Sex hormones: modulators of interhemispheric inhibition in the human brain.

Functional cerebral asymmetries (FCAs), which constitute a basic principle of human brain organization, are supposedly generated by interhemispheric inhibition of the dominant on the nondominant hemisphere. It has repeatedly been shown that FCAs are sex specific: While they are relatively stable in men, they change during the menstrual cycle in women, indicating that sex hormones might play an important role in modulating functional brain organization and brain asymmetries in particular. Modern brain imaging techniques like functional magnetic resonance imaging (fMRI) allow for the noninvasive study of the mechanisms underlying changing FCAs. Imaging data show that in women the inhibitory influence of the dominant on the nondominant hemisphere is reduced with rising levels of sex hormones in the course of the menstrual cycle. Apart from modulating interhemispheric inhibition, sex hormones also seem to change functional organization within hemispheres. These results reveal a powerful neuromodulatory action of sex hormones on the dynamics of functional brain organization in the female brain. They may further contribute to the ongoing discussion of sex differences in brain function in that they help explain the dynamic part of functional brain organization in which the female differs from the male brain.