Laterality and Modularity

One of the reasons why men's and women's creative endeavors might differ is that the modular organization of their brains might differ. Several investigators have provided evidence that, when compared to men, women are more likely to have bilateral hemisphere mediation of language (Gur et al., 2000; McGlone, 1977; Shaywitz et al., 1995). In addition, Gur et al. (2000) demonstrated that it was primarily men who showed right-hemisphere activation with spatial tasks. Other investigators were, however, unable to demonstrate sex differences in the lateralization of language (Frost et al., 1999).

As I noted earlier, Geschwind and Levitsky (1968), Foundas et al. (1994), and many other investigators have reported that there are hemispheric anatomic asymmetries, especially in the perisylvian region, such that the planum temporale is larger in the left hemisphere than in the right hemisphere. Studies have also shown that these asymmetries are more robust in men than women (Good et al., 2001; Kulynych, Vladar, Jones, & Weinberger, 1994). Not all studies, however, showed these sex differences between men and women (e.g., Foundas, Faulhaber, Kulynych, Browning, & Weinberger, 1999). Studies of the size of the major pathway that connects the two hemispheres, the corpus callosum, suggest that there is an inverse relationship such that the greater the asymmetry between the right and left hemispheres, the smaller the size of the corpus callosum and that this inverse relationship seems to be stronger for men than women (Aboitiz, Scheibel, Fisher, Zaidel, 1992). Studies that compared the size of men's and women's corpus callosum found that when corrected for overall brain size, the callosum is relatively larger in women than men, especially in the posterior portions such as the isthmus and splenium (Steinmetz, Staiger, Schlaug, Huang, & Jancke, 1995). Not all studies, however, could replicate these findings, and some have suggested that the smaller the brain the relatively greater the size of the corpus callosum (Jancke, et al., 1997; Pozzilli et al., 1994).

As I also mentioned in the chapter 5 on creativity and hand preference, that the corpus callosum is larger in left-handed people than in right-handed people (Witelson, 1985). A higher percentage of left-handed people have language-speech that is mediated by both hemispheres. Witelson (1985) suggested that the more the hemispheres share cognitive functions, the greater the anatomical connections between the two hemispheres. As also mentioned previously, several studies have suggested that women are more likely to have language bilaterally distributed than are men, and perhaps it is this bilateral distribution that is associated with relatively larger interhemispheric connections.

The corpus callosum contains axons that project from cortical neurons in one hemisphere to neurons in the other hemisphere. Although men have more neurons than do women, the cortex of men and women are equally thick, suggesting that women have a greater number of neuronal connections per neuron. It is this relatively increased connectivity in women that might account for their relatively larger corpus callosum.

Women appear to be more likely to develop degenerative dementia than are men. Women also live longer than men and the older a person is the more likely he or she is to get dementia. When corrected for age, however, the incidence of dementia is higher in woman. In degenerative dementia, such as Alzheimer's disease, there is a loss of cortical neurons, and the higher incidence of Alzheimer's disease in women might be related to a decreased reservoir of cortical neurons (de Courten-Myers, 1999). That men have more neurons than do women might suggest that men's brains might be more modular. As described earlier, the modularity or localizationist hypothesis was first put forth by Franz Gall in the latter part of the 18th century and the early part of the 19th century when he suggested that specific portions of the brain mediate specific functions. He also proposed what might now be termed "anatomically distributed modular cognitive systems." According to this hypothesis, although the different human faculties mediated by the brain are located in different anatomic areas of the cerebral cortex, they are highly interconnected. Thus, if women have a widely distributed language network, it is possible that other modules that mediate other nonlanguage skills would not be as well developed.

Support for this postulate comes from studies that assessed visu-ospatial abilities in women and men. Visuospatial or spatial abilities are primarily nonverbal. Visuospatial abilities include skills such as navigating and finding routes; reproducing angles; estimating relative magnitude of distance, length, and angles; and having the ability to rotate objects in three dimensions. Two of the tests that might require the least amount of verbal mediation are the spatial rotation test and the Judgment of Line Orientation Test, originally devised by Benton and Tranel (1993). In the former test (block rotation), the participants are shown a picture of an object that is made up of a series of attached square blocks (see Figure 6.1) and they have to select from several choices the picture that depicts this block after it has been rotated in space. In the Judgment of Line Orientation Test, participants are shown two line segments that are unattached (see Figure 6.2). Each of the segments is at a different angle from horizontal. After

Figure 6.1. Imagery task. The participant is asked to view the top object and then indicate which of the three (A, B, or C) following objects is the same as the top object.

