Sexual Arousal in the Female Brain: A Review of Neuroimaging Studies
Sophie Rosa van ’t Hof
MSc in Brain and Cognitive Sciences, University of Amsterdam
Student Number: 10713328
Date of Submission: 21-10-2019
Supervisor: Nicoletta Cera, Faculty of Psychology and Educational Sciences, University of Porto Co-assessor: Mark Spiering, Faculty of Social and Behavioral Sciences, University of Amsterdam
Abstract
The underlying mechanisms of sexual arousal in humans can be investigated with
neuroimaging studies. The majority of studies have been conducted with male participants,
leading to men-based models of sexual arousal. Instead of focusing on male studies, this
review discusses sexual arousal in women, differences between women and methods used to
study sexual arousal with neuroimaging techniques in women. Reviewing the current
literature has led to several factors that should be taken into account when conducting a
neuroimaging study with female participants: menstrual phase, hormonal contraception use, a
history of sexual or psychiatric disorders or diseases and medication use. To optimize sexual
arousal during a sexual stimulus, sexual orientation and perhaps additional sexual preferences
are advocated to be assessed. In addition, to optimize sexual arousal, visual sexual stimuli
should be selected by women and there is evidence that duration of the stimuli should maybe
be extended compared to duration commonly used. Overall, there are several aspects to take
into account when researching neural response to sexual stimuli in women. A higher
inclusion of women could lead to more accurate neurobiological models of sexual arousal.
Keywords: Sexual arousal, Women, Brain, Neuroimaging, Functional magnetic
Content
Sexual Arousal 4
Possible Influences on Sexual Arousal 8
Hormone Levels 8
Menstrual phase. 10
Hormonal Contraception. 12
Menopause. 13
Pregnancy. 15
Diseases and Disorders 16
Medication 18
Sexual Preference 20
Affect 25
Additional Factors 26
Method 28
Visual Sexual Stimuli 28
Alternative Sexual Stimuli 33
Neuroimaging Method 37
Conclusion 39
Sexual Arousal
Sexual arousal is a complex set of processes and mechanisms, including social,
psychological and biological processes. It is unlikely the understanding of one particular
process could fully explain sexual arousal. Hence, the investigation of sexual arousal
requires a multimethod and interdisciplinary approach (Woodard & Diamond, 2008). In
general, sexual arousal comprises of genital arousal and subjective sexual arousal. Genital
arousal refers to physiological changes after a sexual stimulus. This includes genital
vasocongestion, vaginal lubrication and clitoral engorgements and is associated with
physiological, extragenital changes, including heart rate, sweating, pupil dilation, hardening
and erection of the nipples and flushing of the skin (Meston & Stanton, 2019). The most
common tool to investigate genital arousal in women is vaginal photoplethysmography. This
tampon-shaped device is inserted into the vagina and measures vaginal blood volume and
vaginal pulse amplitude in response to sexual stimuli. Other methods to measure genital
arousal are doppler ultrasonography to asses clitoral blood flow and magnetic resonance
imaging of the perineum to determine clitoral blood volume and size.
Subjective sexual arousal refers to the subjective rating of sexual arousal. It is often
assessed during a sexual stimulus, with a lever apparatus, or after the stimulus, using
Likert-type items (Chivers, Seto, Lalumière, Laan, & Grimbos, 2010). The vast majority of these
sexual stimuli are visual pictures or film clips, although some studies include tactile,
olfactory, auditory, or cognitive (i.e. a sexual fantasy) stimuli. Genital arousal and subjective
sexual arousal are often measured simultaneously, since the two are not necessarily
correlated. Laan, Everaerd, Bellen, & Hanewald (1994) for example showed that subjective
sexual arousal in women was higher during a womade sexual film compared to a
Although assessing subjective and genital sexual arousal help to clarify physiological
and behavioral responses to sexual stimuli, the neurophysiological correlates of sexual
arousal remain elusive (Mitricheva, Kimura, Logothetis, & Noori, 2019). Understanding the
neurophysiological correlates of sexual arousal could give insight in the underlying
mechanisms of reproductive behavior. Neuroimaging techniques are safe, noninvasive
methods to investigate the central nervous system and have been used to investigate sexual
arousal in the healthy human brain. Neuroimaging techniques can be roughly divided by
structural and functional brain imaging. Structural brain imaging is used to study brain
structure, whereas functional brain imaging can be used to study cognitive and affective
processes. Modalities of functional brain imaging include for functional magnetic resonance
imaging (fMRI), positron emission tomography (PET), electroencephalography (EEG), and
magnetoencephalography (MEG). EEG and MEG have a considerably lower spatial
resolution than fMRI and PET. Since this review will focus on the neural response patterns to
sexual stimuli, results of EEG and MEG will not be discussed. It is appropriate to mention,
however, that EEG and MEG have a much higher temporal resolution and are thus interesting
when investigating the onset of brain response to a sexual stimuli. Studies using
neuroimaging techniques to study neural responses to sexual stimuli have predominantly
included heterosexual male participants. A recent meta-analysis by Mitricheva et al. (2019)
demonstrate an inclusion of 1184 male participants in contrast to 636 female participants. Of
these 1184 male participants, 1054 were heterosexual, making it the largest group to be
included in neuroimaging studies to sexual arousal.
The overrepresentation of heterosexual men have led to meta-analyses including only
male data (Stoléru, Fonteille, Cornélis, Joyal, & Moulier, 2012), or comparing male and
demonstrate small sex differences in neural response in subcortical areas to sexual stimuli,
whereas Mitricheva et al. (2019) did not. Poeppl et al. (2016) included different sensory
modalities as well as different neuroimaging techniques, whereas Mitricheva et al. (2019)
only included studies using visual stimuli and fMRI. Mitricheva et al. (2019) propose that
these different results are due to the inclusion of studies presenting sexual stimuli of different
sensory modalities (visual, olfactory and tactile stimuli) by Poeppl et al. (2016). How this
would lead to different results on a more theoretical level is not discussed. The exact
mechanisms underlying sexual arousal in men and women and how these might or might not
differ thus still remain unclear.
No meta-analysis has been conducted solely in with female participants yet. As a
result, the Neurophenomenological Model of Sexual Arousal as proposed by Stoléru et al.
(2012) is based on data of male participants. In this model, a cognitive, affective,
motivational and autonomic/neuroendocrine component are distinguished. The model does
not demonstrate the overlap of sexual arousal with other processes. For example, brain areas
specific for sexual stimuli processing are not expressed in this model. Affective stimuli
processing might have an extensive of overlap with sexual stimuli processing. It would
therefore be interesting to investigate what components of the model are specific to sexual
stimuli processing and what components express more general processes. Although this
model is much more specific than earlier models of sexual arousal (e.g. the human sexual
response cycle by Georgiadis & Kringelbach, 2012), the model could be more specific and
elaborate.
In addition, the model is based on heterosexual male data but as the name of the
The male results that created the model are thus overgeneralized to the entire population.
