Cerebral lateralization

The lateralization of brain function is the tendency for some neural functions or cognitive processes to be more dominant in one hemisphere than the other. The medial longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. Although the macrostructure of the two hemispheres appears to be almost identical, different composition of neuronal networks allows for specialized function that is different in each hemisphere.

Lateralization of brain structures is based on general trends expressed in healthy patients; however, there are numerous counterexamples to each generalization. Each human’s brain develops differently leading to unique lateralization in individuals. This is different from specialization as lateralization refers only to the function of one structure divided between two hemispheres. Specialization is much easier to observe as a trend since it has a stronger anthropological history. The best example of an established lateralization is that of Broca's and Wernicke's areas where both are often found exclusively on the left hemisphere. These areas frequently correspond to handedness however, meaning the localization of these areas is regularly found on the hemisphere corresponding to the dominant hand (anatomically on the opposite side). Function lateralization such as semantics, intonation, accentuation, prosody, etc. has since been called into question and largely been found to have a neuronal basis in both hemispheres. Another example is that each hemisphere in the brain tends to represent one side of the body. In the cerebellum this is the same bodyside, but in the forebrain this is predominantly the contralateral side.

To get a basic understanding of this complex issue it is easiest to first consider the left (LHS) and right (RHS) hemispheres as distinct but interacting entities. These interactions come in the form of both excitatory and inhibitory signals crossing the corpus callosum and other hemispheric bridges. As LHS and RHS each have unique interpretations of situations these signals allow for discussion and an ultimate decision to be made. This interaction is called hemispheric rivalry. This term is apt as both hemispheres are in conflict yet on the same team. It has been theorized that in humans the reliance on both hemispheres is the basis of a number of functions including consciousness. Despite this theory, however, there have been fully conscious, otherwise healthy individuals who have had half their brain removed due to Rasmussen's Syndrome, with particularly good outcomes if their other hemisphere was removed during childhood.

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