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The Brain and Sexual Orientation

The Brain and Sexual Orientation. Gay, Straight, and the Reason Why by Simon LeVay. All mental traits, including sexual orientation and gender identity, have some durable representation in the brain.

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The Brain and Sexual Orientation

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  1. The Brain and Sexual Orientation Gay, Straight, and the Reason Why by Simon LeVay

  2. All mental traits, including sexual orientation and gender identity, have some durable representation in the brain. • Locating these representations proves difficult – could be specific region or represented by network over large areas. • Even if it’s found, still doesn’t mean we understand why it came into being.

  3. Brain Tour • Forebrain - most of brain’s volume • Brainstem – connects forebrain to spinal cord • Cerebral cortex

  4. Key Areas • Under the white matter - large groups of nuclei-clusters or masses of neurons that are not organized into layers. • Thalamus • Hypothalamus • Amygdala

  5. Thalamus • converts sensory signals into a form that the cerebral cortex can digest; participates in cognitive processes with two-way connection with the cortex.

  6. Hypothalamus • participates in “life preserving” functions; also controls the pituitary gland.

  7. Amygdala • processing of emotion, involved in sexual functions and behavior.

  8. INAH3 • Area in the front of the hypothalamus – medial preoptic area, regulates the male-typical sexual behaviors. • Within this area is one area that is larger in males than females – INAH3

  9. INAH3 Studies • In 1991, LeVay discovered INAH3 was significantly smaller in gay men than in straight men. • Attempt to replicate the study did not prove or disprove LeVay’s results – Byne called it a “trend”.

  10. Other Regions • Anterior commissure – normally larger in women than men; Allen & Gorski found it was larger in gay men than straight men, Byne did not find a difference at all (autopsies)

  11. Other Regions • Corpus callosum – normally larger in women than men; larger in gay men, but only in isthmus connecting parietal and temporal lobes in left/right hemispheres (MRI in living subjects)

  12. Other Regions • Distribution of gray/white matter – typically more gray matter in women than in men; no differences between gay and straight men, but less gray matter in lesbians than straight women. • Size of left/right hemispheres – in women, both sides are the same; in men, right is 2% larger than left. In gay men, both hemispheres were the same size, in lesbians, the right side was just slightly larger than the left.

  13. Amygdale Differences • In straight men, right was more richly connected with other brain regions than left; opposite in straight women AND functional connections were to different brain regions. • In gay men and lesbians, characteristics were sex-atypical

  14. Other Differences • Brain Activity – use of fMRI while watching erotic images • Same brain regions are active when straight men view female images as when gay men view male images; similar for lesbian and straight women • Czech phallometric test for asylum

  15. Pheromone Studies • Several studies using fruit flies and rodents, indicate sex pheromones play an important part in directing males and females to opposite sex. • Controversy as to whether that applies to humans • Olfactory system less well developed in humans • Sex pheromones may be detected by the main olfactory sense organ, not the VNO

  16. Pheromone Studies • Asked straight men, straight women, gay men and lesbians to compare attractiveness of odors from the armpits of these same four groups. • Straight men & women showed no preferences when comparing odors derived from those groups

  17. Pheromone Studies • Straight men, straight women and lesbians all liked odors from gay men less than other subject groups. • Gay men liked the odors from gay men better than those from other subject groups, especially in comparison to odors from straight men or lesbians

  18. Summary • Growing evidence for structural and functional differences between the brains of gay and straight people • Involve brain systems that could be involved in the regulation of sexual attraction to one sex or the other (hypothalamus & amygdala) as well as systems that are unlikely to have any involvement with sexuality.

  19. Summary • Prenatal sex hormones control the sexual differentiation of brain centers involved in sexual behaviors AND this process progresses differently in individuals who become gay than those who become straight. • May be during early development of neurons or later when neurons are elaborating their synaptic connections.

  20. Summary • Differences also exist in areas not concerned with sexuality – compare to cognitive differences like visuospatial abilities, verbal fluency, memory tasks, occupational preferences, etc. • Suggestions that male heterosexuality may involve suppression of attraction to same sex. • Damage to certain parts of the brain remove that inhibition. • Man sues drug company for turning him gay

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