Overgeneralization of male data is not only present in this case but also prevalent in studies
using animal methods (Coiro & Pollak, 2019) or medical clinical methods (Feldman et al.,
2019; Holdcroft, 2007). This overgeneralization can be problematic, illustrated with the
following example: Young women with coronary heart disease represent a high risk,
understudied phenotype driven largely by a male model of coronary heart disease (Beckie,
2014). For instance, the most commonly recognized symptom for a heart attack is chest pain,
although this symptom is typical for men and rarely present in women. The
overgeneralization of male symptoms and the lack of research and information on female
symptoms causes symptoms in women often not to be recognized, which can lead to death.
The overgeneralization of results and models of sexual arousal based on male participants to
women could be problematic for clinical reasons. Disorders and diseases linked to sexual
arousal present mainly or solely in women (e.g. hyposexual desire disorder, dyspareunia)
might for example be less understood. In addition, overgeneralization of male results and
models can also be problematic for theoretical reasons, since an inclusion of female data
could eventually lead to a better understanding of sexual arousal in humans overall.
Hence, in this article the neuroimaging studies on sexual arousal that have included
women will be reviewed. All possible factors that might influence neural response to sexual
stimuli and the methods used in these studies will be discussed. Studies using visual and
tactile sexual stimuli will be compared, since olfactory and auditory stimuli have not been
specifically proven to induce sexual arousal (further discussed in Alternative Sexual Stimuli).
Studies were selected through an extensive search on PubMed and Web of Science, with
keywords for neuroimaging techniques, for female participants and sexual arousal (e.g.
conducted a whole-brain analysis for the separate group of healthy women where neural
response during a sexual stimulus was compared to a control stimulus will be compared on
methodologies.
Possible Influences on Sexual Arousal
Hormone Levels
Hormone levels in women change physiologically during the menstrual cycle, after
menopause, or induced by oral contraceptives. To understand underlying mechanisms of
hormonal influence on neural responses to sexual stimuli, hormones changes that are likely to
interfere with the brain will be discussed. After, the range of different results regarding the
modulation by hormones on neural responses to sexual stimuli will be discussed.
The clearest example of hormone interaction with the brain is in the
hypothalamus-pituitary-gonadal axis. The hypothalamus releases the gonadotropin-releasing hormone
(GnRH), which induces the release of luteinizing hormone (LH) and follicle-stimulating
hormone (FSH) from the anterior pituitary gland. LH then acts on the ovarian theca cells for
the production of testosterone and progesterone and FSH acts on the ovarian granulosa cells
for the production of estrogens, progesterone and inhibin. Estrogen inhibits the release of LH
and inhibin decreases the release of FSH in the anterior pituitary, creating a feedback
mechanism (Clayton & Hamilton, 2010). Both estrogen and testosterone are proposed to
regulate genital arousal. They might regulate the activity of vasoactive intestinal polypeptide
(VIP) and nitric oxide synthase (NOS) during sexual arousal to mediate vasocongestion of
clitoral tissue and subsequent lubrication (Hoyle, Robson, Whitley, Burnstock, & Stones,
transverse the blood-brain barrier to exert an influence on brain structures. Note that the
majority of testosterone is bound to sex hormone-binding globulin, a protein too large to
cross the blood-brain barrier (Clayton & Hamilton, 2010).
Other hormones have also been proposed to interact with sexual arousal. Prolactin and
oxytocin have been found to be mainly involved in orgasm, but they might influence genital
and/or subjective sexual arousal indirectly. Serum prolactin levels increase significantly after
a masturbation-induced orgasms and is proposed to act as a negative feedback signal limiting
sexual arousal and decrease the likelihood of continued sexual activity (Exton et al., 2000).
Oxytocin might contribute indirectly to genital and subjective sexual arousal by a positive
feedback loop (Meston & Stanton, 2019). Dopamine plays a key role in reward-seeking
behaviors and has been linked to sexual arousal based on animal studies (Pfaus, 2009).
However, limited human research has been conducted on the role of dopamine in sexual
arousal. Brom et al. (2016) demonstrate that after dopamine antagonist administration, there
was no effect on conditioned sexual response in women. They did find effects of the
dopamine antagonist on the unconditional genital response to sexual stimulation in women.
Krüger et al. (2018) also found that after administering a dopamine agonist there were no
alterations in subjective sexual arousal, but also not in genital sexual arousal in women.
These studies administer dopamine and not look at natural dopamine in- or decreases. The
exact working mechanisms of the effects of dopamine on sexual arousal in women it thus not
yet clear.
Noradrenaline, cortisol and serotonin also seems to be involved in sexual arousal. A
study from 1979 showed that women had increased plasma noradrenaline during sexual
in plasma noradrenaline after orgasm (Wiedeking, Ziegler, & Raymond Lake, 1979). Cortisol
declines during presentation of sexual stimuli and elevated cortisol levels decrease subjective
sexual arousal, but not genital arousal (Hamilton, Rellini, & Meston, 2008). A review by
Frohlich & Meston (2000) indicates that serotonin is active in several peripheral mechanisms
that are likely to affect female sexual functioning. Serotonin acts as neurotransmitter in the
central nervous system (CNS), but in the periphery acts as vasoconstrictor and vasodilator. In
addition, after contractions of smooths muscles in the genital urinary systems were detected
after administering serotonin.
There are thus many hormones that could be involved in sexual arousal in women, but
a clear integration of all findings and proposed models of hormone interaction with the
central nervous system during sexual arousal has not yet been proposed.
Menstrual phase.
During the menstrual phase, estradiol rises with the growth and maturation of the
dominant follicle to a height at ovulation. Progesterone appears after ovulation and
luteinizing hormone (LH) surges at ovulation. Testosterone mirrors changes in LH over the
menstrual cycle, peaking at ovulation. The majority of research on subjective sexual arousal
throughout the menstrual cycle found an increase in subjective sexual arousal during the late
follicular and ovulatory phases and a decrease during early follicular and luteal phases
(Shirazi, Bossio, Puts, & Chivers, 2018). However, a few studies show that external factors
may have a stronger influence than menstrual phase. For example, Caruso et al. (2014)
demonstrated that menstrual cycle modulates sexual arousal in single women, but not in
into account. Nonetheless, the question if and how menstrual phase influences neural
response to sexual stimuli is still interesting.
Poromaa & Gingnell (2014) conducted a meta-analysis including fMRI studies to
emotional processing during the menstrual phase. They found that differences in sexually
dimorphic cognitive skill tasks are small and difficult to replicate and emotion-related
changes are found more consistently. Overall there was an increased amygdala reactivity in
the luteal phase. These emotion-related changes are better associated with progesterone than
with estradiol, such that high progesterone levels are associated with increased amygdala
reactivity and increased emotional memory. Sexual stimuli processing also involves
emotional processing as will be discussed below.
To date, there only have been three studies investigating the differences in neural
response to sexual stimuli throughout different phases of the menstrual cycle. Gizewski et al.
(2006) found that women in mid-luteal phase compared to menstrual phase showed higher
BOLD response when presented with sexual stimuli in the anterior cingulate, left insula and
orbitofrontal cortex. In contrast, Abler et al. (2013) found no hormonally modulating effect
upon direct and explicit stimulation (i.e. presentation of videos and pictures). They did
however demonstrate differences in frontal brain regions between menstrual phases with less
direct and explicit stimulation, the expectation of a sexual stimulus. During the expectation of
the stimulus, the luteal phase was associated with higher activation in the anterior cingulate
cortex, the dorsolateral and dorsomedial prefrontal cortex compared to the follicular phase.
