DRAFT
Desire emerges from excitement.
A psychophysiological perpective on sexual motivation.
Stephanie Both
Walter Everaerd
Ellen Laan
Department of Psychology, University of Amsterdam, Roetersstraat 15, 1018 WB Amsterdam, The
Netherlands (email: sboth@uva.nl)
In the dominant model of human sexual response sexual desire, excitement, and orgasm are
distinguished as consecutive phases (DSM-IV, 1994). The model is based on the
psychophysiological research of Masters and Johnson (1966), and on the ideas of the psychiatrist
and sex therapist Helen Kaplan (1995) who introduced the desire phase as the phase preceding
sexual excitement. Kaplan was seeing in her clinic many female patients complaining about a lack
of desire for sex. Absence of sexual desire, she reasoned, points to a phase in the normal sexual
response cycle that activates the wanting to experience sexual excitement. “By thus examining the
current sexual experiences of 2,109 patients and couples with chief complaints of deficient sexual
desire I came to the conclusion that the pathological decrease of these patients’ libido is essentially
an expression of the normal regulation of sexual motivation gone awry. More specifically, sexual
motivation or desire, just like other needs or motives, such as hunger and thirst, is regulated by
CNS control mechanisms.” (Kaplan, 1995, p. 3-4). Kaplan conceptualized sexual desire as an
expression of a drive, comparable to hunger and thirst, influenced by sensors that signal changes in
the internal environment of the body. In her view on sexual desire, psychoanalytic thinking can be
heard; thinking that has long dominated ideas about sexual motivation. Even though there is little
evidence for a homeostatic mechanism in human sexual motivation, the essence of the concept of
sexual drive remains in the dominant model of sexual response and in the DSM diagnosis of
hyposexual desire that is based on that model.
In this contribution, first, the concepts libido, lust and drive that were introduced by Freud
will be discussed. Then modern motivation theory and the close relationship between emotion and
motivation mechanisms will be elaborated upon. This relationship will be illustrated by recent
experimental studies that we conducted on motor preparation, sexual arousal and action. Sexual
arousal and desire will be considered in the light of current knowledge about neurobiological
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mechanisms of emotion and motivation. After that we will talk about the interaction of emotion and
motivation circuits in the brain, and the role of dopamine in motivation. We will conclude that
current knowledge about emotion and motivation mechanisms argues against Helen Kaplan’s
assumption that desire and excitement are distinctive sexual response phases. Desire results from
the conscious awareness of the sexually excited state of the body and the brain. This means that the
experience of sexual desire can only come about through sexual excitement.
The drive model versus incentive motivation models of sexual motivation
The concepts of libido, lust, and drive
Sexual motivation is a construct, used to explain the generation of sexual action. Sexual desire is
the (subjective) experience of being attracted to or pushed towards objects or behaviors with
potential rewarding effects. As we noted before, in Freud’s days sexual motivation was seen as a
constant force (Everaerd, Laan, Both, & Spiering, 2001). For sexual desire Freud (1953) preferred
the use of the word libido. "Everyday language possesses no counterpart to the word 'hunger', but
science makes use of the word 'libido' for that purpose." (p. 135). In a footnote, added by Freud, it
is explained that the German language word 'Lust' is ambiguous, because it can either mean the
experience of need and of a gratification. In a note on page 212 the explanation has been elaborated
upon. "The word 'Lust' takes into account the part played by preparatory sexual excitations which
simultaneously produce an element of satisfaction and a contribution to sexual tension". 'Lust' has
two meanings, and is used to describe the sensation of sexual tension ('Ich habe Lust = 'I should
like to', 'I feel impulse to') as well as the feeling of satisfaction. Thus the word lust refers to sexual
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arousal and the wanting to have more of that, as well as to sexual satisfaction. We will come across
these two different meanings later on when concepts of wanting and liking that are important in
recent motivation theories are discussed.
