Do callosal projection neurons reflect sex differences in axon number?

The sensed presence within experimental settings: implications for the male and female concept of self

The sense of "a presence" or of a sentient being during partial sensory deprivation and exposure to very weak, complex magnetic fields across the cerebral hemispheres may be a normal neurocognitive experience that is associated with the brief intrusion of the right hemispheric homologue of the left hemispheric (and strongly linguistic) sense of self into awareness. Within an optimal experimental setting, women reported more frequent experiences of a sensed presence than did men, and men were more likely than women to consider these experiences as "intrusions" from extrapersonal or ego-alien sources. Both effects were predicted by the vectorial hemisphericity hypothesis and the known neurocognitive differences between right-handed men and right-handed women. Sociobiological implications for gender differences in the probability of intercalation between distinctive processes within the left and right temporoparietal lobes are discussed.

Sexually-dimorphic patterns of cortical asymmetry, and the role for sex steroid hormones in determining cortical patterns of lateralization

Cortical asymmetry varies in degree and direction. Sex differences exist for both dimensions of asymmetry: males tend to exhibit more accentuated asymmetries and stronger right hemisphere dominance compared with females, while females typically exhibit more diffuse lateralization patterns and greater left hemisphere bias compared with males. The following review considers the effects of sex steroid hormones on the sexual dimorphism of cortical lateralization patterns in human and nonhuman animals. Several excellent papers exist which consider sexual dimorphism in cortical asymmetries for morphological, physiological or functional asymmetries alone. This paper attempts to consider influences of sex steroid hormones on cortical lateralization in a framework that includes morphology, physiology and function. Converging evidence from studies that incorporate varying methodologies and theoretical backgrounds indicates that sex steroid hormones are important for influencing cortical asymmetry. Although a mechanism has not yet been identified, testosterone is the most likely candidate to influence cortical dominance.

Sex differences in the human corpus callosum: myth or reality?

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.