Fwd: boys' brains, girls' brains

[Scott F. Kiesling]

Hi all -

I'd be interested in your reactions to this article,
especailly those of you who know this literature better!

This is a Scientific American article; the URL is below if
you'd rather read it that way.

Thanks,
Scott


>
> April 25, 2005
>
> His Brain, Her Brain
>
> It turns out that male and female brains differ quite a bit in
> architecture and activity. Research into these variations could lead
> to
> sex-specific treatments for disorders such as depression and
> schizophrenia
>
> By Larry Cahill
>
> On a gray day in mid-January, Lawrence Summers, the president of
> Harvard
> University, suggested that innate differences in the build of the
> male and
> female brain might be one factor underlying the relative scarcity of
> women
> in science. His remarks reignited a debate that has been smoldering
> for a
> century, ever since some scientists sizing up the brains of both
> sexes
> began using their main finding--that female brains tend to be
> smaller--to
> bolster the view that women are intellectually inferior to men.
> To date, no one has uncovered any evidence that anatomical
> disparities
> might render women incapable of achieving academic distinction in
> math,
> physics or engineering. And the brains of men and women have been
> shown to
> be quite clearly similar in many ways. Nevertheless, over the past
> decade
> investigators have documented an astonishing array of structural,
> chemical
> and functional variations in the brains of males and females.
>
> These inequities are not just interesting idiosyncrasies that might
> explain why more men than women enjoy the Three Stooges. They raise
> the
> possibility that we might need to develop sex-specific treatments for
> a
> host of conditions, including depression, addiction, schizophrenia
> and
> post-traumatic stress disorder (PTSD). Furthermore, the differences
> imply
> that researchers exploring the structure and function of the brain
> must
> take into account the sex of their subjects when analyzing their
> data--and
> include both women and men in future studies or risk obtaining
> misleading
> results.
>
> Sculpting the Brain
> Not so long ago neuroscientists believed that sex differences in the
> brain
> were limited mainly to those regions responsible for mating behavior.
> In a
> 1966 Scientific American article entitled "Sex Differences in the
> Brain,"
> Seymour Levine of Stanford University described how sex hormones help
> to
> direct divergent reproductive behaviors in rats--with males engaging
> in
> mounting and females arching their backs and raising their rumps to
> attract suitors. Levine mentioned only one brain region in his
> review: the
> hypothalamus, a small structure at the base of the brain that is
> involved
> in regulating hormone production and controlling basic behaviors such
> as
> eating, drinking and sex. A generation of neuroscientists came to
> maturity
> believing that "sex differences in the brain" referred primarily to
> mating
> behaviors, sex hormones and the hypothalamus.
>
>
>
>
> --------------------------------------------------------------------------------
>
> Several intriguing behavioral studies add to the evidence that some
> sex
> differences in the brain arise before a baby draws its first breath.
>
> --------------------------------------------------------------------------------
>
>
> That view, however, has now been knocked aside by a surge of findings
> that
> highlight the influence of sex on many areas of cognition and
> behavior,
> including memory, emotion, vision, hearing, the processing of faces
> and
> the brain's response to stress hormones. This progress has been
> accelerated in the past five to 10 years by the growing use of
> sophisticated noninvasive imaging techniques such as
> positron-emission
> tomography (PET) and functional magnetic resonance imaging (fMRI),
> which
> can peer into the brains of living subjects.
>
> These imaging experiments reveal that anatomical variations occur in
> an
> assortment of regions throughout the brain. Jill M. Goldstein of
> Harvard
> Medical School and her colleagues, for example, used MRI to measure
> the
> sizes of many cortical and subcortical areas. Among other things,
> these
> investigators found that parts of the frontal cortex, the seat of
> many
> higher cognitive functions, are bulkier in women than in men, as are
> parts
> of the limbic cortex, which is involved in emotional responses. In
> men, on
> the other hand, parts of the parietal cortex, which is involved in
> space
> perception, are bigger than in women, as is the amygdala, an
> almond-shaped
> structure that responds to emotionally arousing information--to
> anything
> that gets the heart pumping and the adrenaline flowing. These size
> differences, as well as others mentioned throughout the article, are
> relative: they refer to the overall volume of the structure relative
> to
> the overall volume of the brain.
>