Libido, according to Freud (1964), is fuelled by the sexual instincts. An instinct arises from a
source within the body. "Its source is a state of excitation in the body, its aim is the removal of that
excitation; on its path from its source to its aim the instinct becomes operative psychically. We
picture it as a certain quota of energy which presses in a particular direction. It is from this pressing
that it derives its name of 'Trieb' (literally 'drive)" (p .96). Freud made clear that libido does not
arise from an external stimulus. "An instinct, then, is distinguished from a stimulus by the fact that
it arises from sources of stimulation within the body, that it operates as a constant force and that the
subject can not avoid it by flight, as is possible with an external stimulus" (p. 96). Freud conceived
of the instinct as energy that pushes towards a certain direction. From that push-factor the German
word ‘Trieb’, drive, is derived. Thus the sexual instinct comes from a source within the body, it
operates as a constant force, and the subject can not escape from it as is possible in the case of an
external stimulus. In this view libido is the result of an internal bodily tension, and there would be a
need to neutralize this state. How the drive builds up remains unclear.
In her discussion of the regulation of sexual motivation Kaplan (1995) used the concepts
libido, lust and drive. She localized the sexual instinct in the hypothalamus, the organ in the brain
where physiological deficits and the restoration of homeostasis are signalled. “ Once again we can
learn from the similarities between eating and sex. More specifically, under normal circumstances,
a state of starvation activates the ventromedial hypothalamic “appetite centers”. This
neurophysiologic activity produces a subjective feeling of hunger” (Kaplan, 1995, p. 17). Because
it is unknown how processes in the hypothalamus contribute to sexual motivation for the time being
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she considered it to be a black box that, in interaction with sexual inciters, activates sexual desire
and lust. There is, however, no evidence of any adverse effects of sexual abstinence. As Beach
(1956) concluded: “No genuine tissue or biological needs are generated by sexual abstinence ….
What is commonly confused with a primary drive associated with sexual deprivation is in actuality
sexual appetite, and this has little or no relation to biological or physiological needs” (p. 4). In the
case of sex there does not seem to be a biological need that demands satisfaction, as is the case in
hunger and thirst. More recently, Herbert (2001) proposed that sexual behaviour may be thought of
as a form of adaptation or response to a perceived deficit. He suggested that the hypothalamus uses
the current levels of gonadal steroids to monitor the current levels of sexual interest and behaviour.
He underlined, however, that sexual behaviour is a complex activity that relies upon the receipt and
analysis of complex social stimuli. Animals and humans will only be brought into sexual readiness
by changes in hormone levels when a sexually attractive stimulus is perceived.
Incentive motivation
According to the drive model we have sex because we feel sexual desire, this is an expression of
Freud’s view that there is a constant force that seeks an outlet. Incentive motivation theories
emphasize that sexual motivation is the result of the activation of a sensitive sexual response
system by sexually competent stimuli that are present in the environment (Bindra, 1974; Singer &
Toates, 1987). Once the sexual system, wherein sensitivity is influenced by hormones and
neurotransmitters, interacts with the stimulus, the individual is pushed towards sexual activity. In
this view sexual motivation does not emerge through a deficit signaled by the hypothalamus, but
through the attractiveness of possible rewards in the environment. Desire is activated, through
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expectations of reward, by attractive and rewarding stimuli in the environment.
Motivation is strongly related to emotion mechanisms. Emotion and motivation mechanisms
interact in such a way that it is sometimes hard to distinguish them; they are two sides of one coin.
According to Bindra (1974) motivation emerges in the interaction of the organism and the
environment. There is no motivation without a stimulus that predicts reward or danger (Schultz,
1998). Our brains are designed to adequately respond to changes in the environment, to avoid
danger and to approach situations that may be rewarding. We learn through experience what is
dangerous and what is rewarding. Emotions can be looked upon as action mechanisms that help the
organism to adapt to aspects of the environment that promote or endanger survival. In this view
emotions serve to satisfy concerns, and therefore result in motivational states or action tendencies
(e.g. Frijda, 1986; Lang, 1993; LeDoux, 2001). The motivational state or action tendency is
reflected in the physiological changes that accompany emotions and which prepare the body for
action.
