Cerebrum

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The cerebrum is the larger of the two cortical subsystems in the mammalian brain, the other, smaller subsystem being the cerebellum. Most of the cerebrum is the cortex, which has two hemispheres, each containing four lobes (frontal, parietal, temporal, and occipital). As a whole, the cerebral cortex is the most recent brain structure to evolve. It normally operates as the highest-level subsystem within the brain and is associated with conscious and volitional control of behavior.

Most basically, the cerebrum is involved with the control and elaboration of reflex homeostatic responses at the brain stem and spinal cord level. This control may be directly from the cerebrum to the brain stem and spinal system, or it may be indirectly through control of cerebellar output. The cerebrum is particularly involved with motor control that is under conscious volition and with discrete and precisely refined movement. These voluntary movements will be controlled by the cerebrum and will rely on feedback to the cerebrum from sensory organs. However, after repeated execution through habit, or through deliberate practice, control of the motor output sequence may be carried out by the cerebellum and may then be performed more quickly and more automatically with less conscious effort. Conscious practice may be seen as subprogramming of the cerebellum by the cerebrum. Such practiced action sequences may thereafter be switched on by the cerebrum and then released automatically from the cerebellum without further involving the cerebrum.

Sensory input direct from the peripheral sensors first reaches the cerebral cortex in the various unimodal pri mary areas. The most basic cerebrocortical processing is therefore concerned with sensory processing limited to a single modality. Within the cerebral cortex there is directionality of information flow from primary sensory areas, through secondary association areas and tertiary amodal regions that integrate information, and eventually to output areas that control motor function. Anatomic, physiological, and behavioral studies have all emphasized that there is a clear sequential processing built into cerebrocor-tical organization. The unimodal primary reception areas of the cerebral cortex result in a process of information abstraction by sensory modality. In addition, the cerebral cortex can selectively control its own input via the thalamus, and most of this control is inhibitory (i.e., of a filtering kind). Through its ability to abstract information (facilitated partly by unimodal primary representation and partly by input filtering) the cerebral cortex is thought to provide the brain with a system for high-level discrimination and for the analytical and abstract processing that is characteristic of logical thought or formal operational reasoning.

Evolution of the cerebral cortex, with its cerebrocortical primary sensory and motor areas, may be viewed as providing the brain with a second cortical subsystem with a higher level of discrimination than the cerebellum, capable of abstracting detail and of analytical sequential processing, and with the ability for more refined and elaborated control. Until recently the prevailing view in contemporary brain research is that cognitive functions are mediated almost exclusively by the cerebral cortex. The cerebrum is thought to be involved in the whole range of cognitive processes, including nonverbal communication, recognition and expression of emotion, visuospatial skills, imagination, mathematical processing, language skills (speaking, reading, writing), problem solving, planning, analytical and logical reasoning, and aspects of memory and recall.

There is evidence that the two cerebral hemispheres represent and process information in different ways and have different roles. The dominant (most often left) cerebral hemisphere is associated with information representation and processing, which is sequential or in series, digital, and abstract. It is characterized by analytical and logical processing that deals with detail. It plays a major role in the processing of verbal information, and in particular digital or abstract linguistic representation. Of the two cerebral hemispheres the dominant one is considered to be involved with consciousness, especially self-consciousness.

The minor (most often right) cerebral hemisphere is associated with information representation and processing, which is simultaneous or in parallel, analog, and holistic. It is characterised by Gestalt, analogical, and integrative processing, which deals with more spatial and global information and with novel or unfamiliar information. It plays a major role in the processing of naturomorphic or imagis-tic representations and in particular the processing of non verbal and emotional information, spatial and pictorial information, and music and other non-language sounds.

One reason given for hemispheric specialization within the cerebral cortex is that it allows for the coexistence of two incompatible but complementary modes of information processing. To some extent this may be true; however, the two cerebral hemispheres have the same fundamental cytoar-chitecture, have unimodal primary reception areas, and receive filtered input. In fact, much of the currently available information points to differences between the cerebral hemispheres that are merely relative or quantitative rather than qualitative. Furthermore, there is evidence from various sources that indicates that the cognitive processes attributed to one or the other of the cerebral hemispheres are not exclusive to that hemisphere. Most basically, the two cerebral hemispheres are both abstract in-series processors, but they appear to have different patterns of development. As a result of this differential development the dominant hemisphere appears to represent and process information at a much higher and relatively incompatible level of abstraction than that of the minor hemisphere.

Association areas within the minor hemisphere temporal lobe, particularly the inferotemporal region, are thought to be involved in emotional processes, experiential memory, and imagination. There is some evidence that the temporal lobe may not be the location of the experiential memory record, but that this information may be transmitted to the temporal lobe from subcerebral areas. Transmission from subcortical regions appears to involve the hippocampus and the brain stem monoamine systems. As noted earlier, the cerebrum projects to the cerebellar cortex and can contribute to cerebellar processing. However, it is also evident that the cerebellum projects to the cerebral cortex, including association areas, and can contribute to cerebral processing in the cognitive domain. Reciprocal interaction between the cerebral neocortex and the cerebellar cortex involves the hippocampus and the brain stem monoamine systems. A range of evidence supports the suggestion that Norepinephrine or its precursor Dopamine faciilitates excitatory output from the cerebellum to the cerebrum whereas 5-Hydroxytryptamine appear inhibit output from the cerebellar to the cerebrum. There is evidence of later-alization of neurotransmitter activity and lateralization of interaction between the cerebellum and cerebrum. It is therefore possible that some of the subcerebral input to association cortex processing is of cerebellar origin and that some of the cognitive processes attributed to the minor hemisphere cerebral hemispheres are of cerebellar origin.

In summary, the cerebrum 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, as a whole, it is the most recent to develop and therefore relates to later stages of cognitive development. It is particularly associated with conscious control and cognition. In information representation and processing terms the cerebrum may be described as a sequential processing abstract subsystem of relatively high discrimination, and from this perspective it is complementary to the cerebellum.

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