There is measure in all things (callosal)

With all the subdivisions agreed upon, finally, students of the corpus callosum are in business. Now they can make dozens of measurements. From the undi­vided CC come dimensions of the total surface area, length, width, and any of these divided by brain volume or weight. From the subdivided CC come named or numbered parts: the anterior one-fifth becomes the genu, the poste­rior one-fifth the splenium, a narrower portion in the center the isthmus. Once researchers have created a measurable object out of the CC, what do they find?

The results summarized in tables £.3, £.4, and £.£ reveal the following: no matter how they carve up the shape, only a few researchers find absolute

sex differences in CC area. A small number report that males and females have differently shaped corpus callosums (females have a more bulb-shaped splenium, making the CC wider, according to these authors), even though the shape does not translate into a size (area or volume) difference. The few stud­ies of fetuses and young children came up with no measurable sex differences; these results suggest that, if there is a gender difference in adult CC’s, it ap­pears only with age.56 Finally, reports about sex differences in corpus callosum size during old age conflict, permitting no firm conclusions about gender differences in the elderly.57

Some researchers have suggested that, if there is a gender difference in the CC, it maybe the opposite of what scientists have commonly assumed it would be. Men generally have larger brains and bodies than women. If it turns out that women and men have similar-sized CC’s but women have smaller brains, then on a relative per volume or per weight basis, do women have larger CC’s?58 Following this logic, many researchers have compared the relative size of the whole and/or parts of the male and female corpus callosum. Table 5.4 sum­marizes these relative measures, and the decision is split: about half report a difference, while half do not.

Although most investigators interested in gender differences focus on the splenium—the more (or less) bulbous-shaped posterior end of the corpus callosum—others have turned their attention to a different segment of the CC named the isthmus (see figure 5.4). While those who measure the sple — nium have tended to look only for differences between men and women, those examining the isthmus believe this part of the brain is linked to several charac­teristics—not only gender, but also left — or right-handedness and sexual ori­entation. Some find that the area of the isthmus is smaller in right-handed than in non-right-handed males, but that women show no such difference. 59 I’ve tabulated these results in table 5.5. Here, too, there is little consensus. Some find a structural difference related to handedness in males but not females; some find no handedness-related differences; one paper even reports that one of the CC regions is larger in right-handed than in left-handed women, but smaller in left-handed than in right-handed men.60

What do scientists do with such diverse findings? One approach uses a special form of statistics called meta-analysis, which pools the data from many small studies to create a sample that behaves, mathematically, as if it were one large study. Katherine Bishop and Douglas Wahlsten, two psychologists, have published what seem to be the unequivocal results of such a meta-analysis. Their study of forty-nine different data sets found that men have slightly larger CC’s than women (which they presume is because men are larger), but no significant gender differences in either absolute or relative size or shape of

# OF STUDIES FINDING ADULT FEMALE LARGER ADULT MALE LARGER ADULTS don’t DIFFER MALE CHILD LARGER CHILDREN NO don’t FETAL DIFFER DIFFERENCE Measurement taken (seefigure 5.4) Callosal area/brain weight or volume 7 8 2 Area: division 1 /brain weight or volume I 2 Area: division 2/brain weight or volume 2 Area: division 3/brain weight or volume I I Area: division 4/brain weight or volume I 2 Splenial width or area/brain weight, volume, or length 3 sc Splenial area/callosal area or length 3 4 Slenderness index (CC length/ideal thickness)b 2d id id

Bulbosity coefficient (average splenial width/average 2d

width of the adjacent region of the corpus callosum)a

Bulbosity coefficient/total callosal area i

Minimal width/total callosal area i

Area 6 (of 7 ’s)/total callosal area i

a. I’ve nicknamed this the ‘‘turkey-baster’’ coefficient, or TBC, because it is based on the idea that a bulbous splenium growing out of a narrow-necked CC gives a turkey — baster shape to the overall structure. See Allen, Richey et al. 1991.

b. Clarke et al. (1989) define the ideal thickness as the corpus callosum area dividedby the length of the median line (calculated to bisect the mid-sagittal corpus callosum area).

c. I calculated one of these results from the data presented by Emory et al. 1991; i of the ^ is based on subdividing the CC into 4 parts.

d.

Find a difference for postmortems but not MRI.

		# OF	STUDIES FINDING
MALES ONLY: RIGHT < LEFT — HANDEDa	MALES ONLY: RIGHT = LEFT — HANDEDa	FEMALES ONLY: RIGHT = LEFT — HANDEDa	FEMALES ONLY: RIGHT > LEFT — HANDEDa	MALES & FEMALES COMBINED: RIGHT < LEFT-HANDEDa	MALES & FEMALES COMBINED: RIGHT = LEFT-HANDEDa

Measurement taken (seefigure 5.4)
Total callosal area	2	3	6	0	I	4
Isthmus: areab	3	I	3	0	2c	I
Isthmus/total callosal area	I
Anterior halfb	I	I	2	0	I	I
Posterior halfb Region 2b CC/brain Splenium/brain	1 2	I I	2 I	0 I	I I	I I I I

b. For regionalization of CC in handedness studies, see figure

a. The definitions of handedness actually used are both more complex and subtler than just left vs. right. £.4. c. LHmales > females.

the CC as a whole or of the splenium. Bishop and Wahlsten recalculated the statistical significance of a finding of an absolute sex difference in splenial area each time they added a new study to their data base. When only a small num­ber of studies with a cumulatively small sample size existed, the results sug­gested the existence of a sex difference in splenial area. As additional data (from newer studies) accumulated in the literature, however, the sex differ­ences diminished. By the time ten studies had appeared, the absolute splenial sex difference had disappeared and nobody has successfully resurrected it.61

Researchers, however, continue to debate the existence of relative differ­ences in CC structure. Bishop and Wahlsten found none, but when a different research team performed a second meta-analysis, they found not only that men have slightly larger brains and CC’s than women, but that relative to overall brain size, women’s CC’s were bigger. This study did not contain enough data, however, to conclude that relative size of male and female sple — niums differed.62

But these meta-analyses run into the same methodological issues experi­enced by individual studies. Is there a legitimate way to establish a relative difference? What factor should we divide by: brain weight, brain volume, total CC size? One research team has called the practice of simply dividing an area by total brain size ‘‘pseudostatistics.’’63 (Them’s fightin’ words!) Another re­searcher countered that it is no wonder colleagues will attack the methodol­ogy behind any study that discovers gender differences, given that ‘‘one end of the political spectrum is invested in the conclusion that there are no differ — ences.’’64 We are left with no consensus.65