How Sexual Desire Works- The Enigmatic Urge

by Frederick Toates

  The class termed ‘oestrogens’, present in both sexes. In women, they are secreted from the ovaries and to a limited extent, the adrenal gland.

  Adrenalin and noradrenalin (known respectively as ‘epinephrine’ and ‘norepinephrine’ in the American literature), secreted from the adrenal glands.

  Oxytocin, which is secreted from the pituitary gland.

  Figure 2.6 Some of the glands of the body

  Starting from about the eighth week following conception, males have a higher concentration of androgens in the body than do females (Hines, 2004). Partly extrapolating from studies on non-human species, the effects of androgens in humans fall into two classes:

  Organizational effects: before birth, in males they act on the brain to sculpt those processes (survival of particular neurons and establishment of connections between them) that will later come to play a role in masculine desire. In females, the relatively low concentration of androgens is not sufficient to cause this masculinization and therefore the processes of desire assume a feminine form (sometimes termed a ‘default position’). These are permanent (or semi-permanent) effects.

  Activational effects: following sexual maturity in both sexes, androgens act on these same brain processes that were formed earlier to sensitize them, so that sexual stimuli and thoughts give rise to desire. This is largely a transient effect, present only so long as the hormone is present at the neurons.

  It is entirely possible that oestrogens act on the female brain to sculpt desire processes (‘active feminization’), but the effects are less clear than for androgens (Hines, 2004; McCarthy, 2008). However, early female development seems to be mainly a consequence of the absence of androgens. In adult females, oestrogens appear to operate in concert with testosterone in activating desire.

  At times of excitement and challenge, adrenalin and noradrenalin are secreted from the adrenal gland, transported in the blood, and have various effects throughout the body, such as to accelerate the heart rate. Oxytocin is released under various circumstances (e.g. nursing) and has effects of calming and strengthening trust and social bonds.

  Basics of the brain

  This section can only highlight a few of the regions implicated in sexual desire. In generating desire, these regions and several more interact in complex ways, but this is beyond the scope of the present study.

  Psychologists and lay folk alike sometimes discuss which brain region is most evident in human behaviour, the primitive (often expressed as ‘animal brain’) or the recently evolved (so-called ‘civilized brain’). The expression ‘he acted like an animal’ implies such a distinction. The discussion often takes a naïve form, since it is safe to assume that all levels of brain organization will be involved at all times. However, as already noted, evidence points to a layered organization of brain function. Different layers can exert different weights of control and weight can shift between them according to various factors such as age, pathology and alcohol ingestion.

  Within one influential perspective, the human brain is described as the ‘triune brain’ (MacLean, 1990), meaning three dynamically interacting layers in one brain (Figure 2.7). This reflects the brain’s emergence in evolution. In evolutionary terms, the reptilian brain, made up of the brain stem and a number of other structures, is the oldest part, something that we share with reptiles and birds. It is concerned with such things as the organization of relatively simple automatic actions and respiration. The newer paleomammalian brain, shared by all mammals, builds upon this. Complex emotions, such as those underlying the subtleties of social bonding and caring, start to emerge at this level, which corresponds to what is known as the ‘limbic system’. Finally, the evolutionarily newest part of the brain, the neomammalian brain, is evident in primates and reaches its most elaborated structure in humans. The neomammalian brain is necessary for what are thought to be peculiarly human attributes such as self-awareness, complex language and the ability to project the imagination backwards and forwards in time. The control of human behaviour requires integration between these brain layers, one especially evolved in humans and the other two largely shared with some other species, something that appears to give rise to more than the occasional problem!

  Figure 2.7 The triune brain

  An analogy, albeit highly simplified, is sometimes used with a building.3 Suppose that the basic construction of a cottage was done in the sixteenth century but plumbing was not installed until the nineteenth century, when a new wing was added. Finally, electricity was added in the twentieth century. It is still the same cottage, with its original foundations and low beams, but the overall functioning of the cottage has changed over the centuries.

