ScienceMasterIf it's your job to develop the mind,
shouldn't you know how the brain works?
The Brain's Hemispheres
- Written by Kenneth Wesson Kenneth Wesson
- Published: 09 October 2003 09 October 2003
Each of the brain's two hemispheres is biologically partitioned into four unequal regions or lobes, each associated with different duties to perform (referred to as the "localization of function"), but all lobes are interdependent.
The frontal lobes occupy approximately 29 percent of the cerebral cortex, is home to such abilities as abstract thinking, planning, decision-making, and creativity. It also houses the Primary Motor Area, which is responsible for muscle control, skilled movements, and complex motor activities. This portion of the brain is most closely associated with the emotional control and our responses to incoming sensory information.
Broca's Area, controlling the indispensable muscle ingredients necessary for speech production, is also located in the frontal lobe. Because, the frontal lobes govern the most complex functions of the mind, they correspondingly take the greatest amount of time to completely develop and they are the last to fully myelinate (more about myelination later).
The sensory areas of the brain that are responsible for reporting pain, touch, heat, proprioception, etc., are located in the parietal lobes. One of the subcortical structures, the thalamus, eventually relays all sensory information to other locations in the cerebral cortex. The parietal lobes (including the Gustatory Areas, which process taste sensations) resemble an arched formation, much like a headband, passing over the center of the cortex.
The human senses processed in the parietal lobes set the boundaries for our experiences. There are neurons (the fundamental communication system) in the human brain that are capable of processing sounds within a particular range. Sounds outside of those limits are also beyond the range in which our sensory systems operate. We can extend those borders, however, with sensory-extending apparatuses like telephones, microscopes, telescopes, eyeglasses, hearing aids, and similar equipment.
In this region of the cerebral cortex, there is a personalized detailed physical map (referred to as the "homunculus") one's entire body. More neurons represent particular parts of the body based initially on their importance to survival, and later modified based on their frequency of use. Each individual's homunculus gets tailor-made based on experience, training and learning.
Tiger Woods' homunculus would show greater representations in the sensorimotor areas that have been well used to develop his incredible proficiency in golf. A professional violinist will show greater amounts of cortical real estate dedicated to finger movement in the left and right primary sensorimotor areas than one might find in a non-musician..
Whether we are looking at words and photographs in a textbook, watching rapidly-moving cars at the Indy 500, or glancing at objects while riding home on a school bus, the occipital lobes, or the Primary Visual Cortex, receive and interpret nearly every aspect of visual information that enters the eyes. The incoming visual information is sent directly to the brain where it is analyzed by the neuromechanisms that process vision. The data is then forwarded to the association areas where an evaluation of the "pieces" of that visual experience is compared with previously stored information to see if it is "recognized." The next step is to decide what action should now be taken based on this elaborate identification process, so the information is forwarded to the decision-making frontal lobe..
Damage anywhere in the Visual Cortex can result in a wide range of quite interesting, although devastating, problems with vision. Stroke victims with occipital lobe impairment often lose their ability to see colors along with the ability to even imagine and dream colors.
The temporal lobes are the place where the Primary Auditory Cortex (PAC) resides. The PAC is the place where all pitch, rhythm and sound features are processed in the brain based on the air vibrations detected by the ears including music, voices, and noises within the human range of hearing. (Twenty-five percent of the auditory cortex is committed to musical processing in the brains of experienced musicians).
The complex nerve fiber networks in the brain for hearing and vision are among the first sensory systems to go "on-line" early in life (along with touch) and they are the first to establish their vital connections with other part of the body and the brain. Wernicke's Area, found in the Auditory Association Cortex, is a localized area of the brain that is necessary for understanding speech. In the early months of infancy, a child begins to dissect and understand the sounds that will become his native language using Wernicke's Area. At about 20 months of age, the neural systems connecting Broca's Area and Wernicke's Area begin to form. It is at this time, that a toddler will begin to produce his first rudimentary sentences. A parent's excitement is owed to the early development and work of these two cerebral areas. Damage to Broca's Area or Wernicke's Area will cause either Broca's aphasia, a speech disorder characterized by one's inability to produce coherent sentences, or Wernicke's aphasia, where speech comprehension is lost.
