Transcript GSRWx
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. • 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. Brain Tour • Forebrain - most of brain’s volume • Brainstem – connects forebrain to spinal cord • Cerebral cortex Key Areas • Under the white matter - large groups of nucleiclusters or masses of neurons that are not organized into layers. • Thalamus • Hypothalamus • Amygdala Thalamus • converts sensory signals into a form that the cerebral cortex can digest; participates in cognitive processes with two-way connection with the cortex. Hypothalamus • participates in “life preserving” functions; also controls the pituitary gland. Amygdala • processing of emotion, involved in sexual functions and behavior. 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 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”. 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) 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) 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. 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 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 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 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 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 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. 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. 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