> Differences in the size of brain structures are generally thought to
> reflect their relative importance to the animal. For example,
> primates
> rely more on vision than olfaction; for rats, the opposite is true.
> As a
> result, primate brains maintain proportionately larger regions
> devoted to
> vision, and rats devote more space to olfaction. So the existence of
> widespread anatomical disparities between men and women suggests that
> sex
> does influence the way the brain works.
>
> Other investigations are finding anatomical sex differences at the
> cellular level. For example, Sandra Witelson and her colleagues at
> McMaster University discovered that women possess a greater density
> of
> neurons in parts of the temporal lobe cortex associated with language
> processing and comprehension. On counting the neurons in postmortem
> samples, the researchers found that of the six layers present in the
> cortex, two show more neurons per unit volume in females than in
> males.
> Similar findings were subsequently reported for the frontal lobe.
> With
> such information in hand, neuroscientists can now explore whether sex
> differences in neuron number correlate with differences in cognitive
> abilities--examining, for example, whether the boost in density in
> the
> female auditory cortex relates to women's enhanced performance on
> tests of
> verbal fluency.
>
> Such anatomical diversity may be caused in large part by the activity
> of
> the sex hormones that bathe the fetal brain. These steroids help to
> direct
> the organization and wiring of the brain during development and
> influence
> the structure and neuronal density of various regions. Interestingly,
> the
> brain areas that Goldstein found to differ between men and women are
> ones
> that in animals contain the highest number of sex hormone receptors
> during
> development. This correlation between brain region size in adults and
> sex
> steroid action in utero suggests that at least some sex differences
> in
> cognitive function do not result from cultural influences or the
> hormonal
> changes associated with puberty--they are there from birth.
>
> Inborn Inclinations
> Several intriguing behavioral studies add to the evidence that some
> sex
> differences in the brain arise before a baby draws its first breath.
> Through the years, many researchers have demonstrated that when
> selecting
> toys, young boys and girls part ways. Boys tend to gravitate toward
> balls
> or toy cars, whereas girls more typically reach for a doll. But no
> one
> could really say whether those preferences are dictated by culture or
> by
> innate brain biology.
>
> To address this question, Melissa Hines of City University London and
> Gerianne M. Alexander of Texas A&M University turned to monkeys, one
> of
> our closest animal cousins. The researchers presented a group of
> vervet
> monkeys with a selection of toys, including rag dolls, trucks and
> some
> gender-neutral items such as picture books. They found that male
> monkeys
> spent more time playing with the "masculine" toys than their female
> counterparts did, and female monkeys spent more time interacting with
> the
> playthings typically preferred by girls. Both sexes spent equal time
> monkeying with the picture books and other gender-neutral toys.
>
> Because vervet monkeys are unlikely to be swayed by the social
> pressures
> of human culture, the results imply that toy preferences in children
> result at least in part from innate biological differences. This
> divergence, and indeed all the anatomical sex differences in the
> brain,
> presumably arose as a result of selective pressures during evolution.
> In
> the case of the toy study, males--both human and primate--prefer toys
> that
> can be propelled through space and that promote rough-and-tumble
> play.
> These qualities, it seems reasonable to speculate, might relate to
> the
> behaviors useful for hunting and for securing a mate. Similarly, one
> might
> also hypothesize that females, on the other hand, select toys that
> allow
> them to hone the skills they will one day need to nurture their
> young.
>
> Simon Baron-Cohen and his associates at the University of Cambridge
> took a
> different but equally creative approach to addressing the influence
> of
> nature versus nurture regarding sex differences. Many researchers
> have
> described disparities in how "people-centered" male and female
> infants
> are. For example, Baron-Cohen and his student Svetlana Lutchmaya
> found
> that one-year-old girls spend more time looking at their mothers than
> boys
> of the same age do. And when these babies are presented with a choice
> of
> films to watch, the girls look longer at a film of a face, whereas
> boys
> lean toward a film featuring cars.
>
>
> Of course, these preferences might be attributable to differences in
> the
> way adults handle or play with boys and girls. To eliminate this
> possibility, Baron-Cohen and his students went a step further. They
> took
> their video camera to a maternity ward to examine the preferences of