Cognitive neuroscience is revealing how emotions, the consequent motivational states or
action tendencies, and emotional feelings appear (Craig, 2002; Damasio, 2003; LeDoux, 2001). The
actual or imagined presence of an emotionally competent stimulus activates the emotion-triggering
sites in the brain. These emotion-triggering sites subsequently activate the emotion-execution sites
that activate changes in the internal organs, and in the musculoskeletal system. Damasio (2003) and
LeDoux (2001), and a long time before them William James (1884), stress that the conscious
experience of emotion, what we call feelings, is the result of the perception of these changes.
Feelings do not precede but follow emotion; feelings are based on the feedback of the emotional
bodily and brain responses to the brain. Feelings are the end result of the whole ‘machinery of
emotion’.
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We propose that the mechanism through which sexual emotional states and feelings of
sexual excitement and desire appear, will be similar as in other emotions that are coupled with
relatively strong bodily reactions. Recently functional imaging studies showed that the subjective
experience of different emotions like anger, disgust, anxiety, and sexual arousal is associated with
activation of the insula and the orbitofrontal cortex (Craig, 2002; Morris, 2002; Sumich, Kumari, &
Sharma, 2003). This indicates that the insula has a generalized role that is not specific to any
particular emotion. Its role seems to be the representation of peripheral autonomic arousal that
provides input to conscious awareness of emotional states.
We suppose that processing of a sexual stimulus automatically results in preparation of the
organism for sexual action. When we become aware of these bodily changes, through feedback of
these bodily responses to the brain, we experience sexual excitement and desire. Thus, sexual desire
does not appear out of the blue. Without a stimulus that activates arousal there will be no desire. In
fact, there is no good reason to assume that desire and excitement are two fundamentally different
things. Perhaps we can phenomenologically distinguish them such that feelings of excitement
represent the subjective experience of genital changes, perhaps combined with a conscious
evaluation that the situation is indeed ‘sexy’, and that feelings of desire represent the subjective
experience of an action tendency, of a willingness to behave sexually.
Processing of emotionally competent stimuli results in physiological changes that prepare
the body for action. In case of threatening stimuli avoidance behavior will be activated, and in case
of attractive (sexual) stimuli appetitive behavior will be triggered. Appetitive behavior includes
locomotor responses to the goal, and occurs in parallel with autonomic and endocrine responses that
prepare for efficient interaction with the goal (Robbins & Everitt, 1999). The generation of sexual
appetitive behavior involves specific genital reactions. However, there will be also changes in the
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somatic motor system. Signals are sent to the muscles to prepare for action. Eventually, these
changes may result in sexual behavior. We will illustrate this process by discussing recent
psychophysiological studies on motor preparation, sexual arousal and action.
Sexual arousal and action tendency
Research in women by Ellen Laan showed that there is an automatic relationship between sexual
stimuli and genital response, and that this response may emerge without the person being aware of
that (Laan & Everaerd, 1995). Recently we focused on somatic motor system changes that result
from the processing of emotionally competent stimuli. More specifically, we searched for an
appropriate measure for motor preparation. One way to do this is to monitor changes in the
amplitude of reflexes. Motor preparation involves heightened activity in the spinal cord, and that
activity will be expressed as a stronger reflex (Brunia & Boelhouwer, 1988). The reflex we use is
the Achilles tendon reflex, for short, the T-reflex. Tendon reflexes are not sensitive to valence of an
affective state, but they are augmented in states of preparation for action, and are modified by
differences in arousal intensity (Bonnet, Bradley, Lang, & Requin, 1995; Brunia & Boelhouwer,
1988; Brunia & van Boxtel, 2000). Therefore investigation of T reflex modulation offers a window
on the generation of action.
Bonnet et al. (1995) hypothesized that stimuli that elicit emotional arousal will facilitate T
reflex magnitude, relative to neutral, low arousal stimuli. They stated that the T reflex, which
functions when the limb is activated for walking, standing, and other activities, is inherently nondirectional (one can run either towards or away from stimulation). Since the T reflex is nondirectional it would be involved both in actions that are appetitevely and defensively motivated.
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They studied the modulation of T reflexes during the presentation of pictures from 'The
International Affective Picture System' designed by Lang, Öhman, and Vaitl (1988). These pictures
were designed to induce emotions varying in valence (positive to negative) and in intensity (low to
high). As expected, T reflexes were significantly augmented when elicited during processing of
highly arousing emotional pictures (either negative or positive) as compared with neutral pictures.
