Abstract
The neurobiology of sexual preference is often discussed in terms of cerebral sex dimorphism. Yet, our knowledge about possible cerebral differences between homosexual men (HoM), heterosexual men (HeM) and heterosexual women (HeW) are extremely limited. In the present MRI study, we addressed this issue investigating measures of cerebral anatomy and function, which were previously reported to show sex difference. Specifically, we asked whether there were any signs of sex atypical cerebral dimorphism among HoM, if these were widely distributed (providing substrate for more general ‘female’ behavioral characteristics among HoM), or restricted to networks involved in self-referential sexual arousal. Cortical thickness (Cth), surface area (SA), subcortical structural volumes, and resting state functional connectivity were compared between 30 (HoM), 35 (HeM) and 38 (HeW). HoM displayed a significantly thicker anterior cingulate cortex (ACC), precuneus, and the left occipito-temporal cortex compared to both control groups. These differences seemed coordinated, since HoM also displayed stronger cortico-cortical covariations between these regions. Furthermore, functional connections within the default mode network, which mediates self- referential processing, and includes the ACC and precuneus were significantly weaker in HoM than HeM and HeW, whereas their functional connectivity between the thalamus and hypothalamus (important nodes for sexual behavior) was stronger. In addition to these singular features, HoM displayed ‘female’ characteristics, with a similar Cth in the left superior parietal and cuneus cortices as HeW, but different from HeM. These data suggest both singular and sex atypical features and motivate further investigations of cerebral midline structures in relation to male homosexuality.
Highlights
One of the more controversial questions in the neurobiology of human behavior relates to the mechanisms of sexual orientation
The parietal lobe cortex, and sensory-motor cortex were thicker in heterosexual women (HeW) than heterosexual men (HeM), whereas the left superior/middle temporal gyrus and the left lateral occipital cortex were thinner
In the present study we investigated whether there are any differences between homosexual men (HoM), HeW, and HeM with respect to various measures of cerebral anatomy and functional connectivity, reported to differ between men and women
Summary
One of the more controversial questions in the neurobiology of human behavior relates to the mechanisms of sexual orientation. Sexual orientation can be viewed as interplay between specific cerebral processes. Most of the hitherto performed studies related to sexual orientation focused on the first level, using various brain imaging methods, and investigating cerebral activation during sexual arousal, elicited by passive viewing of film clips. A majority of these brain imaging studies of cue-induced sexual arousal seem to agree that the cerebral response is invariant to the preferred sex [1,2,3] and is primarily related to whether the stimulus is from the desired or nondesired sex, there are some exceptions [3,4,5,6]. Recently, Zhou et al found that smelling a putative male pheromone enhanced the visual perception of male figures in HoM and HeW, but not in HeM, indicating that hypothalamic activation by the male putative pheromone in HoM and HeW had downstream effects on visual perception with a potential impact on selection of sexual partner [9,10,11]
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