While the locations and general functions inside the brain have been predetermined by genetic programming regardless of the brain region under consideration, one’s experiences not only can further sculpt the developing brain, but they also determine how much each of these lobes or cortical areas will develop and what degree of development will occur. Thus, identifying the consequences of a mental, behavioral, or physical problem is somewhat less challenging when one is aware of the focal point of any existing brain trauma. Conversely, damage to any of the lobes will have a negative effect on the corresponding behaviors and functions of that cortical area.
There is still a considerable degree of controversy surrounding precisely how the brain should be parceled or if the divisions are to some extent subjective, since many of the formations inside the brain, especially the sub-cortical structures are connected in ways that see them fused together almost seamlessly with one another. There are two other parts of the brain that require mentioning in a discussion on human learning and development. They are the limbic system and the corpus callosum.
The limbic (meaning "ring") area is located in the forebrain and is virtually identical in all mammals. It sits just above the brain stem, with the two structures somewhat resembling a bagel with a finger (the brain stem) passing through it. Also known as the "limbic system," this area of the brain is not composed of a single large brain structure. Instead, the “limbic system” is comprised of a large group of complex and oddly shaped smaller structures surrounding the upper portion of the brain stem. Each structure has an immense number of critically important circuits linking them to one another and to the cerebral cortex. These interconnected structures are intimately associated with our basic drives, bodily temperature control, hormone production, and emotions.
The offspring of animals blessed with a limbic system enjoy a number of vital benefits as a result of this unique cluster of sub-cortical formations, some of which are life-saving advantages. Mammals with limbic systems typically engage in a long-term investment with their young by remaining with them until their litter is mature enough to survive on their own. These caring parents will also nurse and protect their young even in life-threatening situations. On the other hand, reptilian mothers experience no grief at the loss of any of it offspring and, due to her cannibalistic nature, will often pose one of the first threats to their survival. Similar emotional disconnections occur when mammals have been subjects to a limbectomy. Not only will these animals demonstrate a complete emotional disengagement from their progeny, but their ability to recognize the existence of other members in their pack or troop will also be impaired. Damage to the cerebral cortex will not lead to the slightest decline in one's maternal instincts. However, damage to or removal of the structures found in the limbic system prompts immediate behavioral changes that promote a disturbing lack of connectedness with others including those to whom one has earlier given life.
Since emotions play a major role in focusing our attention, and what we learn is governed by what we pay attention to, there are three neuroanatomical structures that should be of specific concern to educators, all of which are central players in the inner-working so the limbic system.
1. The hippocampus, an older part of the mammalian brain that is also found in birds, lizards, rodents, and primates, is the brain's main entry point for memory. It is here that the initial encoding of memory elements gets processed for later recall. The hippocampus has genetically-controlled specifications for exactly where in the brain each important element of a memory will be stored. This seahorse-shaped structure is involved in the recognition of novelty and in processing spatial relations, such as the route to school and home, to one's favorite store in a large shopping mall, or to one's office (along with stored information on specifically where objects are located within that office).
The hippocampus plays such an essential role in all memory processing that damage to it can render an individual incapable of forming and storing any new memories or retrieving previously learned information. This consequence is a staple of soap operas and dramas -- various forms of amnesia. While the hippocampus can recover from 5-10% damage, impairment beyond this level would render this component of the brain completely ineffective. Researchers at the University of Illinois have recently shown through a variety of experiments with other mammals that exercise can increase the size and improve the functioning of the
2. The amygdala, considered the brain’s primary emotional center (it is more like an emotional thermostat), communicates with all other sensory input systems and the cerebral cortex through its extensive neural communications tracts. Recall, retention, and long-term memory are all enhanced by the almost "hair-trigger" firing of the amygdala, which performs a key role in processing nearly all emotional events. Emotions assist in deciding what to pay attention to, which impacts what students will ultimately remember.
Should the amygdala be removed surgically or sustain damage, an individual would process events devoid of any emotional input. Eliminating feelings, such as fear, from one’s response arsenal puts his environmental transactions on a perilous course risking his very survival. Connections between the amygdala and the cortex allow emotions to influence or sometimes to prevent nearly all learning as well as long-term memories. Anger, hate, suspicion, love and other emotions provide us with valuable evolutionary benefits. However, enduring negative emotions can reduce the learner’s ability to pay attention, concentrate, learn or remember, during an "emotional high-jacking" of the cerebral cortex sponsored by a hyperactive amygdala.
