|Brain: Cerebral hemisphere|
|The human brain as viewed from above, showing the cerebral hemis. The anterior aspect (front) of the brain is to the right.|
|Lateral surface of human cerebral hemispheres|
A cerebral hemisphere is one of the two regions of the vertebrate brain that are delineated by the median plane, (medial longitudinal fissure). The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is supported by an inner layer of white matter. In eutherian, i.e., placental, mammals (but not in non-eutherian mammals such as marsupials, nor in other vertebrates), the hemispheres are linked by the corpus callosum, a very large bundle of nerve fibers. Smaller commissures also join the hemispheres, both in eutherian and in other vertebrates, including the anterior commissure (which is present in marsupials), posterior commissure, and hippocampal commissure. These commissures transfer information between the two hemispheres to coordinate localized functions.
Macroscopically the hemispheres are roughly mirror images of each other, with only subtle differences (e.g. Yakovlevian torque). On a microscopic level, the architecture, types of cells, types of neurotransmitters and receptor subtypes are markedly asymmetrical between the two hemispheres. However, while some of these hemispheric distribution differences are consistent across human beings, or even across some species, many observable distribution differences vary from individual to individual within a given species.
The cerebral hemispheres are derived from the telencephalon. They arise five weeks after conception as bilateral invaginations of the walls. The hemispheres grow round in a C-shape and then back again, pulling all structures internal to the hemispheres (such as the ventricles) with them. The interventricular foramen (sometimes called the interventricular foramena of munro) allows communication with the lateral ventricle. The choroid plexus is formed from ependymal cells and vascular mesenchyme.
Broad generalizations are often made in popular psychology about certain functions (e.g. logic, creativity) being lateralized, that is, located in the right or left side of the brain. These claims are often inaccurate, as most brain functions are actually distributed across both hemispheres. Most scientific evidence for asymmetry relates to low-level perceptual functions rather than the higher-level functions popularly discussed (e.g. subconscious processing of grammar, not "logical thinking" in general).
Perceptual information is processed in both hemispheres, but is laterally partitioned: information from each side of the body is sent to the opposite hemisphere (visual information is partitioned somewhat differently, but still lateralized). Similarly, motor control signals sent out to the body also come from the hemisphere on the opposite side. Thus, Hand preference (which hand someone prefers to use) is also related to hemisphere lateralization.
Neuropsychologists (e.g. Roger Sperry, Michael Gazzaniga) have studied split-brain patients to better understand lateralization. Sperry pioneered the use of lateralized tachistoscopes to present visual information to one hemisphere or the other. Scientists have also studied people born without a corpus callosum to determine specialization of brain hemispheres.
In some aspects, the hemispheres are asymmetrical, one side is slightly bigger. There are higher levels of the neurotransmitter norepinephrine on the right and higher levels of dopamine on the left. There is more white matter (longer axons) on right and more grey matter (cell bodies) on the left.
Linear reasoning functions of language such as grammar and word production are often lateralized to the left hemisphere of the brain. In contrast, holistic reasoning functions of language such as intonation and emphasis are often lateralized to the right hemisphere of the brain. Other integrative functions such as intuitive or heuristic arithmetic, binaural sound localization, emotions, etc. seem to be more bilaterally controlled.
|Left hemisphere functions||Right hemisphere functions|
|numerical computation (exact calculation, numerical comparison, estimation)
left hemisphere only: direct fact retrieval
|numerical computation (approximate calculation, numerical comparison, estimation)|
|language: grammar/vocabulary, literal||language: intonation/accentuation, prosody, pragmatic, contextual|
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