Neuroscience of sex differences

Neuroscience of sex differences is the study of the characteristics of the brain that separate the male brain and the female brain. Psychological sex differences are thought by some to reflect the interaction of genes, hormones and social learning on brain development throughout the lifespan.

Some evidence from brain morphology and function studies indicates that male and female brains cannot always be assumed to be identical from either a structural or functional perspective, and some brain structures are sexually dimorphic.[1][2]

History

Ideas of differences in the male and female brain circulated during the time of ancient Greek philosophers around 850 B.C. Aristotle claimed that males did not "receive their soul" until 40 days post-gestation and females did not until 80 days. In 1854, Emil Huschke discovered that "the frontal lobe in the male is all of 1% larger than that of the female."[3] As the 19th century progressed, scientists began researching sexual dimorphisms in the brain significantly more.[4] Until around 21 years ago, scientists knew of several structural sexual dimorphisms of the brain, but they did not think that sex had any impact on how the human brain performs daily tasks. Through molecular, animal, and neuroimaging studies, a great deal of information regarding the differences between male and female brains and how much they differ in regards to both structure and function has been uncovered.[5]

Evolutionary explanations

Sexual selection

Females show enhanced information recall compared to males. This may be due to the fact that females have a more intricate evaluation of risk-scenario contemplation, based on a prefrontal cortical control of the amygdala. For example, the ability to recall information better than males most likely originated from sexual selective pressures on females during competition with other females in mate selection. Recognition of social cues was an advantageous characteristic because it ultimately maximized offspring and was therefore selected for during evolution.[1]

Oxytocin is a hormone that induces contraction of the uterus and lactation in mammals. It is also a characteristic hormone of nursing mothers. Studies have found that oxytocin improves spatial memory. Through activation of the MAP kinase pathway, oxytocin plays a role in the enhancement of long-term synaptic plasticity, which is a change in strength between two neurons over a synapse that lasts for minutes or longer, and long-term memory. This hormone may have helped mothers remember the location of distant food sources so they could better nurture their offspring.[1]

Male vs. female brain anatomy

Hemisphere differences

A popular theory regarding language functions suggests that women use both hemispheres more equally, whereas men are more strongly lateralized to the left hemisphere; however a 2008 meta-analysis of 29 studies comparing language lateralization in males and females found no overall difference.[6]

Amygdala

image of Amygdala
The amygdala (red) in a human brain.

A 2016 meta-analysis found that the amygdala is not significantly larger in either sex.[7] A 2008 meta-analysis of fMRI studies of amygdala activation found larger effect sizes in men compared with women.[8]

Amygdala volume correlates positively with fearfulness in girls but not in boys.[9]

In fMRI studies, women display stronger activation in the amygdala and prefrontal cortex than men.[10]

Hippocampus

A 2016 meta-analysis found that the hippocampus does not differ in volume between men and women.[11]

Other regions and not region-specific

There are sex differences in locus coeruleus dendritic structure that allow for an increased reception and processing of limbic information in females compared to males.[9]

Aggressive and defiant behavior is also associated with decreased right anterior cingulate cortex (ACC) volume in boys.[9]

White/grey matter

In a 2013 meta-analysis, researchers found on average males had larger grey matter (GM) volume in bilateral amygdalae, hippocampi, anterior parahippocampal gyri, posterior cingulate gyri, precuneus, putamen and temporal poles, areas in the left posterior and anterior cingulate gyri, and areas in the cerebellum bilateral VIIb, VIIIa and Crus I lobes, left VI and right Crus II lobes.[2] On the other hand, females on average had larger grey matter volume at the right frontal pole, inferior and middle frontal gyri, pars triangularis, planum temporale/parietal operculum, anterior cingulate gyrus, insular cortex, and Heschl's gyrus; bilateral thalami and precuneus; the left parahippocampal gyrus and lateral occipital cortex (superior division).[2] The meta-analysis found larger volumes in females were most pronounced in areas in the right hemisphere related to language in addition to several limbic structures such as the right insular cortex and anterior cingulate gyrus.[2]

Brain networks

A 2014 meta-analysis found that although men and women commonly used the same brain networks for working memory, specific regions were sex specific.[12] For example, both men and women's active working memory networks composed of bilateral middle frontal gyri, left cingulate gyrus, right precuneus, left inferior and superior parietal lobes, right claustrum, and left middle temporal gyrus but women also tended have consistent activity in the limbic regions such as the anterior cingulate, bilateral amygdala and right hippocampus while men tended to have a distributed networks spread out among the cerebellum, portions of the superior parietal lobe, the left insula and bilateral thalamus.[12]

A 2017 review from the perspective of large-scale brain networks, hypothesized that women's higher susceptibility to stress-prone diseases like PTSD and major depressive disorder, in which the salience network is theorized to be overactive and to interfere with the executive control network, may be due in part (along with societal exposure to stressors and the coping strategies that are available to women) to underlying sex-based brain differences.[10]

