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Hemispheric asymmetry of the cerebral cortex

As I was reading the text I came upon a section that I thought to be quite fascinating. It talked about people who have a brain that functions like two different people inside of the brain. This is of course the Split-Brain Personality. As I studied this topic in more detail I found it to be quite broad and yet very detailed. I found that I needed a dictionary to be able to read all of the medical journals and books that are out there, to be able to understand what it was exactly, that I was reading. But with a little study and research I found that this is a precise science that is still largely full of mysteries.

The study of hemispheric asymmetry with in the cerebral cortex had long been a fascination with the human race. The ancient Aztec cultures used to perform a type of brain surgery on humans. This is evident from the human remains that we find with incisions and piece's missing of the skull. Whether or not these primitive surgeries were successful is unknown. The earliest way for man to observe the brain was by noticing brain damage to a particular area of the brain that was damaged. Such observations were first recorded some 5,000 years ago (Myers,1995). The most popular case is that of Phineas Gage a railroad worker that had severe frontal lobe damage. This happened when a rail road spike was shot through his head by a piece of dynamite. Miraculously he lived through the experience, but with a severe change in his personality. From this physiologists learned that personality was largely controlled from this point namely by removing a persons inhibitions.

For the most part the brain has been a mystery that is waiting to be opened. The last two decades have witnessed a period of research on the human cerebral functions comparable to the great era of discovery initiated by Broca in 1861(Young, G,. Segalowitz, S,. Corter, C,. Trehub, S,.1983). We have leaned more in the past 20 years about the brain and it's hemispheric asymmetry than we had learned in combined previous history.(Kosslyn, 1993). Most of this new work has been devoted to the study of cerebral functions in adults, but recently there has been a growing interest in infants and young children most especially among the study of hand preference. About 10 percent of the human population in left-handed(Myers,1995). By looking at ancient writings this right-hand preference has seemed to develop right from the start of the human race. It also is apparent that from ultrasound devises that about 9 in 10 fetuses suck the right hand's thumb(Myers,1995). This would lead us to believe that handedness was an inherited trait. Their was a man by the name of George Michel, who in 1981 did a survey of new born babies and what side of their bodies they liked to lay their heads. He found that about two-thirds of 150 babies preferred to have their heads turned to the right while about only one-third laid their heads to the left. In a follow up survey Michel found that almost all of the right-sided babies were starting to reach with their right hand and again one-third of the left sided babies were reaching with their left hand(Myers,1995). In contrary, it is also found that handedness is one of the few genes that are not shared by genetically identical twins. So what is it exactly that develops handedness? Some speculate that the handedness of a person is evident in the brain and in its specialization concerning hemispheric asymmetry. Tests reveal to us that about ninety-five percent of right-handers process speech primarily in the left hemisphere(Myers,1995). While the study has found that left handed people are more likely to be a little more diverse or ambidextrous in their hemispheric asymmetry. But as we had learned in the first chapter is this a correlation or a causation? I personally feel that it is a correlation and not a causation. The brain is a very flexible and delicate instrument. It has the ability to adapt and change with different stimuli. The brain in left handed people I feel is just adapting to the use of a left hand preference and that is why it is more likely to be ambidextrous

