Neural substrates of sexual arousal are not sex dependent

Abstract

Sexual arousal is a dynamical, highly coordinated neurophysiological process that is often induced by visual stimuli. Numerous studies have proposed that the cognitive processing stage of responding to sexual stimuli is the first stage, in which sex differences occur, and the divergence between men and women has been attributed to differences in the concerted activity of neural networks. The present comprehensive metaanalysis challenges this hypothesis and provides robust quantitative evidence that the neuronal circuitries activated by visual sexual stimuli are independent of biological sex. Sixty-one functional magnetic resonance imaging studies (1,850 individuals) that presented erotic visual stimuli to men and women of different sexual orientation were identified. Coordinate-based activation likelihood estimation was used to conduct metaanalyses. Sensitivity and clustering analyses of averaged neuronal response patterns were performed to investigate robustness of the findings. In contrast to neutral stimuli, sexual pictures and videos induce significant activations in brain regions, including insula, middle occipital, anterior cingulate and fusiform gyrus, amygdala, striatum, pulvinar, and substantia nigra. Cluster analysis suggests stimulus type as the most, and biological sex as the least, predictor for classification. Contrast analysis further shows no significant sex-specific differences within groups. Systematic review of sex differences in gray matter volume of brain regions associated with sexual arousal (3,723 adults) did not show any causal relationship between structural features and functional response to visual sexual stimuli. The neural basis of sexual arousal in humans is associated with sexual orientation yet, contrary to the widely accepted view, is not different between women and men.

Keywords: metaanalysis; neuroimaging; sex differences; sexual arousal.

Fig. 1.

Flow diagram of literature search and study selection.

Fig. 2.

Two-step clustering identifies groupings of categorical and continuous variables by running preclustering first and then by running hierarchical methods. Two clusters of approximately equal size could be obtained (A) which related to stimulus type (i.e., picture or video). Presentation paradigm (i.e., block design or event related) and sexual orientation were found to be weak predictors for classifications (B), while continuous variables such as machine power, age, or biological sex of study participants showed similar density distributions in both clusters (C).

Fig. 3.

ALE analysis suggested that exposure to erotic media, in contrast to neutral stimuli such as sport activities or landscapes, induce significant activations in a number of cortical and subcortical structures.

Fig. 4.

Neural correlates of sexual arousal cluster into 2 distinct patterns associated only to the stimulus type, namely video (A) and pictures (B).

Fig. 5.

Erotic media stimulate numerous cortical and subcortical regions, including insula, middle occipital, anterior cingulate and fusiform gyrus, amygdala, striatum, pulvinar, and substantia nigra. In agreement with the cluster analysis (Upper plot), all regions (marked as orange masks on coronal slices of T1 anatomical image from anterior to posterior) respond to sexual stimuli in both men and women, albeit to different degrees as reflected by the normalized ALE scores. For consistent representation of ALE findings, the scores were normalized within each group since the ALE scores are calculated for each subgroup (e.g., men or women) separately from histograms associated with internal data distribution within the respective group. Larger ALE scores correspond to a stronger effect and as P values of a statistical test are uniquely obtained from distribution.