Zhu et al. (2010) did not compare luteal and follicular phases, but ovulatory and
frontal gyrus, right lateral occipital cortex and postcentral gyrus and bilateral superior parietal
lobule in non-ovulatory phases compared to the ovulatory phase.
Overall, the three studies conducted in women indicate an influence of menstrual
cycle phase on neural responses to sexual stimuli, mainly in cortical regions. The exact
influence is not very clear yet and it is therefore advocated to report menstrual phase of the
participants. Studies could measure neural response to sexual stimuli in women when at the
same menstrual phase. If women in different menstrual phases are included, it is wise to
check for differences between menstrual phase group. An overview of the menstrual phase of
participants in neuroimaging studies to sexual arousal is presented in Table 1. Note that
majority of studies did not report menstrual phase.
Hormonal Contraception.
There are several types of hormonal contraception used by women worldwide. The
most commonly used hormonal contraception method are oral contraceptives (OCs), used by
100 million women worldwide (Petitti, 2003) and by 16% of women in the U.S. (Daniels,
Daugherty, & Jones, 2014). Twenty-five percent of those starting OCs discontinued using
within the first 12 months (Ali, Cleland, Shah, Ali, & Al, 2012). The main cause for this
discontinuation are side effects experienced by women, including sexual side effects
(Sanders, Graham, Bass, & Bancroft, 2001) . A meta-analysis by Lee, Low, & Ang (2017)
shows that OCs cause and worsen female sexual dysfunction in women in the reproductive.
They also show that female sexual dysfunction symptoms are less likely to occur in women
taking newer OCs. The exact working mechanisms behind these findings remain unclear. To
date, only Abler et al. (2013) investigated the neural responses to sexual stimuli in women
the non-contraceptive group in follicular phase demonstrated higher activation compared with
the contraceptive group. So far, there have no studies investigating the influence of other
types of hormonal contraceptives on the neural response to sexual stimuli.
The study by Abler et al. (2013) suggest that OC use could affect neural response to
sexual stimuli but is dependent on menstrual phase. The effects of other hormonal
contraceptives on neural response to a sexual stimulus remain unclear. It is therefore
suggested to report hormonal contraceptive use when studying sexual arousal in women with
neuroimaging techniques. Studies can choose to exclude women using hormonal
contraceptives or only include women using hormonal contraceptives. If both women using
and not using hormonal contraceptives are included, hormonal contraceptive use should be
taken into account as a cofounding variable. An overview of hormonal contraceptive use in
participants of neuroimaging studies to sexual arousal is presented in Table 1. More than half
of these studies did not report hormonal contraceptives use by participants.
Menopause.
Menopause is defined as the permanent cessation of menstruation resulting from the
loss of ovarian follicular activity (World Health Organization, 1996). Post menopause is
characterized with low estrogen and progesterone levels, leading to reduced lubrication of the
vagina, diminished clitoral blood flow and altered sensory perception from peripheral nerves
located in the pelvic region during sexual arousal. This can cause vaginal dryness and
dyspareunia (Bruce & Rymer, 2009). Changes in subjective sexual arousal in menopausal and
postmenopausal women have been reported by several studies. Leiblum, Koochaki,
Rodenberg, Barton, & Rosen (2006) for example demonstrate a lower partner relationship
Stellato, Crawford, Johannes, & Longcope (2000) demonstrate that menopause was
significantly related to lower sexual desire, but a multi regression analysis showed that other
factors such as health, marital status, and smoking had a greater impact on women’s sexual functioning than menopause status. A national probability sample of 1150 women in the U.S.
showed that the overall prevalence of sexual activity after menopausal age declines. Where
sexual activity is prevalent in 73% among respondents who were 57-64 years of age, this is
reduced to 53% for 65-74 years and 26% for 75-85 years of age (Lindau et al., 2007). These
results indicate that postmenopausal women seem to have altered subjective as well as genital
sexual arousal.
Jeong et al. (2005) and Kim & Jeong (2017) compared the neural responses to sexual
stimuli between premenopausal and menopausal women. Jeong et al. (2005) found an overall
8% higher activation ratio of premenopausal women compared to menopausal women. In
addition, the limbic, temporal association areas and parietal lobe showed greater
enhancement of signal intensities in premenopausal women. Menopausal women however,
had dominant signal enhancement of the genu of the corpus callosum and superior frontal
gyrus. Kim & Jeong (2017) found higher activation in the thalamus, amygdala and anterior
cingulate cortex (ACC) in premenopausal women. In addition, a functional connectivity
analysis during resting state was performed in women approaching menopause
(perimenopause) and premenopausal women by Lu, Guo, Cui, Dong, & Qiu (2019). The
analysis revealed that women approaching menopause suffered from aberrant intrinsic
functional connectivity regions related to sexual function in the right medial superior frontal
gyrus, between the left dorsal granular insula and right superior frontal gyrus and the right
functional connectivity or the right area with the right medial superior frontal gyrus and the
functional connectivity of the left dorsal granular insula with the right superior frontal gyrus.
Only one neuroimaging study to sexual arousal is conducted in postmenopausal
women. Archer, Love-Geffen, Herbst-Damm, Swinney, & Chang (2006) demonstrate that
postmenopausal women have reduced overall signal activations when viewing both erotic and
neutral stimuli and limited brain regions that were exclusive to the erotic visual stimulus
compared to premenopausal women. Interestingly, after six weeks of estrogen therapy
(administration of both estradiol and testosterone), there was a global increase in brain
activation in postmenopausal women presented for both neutral and erotic stimulations.
These results suggest a significant effect of estradiol and testosterone in neural response to
sexual stimuli.
Including perimenopausal, menopausal and postmenopausal women could thus lead to
insights in hormone levels on sexual arousal. Overall, these studies show a decreased neural
response to sexual stimuli in perimenopausal, menopausal and postmenopausal women.
Sexual activity has been reported to decline over age. It would therefore be interesting to
investigate if neural response to sexual stimuli also alters with age within the group of
postmenopausal women.
Pregnancy.
Hormonal fluctuations of the menstrual cycle are minor compared to changes during
pregnancy. During pregnancy, estrogen and progestin rise progressively to high levels
(O’Leary, Boyne, Flett, Beilby, & James, 1991). Testosterone rises in the first trimester compared to the nonpregnant state and returns to the nonpregnant range in the second and
third trimesters. Studies have shown that subjective sexual arousal decreases progressively
with pregnancy (Aslan, Aslan, Kizilyar, Ispahi, & Esen, 2005; Senkumwong, Chaovisitsaree,
Rugpao, Chandrawongse, & Yanunto, 2006). It remains unclear if these changes can be
attributed to changes in sex hormones, since other factors like changing body shape,
discomfort during intercourse, or concerns about deleterious effects of sexual activity on the
fetus could also possibly influence sexual arousal (Stuckey, 2008). The difference in neural
response to sexual stimuli in pregnant women compared to nonpregnant women has not been
studied. This is most likely due to concerns about the effects of magnetic field on the fetus. A
recent study has showed that it is safe for women in the first-trimester of pregnancy to
undergo an fMRI scan (Choi et al., 2015) but the effects during the second and third trimester
have not yet been studied.
