No two brains are identical. Some differences are genetically determined; others result from plastic changes caused, for example, by experience and learning or epigenetic factors. Some brain differences are idiosyncratic (unique to a particular person); many others are systematic and common to whole categories of people. In this section, we consider two systematic variations in brain organization, those related to sex and handedness, and one idiosyncratic variation, the fascinating sensory phenomenon synesthesia.

The idea that men and women think differently probably originated with the first men and women. Science backs it up. Books, including one by Doreen Kimura (1999), present persuasive evidence for marked differences between men’s and women’s performance on many cognitive tests. As illustrated in Figure 15-15, paper-and-pencil tests consistently show that on average females have better verbal fluency than males do, whereas males do better on tests of spatial reasoning. Our focus here is on how such differences relate to the brain.

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Neural Bases of Sex Differences

Considerable evidence points to sex differences, both in the brain’s gross cerebral structure and at a neuronal level. Jill Goldstein and her colleagues (2001) conducted a large MRI study of sexual dimorphism in the human brain. They found that women have larger volumes of dorsal prefrontal and associated paralimbic regions, whereas men have larger volumes of more ventral prefrontal regions (Figure 15-16). (Brain size is related to body size, and on average, male brains are bigger than female brains, so the investigators corrected for size.)

Another way to measure sex differences is cortical thickness independent of volume. Figure 15-17 shows that relative to men, women have increased cortical gray matter concentration in many cortical regions. Men’s gray matter concentration, by contrast, is more uniform across the cortex. The MRI studies represented in Figures 15-16 and 15-17 thus point to differences between men’s and women’s cortical organization.

Sex differences in neuronal structure also exist. Gonadal hormones influence the structure of neurons in the prefrontal cortices of rats (Kolb & Stewart, 1991). The cells in one prefrontal region, located along the midline, have larger dendritic fields (and presumably more synapses) in males than in females, as shown in the top row of Figure 15-18 below. In contrast, the cells in the orbitofrontal region have larger dendritic fields (and presumably more synapses) in females than in males, as shown in the bottom row. These sex differences are not found in rats that have had their testes or ovaries removed at birth. Presumably, sex hormones somehow change the brain’s organization and ultimately its cognitive processing.

Stewart and Kolb (1994) found that the presence or absence of gonadal hormones affects the brain in adulthood as well as in early development. In this study, which focused on how hormones affect recovery from brain damage, middle-aged female rats’ ovaries were removed. On examining the brains of these rats and those of control rats some months later, researchers observed that the cortical neurons of the rats whose ovaries had been removed—especially the prefrontal neurons—had undergone structural changes. The cells had grown 30 percent more dendrites, and their spine density increased compared with the control rats’ cells. Clearly, gonadal hormones can affect the brain’s neuronal structure at any point in an animal’s life.

An additional way to consider the neural basis of sex differences is to look at the effects of cortical injury in men and women. If sex differences exist in the neural organization of cognitive processing, differences in the effects of cortical injury in the two sexes should appear. Doreen Kimura (1999) conducted this kind of study and showed that the pattern of cerebral organization within each hemisphere may in fact differ between the sexes.

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Investigating people who had a cortical stroke in adulthood, Kimura tried to match the location and extent of injury in her male and female subjects. She found that men and women were almost equally likely to be aphasic subsequent to left-hemisphere lesions of some kind. But men were more likely to be aphasic and apraxic after damage to the left posterior cortex, whereas women were far more likely to be aphasic and apraxic after lesions to the left frontal cortex. These results, summarized in Figure 15-19, suggest a sex difference in intrahemispheric organization, a conclusion supported by a later study (Figure 15-20) using diffusion tensor analysis of brain networks in over 900 participants. The population, comprising nearly equal numbers of each sex, showed that females have greater interhemispheric connectivity, whereas males’ intrahemispheric connectivity is greater (Ingalhalikar et al., 2013).

