Fine control

My last blog on timing in some neurons in the cerebellum has started a string of thoughts. Here we have a part of the brain with an anatomy that is well mapped as opposed to many other parts. It has more neurons than the rest of the brain put together. It has grown relatively larger in human evolution then any other part of the brain. There are theories about how the system works, and yet, its actions are not understood in detail and new information on one of its important cell types was a surprise. (previous blog)

Abstract (Barton see citation below):

Humans’ unique cognitive abilities are usually attributed to a greatly expanded neocortex, which has been described as “the crowning achievement of evolution and the biological substrate of human mental prowess”. The human cerebellum, however, contains four times more neurons than the neocortex and is attracting increasing attention for its wide range of cognitive functions. Using a method for detecting evolutionary rate changes along the branches of phylogenetic trees, we show that the cerebellum underwent rapid size increase throughout the evolution of apes, including humans, expanding significantly faster than predicted by the change in neocortex size. As a result, humans and other apes deviated significantly from the general evolutionary trend for neocortex and cerebellum to change in tandem, having significantly larger cerebella relative to neocortex size than other anthropoid primates. These results suggest that cerebellar specialization was a far more important component of human brain evolution than hitherto recognized and that technical intelligence was likely to have been at least as important as social intelligence in human cognitive evolution. Given the role of the cerebellum in sensory-motor control and in learning complex action sequences, cerebellar specialization is likely to have underpinned the evolution of humans’ advanced technological capacities, which in turn may have been a preadaptation for language.

This enlargement has been in the neocerebellum which is not primarily concerned with the fine tuning of movements of the whole body and limbs. What appears to have increased is: the ability to learn by being taught, imitating and practice; fine control of the hands as is needed for tool making; find control of the larynx as is needed for speaking; and it might said, fine control of any sequential process including language and some types of thought.

Wikipedia gives a summary of the neocerebral connections. “The lateral zone, which in humans is by far the largest part, constitutes the cerebrocerebellum, also known as neocerebellum. It receives input exclusively from the cerebral cortex (especially the parietal lobe) via the pontine nuclei (forming cortico-ponto-cerebellar pathways), and sends output mainly to the ventrolateral thalamus (in turn connected to motor areas of the premotor cortex and primary motor area of the cerebral cortex) and to the red nucleus. There is disagreement about the best way to describe the functions of the lateral cerebellum: it is thought to be involved in planning movement that is about to occur, in evaluating sensory information for action, and in a number of purely cognitive functions as well, such as determining the verb which best fits with a certain noun (as in “sit” for “chair”).

A computing mechanism for fine control of a process using feedback from the environment has an almost universal usefulness. It does not initiate but controls an action. It evolved to give us balance and posture, to smooth our actions and make them more accurate, to move the eyes and give us a stationary vision from moving eyes, and to steer the eyes to points of attention. What we appear to have gained is extremely fine control of some muscles and the ability to use the same mechanisms for language, music and other forms of thought and social communication. It appears essential to supervised learning. And here is a biggy – it may be responsible for knitting together the fragments of memory and knowledge that produces imagination.

Barton, R., & Venditti, C. (2014). Rapid Evolution of the Cerebellum in Humans and Other Great Apes Current Biology DOI: 10.1016/j.cub.2014.08.056

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