Overall, studies have found a range of different results of neural response to sexual
stimuli influenced by menstrual phase, hormonal contraceptives and menopause. No final
conclusions about hormonal influence on sexual arousal can be drawn from these results.
Hence, it is important that studies report menstrual phase, hormonal contraceptive use,
menopausal state, and pregnancy of participants.
Diseases and Disorders
One fairly obvious group of diseases and disorders were sexual arousal is likely to be
altered is sexual dysfunctions. The most prevalent subtype of sexual dysfunction among
women is hypoactive sexual desire disorder (HSDD), defined by the Diagnostic and
Statistical Manual of Mental Disorders as the persistent or recurrent deficiency or absence of
sexual fantasies and desire for sexual activity that causes marked distress or interpersonal
women with HSDD has been investigated by Arnow et al. (2009), Bianchi-Demicheli et al.,
(2011), and Woodard, Nowak, Balon, Tancer, & Diamond (2013). All studies show a higher
subjective sexual arousal reported by women without HSDD compared to women with
HSDD. Arnow et al. (2009) demonstrate increased neural response in the bilateral entorhinal
cortex in women without HSDD compared to women with HSDD. In addition, they
demonstrate increased neural response in the medial frontal gyrus, right inferior frontal gyrus
and bilateral putamen in women with HSDD compared to women without HSDD. Increased
inferior frontal gyrus response was also found by Bianchi-Demicheli et al., (2011), as well as
increased inferior parietal lobule and posterior medial occipital gyrus response in HSDD
women compared to women without HSDD. Women without HSDD demonstrated stronger
neural response in the intraparietal sulcus, dorsal anterior cingulate gyrus and ento-perirhinal
region to sexual stimuli compared to women with HSDD. Woodard, Nowak, Balon, Tancer,
& Diamond (2013) demonstrate stronger neural response in the right thalamus, left insula,
left precentral gyrus and left parahippocampal gyrus in comparison to women with HSDD,
who exhibited stronger response of the right medial frontal gyrus and left precuneus regions
compared to sexual stimuli. Overall, results thus differ between stimuli but the results do
suggest an altered neural response pattern to sexual stimuli in women with HSDD compared
to women without HSDD.
Besides sexual disorders, altered sexual arousal in women has also been linked to
neurological disorders (e.g. traumatic brain injury), endocrine disorders (e.g. diabetes),
cardiovascular illness (e.g. hypertension), pelvic disease (e.g. urinary incontinence), and other
diseases (e.g. breast cancer) (Clayton & Ramamurthy, 2008). Unfortunately, research
regarding the neural responses to sexual stimuli in women with these disorders or diseases
underestimated problem in psychiatric patients and is reported the highest in depressed
subjects (Cohen et al., 2007). The only study comparing a patient group to healthy women
has been conducted by Yang et al. (2008). They compare depressed patients with healthy
individuals and demonstrate that women with depression had a significant lower subjective
sexual arousal compared to healthy women. In addition, they found that the level of brain
activity in depressive women was more than 50% lower than in healthy women in the
hypothalamus, septal area, anterior cingulate gyrus and parahippocampal gyrus.
Sexual impairment has thus been linked to all types of disorders and diseases. In
addition to sexual dysfunctional and psychiatric disorders or diseases, neurological disorders
and diseases are often excluded in neuroimaging research, since abnormalities in the brain are
likely to influence blood-oxygen-level-dependent (BOLD) response. Since sexual
dysfunctional, psychiatric, and neurological disorders and diseases might influence the neural
response to sexual stimuli, it is advocated to report and potentially exclude or separate these
disorders and diseases from a healthy group of participants when conducting a neuroimaging
study to sexual arousal. An overview of the exclusion criteria regarding these sexual arousal
diseases or disorders, psychiatric and neurological disorders and diseases as reported by
neuroimaging studies to sexual arousal in women is presented in Table 1.
Medication
Medications that contribute to sexual dysfunction include histamine receptor (H2)
blockers, narcotics, NSAIDs, thiazide diuretics, non-selective beta antagonists and
psychotropic such as antidepressants, antipsychotics and benzodiazepines (Clayton &
Ramamurthy, 2008). Yet, research regarding the influence of these medications to neural
receiving antidepressants suffered from more frequent and severe impairment of sexuality
compared with depressed patients not using antidepressants, antipsychotics or opioids. They
found these results for all types of antidepressants, including mirtazapine. This is interesting,
since mirtazapine has been reported to be less associated with sexual dysfunction than SSRI’s (Koutouvidis, Pratikakis, & Fotiadou, 1999). Yang et al. (2013) demonstrate significant
improved subjective sexual arousal and neural response to sexual stimuli in the
hypothalamus, septal area and parahippocampal gyrus were when using mirtazapine. Studies
researching the effects on neural response to sexual stimuli of other antidepressants have
been conducted solely in male participants (e.g. Baldwin, Manson, & Nowak, 2015).
Besides mirtazapine, the influence of medication on neural response to sexual stimuli
remains unknown. In addition, medication might influence other cognitive brain processes as
well. Antidepressants for example, reduce the subcortical-cortical resting-state functional
connectivity in healthy volunteers (McCabe & Mishor, 2011). Since medication can have an
effect on brain processes involved in sexual arousal and the exact effects on neural response
to sexual stimuli of all medication besides mirtazapine remains unknown, it is advocated to
report medication use of participants included or to exclude participants that use medication
that might affect sexual arousal. Since symptoms of decreased sexual arousal during
antidepressants do not immediate stop after patients stop using the drugs (Bala, Nguyen, &
Hellstrom, 2018), it is also advocated to report past antidepressant use of participants. An
overview of neuroimaging studies to sexual arousal that report medication that could possibly
alter sexual arousal specific or more generally psychotropic medication is presented in Table
Sexual Preference
In order to investigate neural responses to sexual stimuli, it is crucial that participants
are aroused by the presented stimuli. The presented stimuli should therefore match the sexual
preference of the participants. However, the majority of studies researching neural response
to sexual stimuli in women have not reported if they included women with a sexual
preference that matches the sexual stimuli. An overview of neuroimaging studies to sexual
arousal that reported sexual orientation of participants and if so, what orientation is included
is presented in Table 1. In studies that do mention the sexual preference of participants, the
majority of studies include heterosexual women.
Homosexual women are only included in studies when they are compared to
heterosexual women, with varying results. Ponseti et al. (2006) compared neural responses to
preferred and non-preferred sexual stimuli (i.e. a female couple for homosexual women and a
heterosexual couple for heterosexual women). They found stronger response in the ventral
striatum, premotor cortex and central median thalamus consistently in all groups. Sylva et al.
