

  The personality characteristics of agreeableness and conscientiousness correlate negatively with the tendency to uncommitted sexuality. The extravert need for variety can perhaps most obviously be satisfied on the basis of a change of partner. Low conscientiousness would surely help to promote short-term mating, especially if infidelity were involved.

  Some combination of an intrinsic personality factor and social context predicts a number of aspects of behaviour better than chance. A general personality factor of ‘intolerance of deviance’ is predictive of resistance to such things as drug-taking in both sexes and early loss of virginity amongst girls (Orford, 2001).

  So, where is the incentive process embodied in the brain? The next chapter addresses this.

  In summary

  Sexual desires arise from raw stimulus features as well as meanings and goals associated with others.

  The control of desire is organized into layers, evolutionarily old and new co-existing.

  There is a common incentive approach system associated with different desires.

  Eight Details of the brain and desire

  For suddenly he was aware of the old flame shooting and leaping up in his loins, that he had hoped was quiescent for ever.

  (D. H. Lawrence, 1928/1993, p. 120)

  Swelling detected at the genitals appears to add to desire arising from the stimulus of a potential partner (Georgiadis et al., 2012) and might even provide the principal stimulus to desire (Laan and Both, 2008). Positive feedback seems to be involved (Mouras et al., 2008); in response to a visual stimulus, desire would contribute to genital arousal and, by means of signals from the genitals to the brain, there would be an amplification of desire.

  The bits that make up the whole

  Knowledge of the brain bases of desire comes from several sources (Chapter 2):

  1 research on non-humans and cautiously extrapolating to humans;

  2 looking at changes in the sexual desire of people following brain damage or disease;

  3 using neuroimaging to examine activity in the brains of people exposed to erotic images.

  Neuroimaging reveals a network of interacting brain region
s, parts of which are excited and others of which are inhibited by erotic visual stimuli (Georgiadis et al. 2010; Redouté et al. 2005). Researchers distinguish regions serving some closely related but nonetheless somewhat conceptually distinct roles:

  1 to make an initial assessment of the sexual value of the content of the image;

  2 to produce a signal that is sent to the genitals to trigger swelling;1

  3 to receive feedback from the genitals on their arousal2 – this signal is thought to contribute to the conscious awareness of the state of the body, particularly the dimension of eroticism;

  4 to create a motivational signal, having unconscious and conscious aspects,3 which tends to direct behaviour towards the sexual stimulus – a feature of this is conscious desire.

  Images are categorized and assessed in terms of their sexual content. Attention is focused upon the image and emotional value (‘intensity’) is attributed to it. If the imagery concerns sexual action, the brain4 is likely to represent this as a simulation of the viewer performing this same action, so-called ‘mirroring’. By extrapolation from evidence on other desires, the insula appears to be a brain region that is sensitive to both external images (in this case, erotic images) and arousal from the body (Craig, 2002). It appears to combine these signals as part of the generation of a desire signal, which then excites the dopamine-rich brain regions, described shortly.

  Asymmetry in the cortex

  Evidence suggests that the left prefrontal cortex has a predominant role in behavioural approach and the right prefrontal cortex has a role in behavioural restraint and inhibition (Sutton and Davidson, 2000). Presenting such incentives as money, a pleasant-tasting food or the opportunity to view a pleasant film causes activation particularly in the left hemisphere. The prefrontal region appears to embody representations of goals and is involved in movement towards them. Those individuals with a higher left-hemisphere activity will be biased towards acquiring incentives and experience high levels of positive affect in their acquisition.

  Similarly, when moving further back in the brain to the temporal lobes, the two hemispheres play somewhat different roles in sexual desire and the exertion of restraint on this. Damage to this lobe in the right hemisphere is often followed by hypersexuality, whereas that to the left is more commonly followed by hyposexuality (Braun et al., 2003).

  The discussion now focuses on the approach system.

  The approach system

  Brain regions underlying approach motivation5 can be distinguished from those involved in remaining passive or general emotional arousal. The role of dopamine in approach and wanting is manifest at both a conscious level of desire and at the level of the unconscious capture of processing by sexual stimuli (Oei et al., 2012).

  Acting ‘like a magnet’, the dopamine-based system underlies attention, wanting, movement towards and pursuit of incentives (Meyer et al. 2012). This provides the common currency of wanting (Berger and Shiv, 2011). For example, the low-level system involving dopamine is excited by anticipation of financial gains by gamblers and drugs by people addicted to them (Breiter et al. 2001).6 Dopamine acting at the level of the nucleus accumbens plays a role in the activation of behaviour and in motivating the individual to overcome obstacles in order to reach a goal (Salamone et al., 2007). It appears that there is a system involving various brain regions and performing a cost–benefit (e.g. instant hedonism versus danger) analysis of behavioural options (Assadi et al., 2009). Dopamine can tilt the bias in favour of immediate gratification, downplaying costs.