Evidence strongly favors a neural basis for gender identity, including transgender identity. Georg Krantz and colleagues (2014) used diffusion tensor analysis to compare brain networks of female-to-male and male-to-female transsexuals with those of gender-typical females and males. The imaging replicated the distinctly different pattern of hemispheric connectivity in gender-typical females and males. The pattern of connectivity in transsexuals, however, falls halfway between those of females and males.

That is, DTIs of the white matter microstructure of transgender females and males falls halfway between that of gender-typical females and males. The investigators take these patterns as evidence that neural white matter microstructure is influenced by the hormonal environment during late prenatal and early postnatal brain development. This is the time, they hypothesize, that determines gender identity. But genital differentiation is determined earlier in development. The timing difference thus makes a genital–gender mismatch possible.

Evolution of Sex-Related Cognitive Differences

Although gonadal hormones have taken center stage in explaining sex differences in cognitive function, we are still left to question how these differences arose. To answer this question, we must look back at human evolution. Mothers pass their genes to both sons and daughters, and fathers do the same. Ultimately then, males and females of a species have virtually all their genes in common.

The only way a gene can affect one sex preferentially is for the animal’s gonadal hormones to influence the gene’s activities. Gonadal hormones are in turn determined by the presence or absence of the Y chromosome, which carries a gene called the testes-determining factor. TDF stimulates the body to produce testes, which then manufacture androgens, which in turn influence other genes’ activities.

Like other body organs, the brain is a potential target of natural selection. We should therefore expect to find sex-related differences in the brain whenever the sexes differ in the adaptive problems they have faced in their species’ evolution. Aggressive behavior is a good example. Males in most mammalian species typically are more physically aggressive than females. This trait presumably improved males’ reproductive success by selecting against individuals with lesser aggressiveness. Producing higher levels of aggression entails male hormones. We know from studies of nonhuman species that aggression is related directly to the presence of androgens and to their effects on gene expression, both during brain development and later in life. In this case, therefore, natural selection has worked on gonadal hormone levels to favor aggressiveness in males.

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Explaining sex-related differences in cognitive processes, such as language or spatial skills, is more speculative than explaining sex-related differences in aggressive behavior. Nevertheless, some hypotheses come to mind. We can imagine, for instance, that in the history of mammalian evolution, males have tended to range over larger territories than have females. This behavior requires spatial abilities, so evolution would have favored these skills in males.

Support for this hypothesis comes from comparing spatial problem solving in males of closely related mammalian species—species in which the males range over large territories versus species in which the males’ range is not extensive. Pine voles, for example, have a restricted range and no sex-related difference in range, whereas meadow voles have a range about 20 times as large as pine voles’, and male meadow voles range more widely than the females.

Meadow voles display far superior spatial skills to those of pine voles. A sex difference in spatial ability among meadow voles favors males, but pine voles have no such sex difference. The hippocampus is implicated in spatial navigation skills. Significantly, the hippocampus is larger in meadow voles than in pine voles, and it is larger in male meadow voles than in females (Gaulin, 1992). A similar logic could help explain sex-related differences in spatial abilities between human males and females (see Figure 15-15).

Explaining sex-related differences in language skills also is speculative. One hypothesis holds that if males were hunters and often away from home, home-based females formed social groups selectively favored to develop tools, one of which is language, for social interactions. We might also argue that females with superior fine motor skills (such as foraging for food and making clothing and baskets) had a selective advantage. The relation between language and fine motor skills may have favored enhanced language capacities in females.

Although such speculations are interesting, they are not testable. We will probably never know with certainty why sex-related differences in brain organization developed.

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Nearly everyone prefers one hand to the other for writing or throwing a ball. Most people prefer the right hand. In fact, left-handedness has historically been viewed as odd. But it is not rare. An estimated 10 percent of the human population worldwide is left-handed. This proportion represents only the number of people who write with the left hand. When broader criteria are used to determine left-handedness, estimates range from 10 percent to 30 percent of the population.