(2013) demonstrated that when women viewed preferred-sex stimuli compared to resting
baseline, small clusters of greater activation in heterosexual women were observed in the left
inferior parietal and right middle temporal cortices. No differences in their response to
nonpreferred-sex stimuli compared to baseline were found. In a region of interest (ROI)
analysis, they found that visual regions, the mediodorsal thalamic nucleus, and left
hypothalamus exhibited a reliable activity for women for preferred-sex stimuli versus
non-preferred sex stimuli. Note that none of these findings persisted after false discovery rate
(FDR) was controlled for. A whole brain analysis in Safron et al. (2018) showed that activity
was greater for heterosexual and bisexual women viewing female compared to male erotic
T abl e 1 P ar ti ci pan t i nf or m at ion for ne ur oi m agi ng s tudi es t o se xual ar ous al (s or te d by y ear of p ub li ca ti on) F ir st A ut hor Y ea r N um be r of Hea lt hy W om en A ge M ens tr ua l P ha se H or m ona l C ont ra ce pt ion Di sor de rs & Di se a se s M edi ca ti on S exua l P re fe re nc e M SD R ange U se rs N on -U se rs S exua l P syc hi at ri c Ne ur ol ogi ca l Inf lue nc e SA P syc hot ropi c He te ros exua l H om os exua l B is exua l P ar k 2001 6 33 - 25 -41 - - - X - - - - - - K ar am a 2002 20 24 3 - N on -O - - - X - - - - - H am ann 2004 14 25.0 - - - - - - - - - X A rc he r 2006 22 42.6 4.2 - - - - - X X - - - - G iz ew ski 2006 5 27 - 20 -35 M id -L M X - - - - - - - P ons et i 2006 26 23.9 4.45 - - - - X X - - X X S avi c 2008 25 31 4.5 - - - - - - - - X X Y ang 2008 9 40.3 - 23 -58 - X X X X X X - - - A rnow 2009 20 29.3 - 18 -30 L X X X X - X X G eor gi adi s 2006 12 32 - 21 -47 - - - X X - - X Z hu 2010 15 26.3 3.8 - O M X - X X - X M ic he ls 2010 15 26 - 22 -34 - - - - X X X X - - - G il la th 2012 20 19.65 - - - - - X X X - - - - K om is ar uk 2012 11 - - 26 -56 - - - - X - - - - -
T abl e 1 ( con ti nu ed) F ir st A ut hor Y ea r N um be r of Hea lt hy W om en A ge M ens tr ua l P ha se H or m ona l C ont ra ce pt ion D is or de rs & D is ea se s M edi ca ti on S exua l P re fe re nc e M SD R ange U se rs N on -U se rs S exua l P syc hi at ri c N eur ol ogi ca l Inf lue nc e SA P syc hot ropi c H et er os exua l H om os exua l B is exua l B ia nc hi -D em ic he ll i 2011 15 30.4 7.09 21 -44 D ay 1 -10 X - X X - X X Y ang 2013 9 40.3 - 23 -58 - - - X X - - X - - - A bl er 2013 24 24 2.0 20 -29 F , N on -O L X X X X X - X K im 2013 23 38.4 10 21 -51 - - - X X X - - - - S yl va 2013 22 22.1 3.1 - - - - - - - - X X W ooda rd 2013 6 29 4.4 - - X X X X - X X X B or g 2014 22 22 2.1 - F , N on -O X X X X X - X X K im 2014 12 22.7 2.9 - - X - X - - - - W ehr um 2014 50 25.4 4.8 - F L X X X X X X X K im 2017 15 39.5 7.1 - F L X - X X X - - - - S af ron 2018 76 29.67 - 21 -46 - - - - - - - X X X X S ta rk 2019 33 25.7 4.6 19 -44 - - - - X - - X X X N ot e: S A , s exua l a rous al , -, no t re port ed, O , ovul at or y pha se , F , fol li cu la r pha se , L , l ut e al pha se , M , m ens trua ti on
showed stronger response in heterosexual women to male pictures compared to
female pictures. For bisexual women however, responses to male compared to female erotic
stimuli were stronger in the posterior midcingulate, right retro splenial cingulate cortices,
supramarginal and angular gyri. For homosexual women, stronger response to female
compared to male erotic pictures extended though the visual system, clustered in the inferior
precuneus, posterior parahippocampal cortex and ventral striatum. No areas had greater
activation for male compared to female erotic pictures in homosexual women.
Besides research to sexual orientation, male to female transgenders have also been
included in neuroimaging studies to sexual arousal. Gizewski et al. (2009) included male to
female transsexuals with ten being heterosexual in respect to their biological gender and two
were attracted to men. They depicted heterosexual couples engaged in sexual activity. No
significant stronger neural response was found when comparing male to female transsexuals
with female controls. Oh et al. (2012) included male to female transsexuals with a sexual
orientation opposite to the genetic sex. They demonstrate that brain response to male sexual
stimuli compared to female, thus non-preferred stimuli, was stronger in cerebellum,
hippocampus, putamen, anterior cingulate gyrus, head of caudate nucleus, amygdala,
midbrain, thalamus, insula, and body of caudate nucleus. Brain response to female sexual
stimuli showed higher response in the hypothalamus and septal area.
The proposed expansion of LGBT to LGBTQIAP might in the future lead to even
more groups to be included. Asexuality for example seems to be prevalent in 1% of the
population and defined as the absence of sexual attraction (Bogaert, 2004). Asexual women
report lower autonomic arousal, sensuality-sexual attraction and positive affect during sexual
sexual arousal compared to non-asexual women. Investigating neural responses to sexual
stimuli in this group of women might thus be very interesting. Another group that is
interesting to include in neuroimaging research to sexual arousal, are intersex individuals. For
instance, Hamann et al. (2014) has investigated women with complete androgen insensitivity
syndrome (CAIS). These women have Y-chromosome, testes, and produce male-typical
levels of androgens. They however lack functional androgen receptors preventing the
receptors to respond to androgens, developing a female physical phenotype and develop into
women. It is unknown if brain structure and function in women in CAIS is more comparable
to women or to men. Interestingly, Hamann et al. (2014) demonstrate similar brain patterns of
typically developing women and women with CAIS compared to men to sexual pictures.
These results not only present interesting findings for women with CAIS but also give
information about the relation between hormones and sexual arousal in general. The elevated
testosterone levels in women with CAIS are aromatized to estradiol, ruling out the
aromatization of testosterone to estradiol as determinate of sex differences in patterns of brain
activation to sexual images.
Sexual paraphilic preferences might also have an effect on neural response to sexual
stimuli. Hamann, Herman, Nolan, & Wallen (2004) revealed that participants preferring
sadomasochistic (SM) content had stronger neural response in the ventral striatum than
participants that did not prefer SM content to SM sexual stimuli. The ventral striatum also
showed stronger response in participants that did not prefer SM content when looking at
sexual pictures compared to SM sexual stimuli.
Overall, these studies show that neural responses to male and female sexual stimuli in
and female erotic pictures. For homosexual women, there seems to be different neural
response to female compared to male sexual stimuli. The two studies including male to
female transsexuals show different results. In addition, other sex, gender and sexual
orientation groups could be included in the future. These findings thus show the importance
of taking the sexual preference of participants into account when selecting sexual stimuli in
neuroimaging studies.
