The cerebellum is one of the two largest structures in the mammalian brain, and, like the cerebum, it has two hemispheres. The cerebellum receives a full range of sensory information directly from the peripheral sensors (including auditory and visual). It also receives motor and sensory information from the cerebral cortex. It is involved with the learning-based control and elaboration of reflex homeosta-tic responses at the brain stem and spinal cord level and may operate in parallel with the cerebral cortex in controlling motor performance. However, the cerebellar system is thought to be particularly important for the control of frequently executed or rehearsed movements that become automatized and are performed without conscious effort.

The cerebellum is considered to be able to represent and process both sensory and motor information and to do so in a unified manner, that is, as a sensorimotor system. It is capable of rapid information processing and real-time regulation of coordinated sensorimotor activity. On the basis of its microstructural and functional characteristics it is con sidered that the cerebellum is essentially a parallel distributed or connectionist type of system that represents and processes information in a probabilistic manner. As a consequence of its parallel distributed amodal representation and processing, the cerebellum is considered to be an analog or holistic type of system and to facilitate associative storage. However, another consequence is that it lacks potential for highly specialized analysis of sensory-specific information: That is, it is a relatively low-discrimination system.

Based on parallel distributed representation and processing models of cerebellar function, a number of other processing or computational features have been inferred. The cerebellum may detect and respond to patterns or features, which may be determined partly on the basis of the relative frequency with which they occur. Parallel processing facilitates the automatic statistical revelation and statistical evaluation of features that are to varying extents common to a number of information inputs or experiences. In this respect the cerebellum may be considered to separate "figure" from "ground" and function like a Gestalt system. When a significant part of a frequently occurring feature or pattern is received as an input, the cerebellar system will recognize that part as though it is the whole, thus in effect completing or generating the expected pattern. This phenomenon may be involved in nonverbal analogical reasoning.

In a similar manner, when part of a familiar sequence is received as an input, the cerebellar system will respond so as to complete the expected temporal pattern. This is a kind of predictive mechanism and facilitates anticipatory responses based on the learned response sequences (sensorimotor schemata). These processing characteristics could facilitate associative learning in the form of classical conditioning or operant conditioning and would allow for sensory stimuli to act as retrieval cues for stored information concerning the response that past experience suggests is the most probably beneficial. The cued or released sensorimo-tor information would automatically, through its motor component, involve a behavioral response. Such responses generated by the cerebellum would appear to be released in an all-or-none fashion—that is, as wholes. It is thought that the cerebellum does not allow for the erasure of recorded information. The only way that information can in effect be forgotten is by functionally overlaying the information to be forgotten with the information to be remembered in preference (similar to the behaviorist notion of extinction).

Over the years there has been a steady accumulation of evidence that the cerebellum is critically involved in the classical conditioning of simple motor responses. Thompson et al. (1997) have shown that the essential memory traces for a range of classical conditioning responses are formed in, and reside in, the cerebellum. It is now generally assumed that the cerebellum plays a major role in sensorimotor learning. This form of learning is evident at the earliest stage in human cognitive development and is typified by the developmental concepts of enactive representation

(Bruner) and sensorimotor intelligence (Piaget). Given this, one might expect that the cerebellum would be involved in cognitive processing at a fundamental level.

A range of research (including Tomographic regional cerebral blood flow [rCBF], regional cerebral metabolic rate [rCMR], and position emission tomography [PET] studies) indicates that in humans the cerebellum contributes to various cognitive process. It appears to be involved with verbal processing, including speech, writing, and reading, not only in the direct motor expression of these skills, but also in the mental simulation and rehearsal of these skills, and in related nonmotor aspects such as word association. It also appears to be involved with visual processing, including imagined motor expression (tennis movements, writing, speech), visual discrimination, mental rotation of simple drawings, and visuospatial organization tasks. Studies have indicated that cerebellum contributes to IQ, mainly visual IQ (performance scale IQ test ability on the Wechsler Adult Intelligence Scale, in particular, picture completion, picture arrangement, and object assembly). Other higherorder cognitive processes in which the cerebellum has been implicated include the skilled manipulation of symbols, conceptual reasoning, and complex planning activities. In summary, several publications have suggested that the cerebellum is involved in a range of cognitive processes, including what has been described as "pure mental activity" and "pure cognitive activity." Data suggest that for verbal processing, including writing and spoken language (verbal working memory and non-motor processing of words such as semantic association tasks), it is the right cerebellar hemisphere that is most involved. The lateralization to the right cerebellar hemisphere is consistent with observations that each cerebellar hemisphere is anatomically and functionally related to the contralateral (in this case the left, language-dominant) cerebral hemisphere. In contrast, complex cognitive spatial operations and visual reasoning are associated with processing in the left cerebellar hemisphere. The cerebellum and cerebrum may be considered complementary and facilitate optimal cognitive performance by the brain as a whole.

In the execution of motor processes, it is considered that the cerebellum does not operate at the level of normal consciousness but, relative to the cerebrum, operates at an unconscious level. Similarly, it is considered that its sensory processing operates not at a conscious level but rather as an unconscious mind's eye. Cerebellar contributions to cognitive processing skills could therefore constitute part of what has been called the "cognitive unconscious."

The cerebellum is one of two cortical subsystems capable of cognitive processing of information from the environment and of controlling the behavior of the organism; in evolutionary terms it is, in part, the earliest to develop and therefore, in part, relates to later stages of cognitive development; it is particularly associated with unconscious control and cognition. In information representation and processing terms it may be described as a parallel processing sensorimotor subsystem of relatively low level of discrimination, and from this perspective it is complementary to the cerebrum.

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