The small difference in the human brain

Can men really not listen, and are women really unable to park? Researchers have long discovered functional differences between the two halves of the brain. It is interesting that this "small difference" is lifted at least once a month for women.

Cognitive gender differences

Some cognitive gender differences have been scientifically proven. For example, women are superior in verbal skills, where the speed of naming target words is important. Men, on the other hand, find it easier to perform some tasks that specifically require spatial imagination.

Gender-specific differences in speech and visual cognition are therefore not a malicious prejudice, but a scientific fact. They could be the result of different educational styles and / or biological factors. The latter suggests that female and male brains differ in about a dozen anatomical features.

Biological factors also indicate test results. With special experimental arrangements sex differences could be detected quite consistently not only in different nations, but also over the last 30-40 years, although the educational styles in these countries and time periods were extremely different. In addition, in males who become women after a sex change, taking female sex hormones increases their language proficiency at the expense of space cognitions. Exactly the reverse development is going through women who become men.

Are the hormones to blame?

There is strong evidence that cognitive differences between men and women may arise, at least in part, from different hormonal factors that are likely to lead to gender-specific brain mechanisms. But would not the hormonal fluctuations during the female monthly cycle also have to produce changes in cognitive performance?

This question has been investigated and asked female subjects who do not take any hormone preparations such as the pill, twice during their cycle tasks in which women usually do worse than men.

A test time was during menstruation (2nd day) when all sex hormones are low. The second task was performed in the luteal phase (22nd day), in which the hormone levels of estradiol and progesterone are very high.

The results were clear: When the female sex hormones reached their low point (Day 2), the performance of women in the mental rotation test was as good as that of men. But if the hormones on the 22nd Day, then the performance dropped dramatically. The examined women were therefore in principle not worse in their visual-spatial ability than the men - it only depends on when they were tested!

The time is important

Since sex hormones have multiple influences on brain functions, it is not easy to find out which of these functions were altered in the subjects. A "promising candidate" are the so-called cerebral asymmetries - the functional differences between the left and right hemispheres.

The left side of the brain shows a superiority of verbal abilities in humans, while the right one has a dominance for visual-spatial 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 cognition would also have to change the brain asymmetries during the monthly cycle.

The asymmetries in humans were examined with a special experiment ("visual half-field technique"), which makes it possible to show pictures of only one half of the brain: If a subject observes a cross in the center of the monitor, the figure to the left of the fixation cross only becomes the right half of the brain seen. As soon as the subject looks to the left and looks at the figure centrally, both halves of the brain naturally perceive this stimulus.

For such a look people need about 200 milliseconds. However, if the lateral figure disappears from the monitor after only 180 milliseconds, while the subject still looks at the central fixation cross, then this lateralized stimulus is only perceived by the right hemisphere.

What comes from the left: quickly recognized

In the next step, the subjects compared different figures. At first, they memorized a centrally presented abstract figure for a few seconds, so that both halves of the brain memorized this stimulus. Then, instead of the central figure, the fixation cross appeared briefly. Subsequently, the same or another figure was shown on the left or right side for 180 milliseconds, while the view remained focused on the cross. The subject decided as quickly as possible at the touch of a button, whether it was the same (G) or an unequal figure (U).

As a rule, the answer will be faster and more correct if the second figure appears on the left monitor because the right hemisphere is superior in visual-spatial tasks. This result was confirmed by male subjects and women during menstruation. In contrast, the performance of both halves of the brain during the luteal phase was the same for the same women. The cerebral asymmetries for visual-spatial tasks actually changed radically during the menstrual cycle!

A reduction of the female sex hormones thus leads both to an increase in performance in mental rotation and to an asymmetric brain organization. There were also left-to-right differences in post-menopausal visual-spatial stimuli, similar to men's and women's during menstruation.

The progesterone is guilty

The investigations showed that the asymmetry changed above all with the fluctuations of the hormone progesterone. Progesterone rises to the 22nd day of the menstrual cycle and then drops again. In the brain, progesterone improves the function of the receptors for the inhibitory messenger GABA and simultaneously reduces the uptake and conversion of the activating messenger glutamate.

Overall, progesterone should thus act on many brain processes dampening. In this case, progesterone could change the cerebral asymmetries primarily by changing the information exchange between the two halves of the brain via the large fiber connection (corpus callosum).

The corpus callosum consists of over 200 million fibers and connects the two halves of the brain. The nerve cells that make up the corpus callosum almost always use glutamate. During the luteal phase, the progesterone could thus reduce the effectiveness of this compound and thus also the cerebral asymmetries. If these considerations are correct, the total excitability within the cerebral cortex would have to fluctuate during the menstrual cycle. But how can one prove this?

Sex hormones dampen activity of nerve cells

The temporal sequence of such a double-stimulus method allows a statement regarding the current inhibitory and excitatory cell activity in a specific brain region. Using a similar TMS technique, signal transduction between the two hemispheres was examined via the corpus callosum. This TMS double-stimulus method has now been used in women at different stages of the menstrual cycle.

The activity of the inhibitory and excitatory neuronal associations showed marked fluctuations in the different cycle phases. Thus, the activity of the excitatory cell clusters at high concentration of the sex hormones estradiol and progesterone in the luteal phase decreased significantly, while the inhibitory cell aggregates were activated simultaneously. This resulted overall in a lower activatability of certain brain regions. At the same time, a change in the information exchange between the two hemispheres was detectable via the corpus callosum: In the luteal phase, the signal transmission decreased, which corresponds to the test results of the visual half-field technique.

The examination results obtained with very different methods impressively demonstrate a hormone-induced changing asymmetry of the brain function during the course of the female cycle. These fluctuations are reflected in day-to-day functions. The research results not only show that "the small difference" in the human brain can be justified objectively, but that this difference varies depending on hormones.

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