Similar to Bonnet et al. we hypothesized emotional stimuli to automatically generate action
tendencies. These action tendencies will result in increased spinal excitability, reflected in
facilitated T reflex magnitude. We studied reflex modulation by appetitive (sexual) and aversive
(anxiety and sexual threat) (Both, Everaerd, & Laan, 2003) and by sexual stimuli varying in
intensity (Both, Everaerd, & Laan, in preparation). In these studies T reflex amplitude, genital
response, subjective sexual arousal and subjective action tendencies were measured.
In the first study subjects were shown 4 film clips of 5 minutes each: a sex stimulus
depicting consensual sexual activity, an anxiety-provoking film, a sexually threatening film, and a
neutral film that was not expected to be emotionally arousing. It was expected that the 3 emotional
stimuli would result in changes in subjective action tendencies, the sexual film in a facilitated
approach tendency and the sexually threatening and the anxiety-provoking film in a facilitated
avoidance tendency. It was also expected that the 3 emotional stimuli would result in stronger Treflexes than the neutral film, and that reflexes during the neutral film would not be higher than
during the preceding rest period. Both the positive and the negative emotional stimuli resulted in
motor preparation, and the subjects reported that during the threatening stimuli they felt the wish to
avoid, and during the consensual sexual stimulus the wish to approach. The increase in reflex
amplitude in response to the sexual stimulus did not differ from the increase in response to the
threatening stimuli.
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In a second study sexual stimuli were used that differed in intensity. The low intensity
stimulus depicted erotic kissing, the medium stimulus showed kissing and caressing, and the high
intensity stimulus depicted intercourse. From a previous study of Ellen Laan (Laan, Everaerd, van
der Velde, & Geer, 1995) we knew that these particular clips evoked distinct, and increasing levels
of genital arousal in women. As expected, T-reflex amplitude increased with increasing levels of
sexual stimulation, and these were significant differences even in a small sample of 10 women.
These studies demonstrate that in interaction with emotionally competent stimuli a tendency
is generated to avoid or approach. Negative or positive emotional stimuli generate an affective
response, and part of this affective response is the generation of an action tendency, which is an
expression of motivation. This action tendency increases as the intensity of the emotional state
increases. To demonstrate that confrontation with a sexually competent stimulus leads to action
tendencies and, eventually, sexual action, we studied sexual activity after laboratory induced sexual
arousal (Both, Spiering, Everaerd, & Laan, under review). Male and female subjects were
randomly assigned to a neutral or sexual condition. In the neutral condition subjects were exposed
to a 15-minute neutral film, and in the sexual condition to a 15-minute erotic film. We expected that
only in the sexual condition genital responses, subjective feelings of sexual arousal, and approach
tendencies would be elicited. Subjects completed, 24 hours after the experimental session, a
questionnaire about their sexual behavior during these 24 hours. We expected that subjects in the
erotic film condition would engage in more sexual activity than the subjects in the neutral film
condition. As expected, the subjects that were exposed to the erotic film had genital responses,
increased T-reflexes, and reported sexual arousal and approach tendencies. Those exposed to the
neutral film did not show these specific reactions. Moreover, the subjects who had seen the erotic
film had engaged in more sexual activity than the subjects who did not see the erotic film. In sum,
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these studies show that confrontation with sexual stimuli results in action tendencies, and these
action tendencies increase the likelihood of actual sexual behavior.
Neurobiological mechanisms
In retrospect, Bindra’s view that emotion and motivation mechanisms are closely intertwined is in
agreement with current ideas about how the brain may work. Sensory information is converted in
the brain to set off motor responses in the autonomic and somatic nervous system. Bindra had no
clear idea of the neurobiological mechanism that may explain the interaction of limbic and motor
systems. Mogenson, Jones and Yim did a proposal in 1980 that still inspires motivation research
(Mogenson et al., 1980; LeDoux, 2001). Recently Holstege (1998) did a similar proposal, the
‘emotional-motor system’. Holstege distinguishes the somatic motor system and the emotional
motor system. The somatic motor system is controlled by the motor cortex and the brainstem.
