Like all scholarship, Allen and her colleagues’ study is necessarily embedded in the context of an ongoing conversation about the broader subject matter it explores—in this case, the corpus callosum. They must rely heavily on preexisting work to establish the validity of their own. Allen and her colleagues note, for example, that even though the CC has a million or more nerve fibers running through it, this enormous number still represents only 2 percent of all the neurons in the cerebral cortex. They note evidence that fibers in the splenium may help transfer visual information from one brain hemisphere to the other. Another region—the isthmus—for which they find no sex difference (but for which others find a complex of differences between gay and straight and left — and right-handed men), carries fibers connecting left and right cortical regions involved with language function.
Allen and colleagues need to keep their discussion pithy. After all, they want to examine their findings, not review all that is known about the structure and function of the corpus callosum. Let’s imagine this aspect of the production of facts about the corpus callosum as a macrame weaving. Here an artist uses knots as links in the creation of intricate, webbed patterns. The connecting threads secure individual knots within the larger structure, even though a single knot in the web may not be all that strong. My drawing of the CC weaving (figure 5.6) includes only contemporary disputes. But each knot also contains a fourth dimension—its social history.84 To locate the knot labeled ‘‘corpus callosum gender differences,’’ Allen et al. have spun out a thread and secured it to a second knot, labeled ‘‘structure and function of the corpus callosum.’’ That tangle is, in turn, secured by a second web of research.
Speculation abounds about the CC’s structure and function. Perhaps more nerve fibers permit faster information flow between left and right brain hemispheres; perhaps faster flow improves spatial or verbal function (or vice versa). Or perhaps larger (or smaller) CC segments slow the flow of electricity between brain halves, thus improving spatial or verbal abilities (or vice versa). But what, exactly, does the CC in general and the splenium in particular do? What kinds of cells course through the CC, where do they go, and how do they function?85 The function/structure knot contains hundreds of papers produced by overlapping research communities, only some of which are interested in sex differences. One team of sociologists calls such groups ‘‘persuasive communities,’’86 whose language choices or use of techniques such as sophisticated statistics may condition how its members envision a problem.87 Work on the structure and function of the corpus callosum links several persuasive communities. One locale, for example, compares the numbers of large and small neurons, some with an insulating coat of myelin, others lying naked in different regions of the CC. These cells perform different functions and thus provide clues to CC function.88
The structure/function node is dense.89 An issue of the journal Behavioural Brain Research devoted entirely to work on the function of the corpus callosum illustrates the point. Some papers in the volume addressed findings and con-
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figure ^. 6: A macrame weaving of knots of knowledge in which the corpus callosum debate is embedded. (Source: Alyce Santoro, for the author)
troversies on hemispheric lateralization, speaking directly to the implications for CC function.90 The laterality work in turn connects to studies of handedness, sex differences, and brain function.91 These also interconnect with a literature that debates the interpretation of studies on humans with damaged CC’s and compares results to studies that try to infer CC function from intact subjects.92 One well-known aspect of lateralization is handedness—how shall we define it, what causes it (genes, environment, birth position?), what does it mean for brain functions, how does it affect CC structure (and how does CC structure affect handedness?), are there sex differences, and do homosexuals and heterosexuals differ? Handedness is a busy knot.93
All of these knots connect at some point with one labeled cognition.94 Sometimes tests designed to measure verbal, spatial, or mathematical abilities reveal gender differences.95 Both the reliability of such differences and their origin provide fodder for unending dispute.96 Some link a belief in gender differences in cognition to the design of educational programs. One essayist, for example, drew a parallel between teaching mathematics to women and giving flying lessons to tortoises.97 Elaborate and sometimes completely opposite theories connect cognitive sex differences with callosal structure. One,
for example, suggests that higher mathematical ability derives from differing numbers of excitatory neurons in the CC, while another suggests that the inhibitory nature of the CC neuron is most important.98
The effects of hormones on brain development form an especially powerful knot in this macrame weaving (I will have a lot more to say about hormones in the next three chapters). Allen and her colleagues wonder whether sex differences in the corpus callosum might be induced by hormones, some other genetic cause, or the environment. After briefly considering the environmental hypothesis,99 they write: ‘‘However, more striking have been the dataindi — cating that nearly all sexually dimorphic structures examined thus far have been shown to be influenced by perinatal gonadal hormone levels.’’100 This brief statement invokes a huge and complex literature about hormones, the brain, and behavior (some of which we have already considered in the context of intersexuality). Standing alone, the corpus callosum research may be weak. But with the vast army of hormone research to back it up, how could claims of a difference possibly fail? Even though there is no convincing evidence to link human corpus callosum development to hormones,101 invoking the vast animal literature102 stabilizes the shaky CC knot.103
Within each of the persuasive communities represented in figure 5.6 by knots on the macrame weaving, one finds scientists at work. They are devising new methods to test and substantiate their favored hypothesis or to refute a viewpoint they believe in error. They measure, use statistics, or invent new machines, trying to stabilize the fact they pursue. But in the end, few of the facts (excised, unsupported knots) about gender differences are particularly robust104 (to use a word favored by scientists) and must, therefore, draw significant strength from their links to the weaving. These researchers work primarily on the science side of things, studying genes, development, parts of the brain, hormones, analyses of brain-damaged people, and more (figure 5.7A). This portion of the nexus appears to deal with more objective phenomena, the realm traditionally handed over to science.105 On the cultural side of the macrame weaving (figure 5.7B) we find that webbed into the sex difference knot are some decidedly political items: cognition, homosexuality, environment, education, social and political power, moral and religious beliefs. Very rapidly we have skated along the strands from science to politics, from scientific disputes to political power struggles.106