> babies that were only one day old. The infants saw either the
> friendly
> face of a live female student or a mobile that matched the color,
> size and
> shape of the student's face and included a scrambled mix of her
> facial
> features. To avoid any bias, the experimenters were unaware of each
> baby's
> sex during testing. When they watched the tapes, they found that the
> girls
> spent more time looking at the student, whereas the boys spent more
> time
> looking at the mechanical object. This difference in social interest
> was
> evident on day one of life--implying again that we come out of the
> womb
> with some cognitive sex differences built in.
>
> Under Stress
> In many cases, sex differences in the brain's chemistry and
> construction
> influence how males and females respond to the environment or react
> to,
> and remember, stressful events. Take, for example, the amygdala.
> Goldstein
> and others have reported that the amygdala is larger in men than in
> women.
> And in rats, the neurons in this region make more numerous
> interconnections in males than in females. These anatomical
> variations
> would be expected to produce differences in the way that males and
> females
> react to stress.
>
> To assess whether male and female amygdalae in fact respond
> differently to
> stress, Katharina Braun and her co-workers at Otto von Guericke
> University
> in Magdeburg, Germany, briefly removed a litter of Degu pups from
> their
> mother. For these social South American rodents, which live in large
> colonies like prairie dogs do, even temporary separation can be quite
> upsetting. The researchers then measured the concentration of
> serotonin
> receptors in various brain regions. Serotonin is a neurotransmitter,
> or
> signal-carrying molecule, that is key for mediating emotional
> behavior.
> (Prozac, for example, acts by increasing serotonin function.)
>
> The workers allowed the pups to hear their mother's call during the
> period
> of separation and found that this auditory input increased the
> serotonin
> receptor concentration in the males' amygdala, yet decreased the
> concentration of these same receptors in females. Although it is
> difficult
> to extrapolate from this study to human behavior, the results hint
> that if
> something similar occurs in children, separation anxiety might
> differentially affect the emotional well-being of male and female
> infants.
> Experiments such as these are necessary if we are to understand why,
> for
> instance, anxiety disorders are far more prevalent in girls than in
> boys.
>
> Another brain region now known to diverge in the sexes anatomically
> and in
> its response to stress is the hippocampus, a structure crucial for
> memory
> storage and for spatial mapping of the physical environment. Imaging
> consistently demonstrates that the hippocampus is larger in women
> than in
> men. These anatomical differences might well relate somehow to
> differences
> in the way males and females navigate. Many studies suggest that men
> are
> more likely to navigate by estimating distance in space and
> orientation
> ("dead reckoning"), whereas women are more likely to navigate by
> monitoring landmarks. Interestingly, a similar sex difference exists
> in
> rats. Male rats are more likely to navigate mazes using directional
> and
> positional information, whereas female rats are more likely to
> navigate
> the same mazes using available landmarks. (Investigators have yet to
> demonstrate, however, that male rats are less likely to ask for
> directions.)
>
> Even the neurons in the hippocampus behave differently in males and
> females, at least in how they react to learning experiences. For
> example,
> Janice M. Juraska and her associates at the University of Illinois
> have
> shown that placing rats in an "enriched environment"--cages filled
> with
> toys and with fellow rodents to promote social interactions--produced
> dissimilar effects on the structure of hippocampal neurons in male
> and
> female rats. In females, the experience enhanced the "bushiness" of
> the
> branches in the cells' dendritic trees--the many-armed structures
> that
> receive signals from other nerve cells. This change presumably
> reflects an
> increase in neuronal connections, which in turn is thought to be
> involved
> with the laying down of memories. In males, however, the complex
> environment either had no effect on the dendritic trees or pruned
> them
> slightly.
>
>
> But male rats sometimes learn better in the face of stress. Tracey J.
> Shors of Rutgers University and her collaborators have found that a
> brief
> exposure to a series of one-second tail shocks enhanced performance
> of a
> learned task and increased the density of dendritic connections to
> other
> neurons in male rats yet impaired performance and decreased
> connection
> density in female rats. Findings such as these have interesting
> social
> implications. The more we discover about how brain mechanisms of
