Cerebral hemisphere
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The cerebral hemisphere forms one half of a brain. Humans (and many other types of animals) have a brain divided into two hemispheres. Each hemisphere has corresponding structures, and structural locations; however, they are not mirror images. Instead, the hemispheres usually are asymmetrical giving specialized function to each hemisphere. Each hemisphere also has an outer layer of grey matter called the cerebral cortex.
Neurologists normally subdivide the cerebral cortex into the following four lobes:
- the frontal lobe - in front of the parietal lobe
- the parietal lobe - above the occipital lobe, behind the frontal lobe
- the occipital lobe - in the rearmost portion of the skull
- the temporal lobe - at the side of the skull
Neurologists also recognize two additional areas of the cerebral cortex:
- the Limbic Cortex - including the cingulate cortex, located above the corpus callosum.
- the Insular Cortex - buried within the lateral sulcus.
In most people, the left hemisphere of the human brain dominates, and specialises (in very broad terms) in speech, writing, detail perception and language. It is quite unclear that the right hemisphere has equivalent broad associations with spatial abilities, coherent form recognition, emotional recognition, holistic perception and visual face recognition. 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.
Pop psychology and popular myths about the brain simplify these distinctions into a very crude binary system whereby a person appears pre-dominantly "left-brained": logical and less "right-brained": creative or vice versa, in order to sell products and prescribe methods for selecting people for specific tasks. For example, the claim that the right hemisphere is creative, whereas the left is not, is, a cornerstone of hemispheric difference mythology, especially as it is applied to management and training. Ned Herrmann, who offers popular and expensive applied creativity work-shops (a half day seminar costs over $2000), bases his program on the assumption that the left brain is logical and the right brain is intuitive, insightful and creative. According to this view, creativity can be enhanced through special training of the right hemisphere. The same line of reasoning has been used to sell courses on mind mapping, speed reading, and psychic development, in addition to expensive machinery for altering brain wave frequency.
Fortunately the neurological sciences can be very helpful in being able to immunize against simplistic pseudoscience claims about brain function, and debunk dubious products and services that are sold using such myths. They have found some intriguing differences: 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.
The hemispheres operate together, 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 commisure. These commissures transfer information very effectively between the two hemispheres thus minimizing the effects of the dominate hemisphere.
Testing the functions of brain lateralization, neuropsychologists have studied split-brain patients. They have also studied people born without a corpus callosum to determine specialization of brain hemispheres.
Reference
- The Emerging Mind (http://www.bbc.co.uk/radio4/reith2003/lectures.shtml); Vilayanur S. Ramachandran; Reith Lectures 2003.
More information on these lobes can be found in a book by Dr. Eric Braverman "The Edge Effect."