Don’t forget the cerebellum

Many theories of humanness rely on a simple idea that the cerebral cortex is enlarged in humans relative to other primates and in primates relative to other mammals. So it must be the cerebral cortex that is the important part of the brain, giving us our smarts and our skills. What is often overlooked is that the cerebellum has also increased in the same proportion. Across the mammals the ratio of neurons in cerebellum to the number in the cerebrum is 3.6 so whatever was happening to the cerebrum was also happening to the cerebellum. In fact our cerebellum may have gained a little – our cerebrum is slightly smaller than earlier homos but our cerebellum is not and maybe a bit bigger.

And what does the cerebellum actually do? It does not appear to initiate anything but modifies what is initiated. In other words, it is not responsible for doing anything, just for doing it much better. It does coordination, timing, accuracy, smoothness, balance. It was once thought to be purely concerned with motor actions but now it appears to also deal in cognition, attention, learning and emotion.

There is a recent paper by Joan Baizer (citation below) on comparisons of the hindbrain. The paper discusses the evidence in the brainstem and cerebellum for (1) structures that are conserved across species but show subtle biochemical differences (2) structures also conserved but showing major differences in overall organization (3) structures found in humans and chimpanzees but not in monkeys or cats (4) structures found only in humans (5) two features that are considered exclusive to the cerebral cortex, individual variability and left-right asymmetries. All these changes mean that the hindbrain has been evolving in step with the forebrain. The cerebellum is doing some very important processing tasks for the rest of the brain.

Here is part of the Discussion:

The human brain is distinguished by parallel and functionally linked expansion of the cerebellum and the cerebral cortex. Our studies show that there are also major changes in the human brainstem, most notably in structures that are known or suspected to project to the cerebellum. It is clear that the expansion of the human brain underlies unique aspects of human cognitive and motor function. What is known about the relative contributions of the cerebellum and the cerebral cortex? The cerebral cortex is critical for both cognitive function and motor control. The traditional view of the cerebellum is that it is critical for motor, but not cognitive, function. That view has been challenged on the basis of anatomical, physiological, and behavioral data, with many supporting role for the cerebellum in cognitive functions.

We will focus on the motor role of the cerebellum and associated brainstem structures. Humans are bipedal and bipedal locomotion imposes very different demands for the control of balance and posture, functions to which the cerebellum contributes. Second, bipedal locomotion frees the forelimbs and hands, allowing the development of fine motor skills, skilled tool use, and the emergence of handedness. There are parallel changes in the visual system, with the evolution of the fovea and parallel changes in voluntary eye movements. The cerebellum also participates in the control of the hands and fingers as well as in the control of eye movements. The specializations of primate brainstem structures may be related to these evolutionary changes.

Here is her observation on the ‘reptilian brain’:

The idea that evolution affects only the cerebral cortex, with brainstem and cerebellum essentially unchanged entered the popular culture of neuroscience through the writings of Paul Maclean, “The Triune Brain” and Carl Sagan’s “reptilian brain”. The concept of the “reptilian brain” maintains that the brainstem and cerebellum are “old” structures that have not changed over evolution. That perspective still colors the understanding of students and the general public today. As shown in this review, it clearly does not reflect the dramatic changes in cerebellar and brainstem structures and their contribution to uniquely human capabilities.

Baizer, J. (2014). Unique Features of the Human Brainstem and Cerebellum Frontiers in Human Neuroscience, 8 DOI: 10.3389/fnhum.2014.00202

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