Over the past thirty years, a hot topic emerging in popular
neuroscience has been the study and anatomical comparison of the male and
female brain. From teachers to researchers, writers and bloggers, everyone's
interested in it - what anatomical differences in the male and female brain
cause the differences between male and female behavior? The answer lies in the
brain. Maybe because it simply prods our interest and fuels curiosity, this
topic has fascinated people for years. A particular area of interest between
gender and the brain is the structure called the corpus callosum, the white matter bridge connecting the brain's
left and right hemispheres.
One proposed male-female discrepancy in neuro anatomy is the
size of this component, as an early study in 1982 showed that females, overall,
had a larger and more bulbous callosal structure. At the time, this newfound
discovery sparked tons of popular interest and speculation over its role in
male-female behavioral differences. People began to hypothesize and propose
that this meant females had more “interconnected” brains, and were better at
things requiring multi-tasking or complex functioning:
“The corpus callosum is 30
percent more highly developed in the female brain… allowing information to flow
more easily from one side of the brain to the other, which allows a woman to
focus on more than one thing at a time.”
Cool, right? As interesting as this sounds, very recently
it’s all been proven to be completely untrue - they didn’t get it quite right.
In all actuality, a recent study shows that rather than women having a larger
corpus callosum, men may just overall have larger brains. The breakthrough in
this study has shown that the corpus callosum is relatively smaller in larger brains, appearing larger in smaller-sized brains. Although
the original observation in 1982 may have shown the female’s callosum to be
larger, it truly just appeared that way because the brain was a smaller size.
So, sorry ladies – the actual anatomical difference in gender, and perhaps the
only one, is that males have larger brains overall.
The Study:
Eileen Luders, et al.
Luders confirmed this theory with a clever technique. In
order to truly investigate the differences between the male and female corpus
callosum, the ideal study was planned to extract measurements of the corpus
callosum in male and female brains of the same size. To our knowledge today, no
other study has compared callosal size in male and females with equal brain
mass – so these results are brand new for further exploration. The lack of info
on this topic is probably due to the difficulty in finding males and females
with similar brain sizes, as cranial capacity differs greatly between sexes.
Fortunately, recent databases encompassing thousands of brain images have been
established, allowing a unique opportunity to select from a vast pool of
subjects. Using the International Consortium for Brain Mapping (ICBM) database,
Luders and colleagues selected 24 male brains and 24 female brains from the
database, perfectly matched for size. They also complemented this sample with
another subset of 24 extra-large male brains and 24 extra-small female brains
to account for wide degree of variation in brain size. Total intracranial
volume was calculated using imaging technology, and then a special process
called “surface-based mesh-modelling” was used to overlay brain images and
measure equidistant spatial points in the corpus callosum, male versus female. What
Luders found was interesting and completely controversial to prior popular
belief. When comparing the brain images divided by sex and accounting for size,
Luders’ data pointed to the opposite - that the corpus callosum was actually always thicker in men than in women.
However, this gender difference was clearly driven by brain size, thus the following conclusion was formed: simply put,
males have larger brains, leading to comparatively thicker corpus callosum than
females.
Altogether, findings suggest that individual differences in
brain size account for the apparent sex differences in the corpus callosum.
Studies like Luders’ will continue to provide important clues about cerebral
differences between men and women, especially if appropriate strategies are
used to account for variations in brain size. Nevertheless, it is also possible
that future studies will use this line of thought to expand on adolescent brain
size between boys and girls, examining brain-imaging technology for gender
differences in how they develop.
Given the freshness and excitement of these popular findings,
and their possible relevance for understanding sex differences in cognition,
emotion, and behavior, the sexual dimorphism of the human corpus callosum has
been and will be a continued object of exploration. So, as fueled by our
neuro-curiosity, the battle of the sexes (the brain battle, that is...) continues
on.
-Madeline Kleypas
Eileen Luders, Arthur
W. Toga, & Paul M. Thompson (2013). Why Size Matters: Differences in Brain
Volume Account for Apparent Sex Differences in Callosal Anatomy Neuroimage DOI:10.1016/j.neuroimage.2013.09.040
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