"here is one mind common to all individual men [and women]."
Poet and essayist Ralph Waldo Emerson (1803-82)
"The lower levels of the brain are also depended on higher level activity in the cerebrum."
The cerebrum is a great mass of neuronal tissue making up a large portion of the human brain. It branches off from the brain stem just in front of the midbrain structures and bulges outwards into two large lobes called the cerebral hemispheres. There is also a mass of biological wires called the corpus callosum which seem to bypass the brain stem completely and make a direct link between the two lobes. These wires are deeply protected beneath and between the lobes and are positioned slightly forward from centre and just behind the frontal lobes.
The two lobes, or cerebral hemispheres, are so large and highly developed in humans that they literally wrap themselves around the upper parts of the brain stem and much of the midbrain sections as if to protect these apparently more sensitive and vital regions.
The surface of the cerebral hemispheres may appear smooth or heavily convoluted like valleys and hills, depending on the amount of brain usage. Upon closer examination, three crevasse-like subdivisions or fissures, called the longitudinal fissure, the lateral fissure and the central sulcus, can be seen. Inside the longitudinal fissure that divides the cerebrum into the two cerebral hemispheres is a mass of some three hundred million separate 'conducting wires' or nerve cell fibres that connect the two cerebral hemispheres together. This bundle of nerve fibres, known as the corpus callosum, shuttles information back and forth between the two hemispheres.
After many years of extensive research and experiments on the brain, there is general agreement by the scientists that the primary objective of the cerebrum is to store on a subconscious and more permanent level (compared to the hippocampus) the longer term patterns derived from information and behaviours through knowledge and experience gathered and learnt over the lifetime of the living organism (the closest analogy would be the massive storage capabilities of the internal hard disk of a computer) and to use this more "unchangeable" (unless deep meditation or hypnosis is applied) patterns as an important source for our cognitive, ideational and imaginative functions as well as the development of our unique and highly refined personality or personalised behaviour.
Although the cerebrum is capable of processing various types of information (including abstract symbolic information of language, pictures and sounds within the frontal cortex), its main purpose is to store and retrieve the longer-term and more relevant and important patterns or symbols needed for lifetime application representing the concepts we learn from language, science and the arts (stored in a visual sense), as well as the essential patterns for understanding auditory information, and the high level of refinement needed to control muscles leading to specific behaviours, acquired over a lifetime of experience and knowledge, and to connect all this information, by bundles of white-coloured nerve fibres called association areas, with itself and with various parts of the brain so as to help create, modify and refine our unique behaviours and interpretation of concepts that make up our unique personality and ideas, including our thoughts and feelings, as well as to handle the great diversity of life experiences.
The storage area for all these patterns is located on the outer surface of the cerebrum and has a thickness of approximately one-eighth of an inch; it is called the cerebral cortex and is where our subconscious mind is generally said to reside (1).
Just below this dense, greyish-looking and highly dendrite-rich storage area lies the white-coloured association areas linking all our memories together and with the rest of human behaviour.
The removal of one or the other cerebral hemisphere has little consequence on the individualised behaviours, or personality traits, of a normal fully grown adult (children, however, are a different matter all together) other than requiring the person to work harder on certain problem-solving activities. But evidence has shown that eventually the remaining hemisphere will take up much of the lost functions after a period of a few years. The main reason why the loss of one hemisphere does not affect personality is because the cerebrum is structured like some networks in society such as the Internet, with information spread out to different parts of the brain in an attempt to preserve and, if necessary, rebuild from this information, and so the chances of losing well-established personality traits following removal of one hemisphere or the other is therefore minimised.
However, the removal of both hemispheres of the brain will lead to a total loss in all individualised behaviours acquired over the lifetime of the individual. What behaviours remain are purely the basic and instinctive types needed for survival (such as activating, maintaining and regulating heart beat and so on). The result of such a major loss or damage to the cerebrum is a person described as a zombie or someone living in the vegetative state (2). An example of this can be seen among patients with Alzheimer's Disease, Parkinson's Disease etc where the cerebral hemispheres have undergone a certain level of degeneration.
Likewise people who have accidents resulting in damage to the cerebral hemispheres tend to experience this state.
It must be understood that no two human brains are exactly the same. Superficially, there are many similarities such as the two cerebral hemispheres and the various fissures that appear to divide the brain, but deep down, there are many elementary differences. The differences appear macroscopically as slight enlargements over certain parts of the brain and in its degree of convolution. Deeper down the differences are even more pronounced, with nerve cells creating and disassembling countless unique and dynamic patterns necessary for remembering or forgetting (for the purposes of acquiring more relevant patterns) various types of information.