Category Archives: cerebellum

Synergy and Modular control

When we learned the simple overview of the nervous system in grade school, we were taught that the brain sent signals to muscles to contract and that is how we moved. And by brain, we assumed the thinking part up high in the head. But it cannot be so.

A little deer is born and in a very short time is standing and in a little longer is taking its first wobbly step. Within a couple of days it is running and frolicking. Deer are not that special; other animals ‘learn’ to get around very quickly too. Even humans babies, if they are held upright with their feet touching a surface will walk along that surface. In a sense, the spinal cord knows how to walk by lifting and moving forward alternate legs. It does not know how to walk well, but the basics are there. Human babies are slower at managing to get around because they are born at a less developed stage and walking on two legs rather than four is trickier. In all sorts of observations and experiments there is evidence that the ability to walk is innate in the spinal cord and does not require the brain.

The spinal cord has some primitive control modules or muscle synergies. Muscle synergies are present in a number of natural behaviors; they are low-level control networks found in the brain stem and spinal cord that coordinate a group of muscles. They make common movements easier to order up. We have the ‘intent to go over there’ and without any more conscious thought we do it in an automatic way. Now if we had to trigger individual muscles in the right time sequence, it would likely take many hours to get not very far with a number of falls along the way. One could say that we would ‘get the hang of it’ as we did it. But that is saying we would make parts of it automatic (create modules and synergies).

This modularization of motor control is layered. The simplest control is in the spinal cord, but it is modified and adapted to conditions by the brain stem and especially the cerebellum. The cerebellum gets instructions from other parts of the brain and finally these modules within modules are able to execute the simple ‘intention to go over there’.

The synergies in a baby’s spinal cord are an ancient set that is similar of all mammals (probably all land vertebrates). The muscles work in a rhythm where each event triggers the next in a circle. There are two primitives that are involved in human walking that we are born with. One is to bend the leg so that the foot leaves the ground and moves forward then goes back down and straightens. Two is a forward push against the ground by the straight leg. These two complexes of muscle contractions and relaxations are wired so that their action in one leg inhibits their action in the other. When the left leg does one, the right leg cannot do one but can do two. And when the left leg does two, the right cannot do two but can do one. They are also wired so that in each leg it is the end of one that triggers the start of two and the end of two triggers the start of one. It is the same in four legged animals except there is another set of inhibitions between the front and hind legs. At this level it is not very adaptive and can only react to sensory information that comes through the spinal cord from the muscles, joints and skin. Babies cannot use this facility to get around because they do not have the strength to maintain the posture needed with such a large heavy head on such a little body, and more importantly, the spinal cord has no information from the ears about balance. Balance is very important for bipedal walking. The baby must create two other synergies: to react to balance information and to use the hips, back and arms to keep the center of gravity over the legs. In the meantime, when they don’t have the strength, they can crawl using the 4 legged modules.

The cerebellum and brain stem add the control of balance and of pace (there are relative changes to the timing of events when the whole process is sped up). They can correct for uneven ground. They can keep the direction of motion toward a target. But the coordination control of the lower brain is not just direct signals to muscles but uses the synergies built into the spinal cord. And it is much more complex than the action in the spinal cord. In fact, the cerebellum has more neurons that the whole rest of the brain. It manages the modules, timing, adjustments to modules, effects from sensory input and feedback and commands from higher levels of the brain, then packages it all for execution. Another great trick of the cerebellum is to do two things at the same time, say walk and throw a ball. Both may be deep seated modules but there are adjustment to be made where they interfere with one another.

The point I am making here is that although movement seems so easy for us to execute, that is because it is not arranged consciously, or even largely in the cerebral hemispheres. It is modularized so that a simple request in the cerebral cortex goes through layers of calculation and fine-tuning to become individual signals to individual muscles. It is synergy/modularization that gives us this flexible but easy to use system. We are surprised that it is easier to create a program to play chess in the abstract (and win) than it is to program a robot to physically move the pieces and operate the time clock in a game. When we do not understand how something is done, it appears easy. It is a common trap.

 

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.
ResearchBlogging.org

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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