Solving the Mystery of the Human Cerebellum

Abstract

The mystery of the human cerebellum is this: Why did it enlarge so dramatically in the last million years of human evolution, concomitantly with the greater enlargement of the cerebral cortex? A solution to this mystery was proposed in the 20th century as a result of research by several groups of scientists who investigated the contributions of the cerebellum to the cerebral cortex. In contrast to the 19th century investigations, which were focused on the motor functions of the cerebellum, the focus of the subsequent investigations was expanded to include some mental functions because evidence was produced that the cerebellum contributes to cognition. It was proposed that the combination in the cerebellum of motor and mental capabilities enables the cerebellum to confer on humans some adaptive advantages of great value, and this ability would explain why the human cerebellum has continued to enlarge so dramatically. A valuable adaptive advantage that is included in the proposal is the possibility that the cerebellum couples the motor function of articulating speech to the mental function that selects the language to be spoken, thus helping to produce fluent human speech and language. The validity of this proposal about linguistic processing has not yet been verified. Therefore the mystery of cerebellar enlargement in humans is not yet solved and requires further research.

Appendices

Appendix

“The Treasure at the Bottom of the Brain.” Henrietta C Leiner and Alan L. Leiner. September 1997, published online [http://www.newhorizons.org/neuro/leiner.htm] and reprinted here with permission from New Horizons for Learning [http://www.newhorizons.org]

One of the most impressive parts of the human brain, named the cerebellum, has been underestimated for centuries. Located at the lower back of the brain, it is a fist-sized structure whose function is now being reappraised. Formerly this structure was thought to have only a motor function, which it performed by helping other motor regions of the brain to do their work effectively. But during the past decade a broader view of its function has emerged as a result of new research, and now the cerebellum is regarded as a structure that can help not only motor but also nonmotor regions to do their work effectively. In fact, the cerebellum has been compared to a powerful computer, capable of making contributions both to the motor dexterity and to the mental dexterity of humans, both of which are required for the emergence of fluent human language.

This powerful mechanism at the bottom of the brain, which every person inherits as a birthright, is immature at birth but develops through childhood and adolescence, reaching its full structural growth by the 15th to 20th year of life. Perhaps the reason why it has traditionally been underestimated is its low-level location in the brain, which contrasts with the high-level location of the structures that are thought to subserve higher mental functions. Such locations in the brain become irrelevant, however, when a structure is regarded as a computer because a computer’s processing power depends not on where it is but on what it contains and to what it is connected.

Judged by what it contains and by its external connections, the human cerebellum is an enormously impressive mechanism. First of all, it contains more nerve cells (neurons) than all the rest of the brain combined. Second, it is a more rapidly acting mechanism than any other part of the brain, and therefore it can process quickly whatever information it receives from other parts of the brain. Third, it receives an enormous amount of information from the highest level of the human brain (the cerebral cortex), which is connected to the human cerebellum by approximately 40 million nerve fibers. To appreciate what a torrent of information these 40 million fibers can send down from the cerebral cortex to the cerebellum, a comparison can be made with the optic fibers in the human brain. The optic tract contains approximately one million nerve fibers, which transmit to the brain the visual information that a human receives via the eyes. Forty times that much information can be sent from the cerebral cortex down to the cerebellum, including information from sensory areas of the cerebral cortex, from motor areas, from cognitive areas, from language areas, and even from areas involved in emotional functions.

As this torrent of information continues to pour into the cerebellum from many other parts of the brain, and as the cerebellum continues to process this information within its neural mechanism, a flow of output information is produced by it which it can send to various other regions of the brain, telling them what to do and when to do it. How can the cerebellum convey such messages? A clue is provided by its internal structure and its output connections, which bear a remarkable resemblance to the design that is employed in organizing modern computing machines.

Resemblance to Computing Machines

In computing machines the processing of information is accomplished by both the hardware in the system (its circuitry) and by the software (the messages transmitted between the various parts of its circuitry). Together the hardware and software can produce a versatile information-processing system, capable of performing a wide variety of functions, including motor, sensory, cognitive, and linguistic ones. Such versatility of function is achieved by organizing the computer hardware in the following way: The basic components are assembled into modular packages that contain similar circuitry, and large numbers of such similar modules are organized into parallel processing networks. This structural organization is exemplified also in the cerebellum: It consists of longitudinal modules containing similar neural circuits, which are arrayed in parallel zones throughout the entire extent of the structure.

