Химия Exercise 5. Read the following article to check some of your answers in Exercise 4. просмотров - 242
Exercise 4. Sex Differences in the Brain.
Men and women display patterns of behavioral and cognitive differences that reflect varying hormonal influences on brain development. Before reading the next text guess if the following statements generally refer more to men or to women:
- Men/Women, on average, have stronger verbal skills (especially in writing).
- Men/Women generally are better at mentally manipulating objects.
- Men/Women have better memory for events, words, objects, faces and activities.
- Men/Women are better at performing certain quantitative tasks that rely on visual representations.
- Men/Women can better recognize emotions and show higher levels of empathy.
Girl Brain, Boy Brain?
The two are not the same, but new work shows just how wrong it is to assume that all gender differences are “hardwired”
By Lise Eliot
As MRI scanning grows ever more sophisticated, neuroscientists keep refining their search for male-female brain differences that will answer the age-old question, “Why can’t a woman think like a man?” (and vice-versa). Social cognition is one realm in which the search for brain sex differences should be especially fruitful. Females of all ages outperform males on tests requiring the recognition of emotion or relationships among other people. Sex differences in empathy emerge in infancy and persist throughout development, though the gap between adult women and men is larger than between girls and boys. The early appearance of any sex difference suggests it is innately programmed—selected for through evolution and fixed into our behavioral development through either prenatal hormone exposure or early gene expression differences. On the other hand, sex differences that grow larger through childhood are likely shaped by social learning, a consequence of the very different lifestyle, culture and training that boys and girls experience in every human society.
At first glance, studies of the brain seem to offer a way out of this age-old nature/nurture dilemma. Any difference in the structure or activation of male and female brains is indisputably biological. However, the assumption that such differences are also innate or “hardwired” is invalid, given all we’ve learned about the plasticity, or malleability of the brain. Simply put, experiences change our brains.
Recent research by Peg Nopoulos, Jessica Wood and colleagues at the University of Iowa illustrates just how difficult it is to untangle nature and nurture, even at the level of brain structure. A first study, published in March 2008 found that one subdivision of the ventral prefrontal cortex - an area involved in social cognition and interpersonal judgment, known as the straight gyrus (SG), - is proportionally larger in women, compared to men. (Men’s brains are about 10 percent larger than women’s, overall, so any comparison of specific brain regions must be scaled in proportion to this difference.) Wood and colleagues found the SG to be about 10 percent larger in the thirty women they studied, compared to thirty men. What’s more, they found that the size of the SG correlated with a widely-used test of social cognition, so that individuals (both male and female) who scored higher in interpersonal awareness also tended to have larger SGs.
In their article, Wood and colleagues speculate about the evolutionary basis for this sex difference. Perhaps, since women are the primary child-rearers, their brains have become programmed to develop a larger SG, to prepare them to be sensitive nurturers. Prenatal sex hormones are known to alter behavior and certain brain structures in other mammals. Perhaps such hormones—or sex-specific genes—may enhance the development of females’ SG (or dampen the development of males’) leading to inborn differences in social cognition.
The best way to test this hypothesis is to look at children. If the sex difference in the SG is present early in life, this strengthens the idea that it is innately programmed. Wood and Nopoulos therefore conducted a second study with colleague Vesna Murko, in which they measured the same frontal lobe areas in children between 7 and 17 years of age. But here the results were most unexpected: they found that the SG is actually larger in boys! What’s more, the same test of interpersonal awareness showed that skill in this area correlated with smaller SG, not larger, as in adults. The authors acknowledge that their findings are “complex,” and argue that the reversal between childhood and adulthood reflects the later maturation of boys’ brains, compared to girls. (Adolescents’ brains undergo a substantial “pruning” or reduction in gray matter volume during adolescence, which happens about two years earlier in girls, compared to boys.)