I would now like to talk about the asymmetry of the hemispheres. First, I will talk about the left side of the brain and then I will talk about the right. For well over a hundred years neuropsychologists have proposed that the left hemisphere plays a special role in both the production and perception of language(Hellige,1993). It has often been said that the left hemisphere is dominant for linguistic or verbal processing. This does not mean that the right does not have linguistic or verbal skills but merely suggests that the left is more capable and therefore more likely to process the language. This conclusion was reached after observation of people with language disorders that occurred after a left hemisphere was damaged. It is now a well documented fact that aphasia (the acquired loss of language) is far more likely after left-hemisphere than after right-hemisphere injury and that specific symptoms depends on which regions of the left hemisphere are injured(Hellige,1993). Studies of patients with unilateral brain injury have led to estimates that the left hemisphere is dominant for speech in approximately ninety-five percent of right-handed adults, with the right hemisphere being dominant for speech in the other 5 percent of right-handed adults(Hellige,1993). Such results demonstrate that the integrity of certain areas within the left hemisphere is necessary for the production of speech and certain other language related activities. Inside of the left hemisphere is a spot called the Broca's area(Myers,1995). This area is named after a French physician named Paul Broca. He reported in 1865 that damage to this area left a person unable to form words, but were still able to sing songs and still could comprehend speech. One would think that these two things are the same, but according to Broca's observation this is not so. Consequently this particular area was named after him. Latter another discovery was made by a man named Carl Wernicke. He discovered that if damage occurred to a specific area in the left temporal lobe this left people able to form words, but unable to make any sense of the words that they are saying(Myers,1995). An example of this is when a patient, with this particular part of the brain damaged, was asked to describe a pitcher of two boys stealing some cookies behind a woman's back, he would say, "mother is away her working her work to get her better, but when she's looking the two boys looking the other part. She's working another time"(Myers,1995) This area was later named after this man and is now known as Wernicke's area. Although damage to the left hemisphere is more likely to cause language disturbance than is damage to the right hemisphere, if left brain damage occurs in childhood recovery may be dramatic and virtually complete. The recovery in these cases is thought to be the result of rapid assumption of language processing in the right hemisphere(Young etal.,1983) When aphasia is associated with a stroke in adulthood, recovery is often a slow and incomplete process. Aphasiologists question whether this form of recovery is the result of gradual left to right switching language dominance, or rather the reorganization of the left hemisphere. Evidence is in favor of the latter. A man by the name of Kinsbourne, who in 1971 did a study on aphasic patients and language compensations. He staged serial unilateral intracarotid amobarbital injections on two right-handed aphasic patients. A third patient had a left side injection only. Left-side injections did not result in speech arrest, but arrest of all vocalization occurred with the right-side injections(Perecman,1983). For Kinsbourne, these results indicate that in these cases dominance for residual language had shifted to the right.

I will now talk about different aspects of the right hemisphere. The right hemisphere has a little less organized principles and the processing elements are not as defined but nonetheless a valuable resource which will often go untapped or underutilized by the average person. In general, the right hemisphere controls the emotions of a person(Perecman,83pg69). In fact there is a theory now that negative emotions are created by the right hemisphere and the positive ones are done by the left. Neuropsychologists have found that motion picture sequences viewed with the left visual field are judged more negative than those viewed with the right field. Questions concerned with negative, rather that positive, produce greater leftward eye movements. Facial motor asymmetries are more likely to be biased to the left side for negative expressions, such as anger, sorrow, or disgust. Where the right side is more likely to favor the positive expressions (Perecman,1983). But in the contrary, studies have found that damage to the right parietal region impairs the identification and production of both positive and negative emotions, even if they are both conveyed verbally. Similar studies have found that there is no difference in the asymmetry of facial expressions when conveying emotions of negative or of positive nature. (Perecman,83pg70). Although some controversy exists as to the relative contribution of each hemisphere to the perception of emotion. The majority of experimental studies with normal subjects have found a right hemispheric superiority for processing a diversity of emotional stimuli including music, and facial expressions. Tonal sequences, invoking both positive and negative moods, are rated more quickly and accurately as well as judged more emotional when listened to on the left ear in contrast to that of the right. The left visual field can also detect emotions of a particular face more quickly as well as more accurately than that of the right visual field.

I, myself am more of a right brain person. The tests that we have taken in class and all previous tests that I have taken tell me this. I think that this is why I make decisions more based on my emotions rather than on logic. This correlates with traits of most right-brained people. This paper has taught me a lot in the field of the brain. Such different aspects of the brain is what make each person distinctly different and human.

Source: Essay UK -

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