Neurochemical differences

Hormones

Steroid hormones have several effects on brain development as well as maintenance of homeostasis throughout adulthood. One effect they exhibit is on the hypothalamus, where they increase synapse formation.[13] Estrogen receptors have been found in the hypothalamus, pituitary gland, hippocampus, and frontal cortex, indicating the estrogen plays a role in brain development. Gonadal hormone receptors have also been found in the basal forebrain nuclei.[14]

Estrogen and the female brain

Estradiol influences cognitive function, specifically by enhancing learning and memory in a dose-sensitive manner. Too much estrogen can have negative effects by weakening performance of learned tasks as well as hindering performance of memory tasks; this can result in females exhibiting poorer performance of such tasks when compared to males.[15]

It has been suggested that during development, estrogen can exhibit both feminizing and defeminizing effects on the human brain; high levels of estrogen induce male neural traits to develop while moderate levels induce female traits. In females, defeminizing effects are resisted because of the presence of α-fetoprotein (AFP), a carrier protein proposed to transport estrogen into brain cells, allowing the female brain to properly develop. The role of AFP is significant at crucial stages of development, however. Prenatally, AFP blocks estrogen. Postnatally, AFP decreases to ineffective levels; therefore, it is probable that estrogen exhibits its effects on female brain development postnatally.[16]

Ovariectomies, surgeries inducing menopause, or natural menopause cause fluctuating and decreased estrogen levels in women. This in turn can "attenuate the effects" of endogenous opioid peptides. Opioid peptides are known to play a role in emotion and motivation. β-endorphin (β-EP), an endogenous opioid peptide, content has been found to decrease (in varying amounts/brain region), post ovariectomy, in female rats within the hypothalamus, hippocampus, and pituitary gland. Such a change in β-EP levels could be the cause of mood swings, behavioral disturbances, and hot flashes in post menopausal women.[14]

Testosterone and the male brain

Testosterone heavily influences male development; a study found that perinatal females introduced to elevated testosterone levels exhibited male behavior patterns. In the absence of testosterone, female behavior is retained.[13] Testosterone's influence on the brain is caused by organizational developmental effects. It has been shown to influence proaptotic proteins so that they increase neuronal cell death in certain brain regions. Another way testosterone affects brain development is by aiding in the construction of the "limbic hypothalamic neural networks".[13]

Oxytocin and Vasopressin

Oxytocin is positively correlated with maternal behaviours, social recognition, social contact, sexual behaviour and pair bonding. Oxytocin appears at higher levels in women than in men.[17] Vasopressin on the other hand is more present in men and mediates sexual behavior, aggression and other social functions.[17][18]

Cognitive tasks

It was once thought that sex differences in cognitive task and problem solving did not occur until puberty. However, as of 2000 evidence suggested that cognitive and skill differences are present earlier in development. For example, researchers have found that three- and four-year-old boys were better at targeting and at mentally rotating figures within a clock face than girls of the same age were. Prepubescent girls, however, excelled at recalling lists of words. These sex differences in cognition correspond to patterns of ability rather than overall intelligence. Laboratory settings are used to systematically study the sexual dimorphism in problem solving task performed by adults.[19]

On average, males excel relative to females at certain spatial tasks. Specifically, males have an advantage in tests that require the mental rotation or manipulation of an object.[20] In a computer simulation of a maze task, males completed the task faster and with fewer errors than their female counterparts. Additionally, males have displayed higher accuracy in tests of targeted motor skills, such as guiding projectiles.[19] Males are also faster on reaction time and finger tapping tests.[21]

On average, females excel relative to males on tests that measure recollection. They have an advantage on processing speed involving letters, digits and rapid naming tasks.[21] Females tend to have better object location memory and verbal memory.[22] They also perform better at verbal learning.[23] Females have better performance at matching items and precision tasks, such as placing pegs into designated holes. In maze and path completion tasks, males learn the goal route in fewer trials than females, but females remember more of the landmarks presented. This shows that females use landmarks in everyday situations to orient themselves more than males. Females are better at remembering whether objects had switched places or not.[19]