3. The cingulate gyrus influences impulsivity, attention, and the control of emotional behaviors. An immature anterior cingulate gyrus is now highly suspect (one of the likely culprits) in incidences of ADD and ADHD. During puberty, the cingulate gyrus and the corpus callosum mature. At that time, the connections from the frontal lobe back to the amygdala come into better balance with the number of connections going the opposite direction. Attention deficits begin to decline at this period in the development of many children diagnosed earlier as hyperactive.
The corpus callosum
is composed of a band of 300 million to as many as 800 million nerve fibers adjoining the left and right cerebral hemispheres. This information superhighway transmits specially coded neural information from one hemisphere to the other producing all integrated thought patterns. There is another smaller fiber bundle, the anterior commissure (found towards the front of the cortex) that connects the two sides of the brain. It too supports bi-hemispheric communications. Through the corpus callosum and the anterior commissure, over 4 billion messages are exchanged between the hemispheres every second, once these parts of the brain have fully matured. .
In the 1980s, thousands of workshops and seminars were offered which detailed the virtues of, as well as the dilemmas presented by, hemisphericity. "Left-brained and Right-brained" books, as well as discussions about "Left-brained/Right-brained" people were the trend. Sadly, these almost cultist notions far exceeded even the most liberal interpretations of the brain research at that time. For all complex functions, the two hemispheres inconspicuously work in tandem with each other, where each side make contributions based on its respective strengths, but never doing the whole job in isolation. The "right brain" (more properly, the "right hemisphere") does not suspend operations to allow for exclusively "left-brained" deliberations. Neither hemisphere is ever even moderately quiescent, while its counterpart completes a task alone. Instead, both make meaningful, although different, contributions to the completion of all cognitive undertakings.
Between nine and twelve years of age, "whole brain" processing becomes feasible offering a neurological meaning to the term "developmentally appropriate" content for learning. In the neurophysiological world, developmentally appropriate translates into a growing young brain that has reached a particular stage in its development, where a task is now "doable" or achievable, because the necessary brain circuitry has been established and now the brain has the wherewithal (the appropriate cognitive "wiring") to process particular kinds of information, activities or concepts.
There are several indicators that the corpus callosum has matured. Among them are
1. The processing of abstractions and more complex thinking becomes considerably easier. This is the best and the most suitable time to introduce concepts such as fractions. At this point in a child's development, ideas of a less concrete nature, multi-step directions and problem solving, as well as concepts for which there is no tangible or observable linkage, can be taught with a relatively high degree of probability that the students will "get it."
2. Fifth-graders suddenly become "insightful" and can understand propositions from a variety of different perspectives. (This stage is often described as the "age of "reason" and the age of morality, because children begin to demonstrate altruistic behaviors and relatively better judgment.)
3. We lose touch with most of our childhood memories. One of the brain's idiosyncratic clean-up and "tune-up" strategies is to implement periodic pruning and purging procedures as it initiates and culminates various developmental stages. During those processes, the brain will discard many of the no longer needed behaviors and skills, allowing for the creations of more beneficial connections that will be important at the next stage of development. Although a skill may have enjoyed an exalted status during an earlier stage, its stock will plummet with the onset of a successive stage in development. While an individual will never forget to crawl, once he begins to walk, crawling loses its lofty position quite suddenly. Now the connections in the frontal and motor cortices focus their efforts on fine-tuning the "walking" program and the crawling ability shifts to a back-burner position. Interestingly, that back burner ability turns out to be located more in the cerebellum, whose location is literally in the back of the brain.
Many fifth-grade teachers are always amazed at the judgmental nature of the comments expressed by their fifth-grade students. Those children will often be quite critical of the behaviors they witness in fourth grade students. However, they often do not recall that they engaged in the identical activities during the preceding year, but they often have suddenly forgotten their own conduct prior to the maturation of the corpus callosum.
Inherent in the process of formal education is one’s emergent awareness of how things are alike or dissimilar and how they fit neatly into logical categories forming critical patterns or groupings. A child though must be cognitively and developmentally astute enough to know that viable cognitive separations or categories can even exist before the distinctions are made. Many of the academic frustrations that educators encounter and endure are clearly rooted in curricular attempts teach concepts at a level of ideational complexity for which the child does not have the requisite neural connections to dissect and reasonably reassemble in a meaningful way on his own cognitive terms
Unfortunately, students are sometimes penalized based our insistence that all children adhere to an arbitrary timeline for mastery.
The corpus callosum is known to change as a result of experience. In non-readers, it is thinner than it is in competent readers of similar age, income and with controls for other characteristics.
Next Time We Will Continue with a discussion of neurons