  Sometimes this layered organization is described as a ‘hierarchy’ (MacLean, 1990). One might logically assume that the neomammalian brain is at the top of the hierarchy and the reptilian brain at the bottom. Things sometimes do work roughly in this way, as in putting conscious intentions, assumed to arise largely in the neomammalian brain, into effect by the muscles of the body. However, the brain does not always work like a well-disciplined army with a commander giving orders that are faithfully followed by lower ranks. At times, the better analogy is with a rebellious army, where the foot soldiers often usurp control. As Price (2002) observes (p. 108):

  When treating patients with depression and anxiety, the clinician finds it obvious that higher centres do not control the lower ones. No patient with his rational brain can command his emotional brain to feel less depressed or anxious.

  The same might be said about the stubbornness of unwanted sexual desires when corrective therapy is tried.

  On the experience of pleasure, Smith et al. (2010) suggest that ‘basic liking’ is embodied in mechanisms shared with other mammals. The neomammalian brain contributes a fine-tuning. Of course, one imagines that the appreciation of a Beethoven symphony is a peculiarly human phenomenon requiring neomammalian structures. However, in all probability the pleasure of orgasm and even that of the symphony depend also upon paleomammalian structures.

  Figure 2.8 shows the human brain, indicating the lobes, and highlighting a part of the frontal lobe: the prefrontal cortex. The outer layer of the brain, the cortex with its wrinkled structure, is the part most strikingly different in humans as compared to any other species. It largely corresponds to the neomammalian brain. All brain structures not in the cortex are termed ‘subcortical’. Figure 2.9 is an X-ray view of the brain highlighting some subcortical (‘limbic system’) structures.

  Figure 2.8 The human brain (a) showing the lobes; (b) highlighting the prefrontal cortex (shaded dark)

  Figure 2.9 The human brain showing some of its deep structures

  Sexual desire arises from a combination of sensory stimuli, that is visual, smell, sound and touch, acting in the brain at both a raw level and in the context of memories and meanings with which they are associated. Information on these sensory stimuli is carried by nerves (bundles of neurons) to the brain.

  A special type of neuron in the brain, known as a ‘mirror neuron’, has come into prominence recently and might be involved in sexual desire, particularly where it involves visual erotica (Mouras et al., 2008) (Chapters 8 and 16). They are located in regions of cortex at the frontal and parietal lobes. A mirror neuron is active when either a particular action is being performed or when another individual is observed to be performing the same action.

  Consider, for example, triggering of desire by visual stimuli. Light strikes the retina of the eye and information on the image is then transmitted by neurons to various brain regions. There is a fast route to the amygdala (there are two amygdalae, one in each half of the brain, though the singular is usually used), where certain basic features of the image are rapidly processed outside conscious awareness (LeDoux, 1999). Another parallel route that information takes is a slower and more refined one to the visual cortex (located at the back of the occipital lobe) and beyond (through the temporal lobe). Here information is processed and detailed meaning on
the nature of the object is extracted for fine-grained image identification. Further detailed processing is carried out in other brain regions, for example the frontal lobe, where the information is put into a context of meanings and personalized memories, associated with conscious awareness (Kringelbach, 2010). A feature of this further processing is to attribute nuanced value, for example erotically desirable or not, to the event in the world. This could depend upon such things as the meaning of any potential sexual interaction and the nature of the social relationship with a given individual.

  The amygdala (Figure 2.9) attaches crude emotional and motivational significance to events in the world (Mahler and Berridge, 2012). Information in both the slow and fast routes projects to the amygdala. For example, an erotic picture would be labelled as such (‘erotic salience attribution’), in part by processing of the information derived from it by the amygdala. The amygdala, in turn, conveys information to other structures, such as the hypothalamus (Georgiadis and Kortekaas, 2010) and the nucleus accumbens (described shortly).

