Neural Correlates of Sexual Orientation in Heterosexual, Bisexual, and Homosexual Women

Abstract

We used fMRI to investigate neural correlates of responses to erotic pictures and videos in heterosexual (N = 26), bisexual (N = 26), and homosexual (N = 24) women, ages 25–50. We focused on the ventral striatum, an area of the brain associated with desire, extending previous findings from the sexual psychophysiology literature in which homosexual women had greater category specificity (relative to heterosexual and bNeural Correlates of Sexual Orientation in Heterosexual, Bisexual, and Homosexual Womenisexual women) in their responses to male and female erotic stimuli. We found that homosexual women’s subjective and neural responses reflected greater bias towards female stimuli, compared with bisexual and heterosexual women, whose responses did not significantly differ. These patterns were also suggested by whole brain analyses, with homosexual women showing category-specific activations of greater extents in visual and auditory processing areas. Bisexual women tended to show more mixed patterns, with activations more responsive to female stimuli in sensory processing areas, and activations more responsive to male stimuli in areas associated with social cognition.

Introduction

Studies using physiological measures have found that women tend to have non-specific patterns of genital arousal1,2,3. That is, in contrast to men, women tend to show similar degrees of arousal to erotic stimuli depicting either sex. For example, heterosexual women have generally shown equivalent arousal to both erotic stimuli featuring men and erotic stimuli featuring women. This has been repeatedly demonstrated with vaginal photoplethysmography3,4. This pattern has also been found using less direct measures such as looking time5, pupil dilation6, and fMRI7. Notably, homosexual women’s arousal patterns are more category-specific than heterosexual women’s, although less so than men’s8.

The fact that women’s sexual arousal patterns are less category-specific than men’s has been interpreted as a potential contributor to gender differences in “erotic plasticity”9, which Baumeister has defined as “the extent to which sex drive is shaped by social, cultural, and situational factors.”

Baumeister offered three lines of evidence when he initially proposed that women may have greater erotic plasticity compared with men: (1) women show larger effects of social and cultural factors on sexual attitudes, desire, and behavior; (2) sexual attitude-behavior consistency is lower in women than in men; (3) individual women exhibit more variation in sexual behavior across time than men. Women’s less specific arousal patterns may also contribute to their increased “sexual fluidity”10, which Diamond has defined as an individual’s “capacity for situation-dependent flexibility in sexual responsiveness, which allows individuals to experience changes in same-sex or other-sex desire across both short-term and long-term time periods”11,12.

One might hypothesize that arousal patterns of bisexual women should be similar to the non-specific arousal patterns of heterosexual women; however, studies of women’s arousal patterns have mostly neglected to include bisexual women. Heterosexual women’s arousal does not appear to favor erotic stimuli of either sex, and thus may be considered to reflect a bisexual pattern. (We do not mean to imply that heterosexual women are confused or in denial about their “real preferences”; rather, the findings in need of explanation are why heterosexual women show non-heterosexual arousal patterns in the laboratory). The implication of women’s non-specific arousal patterns for their sexual orientations is difficult to interpret. Most women, like most men, behave and identify heterosexually13,14,15,16,17,18. However, men are more likely than women to identify as completely heterosexual or completely homosexual, and women are more likely than men to identify as bisexual or “mostly heterosexual”19.

If arousal patterns are similar between heterosexual and bisexual women, the question remains what distinguishes the two groups. One possibility, supported by some research, is that bisexual women tend to have greater sexual motivation, which may increase the likelihood of exploring a capacity for attraction to both sexes20,21. Or, bisexual women may be more aware than heterosexual women of their non-specific arousal22, which could partially contribute to bisexual sexual motivation. Alternatively, bisexual women may be more likely than heterosexual women to interpret their non-specific arousal states in sexual or romantic terms.

It is also possible that bisexual women’s arousal patterns differ from those observed in heterosexual women. Consistent with this possibility, recent studies suggest that women with bisexual interests tend to be more aroused by female than by male erotic stimuli23,24,25. Perhaps for some women with female-biased arousal patterns, this bias can motivate non-heterosexual feelings, behavior, and identity.

Interpretations of non-specific arousal patterns in women are further complicated by the fact that female genital arousal exhibits relatively low correlations with subjectively reported sexual arousal, in contrast to the high correlations observed in men26. Discrepancies between existing genital and subjective measures indicate that some women may report substantial subjective arousal without substantial genital arousal, and vice versa. It has also been suggested that non-specific arousal patterns may not indicate affective responses to erotic stimuli, but may instead reflect a kind of protective preparatory response27.

Neuroimaging assessments may shed light on the neural systems that are involved in responding to a given paradigm. Functional magnetic resonance imaging (fMRI) is a neuroimaging approach that allows for the indirect assessment of brain activity by tracking ratios of oxygenated and deoxygenated blood a proxy for neural firing. When used in the context of presenting erotic stimuli, this non-invasive neural measure could provide a converging line of evidence for interpreting the genital and subjective arousal findings described above. In this study, we used fMRI to specifically focused on the “reward system” in order to address the question: to what extent is there an affective significance to findings from the literature on women’s sexual orientation and genital arousal?

The part of the “reward system” that we focused on is the ventral striatum, a dopamine-sensitive area of the brain that is a reliable measure of reward-related processing–and in particular, wanting and “incentive motivation”28,29–including with respect to sexual orientation30. Most neuroimaging studies of sexual response have focused on men31,32, but the ventral striatum has also been found to reliably activate in studies of women’s responses to erotic stimuli33,34,35. However, until now, no studies have measured neural responses to erotic stimuli in bisexual women.

The present investigation primarily focused on two hypotheses: (1) Homosexual women may show greater category-specificity than non-homosexual women in brain activity, as suggested by the genital arousal literature; (2) Bisexual women may show larger biases towards female stimuli, compared with heterosexual women. We tested these hypotheses with respect to subjective and neural responses to erotic pictures and erotic videos. We used two different kinds of erotic stimuli because of their potentially non-overlapping strengths and weaknesses. Erotic pictures may be particularly well-suited for assessing the initial appraisal of sexual stimuli, but their brevity may not reflect the kinds of experiences that drive sexuality in the real world. Erotic videos may allow for the measurement of more intense states, but their extended duration may also provide opportunities for self-regulatory efforts to modify erotic responses.

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