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COGNITIVE SCIENCE 17 Why Sex is Necessary Part 2 Jaime A. Pineda, Ph.D. Hormonal Control of Reproductive Cycle • Menstrual cycle – Begins with secretion of FSH to stimulate growth of ovarian follicles (epithelial cells surrounding each ovum) – As ovarian follicles mature they secrete estradiol causing the growth of the lining of the uterus (preparation for fertilization) – Increasing levels of estradiol triggers the release of LH causing ovulation (release of ovum) – Ovum enters a Fallopian tube and starts migrating towards uterus. – If it meets sperm and becomes fertilized it begins to divide and then attaches itself to uterus wall – If it is not fertilized, the ruptured ovarian follicle (corpus luteum) and the lining of the uterine wall will be expelled Menstrual Cycle Sexual Behavior • Hormones have organizational effects on sexual behavior • They exert these effects by altering the development of the brain • In most female mammals (other than higher primates) sexual behavior (willingness and ability to mate) is controlled by: – Ovarian hormones: estradiol and progesterone – Pheromones: odor and smell Effects on Human Females • The ability to mate is not strictly controlled by ovarian hormones – No physical barriers (such as lordosis) – Hormones may affect sexual interest (female-initiated sexual activity peaks during ovulation – estradiol levels are highest) • Other factors are important – Pleasure, affection, attraction, and love (emotions) – Desire to get pregnant; confidence in birth control method – Cognition (how a stimulus is interpreted – romantic vs sexual interest?) – Learning (e.g., Fear of rejection) and culture Effects on Human Males • Males (in contrast) resemble other mammals in their response to sex hormones – Normal levels of testosterone potent/fertile – No testosterone reduction in sexual interest, sexual fantasy, and intercourse Sexual activity (even imagining it) affects levels of testosterone which affects levels of sexual activity Neural Control of Sexual Behavior: Role of the Brain Neural Control of Sexual Behavior • Human males – Erection and ejaculation are controlled by spinal reflexes • Spinal nucleus of the bulbocavernosus (SNB) – controls the muscle that attaches to penis • SNB is controlled by the level of androgens present – Brain mechanisms can control reflex circuits • Medial preoptic area (MPA) of the hypothalamus – Sexually dimorphic nucleus (SDN): larger in males than females • Medial amygdala, bed nucleus of the stria terminalis (BNST), periaqueductal gray (PAG) and nucleus paragigantocellularis (PGi) Medial Preoptic Area (MPA) Lesions impair sexual and parenting behaviors Controls mating behavior Contains estrogen receptors What is its role? Involved in both consummatory (execution) and appetitive (motivational) aspects of masculine sexual behavior. Neural Control of Sexual Behavior • Females – Ventromedial nucleus of the hypothalamus (VMH) – Medial amygdala – Periaqueductal gray matter (PAG) – PGi Sexual Orientation • Sexuality is best regarded as a continuum with most people being primarily heterosexual, some bisexual, and others primarily homosexual. • Homosexuality was thought of as a choice or the result of faulty upbringing. • There is now evidence that it is not a choice or the result of upbringing. • Biological events appear to play a major role in determining sexual orientation. – LeVay found evidence of a structure in the hypothalamus (MPA) that is smaller in women and in homosexual males than in heterosexual males. – This study is supported by animal research linking atypical sexual behavior when the MPA was removed. – These and other data suggest a biological predisposition for some people to become homosexual. The evidence for biological underpinnings to sexual orientation is not sufficient to rule out a role for the environment. Sex Differences in Cognitive Function • Female superiority: tests that emphasize perceptual, psychomotor abilities, face recognition, object memory, object location memory, language and processing speed • Male superiority: tests that emphasize visualspatial functions, mental rotation, navigation, targeting, mechanical reasoning, and mathematical skills Fig. 1. Targeting accuracy with no delay in the targeting response. Data are mean (+ SEM). Figure 9. The four EVITA conditions are shown in the top two rows. Color-EVITA (bottom row) uses a white ball that changes randomly on 50% of the trials to a shade of red, blue or yellow just before it goes under the masking shade. Fig. 4 Color identification accuracy under slow (left panel) and fast (right panel) ball speeds. Females were significantly (p<0.05) more accurate than males when ball speed was slow. Data are mean (+ SEM). Anatomic Differences & Behavior • Lateralization of verbal functions to the left hemisphere and visual-spatial functions to right is more pronounced in males, beginning in childhood • Less hemispheric lateralization in females: greater need for interhemispheric communication (larger corpus callosum?) Theories of Functional Differences • Perhaps specific functions that show male superiority require relatively large volumes of brain tissue • Whereas functions that show female superiority may be independent of tissue volume but critically dependent on circuit organization. • Neuronal density may be normally greater in certain critical areas of the female brain, reducing volume without reducing efficacy • Males greater hemispheric lateralization may confer certain functional advantages but result in male intrahemispheric brain tissue redundancy for other functions Sagittal MRI