Can men really not listen, and are women really unable to park? Researchers have long discovered functional differences between the two hemispheres of the brain. What’s interesting is that this “little difference” is picked up at least once a month in women.
Cognitive sex differences
Some cognitive sex differences have been scientifically proven. For example, women are superior at verbal skills that involve quickly naming target words. Men, on the other hand, find some tasks easier, especially those that require spatial awareness. Gender-specific differences in language ability and visual spatial cognition are therefore not a malicious prejudice, but scientific fact. They could be the result of different educational styles and/or biological factors. In favor of the latter is the fact that female and male brains differ in about a dozen anatomical features. Test results also point to biological factors. Using special test setups, sex differences have been demonstrated to be quite consistent not only in different nations but also over the past 30-40 years, although parenting styles have varied extremely across these countries and time periods. Moreover, in males who become females after sex reassignment, taking female sex hormones increases language skills at the expense of spatial cognition. Women who become men undergo exactly the opposite development.
Are hormones to blame?
There is much to suggest that the cognitive differences between men and women may arise, at least in part, from different hormonal factors, which are then likely to entail gender-specific brain mechanisms. But then shouldn’t hormonal fluctuations during the female menstrual cycle also produce changes in cognitive performance? This question has been investigated and female test subjects who do not take hormonal preparations such as the pill have been given tasks twice during their menstrual cycle in which women usually perform worse than men. One test time was during menstruation (day 2), when all sex hormones are at their lowest point. The second task was set during the luteal phase (day 22), when hormone levels of estradiol and progesterone are very high. The results were clear: when female sex hormones reached their nadir (day 2), women’s performance on the mental rotation test was similar to that of men. However, when hormones rose to day 22, performance dropped dramatically. Thus, the women studied were not in principle worse than the men in their visuospatial ability – it just depended on when they were tested!
Timing matters
Because sex hormones have multiple influences on brain functions, it is not easy to figure out which of these functions were altered in the subjects. One “promising candidate” is the so-called cerebral asymmetries – the differences in function between the left and right sides of the brain. In humans, the left side of the brain shows a predominance of verbal abilities, while the right has a dominance for visuospatial functions. These functional left-right differences are more pronounced in men than in women. Could it be that women and men differ cognitively because the asymmetries of their brains are different? But then, along with cognition, brain asymmetries would also have to change during the menstrual cycle. The asymmetries in humans were investigated using a special experiment (“visual hemifield technique”), which makes it possible to show images to only one half of the brain, so to speak: When a test person looks at a cross in the center of the monitor, the figure to the left of the fixation cross is seen only by his right brain hemisphere. As soon as the subject looks to the left and looks at the figure centrally, both hemispheres of the brain naturally perceive this stimulus. People need about 200 milliseconds for such a gaze movement. However, if the lateral figure disappears from the monitor after only 180 milliseconds while the subject is still looking at the central fixation cross, then this lateralized stimulus is perceived only by the right hemisphere.
What comes from the left: quickly detected
In the next step, the subjects compared different figures.First, they memorized a centrally presented abstract figure for a few seconds so that both hemispheres of the brain stored this stimulus. Then, instead of the central figure, the fixation cross appeared briefly. Subsequently, the same or a different figure was displayed to the left or right for 180 milliseconds, while the gaze remained focused on the cross. The test subject now decided as quickly as possible by pressing a key whether the figure was the same (G) or different (U). As a rule, the answer followed more quickly and correctly if the second figure appeared on the monitor on the left, since the right hemisphere is superior in visuo-spatial tasks. This result was confirmed by male subjects as well as women during menstruation. In contrast, in the same women, the performance of their two cerebral hemispheres was laterally equal during the luteal phase. Cerebral asymmetries for visuospatial tasks had indeed radically changed during the menstrual cycle! Thus, a reduction in female sex hormones leads both to an increase in mental rotation performance and to asymmetric brain organization. Postmenopausal women were also found to have left-right differences for visuospatial stimuli that matched those of both men and women during menstruation.
Progesterone is to blame
The studies showed that the asymmetry changed primarily with fluctuations in the hormone progesterone. Progesterone rises to day 22 of the menstrual cycle and then falls again. In the brain, progesterone improves the function of receptors for the inhibitory neurotransmitter GABA while reducing the uptake and conversion of the activating neurotransmitter glutamate. Overall, progesterone should thus have a dampening effect on many brain processes. In this context, progesterone could alter cerebral asymmetries primarily by altering the exchange of information between the two cerebral hemispheres via the large fiber connection (corpus callosum). The corpus callosum consists of more than 200 million fibers and connects the two cerebral hemispheres. The neurons that make up the corpus callosum use glutamate almost exclusively. Thus, during the luteal phase, progesterone could reduce the effectiveness of this connection and, at the same time, reduce cerebral asymmetries. If these considerations are correct, the total excitability within the cerebral cortex should fluctuate during the menstrual cycle. But how can this be demonstrated?
Sex hormones dampen activity of neurons
The timing of such a double-stimulus method allows a statement regarding the current inhibitory and excitatory cell activity in a particular brain region. A similar TMS technique was used to study signal transmission between the two hemispheres via the corpus callosum. This TMS dual-stimulus method has now been used in women at different stages of the menstrual cycle. The activity of the inhibitory and excitatory neuron associations showed marked variations in the different phases of the cycle. Thus, the activity of the excitatory cell assemblies decreased significantly at high concentrations of the sex hormones estradiol and progesterone in the luteal phase, while the inhibitory cell assemblies were activated at the same time. This resulted in an overall lower activability of certain brain regions. At the same time, a change in the exchange of information between the two hemispheres via the corpus callosum was detectable: In the luteal phase, signal transmission decreased, which corresponds to the test results of the visual hemifield technique. The test results obtained with very different methods impressively prove a changing asymmetry of brain function caused by hormones in the course of the female cycle. These fluctuations are reflected in daily functions. The research results not only show that “the little difference” in the human brain can be objectively accounted for, but that this difference fluctuates in a hormone-dependent manner.