Affect
Exposure to visual sexual stimuli can result in positive affective reaction, for example
excitement, and/or negative reactions, for example shame. A study by Peterson & Janssen
(2007) demonstrate that positive affect has found to be positively correlated with subjective
sexual arousal in women, but not necessarily with genital responses. In addition, they
demonstrate no positive correlation between negative affect and subjective sexual arousal,
although negative affect was positively correlated with genital responses. Note that this study
analyzed group differences, even though the relation between affect and sexual arousal might
vary among individuals (Bancroft, Janssen, Strong, Carnes, & Vukadinovic, 2003; Lykins,
Janssen, & Graham, 2006). Peterson & Janssen (2007) did find that participants reported both
positive and negative affect during visual sexual stimuli. Sexual stimuli can thus convey more
than one meaning and positive and negative affect might even co-occur simultaneously. It is
difficult to examine this subjectively, since affect is often measuring on a bipolar dimension.
For instance, valence and arousal, considered dimensions of affect, can be examined with the
commonly used Self-Assessment Manikin (SAM) questionnaire (Bancroft, Janssen, Strong,
Carnes, & Vukadinovic, 2003; Lykins, Janssen, & Graham, 2006). The bipolar dimension of
negative affect. A solution has been proposed with the ‘Evaluative Space Model’ (Bradley &
Lang, 1994) presenting positive and negative affect on two distinct dimensions.
Since both positive and negative affect are likely to occur when watching sexual
stimuli, it is interesting to look at the neural overlap between sexual arousal and affect. When
using positive and negative affective control stimuli, neural response specifically associated
with sexual arousal can be extracted. (Cacioppo, Gardner, & Berntson, 1997) has applied this
idea and found that hypothalamic and ventral striatal neural response is linked to sexual
arousal specifically. When choosing control stimuli, it is therefore important to take into
account that neutral stimuli (e.g. a nature documentary) not only reveals sexual stimuli
processing, but also general arousal and valence that accompany emotions per se. Finding
affective stimuli that don’t induce any sort of sexual arousal can however be challenging.
Additional Factors
Neural responses can be influenced by many other factors. When conducting a
neuroimaging experiment, it is important to check for attention during the presentation of a
stimulus. It is possible to check for attention by implementing a task during the presentation
of sexual stimuli, for example the oddball task (implemented by Ponseti et al., 2006), the
one-back task (implemented by Bianchi-Demicheli et al., 2011), or questions (implemented by
Zhu et al., 2010). However, a task might lead to less sexual arousal, since a task might
distract the participant from fully immersing into the sexual stimuli. Presenting questions
after the stimuli lengthens the entire scan duration, which might lead to less attention at the
end of the experiment. The importance of checking for attention might not be as relevant for
sexual stimuli as it is for alternate stimuli, since sexual stimuli have been linked to a
stimuli that induces the same level of attention and arousal as a sexual stimulus is a very
difficult task. When comparing neural response to control stimuli and sexual stimuli,
attention and arousal are thus likely to be higher during sexual stimuli. In order to understand
the brain processes of attention and general arousal, it might be interesting to include an extra
control stimulus that elicits higher general arousal and attention levels, without sexual
content. The overlap between sexual arousal and general arousal stimuli could show the brain
process of arousal, whereas the differences might show the attention and arousal specific for
sexual stimuli.
Factors that have been proposed to influence sexual arousal not yet discussed for
example are stress, mood and relationship problems. In addition, the novelty of an employed
stimuli, experimental setting, or procedure could potentially influence neural responses to
sexual and control stimuli. To test if these additional factors have significant influences,
Walter et al. (2008) measured neural responses to sexual stimuli at two separate time points,
with an interval of 1 to 1.5 years. They found high stability of valence and sexual arousal
ratings of sexual stimuli on group level. When looking at interindividual differences however,
arousal rating was not correlated significantly. The neural network associated with the
processing of sexual stimuli (OCC, parietal cortex, ACC, OFC and left insula) was found to
be stable on a group level at different time points, except for the hypothalamus. However, the
interindividual stability showed that in women, significant correlations between timepoint 1
and 2 for all contrasts of interest were restricted to the parietal cortex. These findings thus
suggest that additional factors that are difficult to account for can be different for women at
different timepoints. However, it also shows that when looking at a group level, these
A factor that is often mentioned in neuroimaging is the handedness of participants.
Within cognitive neuroscience, left handed people are often not included due to the reversed
lateralization of the brain (Willems, Der Haegen, Fisher, & Francks, 2014). The majority of
neuroimaging studies to sexual arousal presented in Table 1 exclude left handed participants
although it was not reported by Archer et al. (2006) and Hamann et al. (2004) and one or two
left handed people were include by Abler et al. (2013) and Borg et al. (2014).
Method Visual Sexual Stimuli
When investigating sexual arousal with neuroimaging techniques, sexual arousal
needs to be induced during a neuroimaging scan. The majority of studies have used visual
stimuli to evoke sexual arousal, with varying results. Besides the possible influences of
participants in and exclusion criteria as previously discussed, the differences in results might
be due to the differences in type, intensity, duration, selection method and content of the
visual sexual stimuli. There are two types of visual sexual stimuli used in studies:
photographs or film clips. Sexual photos might be well-suited for assessing the initial
appraisal of sexual stimuli but are static and presented briefly. Abler et al. (2013) used both
photos and film clips and found that static erotic pictures showed the same brain regions to be
activated as during video clips, but at a considerably smaller spatial extent. Note that the film
clips were presented for 20 seconds, which might also induce primarily initial appraisal.
In addition to type of visual sexual stimulus, the intensity and content of the visual
stimulus in both photos and films differ between studies. The content of visual stimuli is very
important, as already discussed in the section Sexual Preference. In addition to matching
results. With soft stimuli, such as naked pictures, differences between groups or individuals
might be hard to detect. More explicit sexual stimuli, for instance a film clip presenting
couples in sexual activity, might elicit a higher sexual arousal reaction and thus more clear
neural response findings. However, the more ‘hard-core’ the visual sexual stimuli does not
necessarily mean the higher sexual arousal or higher neural response. Borg, Georgiadis, et al.
(2014) demonstrated a considerable co-joint brain activity in response to penetration and
disgust pictures (e.g. mutilation or rotten food). Note that these results might have been
different for women that prefer SM content, as demonstrated by Stark et al. (2005). More
explicit sexual stimuli might thus overall elicit a higher neural response, but some women
might be more aroused by soft sexual stimuli, whereas others are more aroused by more
explicit sexual stimuli.
A few studies have reported if women have experience with watching visual sexual
stimuli (Archer et al., 2006; Hamann et al., 2004; Woodard et al., 2013). This is interesting,
since women experienced with watching visual sexual stimuli might be more aroused by
explicit sexual stimuli, whereas women that do not have experience might induce more
negative affective feelings (e.g. disgust) and could possibly be more aroused by soft stimuli.
An overview of the type of visual stimulus and the content is presented in Table 2. The
individual differences of eliciting sexual arousal in participants can be challenging. A method
to check if participants did subjectively experienced sexual arousal is with subjective rating
of sexual arousal. Some studies do not report if they assessed subjective sexual arousal (e.g.
Archer et al., 2006; Park et al., 2001; Zhu et al., 2010), although the majority assessed
subjective sexual arousal during (e.g. Arnow et al., 2009; Gillath & Canterberry, 2012;
Wehrum-Osinsky et al., 2014) or after the neuroimaging scan (e.g. Borg, de Jong, &
As mentioned before, Laan et al. (1994) demonstrate that women reported to be less
sexually aroused by a man-made sexual film compared to a woman-made film, although there
were no differences in genital arousal. The differences in neural response to man-made and
woman-made sexual films have not yet been investigated in women. These results do already
show the importance of selecting the right stimuli in order to sexually arouse the participants.
