Specific factors and methodological decisions influencing brain responses to sexual stimuli in women

Specific factors and methodological decisions influencing brain responses to sexual stimuli in women. Sophie Rosa van’t Hof, Nicoletta Cera. Neuroscience & Biobehavioral Reviews, September 21 2021. https://doi.org/10.1016/j.neubiorev.2021.09.013

Highlights

• Several female-specific factors important for sexual arousal neuroimaging research

• Stress and mood could be assessed when analyzing on individual level

• Methodologies should focus on optimizing sexual arousal

• Sexual stimuli should be selected by women and optimal duration should be piloted

• Brain models of sexual arousal should be updated with data of women

Abstract: Most of the neuroimaging studies on sexual behavior have been conducted with male participants, leading to men-based models of sexual arousal. Here, possible factors and methodological decisions that might influence brain responses to sexual stimuli, specifically for the inclusion of women, will be reviewed. Based on this review, we suggest that future studies consider the following factors: menstrual phase, hormonal contraception use, history of sexual or psychiatric disorders or diseases, and medication use. Moreover, when researching sexual arousal, we suggest future studies assess sexual orientation and preferences, that women should select visual sexual stimuli, and a longer duration than commonly used. This review is thought to represent a useful guideline for future research in sexual arousal, which hopefully will lead to a higher inclusion of women and therefore more accurate neurobiological models of sexual arousal.

Keywords: sexual arousalwomenbrainneuroimagingfunctional magnetic resonance imagingpositron emission transmission

1. INTRODUCTION

During the last twenty years, several studies investigated the cerebral correlates of human sexual behavior, with the majority using external sexual stimuli to evoke sexual arousal (for meta-analyses and reviews, see: Stoléru et al., 2012; Georgiadis & Kringelbach, 2012; Poeppl et al., 2016; Mitricheva et al., 2019). Human sexual arousal refers to a complex set of social, psychological, and biological processes and therefore investigation of sexual arousal requires a multi-method and an interdisciplinary approach (Woodard & Diamond, 2008).

Sexual arousal can be induced by both internal cues, represented by sexual interest, autobiographical memories, fantasies, or, simply thoughts, and external sexual stimuli. External sexual stimuli, of different sensory modalities, have been considered a reliable tool to study the brain underpinnings of sexual arousal in both men and women. Sexual arousal is usually operationalized through the measurement of genital responses and self-reported (i.e., subjective) sexual arousal. Since both genital responses and subjective sexual arousal are activated, and regulated, by brain circuits responding to internal and external stimuli, sexual arousal has been measured by functional neuroimaging as well. Modalities of functional brain imaging include 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 brain response patterns to sexual stimuli, results of EEG and MEG will not be discussed.

A wide array of brain regions is involved in processing and experiencing sexual arousal, not surprising for a complex task involving multiple sensory modalities and several cognitive functions as focused attention, working and long-term memory, and emotional appraisal (Stoléru et al., 2012; Georgiadis & Kringelbach 2012). Two recent meta-analyses showed different results regarding the brain regions involved during visual sexual stimulation (VSS) in women and men. A meta-analysis by Poeppl et al. (2016) showed small between-gender differences in brain response in subcortical areas to sexual stimuli, whereas Mitricheva et al. (2019) did not find any differences in brain response to sexual stimuli between men and women. According to Mitricheva et al. (2019), this discrepancy in the meta-analyses results could depend on the inclusion of studies using different sensory modalities sexual stimulation (visual, olfactory, and tactile stimuli).

Although there is a common assumption of large sex differences in brain responses to sexual stimuli, and the evoked sexual arousal, these meta-analyses show small or null between genders differences. However, previous behavioral and psychophysiological studies found a significantly higher level of agreement between self-reported sexual arousal and genital response in men than in women (Chivers et al., 2010). Methodological issues, such as differences in devices and procedures used, or fundamental differences, might modulate this. An fMRI study by Parada et al. (2016; 2018) examined both self-reported sexual arousal and genital responses in relation to brain responses in both men and women. Various subregions of the parietal cortex show significant changes in brain responses corresponding to the degree of self-reported sexual arousal, with no gender differences. The strength of the correlation between brain activation and genital response shows that women had a stronger brain-genital relation than men in the insula, amygdala, posterior cingulate cortex, lateral occipital cortex, and bilateral cerebellum. Conversely, in men, no brain regions showed a strong brain-genital correlation. This study presents that fMRI studies can be an important addition to psychophysiological and behavioral research in understanding complex questions, such as the gender differences in concordance between genital response and self-reported sexual arousal.

Previous neuroimaging studies on sexual arousal have predominantly included heterosexual male participants. The recent meta-analysis by Mitricheva et al. (2019) demonstrated the 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. Due to the large inclusion of men, one of the most recent and influential models of brain responses to sexual stimuli is based on data of male participants (Stoléru et al., 2012). The overrepresentation of male participants and overgeneralization of theories and models based on male data is not limited to neurosexology but for instance also present in animal studies (Coiro & Pollak, 2019) or clinical trials (Feldman et al., 2019; Holdcroft, 2007). By including more women, but also more non-heterosexual and non-cis participants, the specificity and clinical utility of future theoretical models could be improved. Besides theoretical reasons, the larger inclusion of women could lead to a better understanding of female-specific sexual disorders and diseases (e.g., female sexual arousal disorder, genito-pelvic pain/penetration disorder).

It is not clear why there is an overrepresentation of men in previous studies. A potential reason might be female-specific factors and methodological decisions, which could be seen as an obstacle. Hence, the present review will examine factors and methodological decisions that could potentially influence brain responses to sexual stimuli when women are included in neuroimaging studies to sexual arousal and genital response. Moreover, we will assess whether previous neuroimaging studies considered these factors.