Figure 6.1. Imagery task. The participant is asked to view the top object and then indicate which of the three (A, B, or C) following objects is the same as the top object.

Figure 6.2. Visuospatial task. The participant is first shown the left figure (A), which has two lines at different angles from horizontal. This figure (A) is then removed, and the participant is shown a figure (B) that has lines at angles that vary from 1 to 180 degrees. The participant is asked to name or point to the figure that has the same two lines that she or he just viewed.

Figure 6.2. Visuospatial task. The participant is first shown the left figure (A), which has two lines at different angles from horizontal. This figure (A) is then removed, and the participant is shown a figure (B) that has lines at angles that vary from 1 to 180 degrees. The participant is asked to name or point to the figure that has the same two lines that she or he just viewed.

viewing these segments, the participants view a protractor-like image that contains multiple line segments that range from 0 to 180 degrees. The participants have to pick out the segments on this protractor that are the same angles as those seen in the target segments. Studies of gender differences in the ability to rotate three-dimensional objects and the ability to reproduce angles demonstrate that men are superior to women in these areas (Collaer & Nelson, 2002; Voyer, Voyer, & Bryden, 1995). What is unclear, however, is why men are superior to women in these types of visuospatial tasks and why such skills would lead to differences in creativity.

One of the teleological explanations for the gender differences in spatial skills is related to survival. Men were primarily the hunter-gatherers, hence navigational skills were critical for survival. Studies have shown that men perform some spatial-motor abilities better than women, such as accurately throwing projectiles (Kimura, 1999), and findings such as these do suggest that spatial abilities might have had survival benefit. But this type of teleological reasoning does not explain what it is about men's brains that makes them superior in these visuospatial skills. Studies of brain-damaged people have revealed that right-hemisphere injury impairs both mental rotation (Ditunno & Mann, 1990) and line orientation (Hamsher, Capruso, & Benton, 1992) more than left-hemisphere injury does. Functional imaging studies have provided converging evidence (Deutsch, Bourbon, Papanicolaou, & Eisenberg, 1988; Harris et al., 2000) for the postulate that the right hemisphere is dominant for performing visuospatial computations.

Although spatial skills are of minor importance in writing poetry and novels, these skills appear to be important in physics, chemistry, and engineering as well. Other disciplines such as painting and sculpting are also are very dependent on spatial skills. Whereas women do better at arithmetic (Kimura, 1999), men in general appear to be superior in higher forms of mathematics (Kegel-Flom & Didion, 1995), and several investigators have found that visuospatial skills are important in higher forms of mathematics (Casey, Nuttall, & Pezaris, 1997; Geary, Saults, Liu, & Hoard, 2000).

Although the influence of culture on gender differences has not been entirely eliminated, one possible reason why men do better on these spatial tasks is that many women have language distributed in both hemispheres, but men's language is mediated almost entirely by their left hemisphere. Support for this gender difference postulate come from morphological and lesion studies and functional imaging. As mentioned, some studies of the planum temporale (see Figure 3.8), an area critical in language processing, have suggested that men have a larger asymmetry than women (Good et al., 2001). If women's right hemisphere had a greater potential to mediate language than do men's right hemisphere, we might expect that right-handed women would have a much higher incidence of crossed aphasia (aphasia in a person who prefers his or her right hand from an injury to the right hemisphere) and a better prognosis for recovering from aphasia after a left-hemisphere injury. Overall, when I looked at this literature, I did not find that it fully supported these predictions. The failure to find support for this gender difference in the hemispheric distribution of language might be related to confounding factors. In contrast to the data from lesion studies, functional imaging studies in normal individuals have demonstrated then when performing language tasks, men primarily activate their left hemisphere, but women are more likely to activate both hemispheres (Jaeger et al., 1998). According to a limited capacity model, if women's right-hemisphere networks are more likely to store language representations and mediate language functions, then the remaining right-hemisphere processing networks that are available to mediate spatial cognition might be limited and thus not be able perform in a manner comparable to men whose right hemisphere is not occupied by language representations.

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