However, in both studies, Wood and colleagues added another test that reminds us to be cautious when interpreting any finding about sex differences in the brain. Instead of simply dividing their subjects by biological sex, they also gave each subject a test of psychological “gender:” a questionnaire that assesses each person’s degree of masculinity vs. femininity—regardless of their biological sex—based on their interests, abilities and personality type. And in both adults and children, this measure of “gender” also correlated with SG size, albeit in just as complicated a way as the correlation between “sex” and SG size. (Larger SG correlated with more feminine personality in adults but less feminine personality in children.)
In other words, there does seem to be a relationship between SG size and social perception, but it is not a simple male-female difference. Rather, the SG appears to reflect a person’s “femininity” better than one’s biological sex: women who are relatively less feminine show a correspondingly smaller SG compared to women who are more feminine, and ditto for men.
This finding—that brain structure correlates as well or better with psychological “gender” than with simple biological “sex”—is crucial to keep in mind when considering any comparisons of male and female brains. Yes, men and women are psychologically different and yes, neuroscientists are uncovering many differences in brain anatomy and physiology which seem to explain our behavioral differences. But just because a difference is biological doesn’t mean it is “hard-wired.” Individuals’ gender traits—their preference for masculine or feminine clothes, careers, hobbies and interpersonal styles—are inevitably shaped more by rearing and experience than is their biological sex. Likewise, their brains, which are ultimately producing all this masculine or feminine behavior, must be molded—at least to some degree—by the sum of their experiences as a boy or girl.
And so, any time scientists report a difference between male and female brains, especially in adults, it begs the question, “Nature or nurture?” Is women’s larger SG the cause of their social sensitivity, or the consequence of living some 30 years in a group that practices greater empathetic responding? Wood and colleagues are among the few neuroscientists to analyze male-female brain differences for their relationship to gender type, as opposed to strict biological sex. Their findings do not prove that social learning is the cause of male-female differences in the brain, but they do challenge the idea that such brain differences are a simple product of the Y chromosome. (From Scientific American Online, September 8, 2009)
Exercise 6. Answer the questions using the information from the text:
- Why is social cognition regarded a fruitful area of investigation?
- What is nature/nurture dilemma?
- What is straight gyrus (SG) and what is it responsible for?
- How do researchers explain larger SG in women from the evolutionary perspective?
- How did the results of the second study contradict those of the first?
- What correlation between SG size and sex/gender was finally established?
- How can it be interpreted?
Exercise 7. Divide into two groups. Each group should read either Text A or Text B about differences between the sexes. Then tell other students what you have read about.
Text A. Enzyme Lack Lowers Women's Alcohol Tolerance
By Harald Franzen
An international team of researchers may have found one of the reasons why alcohol harms women more than men: women, it appears, are deficient in an enzyme that helps metabolize alcohol. The findings appear in the April issue of Alcoholism: Clinical and Experimental Research. "It has been known for a long time that, in general, both women and female animals are more susceptible to the negative or toxic effects of alcohol," team member Steven Schenker of the University of Texas at San Antonio says. "This is true for the liver, heart muscle and skeletal muscle, and it may be true for the pancreas and the brain. In other words, there is something about the female gender that makes them more susceptible to toxic amounts of alcohol."
In the past scientists attributed this susceptibility to women's smaller body size and their relatively higher percentage of fatty tissue. For this study, however, the researchers focused on what is known as first-pass metabolism. Before alcohol reaches the blood stream, it goes through the stomach, where so-called gastric alcohol dehydrogenase (ADH) isozymes break some of it down. "In an earlier study we found that women have less of this ADH activity than men do," notes lead author Charles Lieber of the Mount Sinai School of Medicine. "Accordingly, women have a lesser first-pass metabolism and, therefore, for a given dose of alcohol, their blood level is higher than it is for men."
Following up on that research, the team recently turned their attention to the makeup of ADH. They found that one of the enzyme's three components, glutathione-dependent fomaldehyde dehydrogenase (x-ADH), is deficient in women, thus explaining their lower ADH activity levels. To Schenker, the take-home message is clear: "Women simply need to be more cautious than males in terms of the amount of drinking they do." (From Scientific American Online, April 16, 2001)