  Dopamine appears to be involved in only the seeking (‘goal-directed’) aspect of behaviour. If we can extrapolate from rat studies, when the goal is reached dopamine is deactivated (Blackburn et al., 1992).

  Different individuals can be compared in this regard. Generalizing from other tasks to sexual desire, high levels of dopamine activation underlie high degrees of persistence in the face of obstacles, delays and uncertainty of getting a reward (Treadway et al. 2012). The extreme persistence of some individuals in sexual pursuit, for example stalking, appears to have a basis in abnormally activated dopamine levels accompanied by relatively low levels of serotonin (Meloy and Fisher, 2005). By contrast, the dopamine-deficient individual is lacking in energy and appears apathetic. Given that many situations of sexual and romantic attraction are those of high arousal, we might expect hormones associated with stress to be active at such times (Fisher, 2004). Stress hormones are known to excite dopamine activity.

  In males, even so-called subliminal sexual images (i.e. not triggering conscious awareness) activate various brain regions containing dopamine, including the nucleus accumbens, compared to neutral or unpleasant images (Oei et al., 2012).7 Boosting dopamine levels artificially was associated with still higher levels of activation in these regions in response to the erotic stimuli. Such unconscious triggering of desire, in a world full of sexual imagery (termed a ‘running start’ by the authors) is surely relevant to the difficulties some people experience in resisting sexual desire.

  By comparison with drug-taking, the notion of incentive salience (Robinson and Berridge, 1993) is relevant here. This term refers to the ‘magnet-like power’ of incentives to engage and attract attention and behaviour. Acting through dopaminergic brain systems, repeated use of certain drugs, such as nicotine or heroin, increases their incentive salience. Their sensory representation in the brain become ‘tagged’, such that they exert an increasing control of behaviour and increasing occupation of the conscious mind with thoughts of the drugs. Although, say, sex and gambling do not, of course, involve taking a chemical into the body, their effects seem similar to that of drugs (Pitchers et al., 2010). The consequences of such behaviour involve intrinsic chemical changes in the brain that underlie increasing incentive salience. An exaggeration of this process might underlie hypersexuality (Oei et al., 2012), as in sexual addiction (Chapter 17).

  Knutson et al. (2008) presented erotic images to heterosexual males, while observing (a) activity in different brain regions by means of neuroimaging and (b) the choice made in a subsequent gambling task, whether opting for high risk or low risk. Seeing erotic images tended to move participants towards the high-risk choice. Other studies also point to future discounting after exposure to erotic images. Compared to viewing neutral images, activity triggered by exposure to erotic images was pronounced in the region of the nucleus accumbens, a good predictor of the move to a risky choice.

  Relatively high levels of dopamine activity at both a prefrontal cortical location8 and a subcortical location9 correlate with the strength of an individual’s willingness to exert effort to obtain monetary reward under uncertain conditions (Treadway et al., 2012). Similarly, dopaminergic activity in these regions might be necessary for a person to be motivated to pursue sexual incentives where effort is needed and there is an uncertain outcome. In early human evolution, considerable effort and uncertainty were probably involved in securing sexual partners. This should be contrasted with the experience of some in today’s culture, where no more than a telephone call is needed to obtain ‘room service’. The notion of evolutionary mismatch (Chapter 2) seems to be applicable.

  The high-level conscious and intention system of wanting seems to be less dependent upon dopamine than the low-level system (Meyer et al., 2012). Nonetheless, there is evidence for dopamine acting there. It is commonly assumed that human action is motivated by the conscious future expectation of pleasure and pain (Gilbert and Wilson, 2007; Sharot et al., 2009). An experiment investigated the effect of artificially boosting dopamine levels. When a stimulus (a holiday location) was simply imagined under a condition of elevated dopamine level, subsequently the person estimated a higher hedonic rating of experiencing that stimulus (being in the location) in the future. This suggests a role of dopamine in so-called ‘anticipatory pleasure’.

  Hormones

  What is the role of hormones in sexual desire? At puberty, do they literally ‘drive’ the unfortunate teenager to distraction? So
me might argue that hormones are what give drive to sexual behaviour. However, should we not expect that their level would fall with satiety? There is no evidence for this. As an alternative, how might hormones fit an incentive interpretation? This section will suggest that certain so-called ‘sex hormones’ modulate the strength of incentive motivation.

  Comparison with non-human species

  Insight might be gained from non-human species (Beach, 1947), though there is a risk of uncritical extrapolation to humans. The female rat shows a four-day cycle of changes in sexual activity, from interest through lack of interest, and this depends upon her four-day cycle of hormones. However, unlike primates including humans, she needs a minimum level of hormone to be able physically to assume the posture for mating. Humans, of course, do not need to adopt one particular position to facilitate intromission, the imagination being the most obvious parameter to set limits.