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Thursday, September 23, 2010

9.7 Cerebral Cortex 3: Specialization of Function

Physician's Notebooks 9 - See Homepage - http://physiciansnotebook.blogspot.com


7. The Lobes of the Brain: Specialization of Function - Update 14 April 2018. Note the below descending column of headings in order as they appear in this chapter. Use search & find or scroll down to them as desired.
 Each cerebral hemisphere almost a Mirror of the other
Cerebral Sidedness and Handedness
Brain strokes
The brain's language areas 
Left-handers have lower rate of language-damaging stroke
Left and Right Brain Connection
Left and Right Brain Persons
Aphasia is the word for loss of language ability
Other Cerebral Function - Localized or Distributive?
Keep in mind the next chapter is about the Pre Frontal Cerebral Cortex.
Each cerebral hemisphere almost a Mirror of the other: Each hemisphere (right & left) has a frontal lobe, a parietal lobe, an occipital lobe from front to back, and a temporal lobe (like a rabbit ear) on the side; and the 2 hemispheres are separated by a mid-line deep fissure crossed in its depths by a long band of nerve fibers called corpus callosum that keep each hemisphere in communication with the other. Both hemispheres, on inner aspect,have a medial cortex which outlines the ill-defined limbic lobe that controls emotion in lower and rear part the famous hippocampus area for long term memories. So at a glance it appears that each hemisphere is a mirror image of the other. But neurologists discovered that each hemisphere has important differences from its other-sided one. This may be brought out after a brain stroke because, in most patients, a damaged left hemisphere causes loss of speech while a right-side brain stroke does not much affect speech.
Cerebral Sidedness and Handedness: One side of your brain makes a preference for and a greater skill in the use of the hand, foot and eye on the opposite side of the body. And most language processing goes on in this, the dominant hemisphere. In about 98% of right-handed persons, the left cerebral hemisphere is dominant. Most persons show right-handedness from about age 2 years. (A child that shows one-sided handedness at 1 year or earlier should be suspected of having same-sided motor brain damage from birth)  
Brain strokes that involve one side of the Brain often cause paralysis or weakness on the opposite side of the body; called hemiplegia). The speech centers of the brain in right-handed persons are in the left brain. The extent to which a left brain stroke causes loss of language depends on the degree to which the stroked-out person shows right-handedness (Some right-handers are more dominant on the right side than others) and by the number of left-handed persons in the stroked person's family. About 60 to 65% of left-handed individuals are left brain dominant; about 15 to 20% are right dominant; and the remainder appear to use both sides for language processing.
The brain's language areas  are alongside the large lateral (Sylvian) fissure and, in 95% or more of people, on the left side; the area (Broca's) that controls vocal expression and grammar is in the lateral frontal lobe, and the area (Wernicke's) that allows comprehension of the spoken, written  or thought-out language is to the rear of it in the parieto-temporal lobes interface. Brain strokes affecting Broca's area cause an inability or difficulty to speak clearly, and even when some speech is preserved it is ungrammatical, with unusual pronunciation and accent. But the Broca's area stroke person can hear and understand what he hears, and think out the words clearly and grammatically and may read and write OK. In the case of a Wernicke's area stroke, the ability to vocalize words is not as much affected as in Broca's but the person speaks somewhat unintelligibly. He cannot well appreciate what he is saying because he does not understanding well what he hears himself saying or thinking. Such patients are said to confabulate, they talk a lot using correct words but much of it is nonsense. And they have trouble with reading. Many strokes are of varying degrees of speech disability and within weeks of the acute phase the speech improves or can be improved by speech therapists.
  