Through this system voluntary movements are controlled. The emotional motor system is
controlled by structures that are part of or are connected with the emotional circuit in the brain, the
limbic system. Through the emotional motor system specific emotional behaviors, like for example
mating behavior, and more general changes, like increased muscle tension, are initiated.
Mogenson, et al. (1980), and more recently LeDoux (2001), present the following sketch of
the interaction between the limbic system, the emotion circuit, and the motor system, the motivation
circuit. The interface between the amygdala, part of the limbic system, and the motor system is the
nucleus accumbens. The nucleus accumbens receives direct input from the amygdala, and indirect
input from the ventral tegmental area. This area is the source of the dopaminergic connections to the
nucleus accumbens. This nucleus accumbens passes the information on to the globus pallidus,
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which is connected to cortical and brainstem areas that control movement. This interface between
the emotional and the motor circuit is necessary because primary (emotional) needs need to be
transformed into actions, to satisfy those needs. Dopamine has an important role in the motivational
circuit; it seems to be involved in both reward signalling and in the initiation of motor responses
(Kalivas & Nakamura, 1999; Phillips et al, 2003). Dopamine has long been regarded as the
transmitter responsible for the experience of satisfaction. However, studies in rats have shown that
dopamine is not involved in the valence or appreciation of a stimulus, but in the tendency to
approach a stimulus (Berridge, 1996).
Berridge introduced the concepts of ‘wanting’ and ‘liking’. Liking represents the affective
response to a stimulus. Both rats and humans exhibit specific facial expressions with the negative or
positive appreciation of food, from these specific facial expressions the affective response can be
derived. Wanting represents the tendency to approach a stimulus. This tendency may express itself
in the frequency and intensity with which a rat presses a lever to obtain food. Berridge showed that
manipulation of the dopamine system affects the instrumental behavior (wanting), and not the facial
expressions (liking). The difference between wanting and liking is relevant for understanding
disorders related to motivation. It explains why a person who is addicted to some substance can
crave for the drug intensely, whereas subsequent intake of the drug is not very satisfying.
Animal studies have shown facilitating effects of dopamine on sexual motivation (Melis &
Argiolas, 1995), and the very few studies in humans also showed a facilitating effect of dopamine
on sexual motivation and sexual arousal (Meston & Frohlich, 2000). We reasoned that measuring
somatic motor system activity through means of T reflex modulation may offer a sensitive measure
to investigate the effects of psychomotor stimulant drugs like dopamine on sexual arousal and
sexual motivation in humans. We wanted to find out whether a dopamine agonist, that increases
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dopamine levels in the brain, would affect sexual response, particularly action tendencies. We
expected dopamine to affect the wanting component of sexual motivation, specifically the
instigation of action, and therefore to result in stronger T reflex magnitudes in response to sexual
stimulation. Secondly, based on the evidence for dopaminergic influences on penile response in
men (e.g. Giuliano & Allard, 2001; Heaton, 2000), we expected levodopa to facilitate genital
response. Conscious emotional feelings, and subjectively experienced tendencies for approach
behavior may also be affected by levodopa. Although Berridge (1996) underlines that awareness of
a motivational state may be dissociable from the underlying motivational processes that give rise to
the conscious experience, it may be expected that substantial changes in the underlying processes
will enter consciousness and will be reflected in subjective report. Both men and women
participated to allow for investigation of gender differences in the effects of dopamine on sexual
responses.
Subjects (19 men and 28 women) were given a single dose of levodopa (100 mg), which
increases dopamine levels in the brain (Both, Everaerd, Laan, & Gooren, in preparation).
Participants visited the laboratory 2 times, receiving at one visit a placebo and at the other visit
levodopa, following a double blind, crossover protocol. Fifty minutes after placebo/drug
administration, when substantial dopamine levels were expected, subjects were asked to engage in
a pleasant sexual fantasy, and then they were shown an erotic film. T reflexes, genital responses,
subjective action tendencies and subjective sexual arousal in response to erotic stimulation were
measured. We found that levodopa did not affect the subjective and the genital responses. There
was a significant effect of levodopa on T-reflexes. In men, T-reflexes were stronger with levodopa,
both in the sexual fantasy and in the erotic film condition. In women there were no effects of
levodopa at all. Thus in accordance with evidence from studies in male rats, an increased level of
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dopamine resulted in stronger instigation of action in response to sexual incentives in human males.