  The hypothalamus (Figure 2.9) consists of several sub-regions, each concerned with the organization of one or more particular form of behaviour and control of hormonal events in the body (Hines, 2004). Certain hormones are released from the pituitary gland under the influence of the hypothalamus and they in turn exert a role over the release of oestrogens and androgens from the ovaries and testes.

  A region of the hypothalamus, the ‘anterior hypothalamus/preoptic area’ (AH/POA) has a pivotal role in the control of sexual behaviour. Neurons in the AH/POA are responsive to sexual stimuli and contain receptors that are occupied by testosterone brought via the blood. On engaging with the receptors, testosterone sensitizes these neurons, so that they are particularly responsive to sexual stimuli, underlying presumably part of sexual desire and possibly sexual pleasure (Georgiadis and Kortekaas, 2010). Conversely, loss of testosterone (e.g. following disease) is associated with desensitization of these neurons, with some loss of desire. A sub-region of the AH/POA, termed the INAH-3, is larger in men than women and contains more neurons (described later). In response to erotic stimuli, activation of the hypothalamus is lower in women than in men (Hamann et al., 2004), which might say important things about the roots of desire.

  Figure 2.10 shows pathways of neurons that use dopamine as their neurochemical. They project to several regions of interest here. One place at which they terminate and release dopamine is known as the ventral striatum, which contains the nucleus accumbens. Activity of part of the nucleus accumbens underlies the raw pull that certain events, such as sex and drugs, can exert on behaviour. A different part of the nucleus accumbens has a role as a basis of pleasure (Kringelbach and Berridge, 2010). The nucleus accumbens is a hub that receives information about such things as food and sexually attractive targets and then transmits signals to parts of the brain that organize action based on this (Robbins and Everitt, 1996) (Figure 2.11). The nucleus accumbens is sensitive to conditional stimuli associated with rewards. For example, a conditional stimulus such as a tone sounded just before the appearance of a sexual partner would excite activity there, leading to directed attention and searching. Neurons using dopamine also project to the prefrontal cortex and to the amygdala.

  Figure 2.10 Pathways of neurons that use dopamine: (a) human; (b) rat

  Figure 2.11 The nucleus accumbens

  A region of the prefrontal cortex that is associated with attributing value to events is the orbitofrontal cortex (OFC). Neurons that are active here encode the hedonic value of particular stimuli (Rolls, 2012). For example, in the case of taste, early processing (before the OFC) detects, say, a curry taste and its raw hedonic value, whereas later stages of processing by the OFC attribute a more refined hedonic value of liking or not to the taste. The liking value arising here depends in part upon the intrinsic taste but also (a) the level of nutrients in the body, (b) how much food of this particular taste has been recently ingested and (c) high-level associations such as any verbal descriptions and cultural meanings related to the taste. So, the OFC, constituting a nexus, is informed of all these factors. Taking them into account, the activity of particular OFC neurons is part of the brain embodiment of the pleasure derived from a taste. Other OFC neurons encode unpleasant events such as unpleasant touch or smell.

  Extrapolating to sex, a combination of, say, touch, perceived attractiveness of a partner and meaning would similarly converge onto a different set of neurons in an adjacent OFC region. If one can extrapolate from monkeys, neurons in the OFC are responsive (‘selectively active’) to particular faces (Rolls, 2012). Regions of human OFC are more strongly activated by what are judged as attractive faces, something of obvious relevance to human sexual desire. The OFC is then an integrator, which extracts signals corresponding to the holistic quality of events put into context. Other neurons encode when an expected event fails to occur. Neurons project from the OFC to the nucleus accumbens (Rolls, 2012), where they appear to inhibit or enhance the strength of signals arising from low-level processing (Figure 2.11).