Left-handers have lower rate of language-damaging stroke and less serious language effects from strokes than right-handers. The reason for this seems to be that most left-handers are either born with both sides of brain equally balanced or else have trained the originally non-controlling side of the brain to take over the control.
Left and Right Brain Connection: Relating to separate functions of each side of brain, the hemispheres are connected across the middle by a wide (in the brain's front to back aspect) band of neuron fibers called corpus callosum. When it is cut by surgery, tests show that the right side of brain loses comprehension of its left side (and vice-versa) so a normally known object by feel, like a key, placed in left hand cannot be named unless the person looks at it, because Broca’s area that controls speech is almost always in left frontal lobe and, with the corpus callosum cut, it can’t receive signal of the meaning and sound of the word from what the key in left hand feels like. It is an example of consciousness for a single item in mind being lost while general consciousness is retained.   
Left and Right Brain Persons:  There has been much written popularly about right brain vs left brain individuals. What is proven is that, in most persons, the left side of the brain affects skilled motor activities and speech while the opposite side shows superiority in visual-spatial skills and also is more controlling in one’s emotional life. But the idea some persons are right brain types and better artists and more emotional is unproven and exaggerated.
Aphasia is the word for loss of language ability: The Broca area in the left frontal lobe gives a nearly pure motor aphasia, meaning that the patient understands spoken and internal thought language and knows what she wants to say but just cannot say it and not because of muscle paralysis but because of a loss of the learned ability to make speech with the voice muscles. Typical Broca aphasia might occur in a young right-handed woman who had a blood clot on her mitral heart valve that one morning as she got out of bed and coughed, or strained at stool, broke off a piece (embolus) and it passed from left heart chamber up into ascending aorta, which turns left at its arch, and followed its blood current into left carotid artery in neck and thence went up into the left middle cerebral artery and got stuck in branch that feeds Broca area of frontal lobe cortex. In such a patient the symptom starts at once. Suddenly the young woman finds she is unable to say words that she wants to say. But she would understand what is spoken to her and retain the internal speech of thought. The aphasia is “motor” because the left Broca frontal lobe cortex neurons control the speech muscle facility. Broca’s aphasia can start off with total loss or be partial. Gradually these patients can be taught to speak again but typically their recovered speech, which is probably coming from newly trained other side cerebral cortex, is lacking in the articles “the” and “a” and many appropriate verb forms. Broca’s aphasia often has with it right-sided lower facial and upper arm weakness because of involvement of nearby cerebral cortex. It should be noted here that mild speech defect called dysarthria is frequent after many strokes due to loss of muscle power in larynx but it differs from aphasia, which is a loss of learned language skill and not just from weakness of muscles.
   The contrasting Wernicke’s aphasia occurs in a stroke from blockage of artery that feeds the more posterior left lateral parieto-temporal lobe area. It might occur in a 75 y/o with brain tumor compressing left Wernicke area. He might be speaking with his son when the effect begins and what his son would observe is his dad’s speech becoming unintelligible although single words retain meaning and enunciation. But he - the dad -  may not realize it. In Wernicke’s aphasia comprehension of one's spoken language is compromised while ability to voice the words is retained. The patients with Wernicke’s aphasia speak language that may make sense but it is disconnected from input so if you actually investigate what a Wernicke aphasic seems to be saying it turns out partly nonsense. Also slight changes in sounds occur (eg, “The grass is greel”)
Other Cerebral Function - Localized or Distributive?   I have emphasized speech function in this chapter, but, you should not forget that several other important functions like vision are localized in other parts of the cerebral cortex. Also, although I emphasize localization and labeled lines (cf. previous chapter), keep aware that the CNS also functions in a distributive way. This is especially seen with vision where several parts of the cerebral cortex cooperate in assembling images and interpreting them and one cannot point to a single area as visual center.
Example: vision starts bottom-up from points of light on the visual image in the space before the eyes. At this early stage vision is transmitted in labeled lines, ie, a retinal photoreceptor cell - a rod or cone - transmits its light signal in a single fiber of the optic nerve from the eye globe and then the optic tract through the brainstem up to the occipital lobe in rear of the cerebral cortex and each fiber is separate from its neighboring fiber till the cortex V1 granular neuron layers. But from the occipital cortex then each labeled line splits and also the lines combine (diverge and converge) depending on the information needed to be accessed from the combined points on the visual image. Occipital neurons that combine to identify the image, eg, a familiar face, take a top-down course to neurons in the inferior temporal lobe to identify faces and other objects.  Other occipital neurons take another top-down course to the superior parietal cortex neurons to give the information that allows the brain to make a spatial relationship between the visual image and another point in space. These are 2 examples of the splitting of the original labeled line visual image info that takes the very simple elements of the image (color, movement direction, simple form) and combines them for the various purposes (identification, spatial relating). This example now introduces you the reader to the method the brain uses to combine initial data, distribute its many parts and derive complex information from it.






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