Evolving Brain Stuff, Y’all

The Authors: The article, entitled “Evolution of the Cerebellar Cortex: The Selective Expansion of Prefrontal-Projecting Cerebellar Lobules,” was researched and written by Dr. John Balsters, E. Cussans, Jörn Diedrichsen, Dr. Kathryn A. Phillips, Dr. Todd M. Preuss, Dr. James K. Rilling, and Dr. Narender Ramnani. All of these people have interests in the cerebellum and motor functions.


Hypothesis: The authors predicted that since the prefrontal cortex has evolved to be larger in relation to the motor cortex in humans, there should also be enlargements in the cerebellum, specifically those parts that are associated with the prefrontal cortex, in relation to the lobules of the cerebellum associated with the motor cortex.


This shows where the cerebellum is located. The prefrontal cortex is located at the front of the cerebrum and the motor cortex in about the middle of the cerebrum.


The Experiment: They decided to test their hypothesis by examining three different primate species, humans, chimpanzees, and capuchin monkeys.  They took ten different subjects for each species, five of these were males and five were females.  All of the subjects were of an age where the brain would have reached full maturity.  High-resolution MRI scans were taken of each of the subject’s brains, as well as structural images. Using various programs, the scans and images were oriented in the same direction, and the cerebellum was eventually isolated from the rest of the brain, so that the scientists were left with only images of the part of the brain they were interested in (those lobules that were associated with the motor loop or the prefrontal loop). They then extracted images for the cerebellar lobules using the FSLView program. The volumes of each of the images of the cerebellar lobules were then calculated. The specific parts of the cerebellum they isolated were Lobule V, Lobule VI, Crus I, Crus II, Lobule VIIb, and Lobule VIIIa. After completing the calculations of the volumes of the lobules, they decided to compare the volumes measured against the volume of the whole cerebellum and against the sum of the volumes that had been masked, which are those related to the motor and prefrontal cortex.


A capuchin monkey


Results: In reference to the lobules of the cerebellum when compared to the whole cerebellum, the largest differences across the species came from the comparisons with Crus I and Crus II, in which humans were found to have the greatest proportion, followed by chimpanzees and then the capuchin monkeys.

A chimpanzee


In reference to the lobules of the cerebellum when compared to the masked volumes, it was found that the volumes of the masked lobules occupied the greatest portion of the cerebellum in humans, followed by chimpanzees and then capuchin monkeys respectively. Through this comparison, it was again shown that the greatest differences between species came from the Crus I and Crus II sections.


Difference in size of a human brain (left) and a chimpanzee brain (right).

Discussion: They have shown that the evolution of the cortical lobules is directly related to the evolution of the neocortical areas that are associated with them. Crus I and Crus II specifically are much larger than other lobules associated with the primary motor cortex. It was also discovered that Crus I and Crus II in capuchin monkeys are significantly smaller than Crus I and Crus II in humans and chimpanzees. The enlargements in the cerebellar cortex relate to those of the prefrontal cortex in all of the species. These enlargements in the human brain correlate specifically to its functional specializations.

The scientists compare their data to that of the brains of Old World monkeys and hypothesize that the volumes of Old World monkeys should fall in between those of the chimpanzees and the capuchin monkeys, which upon further examination proves to be accurate in the observation of one macaque monkey.

The allometric trends that could arise due to these differences between the species are an area the scientists think needs more study and that they did not examine specifically.  They do, however, state that humans definitely depart from the isometric trends, which they attribute to the differences in the cerebellum.

The enlargement of the prefrontal cortex in humans had been attributed to white matter expansions as opposed to grey matter.  This study suggests the opposite. It is mentioned that the cerebellum is largely made of white matter, but the lobules that were examined in this study were largely comprised of grey matter, which is the reason for the shift.


My comments: I found this article to be incredibly difficult to understand, which was surprising to me because I do not usually find myself struggling to read articles about research experiments.  That being said, research articles such as this are very often written with a specific audience in mind and are therefore fairly exclusionary to the general public. The problem with this is that laymen can not find materials to read on subjects like this because they are largely unreadable to the public. It can cause a lot of misunderstanding or loss of interest in subjects such as this because most people will not usually try to wrestle with and understand much of the scientific jargon used in the article.

7 thoughts on “Evolving Brain Stuff, Y’all”

  1. Hey all,
    The post states that, unlike the prefrontal cortex, which has expanded with white matter, the lobules of the cerebellum expanded with grey matter. I was wondering about what the implications of this might mean. Why has the cerebellum expanded with grey matter instead of white matter?
    -Genevieve Miller

  2. Stephanie, take a look at the blogging instructions again please & edit this accordingly. Make sure Taylor sees it too. You’re collaborating.

  3. At your next social gathering, try to slip this into casual conversation: “the petrosal lobule is a ‘reduced accessory paraflocculus of the great apes and man’.”

  4. Balsters et al suggest that OWM should be between humans & capuchins in cortico-cerebellar encephalization, but MacKinnon & Fuentes point out that capuchins are uniquely intelligent NWM. What do you think?

  5. The fact that the neocortex and the cerebellum both advanced structurally prompts an intriguing thought for me along the vein of the chicken or the egg adage. Did the neocortex increase in size and complexity because of the the growth and developement of the cerebellum or vice versa? They must not have grown in completely in tandem. My thought is that the cerebellum increased in capacity thereby allowing the neocortex to grow or conversely, the neocortex put a strain on the cerebellum initiating a spurt of advancement in the old region of the brain. I am unsure as to whether or not this is a question that has defined answer as opposed to being a topic that is up for debate.

    1. I totally agree with everyone else that this was a rough one to get through. My proposal had to do with an article that that talked about the reduction of grey matter in PTSD sufferers. The functional impairment that causes can be extremely severe. I know gcmiller had a question about what the implications of that might be so this is my little theory, that the grey matter increase increases cognitive functionality thereby bettering the cognitive capabilities of the brain.

  6. The second time around was a little easier to understand, but not by much. For me, one of the greatest things that I got out of this paper was the give and take that occurred when the brain evolved. We only have so much room in our heads, and when one section of our brain decides to expand another section must retract. Our cerebellum was adapted for our physical balance, but the need for mental balance outweighed those for physical balance in humans. If we can’t have everything, I would much rather have mental abilities than physical balance. I can see this trade-off in my own life, for I am very intelligent and very clumsy. This is just a small scale example, and of course this concept applies to species rather than individuals.

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