Abstract:It has been claimed that the human corpus callosum shows sex differences, and in particular that the splenium (the posterior portion) is larger in women than in men. Data collected before 1910 from cadavers indicate that, on average, males have larger brains than females and that the average size of their corpus callosum is larger. A meta-analysis of 49 studies published since 1980 reveals no significant sex difference in the size or shape of the splenium of the corpus callosum, whether or not an appropriate adjustment is made for brain size using analysis of covariance or linear regression. It is argued that a simple ratio of corpus callosum size to whole brain size is not an appropriate way to analyse the data and can create a false impression of a sex difference in the corpus callosum. The recent studies, most of which used magnetic resonance imaging (MRI), confirm the earlier findings of larger average brain size and overall corpus callosum size for males. The widespread belief that women have a larger splenium than men and consequently think differently is untenable. Causes of and means to avoid such a false impression in future research are discussed. Keywords: allometry, brain size, effect size, literature review, meta-analysis, morphometry, ratio measures, statistical power Article:IF men and women think differently, their brains must also differ in some way. This holds true, even if the difference emanates largely from experience, because experience changes the brain (39,114). The size of cognitive gender differences has never been very large (58,69), and the difference has almost vanished for several abilities in more recently published reports (47,70). Of course, not all measures of behavior show small sex differences. For example, in the USA, boys possess considerably more knowledge about electronics, automobiles and machinery (58). Among 21 indicators of sexual behavior and sexuality, two (attitude towards casual intercourse and incidence of masturbation) exhibit male-female differences greater than 0.8 standard deviation (95), which is the conventional criterion for a large group difference in psychological research (27), although most (18 out of 21) show differences less than 0.5 standard deviation (SD). However, on tests of cognitive abilities, sex differences are usually small (58). In a review of 287 effect sizes for spatial visualization abilities (118), the average sex difference was 0.37 SD and only one kind of test (mental rotation) met the criterion for even a moderate effect size. For 254 effect sizes involving mathematical performance in 100 published studies, the male average exceeded the female average by only 0.2 SD, but for studies in which the samples were drawn from the general population, females scored slightly better than males (70). Given such small cognitive gender differences, it seems likely that only the most sensitive and refined neurological techniques will be able to locate the relevant difference in brain tissue.Neuroanatomists have scrutinized thousands of pre...

The inbred strains BALB/cWah1 and 129P1/ReJ both show incomplete penetrance for absent corpus callosum (CC); about 14% of adult mice have no CC at all. Their F1 hybrid offspring are normal, which proves that the strains differ at two or more loci pertinent to absent CC. Twenty‐three recombinant inbred lines were bred from the F2 cross of BALB/c and 129, and several of these expressed a novel and severe phenotype after only three or four generations of inbreeding – total absence of the CC and severe reduction of the hippocampal commissure (HC) in every adult animal. As inbreeding progressed, intermediate sizes of the CC and the HC remained quite rare. This striking phenotypic distribution in adults arose from developmental thresholds in the embryo. CC axons normally cross to the opposite hemisphere via a tissue bridge in the septal region at midline, where the HC forms before CC axons arrive. The primary defect in callosal agenesis in the BALB/c and 129 strains is severe retardation of fusion of the hemispheres in the septal region, and failure to form a CC is secondary to this defect. The putative CC axons arrive at midline at the correct time and place in all groups, but in certain genotypes, the bridge is not yet present. The relative timing of axon growth and delay of the septal bridge create a narrow critical period for forming a normal brain.

Sex differences in the forebrain commissures (corpus callosum, hippocampal commissure, and anterior commissure) were examined in B6D2F2 hybrid mice and Sprague-Dawley rats. Twenty-four male-female littermate pairs of mice were perfused at each of 21, 42 and 63 days of age and the midsagittal area of the commissures was measured from en bloc stained tissue. Twenty-two male-female littermate pairs of rats were examined at 110 days of age using the same methods. Male mice had larger bodies than females but no sex differences were found for mouse brain weight or commissure areas. In contrast, a significant sex difference was found for rat body, brain, corpus callosum and hippocampal commissure sizes. Four methods were used to adjust for differences in brain size (ratio, geometric, linear regression, and allometric). When the two species were analysed separately, neither mice nor rats showed significant sex differences in commissure areas relative to brain size if regression or allometric adjustments were made. Even when data from mice and rats were combined into one large group with a wide range of values, no species or sex differences were apparent after adjustments were made for brain size with either the regression or allometric methods. The use of ratios to adjust for differences in overall size is not recommended, especially because this method does not effectively remove the influence of brain size from commissure size; a substantial correlation is often present between the ratio and brain size.

The design of a rodent running wheel with adjustable drag and ability to measure accurately the direction as well as the speed of running is described. A computer program in the C language enables a single microcomputer to monitor several wheels simultaneously. Drag is conveniently calibrated by the rate of deceleration from 1 revolution per second when no animal is present, and the calibration procedure can also assess the smoothness of the drag mechanism. Results of tests of the setability, repeatablility, and durability of the drag are presented. A method is outlined to determine the physical work required to accelerate the wheel and to maintain its rotation against the frictional drag.Running wheels have long been a useful tool in examining a variety ofphysiological and psychological phenomena such as instrumental responses in learning (Brogden,

Humans born without a corpus callosum (CC) are often comparatively slow and clumsy on tasks requiring bilateral motor coordination. In this study, we attempted to identify correlates of CC agenesis in mice by examining an ecologically valid motor behavior: running-wheel performance. Mice with varying degrees of congenital CC deficits were tested on a running wheel apparatus for 7 consecutive days. The mice became more proficient at running with experience (Le., rotations, time spent running, length of running bouts, and maximum and modal running speeds increased while variability of rotation times decreased). Multiple regression analysis suggested that CC deficiency may be related to 2 out of 20 measurements of running (decreased variability of rotation times and shorter maximum running bout duration), but at levels below the traditional criteria for significance. Although these variables predicted CC deficiency at a level significantly greater than chance (71% correct) in a discriminant analysis, examination of the meaning of their relationship with CC size suggested that any connection may be spurious.Axons of the corpus callosum (CC) connect the cerebral hemispheres in placental mammals. Agenesis ofthe CC occurs in the embryo when axons are unable to cross the cerebral midline but instead grow longitudinally in the novel Probst bundle (Ozaki, Murakami, Toyoshima, & Shimada, 1987;Ozaki & Shimada, 1988) and soon return to the ipsilateral cerebral cortex (Ozaki & Wahlsten, 1993). In humans, the incidence of callosal agenesis is estimated to be 0.0005%-0.7% (Wisniewski & Jeret, 1994), and its occurrence is often associated with other neurological deficits (Aicardi & Chevrie, 1994;Andermann & Andermann, 1994). It is paradoxical that humans born without a CC exhibit comparatively few behavioral or performance deficits, especially in comparison with patients who have had their CC surgically split (Sauerwein & Lassonde, 1994;Sperry, 1982). Unlike callosotomy patients, acallosals can transfer information from one hemisphere to another, suggesting that plasticity exists in the formation of forebrain cortical connections. The most consistent functional finding is that acallosals often exhibit deficits on motor tasks requiring bimanual coordination, especially under speed stress (Silver & Jeeves, 1994).Four strains of mice (BALB/c, I/LnJ, 129, and ddN) frequently exhibit congenital agenesis of the CC to vary-