  Other regions of the prefrontal cortex are also relevant here (Figure 2.10). Neurons employing dopamine (‘dopaminergic’ neurons) make projections from the VTA (ventral tegmental area) to the dorsolateral region of the prefrontal cortex, which is involved with holding memories in awareness and using them in the control of behaviour (Luciana, 2001).4 Extrapolating from studies on monkeys, this area might be expected to be activated when a sexual incentive not physically present, that is distant in space and time, is being pursued, and during sexual fantasy. Dopamine most likely gives strength (‘salience’) to the activated memory so that it can occupy consciousness and on occasion take command of the control of behaviour.

  Of course, there are also inhibitions on sexual desire and its expression. Inhibition arises from, amongst other things, sexual satiety following orgasm and the assessment of risks attached to sexual advance. Inhibition can be linked to particular brain regions. Damage to the temporal lobes is sometimes associated with heightened sexuality, suggesting that parts of this cortical region normally have an inhibitory effect (Georgiadis and Kortekaas, 2010). One might speculate that the arousal some find in autoerotic asphyxiation, by strangling, could arise from a cut-off of blood supply and hence relative inactivation of the cortex as compared to other brain regions. Parts of the OFC (as distinct from those described earlier) are activated at times of sexual satiety, suggesting a restraining role. The OFC is associated with ‘urge suppression’; it plays a role in restraining behaviour motivated by short-term gain in order to avoid long-term costs (Bechara et al, 2000). Damage there can be followed by the manifestation of inappropriate sexual desires.

  The parts of the brain do not develop at the same rate. Of interest here, the prefrontal cortex is slow to develop, not reaching full maturity until well into the twenties (Steinberg, 2007). Corresponding to this reorganization, new processing properties become possible, such as the capacity to hold items in memory, to plan and to inhibit behaviour.

  The autonomic nervous system

  The ‘autonomic nervous system’ is a division of the nervous system that controls the activity of the inside of the body, such as the stomach, intestine, genitals and heart (Figure 2.12). It controls the flow of blood to the various parts of the body, by means of neurons that contact the muscles located at the local blood vessels.

  Figure 2.12 The autonomic nervous system

  It is called ‘autonomic’ because it is to a considerable degree self-governing, for example, we do not consciously decide when to start digestion or accelerate the heart beat. As many soon discover, an exertion of conscious will can prove quite ineffective in diverting blood to the genitals. Signals to the autonomic nervous system and hence to the bodily organs are computed in the hypothalamus and other brain regions. There is a strong link between activity in the hypothalamus and penile erection (Georgiadis and Kortekaas, 2010).

  There are two divisions of the autonomic nervous system:
the sympathetic and parasympathetic. The sympathetic is active at times of challenge and is responsible for activation, for example increased heart rate and blood pressure. The parasympathetic is dominant at times of relaxation and has effects such as to lower heart rate. Neurons with their tips in the organs such as the heart and genitals transmit signals back to the brain concerning such events as respectively heart rate or touch.

  In seeking insight, the book will look at sexual desire in the context of a range of other desires, discussed next.

  Insight through comparison with other desires

  But is it not equally difficult to understand fully anybody who is wholly dedicated to any other single interest, be it money, power, beauty, study, or some special hobby, when one does not feel capable of being so absorbed oneself? To some men there is, for instance, nothing more interesting than, say, sports cars or horse races.

  (Kronhausen and Kronhausen, 1967, p. 187, on Walter)

  One approach to understanding sex is to employ analogies, metaphors or comparisons with other desires. Does sex work in a similar way to anything else, so investigators in different areas can pool insights? In psychology textbooks, sex is often grouped with feeding, drinking and drug-taking, under the general heading of ‘motivation’. Some similar, if not identical, brain processes underlie each (Robbins and Everitt, 1996). Everyday expressions compare sexual desire with other desires, as in ‘hungry for sex’ or ‘you are like a drug to me’. These are more than colourful metaphors, but rather they tap into a common feature of all such desires and dopamine systems (Figure 2.10) are central to how desires work. In each case, there can be strong craving to answer the desire and pleasure associated with its satiety (Georgiadis et al., 2012).