Because the left hemisphere controls the right hand, the general assumption is that right-handedness is somehow related to the presence of speech in the left hemisphere. If this were so, language would be located in the right hemisphere of left-handed people. This hypothesis is easily tested, and it turns out to be false.

In the course of preparing patients with epilepsy for surgery to remove the abnormal tissue causing their seizures, Ted Rasmussen and Brenda Milner (1977) injected the left or right hemisphere with sodium amobarbital. This drug produces a short-acting anesthesia of the entire hemisphere, making it possible to determine where speech originates. As described in Clinical Focus 15-5, Sodium Amobarbital Test, if a person becomes aphasic when the drug is injected into the left hemisphere but not when the drug is injected into the right, then speech must reside in that person’s left hemisphere.

Rasmussen and Milner found that in virtually all right-handed people, speech was localized in the left hemisphere, but the reverse was not true for left-handed people. About 70 percent of left-handers also had speech in the left hemisphere. Of the remaining 30 percent, about half had speech in the right hemisphere and half had speech in both hemispheres. Findings from neuroanatomical studies have subsequently shown that left-handers with speech in the left hemisphere have asymmetries similar to those of right-handers. By contrast, in left-handers with speech originating in the right hemisphere or in both hemispheres—known as anomalous speech representation—the anatomical symmetry is reversed or absent.

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Sandra Witelson and Charlie Goldsmith (1991) asked whether any other gross differences in the brain structure of right- and left-handers might exist. One possibility is that the connectivity of the cerebral hemispheres may differ. To test this idea, the investigators studied the hand preference of terminally ill subjects on a variety of one-handed tasks. They later performed postmortem studies of these patients’ brains, paying particular attention to the size of the corpus callosum. They found that the callosal cross-sectional area was 11 percent greater in left-handed and ambidextrous (little or no hand preference) people than in right-handed people.

Whether this enlarged callosum is due to a greater number of fibers, to thicker fibers, or to more myelin remains to be seen. If the larger corpus callosum is due to more fibers, the difference would be on the order of 25 million more fibers. Presumably, such a difference would have major implications for the organization of cognitive processing in left- and right-handers.

Some variations in brain organization are idiosyncratic rather than systematic. Synesthesia is an individual’s capacity to join sensory experiences across sensory modalities, as discussed in Clinical Focus 15-6, A Case of Synesthesia. Examples include the ability to hear colors or taste shapes. Edward Hubbard (2007) estimated the incidence of synesthesia at about 1 in 23 people, although for most it likely is limited in scope.

Synesthesia runs in families—the family of Russian novelist Vladimir Nabokov, for example. As a toddler, Nabokov complained to his mother that the letter colors on his wooden alphabet blocks were all wrong. His mother understood what he meant, because she too perceived letters and words in particular colors. Nabokov’s son is synesthetic in the same way.

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If you’ve shivered on hearing a particular piece of music or the noise of fingernails scratching across a chalkboard, you have felt sound. Even so, other sensory blendings may be difficult to imagine. How can sounds or letters possibly produce colors? Studies of synesthetes show that the same stimuli always elicit the same experiences for them.

The most common form of synesthesia is colored hearing. For many synesthetes, this means hearing both speech and music in color—perceiving a visual mélange of colored shapes, movement, and scintillation. The fact that colored hearing is more common than other types of synesthesia is curious.

The five primary senses (vision, hearing, touch, taste, and smell) all generate synesthetic pairings. Most, however, are in one direction. For instance, whereas synesthetes may see colors when they hear, they do not hear sounds in colors. Furthermore, some sensory combinations occur rarely, if at all. In particular, taste or smell rarely triggers a synesthetic response like Michael Watson’s.

Because each case is idiosyncratic, synesthesia’s neurological basis is difficult to investigate. Few studies have related it directly to brain function or brain organization, and different people may experience it for different reasons. Various hypotheses have been advanced to account for synesthesia:

Whatever the explanation, when it comes to certain sensory inputs, the brain of a synesthete certainly works differently from other people’s brains.

Variations in Cognitive Organization

Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Answers appear in the Self Test section of the book.