Abler et al. (2013) for example selected film clips for a male audience. Their results might
have been different if they had selected film clips specifically selected for women. Many
studies did not report how the visual sexual stimuli were selected or if the stimuli were
selected by women or men. An overview of the selection methods for sexual stimuli is
presented in Table 2.
The length of the presented visual sexual stimuli also differs between studies, which is
presented in Table 2. The longest duration of a visual sexual stimulus has been presented by
Kim & Jeong (2017). They analyzed the time course of BOLD response to sexual visual
stimuli with a duration of 9 minutes. They demonstrate that premenopausal women show the
largest BOLD signal in 11 ROIs between 2.30 and 3.30 seconds for the putamen and the
insula; between 5.30 and 6.30 for the hippocampus, parahippocampal gyrus, amygdala, septal
area, globus pallidus, head of caudate nucleus, thalamus and hypothalamus; between 7.30 and
8.30 minutes for the ACC. The greatest signal change was thus found between 5.30 and 6.30
minutes. Interestingly, menopausal women show the greatest signal change between 8 and 9
minutes. These findings are very interesting, since they raise the question if optimal signal
change is measured in all other studies, since all other studies have a photo of film clip length
below the 6.30 seconds. In addition, these results show differences in the optimal signal
have a longer duration in future research in order to induce optimal sexual arousal and to find
significant differences between groups or conditions.
Besides the length of the stimuli, studies also vary in their experimental design. For
example, some studies use a block design, whereas others use an event-related design. In
addition, the order of stimulus presentation and intervals in between stimuli vary. A
schematic overview of the different experimental set-ups used by neuroimaging studies using
visual sexual stimuli is presented in Figure 1. Note that many studies present the sexual and
control stimuli without a long interval between the stimuli. When stimuli are presented
quickly after one and other, a cross-over effect might happen. It is however unclear how long
sexual arousal effects might persist after a visual sexual stimulus, making it difficult to
determine the interstimulus interval time period.
At last, the duration, selection method and content of the visual control stimuli also
vary tremendously between studies (for an overview see Table 2). The content of the control
stimuli is very important to keep in mind when reviewing results. Some studies do not even
mention the content of the control stimuli. A commonly used control stimuli is a nature
documentary, often subjectively rated with low valence and general arousal. When the results
of the sexual stimuli are compared to these types of control stimuli, the results do not solely
represent sexual arousal, but also other processes (e.g. general arousal, face processing,
affective processing). Note that comparable to sexual stimuli, there might also be large
individual differences in attention, valence and arousal to control stimuli. For example, some
participants might be very excited watching a documentary about trains, whereas others
might be bored. The lack of valence, general arousal, or other types of processing is even
Figure 1. This image presents a schematic simplified representation of the experimental
the stimulus or interval period, either a blank screen, fixation cross or question, with the
duration in seconds (rounded up). Control (blue) and sexual (pink) stands for the type of
stimulus presented, see Table 2 for content of the stimulus. Control/sexual (purple) means
that either a control or a sexual stimulus is presented in a counterbalanced or
pseudorandomized order. The amount of repetition is presented in the arrow. A question
mark (?) entails that the number of repetitions is not clearly stated in the study.
* Gillath & Canterberry (2012) present a cue for 500 milliseconds (ms), followed by a
forward mask for 476 ms, a sexual prime for 24 of 524 ms, and a backward mask for 500 ms
before presenting the target, ** 50 fixation periods of 5 seconds pseudorandom interspersed
within each run, *** Safron et al. (2018) presented two types of experimental set-ups. First,
sexual and control photographs were shown, followed by sexual and control film clips, ****
Stark et al. (2019) present a fixation cross for 0 -1550 ms, then a text message announcing the
stimulus for 400 ms, followed by a black screen for 3-5 seconds before presenting the
stimulus.
On the other hand, control stimuli showing clothed couple interactions without erotic
meaning could be interpreted in for example a romantic way, inducing sexual arousal. Again,
interpretation can differ between individuals. Control stimuli showing people but not couples,
for example a vacuum infomercial, again could have low arousal and valence. There is a
variety of control stimuli used in neuroimaging studies to sexual arousal. When reviewing
results, it is important to consider what possible processes are reflected by the results.
Alternative Sexual Stimuli
Sexual arousal can also be induced from olfactory, tactile, auditory, taste and even
instructions have been used in subjective and genital sexual arousal assessment. In men,
subjective and genital arousal was higher after visual sexual stimuli compared to induced
sexual fantasy stimuli (Koukounas & Over, 1997) but this has not been demonstrated in
women. Note again that the induced sexual arousal by different kinds of sensory modalities
might differ per individual. Some women, for example, might not be aroused by visual sexual
stimuli, whereas others won’t get aroused by fantasies. To date, no studies have implemented sexual fantasies in neuroimaging studies. However, another form of a more ‘cognitive’ sexual stimulation was presented by Abler et al. (2013). They analyzed the expectation period before
a sexual picture. They demonstrate no differences in neural response to both pictures and film
clips in women using or not using hormonal contraceptives and between different menstrual
phases. Interestingly, they did find differences in neural response during the expectation of
the sexual pictures between these groups. These result shows that investigating sexual arousal
with cognitive stimuli rather than solely using visual sexual stimuli are valuable for detecting
differences between groups.
Most studies don’t mention if they presented audio during the visual sexual stimuli. If
presenting audio during visual sexual stimuli has an effect of neural response has not yet been
investigated. A study by Polan et al. (2003) demonstrate that subjective and genital sexual
arousal was not different during the presentation of visual sexual stimuli with or without
audio. However, for neural response patterns, it is plausible that at least the auditory cortex
shows different response to visual sexual stimuli with and without audio. Presenting solely
auditory stimuli to induce sexual arousal has been applied only once in a neuroimaging study.
In this study, happy, angry, fearful, erotic and neutral prosodies were presented during an
fMRI experiment (Ethofer et al., 2007). They found no main effects but did find a significant
both males and females. This interaction was stronger for erotic than all the other categories.
If sexual arousal is induced during auditory sexual stimuli is thus not yet clear.
Olfactory stimuli have also been used in neuroimaging studies. A type of
progesterone (4,16-androstadien-3-one) and a type of estrogen (estra-1,3,5(10),16-3-ol) have
both been proposed as candidate compounds for human pheromones. Oliveira-Pinto et al.,
(2014) found to that both olfactory stimuli activate the anterior hypothalamus in heterosexual
women in a sex-differentiated manner. Ciumas, Hirschberg, & Savic (2009) however found
that progesterone activated the anterior hypothalamus, and the estrogen the amygdala,
piriform and anterior insular cortex in heterosexual women. Homosexual women processed
the progesterone by olfactory networks and not the anterior hypothalamus. However, when
smelling the estrogen, they partly shared activation of the anterior hypothalamus congruently
with heterosexual men (Berglund, Lindström, & Savic, 2006). Non-homosexual (from their
biological sex) male to female transsexuals show a pattern of activation away from their
biological sex (Berglund, Lindström, Dhejne-Helmy, & Savic, 2008). Menstrual cycle might
also play a role on neural responses to olfactory cues, since Graham, Janssen, & Sanders
(2000) demonstrate that effect of male fragrance on genital arousal during erotic fantasy was
only apparent in the follicular phase and not during the periovulatory phase.
