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Developmental Biology - Human Cerebellum

The Thin Crepe Which Is Our Cerebellum

High-res imaging shows the cerebellum is a long strip of thin "crepe" like tissue and is 80% the size of the cortex...

"A quick mind" may be our expectation for our expanded cerebellum, which distinguishes our human brain from those of macaque monkeys. High resolution imaging shows the human cerebellum is 80% the area of our cortex, perhaps indicating it's grown as human behavior and cognition evolve. It plays a role in sensory perception, motor output, balance and therefore posture.

The Latin origin of cerebellum means "little brain". Located close to our brainstem, it sits under the cortex in our hindbrain. Now research from San Diego State University (SDSU), calls the diminutive "little" into question.
The cerebellum contributes to pain, movement, thought, our five senses and emotion, yet is essentially a flat sheet the thickness of a crepe. It is crinkled into hundreds of folds to make it fit into a compact space about one-eighth the volume of our cerebral cortex.

Yet, its surface area was thought to be considerably smaller than that of the cerebral cortex. However, uing an ultra-high-field 9.4 Tesla MRI machine to scan the brain, and custom software to process the images, SDSU neuroimaging discovered these tightly packed folds actually contain a surface area equal to 80% of the cerebral cortex's surface area. In comparison, the macaque's cerebellum is about 30% the size of its cortex.
"The fact that it has such a large surface area speaks to the evolution of distinctively human behaviors and cognition. It has expanded so much that the folding patterns are very complex."

Martin Sereno PhD, Professor, Psychology & Cognitive Neuroscience, Director, San Diego State University MRI Imaging Center, San Diego, California, USA.

Unprecedented Insights

Collaborating with imaging and cerebellum experts from the United Kingdom, Netherlands and Canada, Sereno used customized open source software (FreeSurfer) he developed with colleagues while at the University of California San Diego. With 'FreeSurfer' he computationally reconstructed the folds of the cerebellum. The software unfolded and flattened the cerebellar cortex to visualize each individual folia - or thin leaf like fold.

When the cerebellum is completely unfolded, it forms a thin "crepe" four inches wide and three feet long.

The findings are published in PNAS (Proceedings of the National Academy of Sciences).

Sereno: "Until now we only had crude models of what it looked like. We now have a complete map or surface representation of the cerebellum, much like cities, counties, and states."

Puzzle Pieces

Previous research discovered that while there were many similarities between the cortex and the cerebellum, there was one key difference. In the cerebral cortex, regions representing different parts of the body are arranged roughly as they are in the body: juxtaposed and orderly. In the cerebellum, they are placed more randomly.

"You get a little chunk of the lip, next to a chunk of the shoulder or face, like jumbled puzzle pieces," Sereno explains.

Those parts of the cerebellum are therefore set up to pull in and coordinate information from disparate parts of the body.
"It is intriguing to think there might be analogs of 'fractured somatotopy' in cognitive parts of the cerebellum that could help support highly complex, sophisticated cognitive functions such as language or abstract reasoning. When you think of the cognition required to write a paper or explain a concept, you pull in information from many sources. That is just how the cerebellum is set up."

Martin Sereno PhD

Until now, the cerebellum was thought to be involved mainly in basic functions like movement, but its expansion over time and its new inputs from cortical areas involved in cognition suggest that it can also process advanced concepts like mathematical equations.

"Now that we have the first high resolution base map of the human cerebellum, there are many possibilities for researchers to start filling in what is certain to be a complex quilt of inputs, from many different parts of the cerebral cortex in more detail than ever before," Sereno adds.

For example, there is recent evidence people who suffer cerebellum damage have difficulty processing emotion.
"The 'little brain' is quite the jack of all trades. Mapping the cerebellum will be an interesting new frontier for the next decade."

Martin Sereno PhD

The cerebellum has long been recognized as a partner of the cerebral cortex, and both have expanded greatly in human evolution. The thin cerebellar cortex is even more tightly folded than the cerebral cortex. By scanning a human cerebellum specimen at ultra-high magnetic fields, we were able to computationally reconstruct its surface down to the level of the smallest folds, revealing that the cerebellar cortex has almost 80% of the surface area of the cerebral cortex. By performing the same procedure on a monkey brain, we found that the surface area of the human cerebellum has expanded even more than that of the human cerebral cortex, suggesting a role in characteristically human behaviors, such as toolmaking and language.

The surface of the human cerebellar cortex is much more tightly folded than the cerebral cortex. It was computationally reconstructed for the first time to the level of all individual folia from multicontrast high-resolution postmortem MRI scans. Its total shrinkage-corrected surface area (1,590 cm2) was larger than expected or previously reported, equal to 78% of the total surface area of the human neocortex. The unfolded and flattened surface comprised a narrow strip 10 cm wide but almost 1 m long. By applying the same methods to the neocortex and cerebellum of the macaque monkey, we found that its cerebellum was relatively much smaller, approximately 33% of the total surface area of its neocortex. This suggests a prominent role for the cerebellum in the evolution of distinctively human behaviors and cognition.

Martin I. Sereno, Jörn Diedrichsen, Mohamed Tachrount, Guilherme Testa-Silva, Helen d’Arceuil and Chris De Zeeuw.

The authors thank Alex de Crespigny for providing the macaque monkey scan and Anders Dale for writing the core of the original FreeSurfer code that was extended here. This research was funded by NIH Grant R01 MH081990, a UK Royal Society Wolfson Fellowship (to M.I.S.), and Canadian Institutes of Health Research Grant PJT159520 (to J.D.)

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Aug 5 2020   Fetal Timeline   Maternal Timeline   News

Cerebellar slices, folded and inflated of flattened cerebellar surfaces, all at same scale. Starting in the Upper Left, slice images are tessellated and the resulting surface was unfolded, cut (Upper Right), and flattened. Each surface is shown twice, first color-coded by FreeSurfer average convexity (“sulc,” which marks lobules) and then by local curvature (“curv,” which marks much smaller folia). At Lower Left, a macaque monkey cerebellum is shown at the same scale. (Movie S1).
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