PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects

Abstract

Cerebral responses to putative pheromones and objects of sexual attraction were recently found to differ between homo-and heterosexual subjects. Although this observation may merely mirror perceptional differences, it raises the intriguing question as to whether certain sexually dimorphic features in the brain may differ between individuals of the same sex but different sexual orientation. We addressed this issue by studying hemispheric asymmetry and functional connectivity, two parameters that in previous publications have shown specific sex differences. Ninety subjects [25 heterosexual men (HeM) and women (HeW), and 20 homosexual men (HoM) and women (HoW)] were investigated with magnetic resonance volumetry of cerebral and cerebellar hemispheres. Fifty of them also participated in PET measurements of cerebral blood flow, used for analyses of functional connections from the right and left amygdalae. HeM and HoW showed a rightward cerebral asymmetry, whereas volumes of the cerebral hemispheres were symmetrical in HoM and HeW. No cerebellar asymmetries were found. Homosexual subjects also showed sex-atypical amygdala connections. In HoM, as in HeW, the connections were more widespread from the left amygdala; in HoW and HeM, on the other hand, from the right amygdala. Furthermore, in HoM and HeW the connections were primarily displayed with the contralateral amygdala and the anterior cingulate, in HeM and HoW with the caudate, putamen, and the prefrontal cortex. The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects. The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities.amygdala ͉ homosexuality ͉ cerebral lateralization ͉ cerebral connectivity ͉ magnetic resonance volumetry O ne of the more controversial questions in the neurobiology of human behavior relates to the mechanisms of sexual orientation. This issue received increasing interest over the last decade and was further substantiated by recent results from imaging studies of cerebral activation in homo-and heterosexual subjects. During judgment of face attractiveness and when viewing sexually arousing films the cerebral response was found to be invariant to the preferred sexual stimulus/face [male in heterosexual women (HeW) and homosexual men (HoM), and female in heterosexual men (HeM) and homosexual women (HoW)] and located in certain core regions of the reward circuitry and the motor cortex (1, 2). Furthermore, in a series of PET activation studies during smelling of putative pheromones we detected a sex differentiated activation of the anterior hypothalamus in HeM and HeW (3) and a sexatypical (almost reciprocal) pattern of activation in HoM and HoW (4, 5). While intriguing, none of these studies provided conclusions about the underlying mechanisms because they imaged perceptional processes, which could be innate, as well as learned. By indicating a link between sexually dimorphic regions of the midbrain and the sexual orientation, they fueled,...

Sexual differentiation of the human brain in relation to gender identity and sexual orientation

“…In homosexual women and heterosexual men, on the other hand, they were more widespread from the right amygdala. Furthermore, in homosexual men and heterosexual women the connections displayed were primarily with the contralateral amygdala and the anterior cingulate, while in heterosexual men and homosexual women the connections displayed were primarily displayed with the caudate, putamen, and the prefrontal cortex (53). Dichotic listening performance has also been found to show a greater right ear advantage in heterosexual men as compared to heterosexual women, while lesbian women were somewhat masculinized in their functional cerebral asymmetry (130).…”

“…In the adult human brain structural sex differences can be found from the macroscopic level (47) down to the ultramicroscopic level (48). Functionally, too, a large number of sex differences in different brain regions have recently been described (49)(50)(51)(52)(53). Sexual differentiation of the human brain is also expressed in behavioral differences, including sexual orientation (homo-, bi-and heterosexuality) and gender identity (22,(54)(55)(56)(57), and in differences at the level of brain physiology and in the prevalence of neurological and psychiatric disorders (57)(58)(59).…”

“…Savic's previous studies raised the question of whether certain sexually dimorphic features in the brain, which are unlikely to be directly involved in reproduction, may differ between homosexual and heterosexual individuals. This issue was explored by studying hemispheric asymmetry, using volumetric MRI, and functional connectivity of the amygdala, using PET measurements of cerebral blood flow (53). Volumetric measurements in heterosexual men and homosexual women showed a rightward cerebral asymmetry, whereas the volumes of the cerebral hemispheres were symmetrical in homosexual men and heterosexual women (53).…”

“…This issue was explored by studying hemispheric asymmetry, using volumetric MRI, and functional connectivity of the amygdala, using PET measurements of cerebral blood flow (53). Volumetric measurements in heterosexual men and homosexual women showed a rightward cerebral asymmetry, whereas the volumes of the cerebral hemispheres were symmetrical in homosexual men and heterosexual women (53). Moreover, homosexual subjects also showed sex-atypical amygdala connections.…”

The structure of the amygdala associates with human sexual permissiveness: Evidence from voxel‐based morphometry

“…However, even though previous studies associated basic human physiological sexual functions with studies of human sexual orientations [Savic and Lindstr€ om, 2008], the neural mechanisms that underlies the individual differences in human sexual attitudes and determine complex human attitudes toward various aspects of sexuality in social relationships have not been well investigated.…”