On the basis of this known cerebellar “hardware”, it is possible to investigate the “software” capabilities of such an organization of modules. In investigating the part of the cerebellum that is greatly enlarged in the human brain, investigators found that each module in this part of the cerebellum (the lateral part) is able to communicate with the cerebral cortex by sending out signals over a segregated bundle of nerve fibers. This is a particularly powerful way of communicating complex information. It is exemplified also in computing machines, where the “fibers” (i.e., the wires connecting the modules) also are organized into segregated bundles. The benefits of such bundling of fibers are linguistic; such organization enables the cerebellum to communicate with the cerebral cortex at a high level of discourse, by using internal languages that are capable of conveying complex information about what to do and when to do it.

Functions of the Cerebellum

Given that the cerebellum seems well organized to convey complex information to many other regions of the brain, where does it actually send this information? Each module of the cerebellum seems to be uniquely connected, both through its input and output connections, with different regions of the brain. Modules in the middle of the cerebellum (in the medial part) receive different input and send information to different output targets than do the modules in the lateral part of the cerebellum. Despite such differences in input and output, however, the circuitry within each module seems to be similar to that in every other module. For this reason, the basic processing that every module can perform on the incoming information would seem to be similar, no matter whether this incoming information represents motor, sensory, cognitive, linguistic, or any other kind of information.

What does this basic processing do? More specifically, what computations are performed by the similar cerebellar circuits in each module, and to what use are these computations put when the results are sent to the different target structures in the other regions of the brain? Many theories about such cerebellar functions are under investigation, but definitive answers are not yet available. They await further research.

Although many of these theories are considered controversial at present, it seems possible that each of them may be at least partially correct and that the present controversies can therefore be reconciled in the future. The present proposals encompass not only the traditional view that the cerebellum is involved in skilled motor performance but also the broader view that it is involved in skilled mental performance, and is also involved in various sensory functions including sensory acquisition, discrimination, tracking and prediction. A recent theory that is broad enough to encompass all of these motor, mental, and sensory functions has proposed that the cerebellum does the following basic processing: It makes predictions (based on prior experience or learning) about the internal conditions that are needed to perform a sequence of tasks in other regions of the brain, and it sets up such internal conditions in those regions automatically, thus preparing those regions for the optimal performance of the tasks. By doing this, the powerful and versatile computing capabilities of the cerebellum would be used for providing automatic help to various other regions of the brain, helping them to do their work better.

The Advantages of Automation

Experimental evidence has shown that the cerebellum is involved in the process by which novel motor tasks can, after some practice, be performed automatically. Through such automation, the performance can be improved: Sequences of movements can be made with greater speed, greater accuracy, and less effort. The cerebellum also is known to be involved in the mental rehearsal of motor tasks, which also can improve performance and make it more skilled.

Because the cerebellum is connected to regions of the brain that perform not only motor but also mental and sensory tasks, it can automatize not only motor but also mental and sensory skills in the human brain. As with motor skills, several advantages accrue from learning to perform the other skills automatically, without conscious attention to detail.

The skills involved in human communication, for example, require both motor and mental activity: the motor activity of speech or gesture, and the mental activity that formulates what is to be said. In the course of learning these skills, an individual’s performance can be improved incrementally through practice so that the skills eventually can be performed without conscious attention to detail. For example, in recalling words stored in the memory, the activity can be performed without conscious attention to the details of how the words are selected by the brain during the retrieval process.

To the extent that an individual can perform some mental activities without conscious attention to detail, the conscious part of the brain is freed to attend to other mental activities, thus enlarging its cognitive scope. Such enlargement of human capabilities is attributable in no small part to the enlarged human cerebellum and its contribution to the automation of mental activities, which appears to have been a prerequisite for the emergence of human language. Because such language confers a unique and inestimable advantage on humans, the cerebellum can be regarded as an underestimated treasure submerged at the bottom of the brain.