> learning
> differ between the sexes, the more we may need to consider how
> optimal
> learning environments potentially differ for boys and girls.
>
> Although the hippocampus of the female rat can show a decrement in
> response to acute stress, it appears to be more resilient than its
> male
> counterpart in the face of chronic stress. Cheryl D. Conrad and her
> co-workers at Arizona State University restrained rats in a mesh cage
> for
> six hours--a situation that the rodents find disturbing. The
> researchers
> then assessed how vulnerable their hippocampal neurons were to
> killing by
> a neurotoxin--a standard measure of the effect of stress on these
> cells.
> They noted that chronic restraint rendered the males' hippocampal
> cells
> more susceptible to the toxin but had no effect on the females'
> vulnerability. These findings, and others like them, suggest that in
> terms
> of brain damage, females may be better equipped to tolerate chronic
> stress
> than males are. Still unclear is what protects female hippocampal
> cells
> from the damaging effects of chronic stress, but sex hormones very
> likely
> play a role.
>
>
> The Big Picture
> Extending the work on how the brain handles and remembers stressful
> events, my colleagues and I have found contrasts in the way men and
> women
> lay down memories of emotionally arousing incidents--a process known
> from
> animal research to involve activation of the amygdala. In one of our
> first
> experiments with human subjects, we showed volunteers a series of
> graphically violent films while we measured their brain activity
> using
> PET. A few weeks later we gave them a quiz to see what they
> remembered.
>
> We discovered that the number of disturbing films they could recall
> correlated with how active their amygdala had been during the
> viewing.
> Subsequent work from our laboratory and others confirmed this general
> finding. But then I noticed something strange. The amygdala
> activation in
> some studies involved only the right hemisphere, and in others it
> involved
> only the left hemisphere. It was then I realized that the experiments
> in
> which the right amygdala lit up involved only men; those in which the
> left
> amygdala was fired up involved women. Since then, three subsequent
> studies--two from our group and one from John Gabrieli and Turhan
> Canli
> and their collaborators at Stanford--have confirmed this difference
> in how
> the brains of men and women handle emotional memories.
>
> The realization that male and female brains were processing the same
> emotionally arousing material into memory differently led us to
> wonder
> what this disparity might mean. To address this question, we turned
> to a
> century-old theory stating that the right hemisphere is biased toward
> processing the central aspects of a situation, whereas the left
> hemisphere
> tends to process the finer details. If that conception is true, we
> reasoned, a drug that dampens the activity of the amygdala should
> impair a
> man's ability to recall the gist of an emotional story (by hampering
> the
> right amygdala) but should hinder a woman's ability to come up with
> the
> precise details (by hampering the left amygdala).
>
> Propranolol is such a drug. This so-called beta blocker quiets the
> activity of adrenaline and its cousin noradrenaline and, in so doing,
> dampens the activation of the amygdala and weakens recall of
> emotionally
> arousing memories. We gave this drug to men and women before they
> viewed a
> short slide show about a young boy caught in a terrible accident
> while
> walking with his mother. One week later we tested their memory. The
> results showed that propranolol made it harder for men to remember
> the
> more holistic aspects, or gist, of the story--that the boy had been
> run
> over by a car, for example. In women, propranolol did the converse,
> impairing their memory for peripheral details--that the boy had been
> carrying a soccer ball.
>
>
> In more recent investigations, we found that we can detect a
> hemispheric
> difference between the sexes in response to emotional material almost
> immediately. Volunteers shown emotionally unpleasant photographs
> react
> within 300 milliseconds--a response that shows up as a spike on a
> recording of the brain's electrical activity. With Antonella Gasbarri
> and
> others at the University of L'Aquila in Italy, we have found that in
> men,
> this quick spike, termed a P300 response, is more exaggerated when
> recorded over the right hemisphere; in women, it is larger when
> recorded
> over the left. Hence, sex-related hemispheric disparities in how the
> brain
> processes emotional images begin within 300 milliseconds--long before
> people have had much, if any, chance to consciously interpret what
> they
> have seen.
>
> These discoveries might have ramifications for the treatment of PTSD.
> Previous research by Gustav Schelling and his associates at Ludwig
> Maximilian University in Germany had established that drugs such as
