Cerebral hemisphere
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A cerebral hemisphere (hemispherium cerebrale) is defined as one of the two regions of the brain that are delineated by the body's median plane. 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. The hemispheres are linked by the corpus callosum, a very large bundle of nerve fibers, and also by other smaller commissures, including the anterior commissure, posterior commissure, and hippocampal commissure. These commissures transfer information between the two hemispheres to coordinate localized functions. The architecture, types of cells, types of neurotransmitters and receptor subtypes are all distributed among the two hemispheres in a markedly asymmetric fashion. However, it must be noted that, 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.
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[edit] Embryological development
The cerebral hemispheres are derived from the telencephalon. They arise 5 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.
[edit] Hemisphere lateralization
Popular psychology has suggested that the right brain is responsible for creativity and emotion, and the left brain is responsible for logic, analysis, and spatial reasoning. However, these are broad generalizations with little support. Both hemispheres process the same data. The difference is that the left brain processes in a linear, or sequential manner. The right brain processes data simultaneously, and because it is mute, does not connect plausible explanations immediately to each step in the process, but instead trusts the left brain to later link the reasoning behind beliefs or decisions.
The best evidence of lateralization for one specific ability is language. Both of the major areas involved in language skills, Broca's area and Wernicke's area, are in the left hemisphere. Perceptual information from the eyes, ears, and rest of the body is sent to the opposite hemisphere, and motor information sent out to the body also comes from the opposite hemisphere (see also primary sensory areas).
If these specific pieces of the brain are injured or destroyed, their functions can sometimes be recovered by neighboring brain regions - even opposite hemispheres. This depends more on the age and the damage occurred than anything else.
The magnocellular pathway of the visual system sends more information to the right hemisphere, while the parvocellular pathway sends more information to the left hemisphere. 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 (Carter, 2004).
Studying the brain has shown that simplistic pseudoscience claims about brain function tend to miss or skew vital information. It is important to stress that there is much about the brain that is not understood by scientists, but it is clear at this point that processes like creativity, emotion, spatial reasoning, and logical reasoning involve regions spread across the brain.
Researchers neuropsychologists like Roger Sperry have studied split-brain patients to better understand lateralization. Sperry has also used 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.
[edit] References
- The Emerging Mind; Vilayanur S. Ramachandran; Reith Lectures 2003.
- R. Carter, Mapping the Mind, Phoenix, London, 2004, Originally Weidenfeld and Nicolson, 1998.
- More information on these lobes can be found in a book by Dr. Eric Braverman "The Edge Effect."