See also

References

  1. 1 2 3 Cahill L (June 2006). "Why sex matters for neuroscience". Nature Reviews. Neuroscience. 7 (6): 477–84. doi:10.1038/nrn1909. PMID 16688123.
  2. 1 2 3 4 Ruigrok, Amber N. V.; Salimi-Khorshidi, Gholamreza; Lai, Meng-Chuan; Baron-Cohen, Simon; Lombardo, Michael V.; Tait, Roger J.; Suckling, John (2014-02-01). "A meta-analysis of sex differences in human brain structure". Neuroscience & Biobehavioral Reviews. 39: 34–50. doi:10.1016/j.neubiorev.2013.12.004. PMC 3969295. PMID 24374381.
  3. ↑ Swaab DF, Hofman MA (1984). "Sexual differentiation of the human brain. A historical perspective". Progress in Brain Research. Progress in Brain Research. 61: 361–74. doi:10.1016/S0079-6123(08)64447-7. ISBN 9780444805324. PMID 6396708.
  4. ↑ Hofman MA, Swaab DF (1991). "Sexual dimorphism of the human brain: myth and reality" (PDF). Experimental and Clinical Endocrinology. 98 (2): 161–70. doi:10.1055/s-0029-1211113. PMID 1778230.
  5. ↑ McCarthy, Margaret M. (2016-02-19). "Philosophical Transactions of the Royal Society B: Biological Sciences: 371 (1688)". Phil. Trans. R. Soc. B. Theme issue ‘Multifaceted origins of sex differences in the brain’. 371 (1688). doi:10.1098/rstb/371/1688. ISSN 0962-8436.
  6. ↑ Sommer IE, Aleman A, Somers M, Boks MP, Kahn RS (April 2008). "Sex differences in handedness, asymmetry of the planum temporale and functional language lateralization". Brain Research. 1206: 76–88. doi:10.1016/j.brainres.2008.01.003. PMID 18359009.
  7. ↑ Marwha, Dhruv; Halari, Meha; Eliot, Lise (2017-02-15). "Meta-analysis reveals a lack of sexual dimorphism in human amygdala volume". NeuroImage. 147: 282–294. doi:10.1016/j.neuroimage.2016.12.021.
  8. ↑ Sergerie, K; Chochol, C; Armony, JL (2008). "The role of the amygdala in emotional processing: a quantitative meta-analysis of functional neuroimaging studies". Neuroscience and biobehavioral reviews. 32 (4): 811–30. doi:10.1016/j.neubiorev.2007.12.002. PMID 18316124.
  9. 1 2 3 Kret, ME; De Gelder, B (June 2012). "A review on sex differences in processing emotional signals". Neuropsychologia. 50 (7): 1211–21. doi:10.1016/j.neuropsychologia.2011.12.022. PMID 22245006.
  10. 1 2 Homberg, Judith R; Kozicz, Tamas; Fernández, Guillén (April 2017). "Large-scale network balances in the transition from adaptive to maladaptive stress responses". Current Opinion in Behavioral Sciences. 14: 27–32. doi:10.1016/j.cobeha.2016.11.003.
  11. ↑ Tan, A; Ma, W; Vira, A; Marwha, D; Eliot, L (1 January 2016). "The human hippocampus is not sexually-dimorphic: Meta-analysis of structural MRI volumes". NeuroImage. 124 (Pt A): 350–366. doi:10.1016/j.neuroimage.2015.08.050. PMID 26334947.
  12. 1 2 Hill, Ashley C. (2014). "Gender differences in working memory networks: A BrainMap meta-analysis" (PDF). Biological Psychology. 102: 18–29. doi:10.1016/j.biopsycho.2014.06.008. PMC 4157091. PMID 25042764.
  13. 1 2 3 Simerly RB (February 2005). "Wired on hormones: endocrine regulation of hypothalamic development". Current Opinion in Neurobiology. 15 (1): 81–5. doi:10.1016/j.conb.2005.01.013. PMID 15721748.
  14. 1 2 Genazzani AR, Pluchino N, Luisi S, Luisi M (2007). "Estrogen, cognition and female ageing". Human Reproduction Update. 13 (2): 175–87. doi:10.1093/humupd/dml042. PMID 17135285.
  15. ↑ Korol DL (November 2004). "Role of estrogen in balancing contributions from multiple memory systems". Neurobiology of Learning and Memory. 82 (3): 309–23. doi:10.1016/j.nlm.2004.07.006. PMID 15464412.
  16. ↑ Bakker J, Baum MJ (January 2008). "Role for estradiol in female-typical brain and behavioral sexual differentiation". Frontiers in Neuroendocrinology. 29 (1): 1–16. doi:10.1016/j.yfrne.2007.06.001. PMC 2373265. PMID 17720235.
  17. 1 2 Carter, C.Sue (2006). "Sex differences in oxytocin and vasopressin: Implications for autism spectrum disorders?" (PDF). Behavioural Brain Research. 176 (1): 170–186. doi:10.1016/j.bbr.2006.08.025. PMID 17000015.
  18. ↑ Skuse, David H. (2006-11-01). "Sexual dimorphism in cognition and behaviour: the role of X-linked genes". European Journal of Endocrinology. 155 (suppl 1): S99–S106. doi:10.1530/eje.1.02263. ISSN 0804-4643.
  19. 1 2 3 Kimura, Doreen (July 31, 2000). Sex and Cognition. A Bradford Book. p. 28. ISBN 0262611643.
  20. ↑ Miller, DI; Halpern, DF (January 2014). "The new science of cognitive sex differences". Trends in cognitive sciences. 18 (1): 37–45. doi:10.1016/j.tics.2013.10.011. PMID 24246136.
  21. 1 2 Roivainen, Eka (2011). "Gender differences in processing speed: A review of recent research". Learning and Individual Differences. 21 (2): 145–149. doi:10.1016/j.lindif.2010.11.021.
  22. ↑ Li, Rena (2014-09-01). "Why women see differently from the way men see? A review of sex differences in cognition and sports". Journal of Sport and Health Science. 3 (3): 155–162. doi:10.1016/j.jshs.2014.03.012. PMC 4266559. PMID 25520851.
  23. ↑ Wallentin, Mikkel (2009). "Putative sex differences in verbal abilities and language cortex: A critical review". Brain and Language. 108 (3): 175–183. doi:10.1016/j.bandl.2008.07.001. PMID 18722007.
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