Studies using olfactory stimulation have also performed functional connectivity
analysis. Heterosexual women displayed connections with the contralateral amygdala,
cingulate and hypothalamus and were more widespread in the left amygdala. These findings
were the same for heterosexual women with congenital adrenal hyperplasia (high fetal
testosterone) (Ciumas et al., 2009). In homosexual women connections were primarily
homosexual women showed a rightward cerebral asymmetry, congruent with heterosexual
men, whereas volumes of cerebral hemispheres were symmetrical in heterosexual women,
congruent to heterosexual men. These studies thus show interesting neural response patterns
to olfactory cues for hetero-, homo- and transsexual women. Olfactory cues might thus
induce a neural response linked to sexual orientation. Olfactory cues thus seem to be related
to sexual attraction, but this not necessarily means that sexual arousal is induced by these
olfactory stimuli.
A more likely stimuli to induce sexual arousal, is tactile genital stimulation, although
it might depend on experimental set up if sexual arousal is also induced in the laboratory with
genital tactile stimulation. Neural responses to tactile stimulation of clitoral, vaginal or
cervical regions have been examined by a handful of studies. Komisaruk et al. (2011) and
Michels, Mehnert, Boy, Schurch, & Kollias (2010) both focused on the representation of
genital areas in somatosensory cortex. Michels et al. (2010) used a block design of 18
seconds rest, alternating with 12 seconds of electrical stimulation on the clitoris. Each block
was repeated 10 times. Although stimulation with short intervals might be arousing for some
women, the average score on a scale from -10 to 10 for subjective sexual arousal was 0.
These subjective rating results demonstrate that women might have not been sexually aroused
during this experiment. Note that it is possible that they were genitally aroused, but this was
not assessed during scanning. In addition, the scores ranged from -7.5 to 8 (-2 was 25%
percentile and 2.5 was 75% percentile), demonstrating that some women might have been
aroused during the experiment and others not. Komisaruk et al. (2011) also used a block
design of 30 second rest and 30 second of stimulation, repeated 5 times. Unfortunately, they
did not measure subjective sexual arousal. These two studies were thus interested in the
designed to induce sexual arousal. Georgiadis et al. (2006) focusses exclusively on orgasm,
but extended analysis from Georgiadis et al. (2009) also pays attention to at neural responses
during tactile stimulation before orgasm. They present a more complex design, where the
clitoral tactile stimulation has a duration of 2 minutes for 2 to 5 times, eventually leading to
orgasm. The participants were encouraged to enjoy the stimulation. Subjective sexual arousal
also shows to be significantly elevated during stimulation. In this study, sexual arousal was
very likely to be induced during the experiment. They found co-joint activation in the left
primary and secondary somatosensory cortices and deactivation in the right amygdala, and on
the ventral aspect of the temporal lobe. They compared results of men and women and found
more activation in the right posterior claustrum, ventral occipitotemporal region and posterior
lobe of the cerebellar vermis, and more activation in women compared to men in the parietal
areas in the left hemisphere, posterior parietal cortex, somatosensory area 2, left primary
motor cortex, right premotor cortex, and left precuneus. With the right experimental set-up,
the use of tactile genital stimuli is a promising method to induce sexual arousal during a
neuroimaging scan.
Neuroimaging Method
The most commonly used neuroimaging method when studying sexual arousal is
BOLD fMRI. BOLD fMRI provides a nice balance between spatial resolution, temporal
resolution and invasiveness and has become the dominant functional imaging modality in the
past decade. It is important to note that BOLD fMRI doesn’t measure neuronal activity
directly, but measures the metabolic demands, the oxygen consumption, of active neurons. In
addition, one study has been conducted using PET (Georgiadis et al., 2006) and one using
functional magnetic resonance spectroscopy (fMRS) (Kim et al., 2013). The latter can
brain metabolites involved in sexual stimuli processing focusing on the anterior cingulate
cortex. They found several metabolites (-Glx, --Glx, Cho, and Lac) to be significantly
increased during visual sexual stimuli compared to before and most metabolites were
recovered to equilibrium state. PET neuroimaging is based on the assumption that areas on
high radioactivity are associated with brain activity (Woodard & Diamond, 2008). It
measures indirectly blood flow to different parts of the brain. The image shows the metabolic
activity of the cerebral tissues in order to localize functional response. Georgiadis et al.
(2006) choose to use PET because it is more robust to motion artifact than fMRI and they
wanted to include orgasm.
Compared to PET, fMRI is less invasive since no tracer needs to be injected and fMRI
devices have a higher spatial and temporal resolution. The spatial resolution is higher with a
higher magnetic field strength. A higher magnetic field strength has a better the
signal-to-noise ratio and blood-oxygen-level-dependent signal, making it easier to identify and quantify
small brain areas such as the hypothalamus. The field strength differs between studies due to
increased availability of 3.0 Tesla (T) scanners. Neural response to sexual stimuli in women
using fMRI have used a magnetic field strength of 1.5 or 3.0 T (for an overview see Table 2).
No studies have been conducted yet with 7.0 T or higher in women. A study with 7.0 tesla in
men show increased activation in the subnuclei of the thalamus (Walter, Stadler,
Tempelmann, Speck, & Northoff, 2008). It would be interesting to use 7.0 T or higher field
magnetic strength in women to reveal subcortical regions involved in sexual stimuli
processing. Note that there are also downsides to a higher magnetic field strength: the costs of
a scan are much higher, there are more distortions and signal inhomogeneities and analysis
Besides differences in magnetic field strength, studies also analyze the fMRI data in
various ways. For example, some studies show results of a whole brain analysis with an
uncorrected p-value of less than 0.001, whereas others show a corrected p-value of less than 0
(for an overview see Table 2). In addition, several studies do not report significant lower
BOLD response, even though they could be as important as the significant higher BOLD
response (van der Zwaag, Schäfer, Marques, Turner, & Trampel, 2016). Neuroimaging
studies to sexual arousal thus vary in neuroimaging methods. The majority use fMRI, but
PET and fMRS studies are also conducted. Magnetic field strength varies between 1.5 and
3.0 T, studies choose different p-values for the whole-brain analysis, and significant lower
BOLD responses are not presented in a many neuroimaging studies to sexual arousal.
Conclusion
This paper discusses possible factors that are advocated to be taken into account when
investigating neural response to sexual stimuli in women: menstrual phase of the participant,
hormonal contraception use, history of sexual, psychiatric or neurological disorder or
diseases, and medication and history of medication use. The average age of participants in
neuroimaging studies to sexual arousal discussed in this study is 27.5. Future studies might
include menopausal or postmenopausal women to understand the differences in sexual
arousal in relation to age. Including menopausal and postmenopausal women could also help
understand the complicated the relation between hormones and sexual arousal (Archer et al.,
2006). Due to ethical concerns, studies to sexual arousal in women under the age of 18 has
not been conducted, although it could be for instance interesting to study neural responses to