> propranolol diminish memory for traumatic situations when
> administered as
> part of the usual therapies in an intensive care unit. Prompted by
> our
> findings, they found that, at least in such units, beta blockers
> reduce
> memory for traumatic events in women but not in men. Even in
> intensive
> care, then, physicians may need to consider the sex of their patients
> when
> meting out their medications.
>
> Sex and Mental Disorders
>
> ptsd is not the only psychological disturbance that appears to play
> out
> differently in women and men. A PET study by Mirko Diksic and his
> colleagues at McGill University showed that serotonin production was
> a
> remarkable 52 percent higher on average in men than in women, which
> might
> help clarify why women are more prone to depression--a disorder
> commonly
> treated with drugs that boost the concentration of serotonin.
>
> A similar situation might prevail in addiction. In this case, the
> neurotransmitter in question is dopamine--a chemical involved in the
> feelings of pleasure associated with drugs of abuse. Studying rats,
> Jill
> B. Becker and her fellow investigators at the University of Michigan
> at
> Ann Arbor discovered that in females, estrogen boosted the release of
> dopamine in brain regions important for regulating drug-seeking
> behavior.
> Furthermore, the hormone had long-lasting effects, making the female
> rats
> more likely to pursue cocaine weeks after last receiving the drug.
> Such
> differences in susceptibility--particularly to stimulants such as
> cocaine
> and amphetamine--could explain why women might be more vulnerable to
> the
> effects of these drugs and why they tend to progress more rapidly
> from
> initial use to dependence than men do.
>
> Certain brain abnormalities underlying schizophrenia appear to differ
> in
> men and women as well. Ruben Gur, Raquel Gur and their colleagues at
> the
> University of Pennsylvania have spent years investigating sex-related
> differences in brain anatomy and function. In one project, they
> measured
> the size of the orbitofrontal cortex, a region involved in regulating
> emotions, and compared it with the size of the amygdala, implicated
> more
> in producing emotional reactions. The investigators found that women
> possess a significantly larger orbitofrontal-to-amygdala ratio (OAR)
> than
> men do. One can speculate from these findings that women might on
> average
> prove more capable of controlling their emotional reactions.
>
> In additional experiments, the researchers discovered that this
> balance
> appears to be altered in schizophrenia, though not identically for
> men and
> women. Women with schizophrenia have a decreased OAR relative to
> their
> healthy peers, as might be expected. But men, oddly, have an
> increased OAR
> relative to healthy men. These findings remain puzzling, but, at the
> least, they imply that schizophrenia is a somewhat different disease
> in
> men and women and that treatment of the disorder might need to be
> tailored
> to the sex of the patient.
>
> Sex Matters
> in a comprehensive 2001 report on sex differences in human health,
> the
> prestigious National Academy of Sciences asserted that "sex matters.
> Sex,
> that is, being male or female, is an important basic human variable
> that
> should be considered when designing and analyzing studies in all
> areas and
> at all levels of biomedical and health-related research."
>
>
> Neuroscientists are still far from putting all the pieces
> together--identifying all the sex-related variations in the brain and
> pinpointing their influences on cognition and propensity for
> brain-related
> disorders. Nevertheless, the research conducted to date certainly
> demonstrates that differences extend far beyond the hypothalamus and
> mating behavior. Researchers and clinicians are not always clear on
> the
> best way to go forward in deciphering the full influences of sex on
> the
> brain, behavior and responses to medications. But growing numbers now
> agree that going back to assuming we can evaluate one sex and learn
> equally about both is no longer an option.
>
>
>
>  1996-2005 Scientific American, Inc. All rights reserved.
> Reproduction in whole or in part without permission is prohibited.
>
>
> http://www.sciam.com/print_version.cfm?articleID=000363E3-1806-1264-980683414B7F0000
>
>
>



----- End forwarded message -----

--
Scott F. Kiesling

Assistant Professor
Department of Linguistics
University of Pittsburgh

2816 Cathedral of Learning     Phone: 1-412-624-5916
Pittsburgh, PA 15260 USA       Fax: 1-412-624-6130

kiesling at pitt.edu
http://www.pitt.edu/~kiesling/skpage.html
http://www.linguistics.pitt.edu

----- End forwarded message -----

--
Scott F. Kiesling

Assistant Professor
Department of Linguistics
University of Pittsburgh

2816 Cathedral of Learning     Phone: 1-412-624-5916
Pittsburgh, PA 15260 USA       Fax: 1-412-624-6130

kiesling at pitt.edu
http://www.pitt.edu/~kiesling/skpage.html
http://www.linguistics.pitt.edu