The absence of an effect of levodopa on T reflex magnitude in women is in line with the conflicting
reports about the effects of dopamine on sexual motivation in female rats (Meston & Frohlich,
2000) and warrants further study. The fact that levodopa increased male T reflex magnitude during
sexual stimulation shows that, in line with the expectations, dopamine is involved in the energetic
aspects of motivated behavior, and that T reflex modulation offers a sensitive measure for
dopaminergic effects on the generation of sexual appetitive behavior in humans. About the sex
difference in the effect of levodopa we can only speculate. As noted before, studies on
dopaminergic effects on sexual motivation in female rats revealed conflicting results. These
conflicting results are attributed to possible interactions of dopamine with the hormonal treatments
that are used to induce estrus in female rats. There is evidence for steroid-dopamine interactions
(e.g. Balthazart, Baillien, & Ball, 2002; Becker, 1999; Giuliano & Allard, 2001). The gender
difference in the effect of levodopa in the present study might be due to differences in sex steroid
levels in the brain. Perhaps the effect of dopamine depends on testosterone levels, which are higher
in males. It would be informative to include hormonal status as a variable in future studies on
dopaminergic effects on sexual responses in women.
Conclusion
The mechanism of sexual motivation can be summarized as follows. Processing of a sexually
competent stimulus (actually present or imagined) automatically energizes emotional systems,
resulting in bodily changes that prepare for sexual action. The elicited action tendency increases as
the intensity of the emotional state increases. Dopamine is important for the transformation of
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emotion into action. It seems to be involved in both reward signalling and in the initiation of
behavioural action. The bodily changes that are elicited by a sexually competent stimulus include
motor responses involved in general approach behaviour, and motor responses involved in sexspecific responses; e.g. relaxation of genital smooth muscles. The processing of a sexually
competent stimulus is largely involuntary and unconscious. Feelings of sexual excitement and
desire emerge when motor responses enter consciousness through the feedback of the bodily and
brain responses to the brain.
From this follows that the experience of sexual feelings is a consequence of an incentive
energizing the sexual system. Feelings of sexual desire and excitement result from the awareness of
the sexually excited state of the body and the brain. Hence, the experience of sexual desire can only
come about through sexual excitement. Contrary Kaplan’s proposal sexual desire does not precede
sexual excitement; excitement precedes sexual desire. However, the concepts of libido and drive
can be replaced by more precise concepts derived from cognitive neuroscience. In addition, we
know that reports of patients about sexual desire may be as much a reflection of current ideas or
myths about sexual desire as to what actually happens in the motivational process.
The consequences of our view on sexual desire are important for practice. From an
incentive motivation model it should be concluded that hyposexual desire is not a manifestation of
a malfunctioning instinct, but an indication that the emotion-motivation mechanism is not activated.
In some cases hypoactive sexual desire may be caused by a lack of brain chemicals, but in most
cases the cause will be the absence of attractive stimuli. In fact Kaplan was aware of that, since an
important part of her treatment for hyposexual desire consisted of what she called: “Libido
enhancing sexual homework assignments: fantasy and friction” (Kaplan, 1995, p. 6). That
homework could be sexual fantasy, explicit erotic material, masturbation, or other methods of
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erotic stimulation. Secondly, an incentive motivation view emphasizes the importance of the
evaluation of the stimulus, which is determined by individual histories of sexual rewards. When a
patient has little or no experience with sexual rewards or a mainly negative sexual or relational
history, pharmacological enhancement of the sensitivity of the sexual system (be it with androgens,
dopamine, or other pro-sexual drugs) will not facilitate sexual desire.
We expect that the study of somatic motor system activity through means of T reflex
modulation may offer a sensitive measure to investigate (disorders in) the signalling of sexual
reward and the instigation of action tendencies in humans. In conclusion, it’s obvious that our
knowledge about sexual motivation, about hypoactive as well as hyperactive sexual desire, will
only increase by further studies into the behavioural mechanisms through which sexual action is
instigated and regulated.
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