DEFINING THE CORPUS CALLOSUM
Scientists don’t measure, divide, probe, dispute, and ogle the corpus callosum per se, but rather a slice taken at its center (figure 5.2). This is a twodimensional representation of a mid-saggital section of the corpus callosum.43 This being a bit of a mouthful, let’s just call it CC. (From here on, I’ll refer to the three-dimensional structure—that ‘‘bird with complicated wing formation’’—as the 3-D CC.) There are several advantages to studying the twodimensional version of the CC. First, the actual brain dissection is much easier. Instead of spending hours painstakingly dissecting the cerebral cortex and other brain tissues connected to the 3-D CC, researchers can obtain a whole brain, take a bead on the space separating left and right hemispheres, and make a cut. (It’s rather like slicing a whole walnut down the middle and then measuring the cut surface.) The resulting half brain can be photographed at one of the cut faces. Then researchers can trace an outline of the cut CC surface onto paper and measure this outline by hand or computer. Second, because tissue preparation is easier, the object can be more handily standardized, thus assuring that when different laboratory groups compare results, they are talking about the same thing. Third, a two-dimensional object is far easier to measure than a three-dimensional one.44
But methodological questions remain about this postmortem (PM) technique. For example, to prepare the brains, one must pickle them (a process of preservation called fixation). Different laboratories use different fixation methods, and all methods result in some shape distortion and shrinkage. Thus, some doubt always exists about the relationship between living, functioning structure and the dead, preserved brain matter actually studied. (For example, one could imagine that a size difference between two groups could result from different quantities of connective tissue that might show different shrinkage responses to fixation.)45
Although researchers disagree about which techniques for obtaining brain samples cause the least distortion, they rarely acknowledge that their data, based on two-dimensional cross sections, might not apply to brains as they actually exist: three-dimensionally in people’s heads. In part, this may be because researchers are more interested in the relative merits ofthe postmortem technique and techniques made possible by a new machine, the Magnetic Resonance Imager (MRI). Some hope that this advanced technology will allow a unified account of the CC to emerge.46
figure £. 2 : The transformation of the 3 – D corpus callosum to a version represented in only two dimensions. (Source: Alyce Santoro, for the author)
MRI’s (figure £.3) offer two major advantages. First, they come from living, healthy individuals; second, living, healthy individuals are more available than autopsied brains.47 Hence larger samples, better matched for possibly confounding factors such as age and handedness, can be used. But there is no free lunch. The neuroscientists Sandra Witelson and Charles Goldsmith point out that the boundaries between the CC and adjacent structures appear less clearly in MRI’s than PM’s. Furthermore, the scans have a more limited spatial resolution, and the optical slices taken are often much thicker than the manual slices taken from postmortems.48 Jeffrey Clarke and his colleagues note that ‘‘the contours of the CC’s were less sharp in the MRI graphs than in the postmortem’’ while others cite difficulties in deciding just which of the many optical slices was the true mid-saggital slice.49 Finally, studies using MRI’s are
figure 5.3: An MRI image of a mid-saggital section of a human head. The convolutions of the cerebral cortex and the corpus callosum are clearly visible. (Courtesy of Isabel Gautier)
hard to standardize with respect to brain weight or size. Thus, because, MRI’s, like PM’s, represent certain brain features, researchers using either technique study the brain at an interpretive remove.