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December 9, 1998
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The "notion" of people being divided into right-brain and left-brain thinkers is rampant in popular psychology and Western culture as a whole. Those people who are very logical and exhibit verbal fluency are thought to be left-brained, while those creative, intuitive souls, with an eye for visual relationships, are termed right-brained. Now the creative ones, feel their brethren are underrepresented, therefore, an educational and self-help crusade was launched to "balance" excesses of the left-brainers, with the under appreciated virtues of the right. Not surprisingly, handedness is thought to indicate the cerebral hemisphere that predominately runs things in a particular person. That is, the right-hander is said to be left-brain dominant, while the leftie is said to be right-brained. (1) Furthermore, when a person performs a function that is associated with a particular hemisphere, that hemisphere is thought to be the focus of conscious control and action, while the other side waits idly. One could easily conclude that people frequently waste large parts of their brain. Yes, our logical/verbal friends are wasting their visiospatial right hemispheres even now; if only they could be educated to tap into that unused brain power: hence the crusade (Coren 1992; Levy 1985). The "notion" can be summarized in two assumptions (based on neuroscientific research), and a single logical leap: One, our single brain is actually two independent "brains." Two, these brains each have their own specialized function. Leap: Depending on the task, only one "brain," or rather, side of the whole brain, is actively engaged at once (Levy 1985). The "reality," both lends kernels of truth to the "notion," while at the same time contradicting its assumptions and conclusion in a way that is just as fascinating, and more profound. Let us glance at three areas of study to illustrate: Cerebral Dominance, Brain Lateralization, and Handedness. Simple explanations and provocative research aside, there is not yet a unified concept of how these three aspects of our minds work together. An important consideration is that a great many of the findings on this topic point to possibilities, not to certain causes, with certain effects, defining certain fixed relationships. The temptation to over-generalize and oversimplify is great; many have so "sinned."
Cerebral dominance is a concept which was formed on the basis of the work of Paul Broca, and Karl Wernicke. These neurologists of the late 19th century discovered, through the observation of brain-injured people and cadavers, that speech is processed primarily in the left hemisphere of the brain. (2) This discovery inspired a flurry of activity and a major shift in neurological thinking. As the results accumulated, the left-hemisphere grew in importance. Meanwhile, the right-hemisphere demonstrated no special measurable talents. Indeed, it seemed as if the right-brain served no other purpose except to switch signals into the left-brain for higher-order processing. So it came about that the left-hemisphere was said to dominate the brain. This fit well with the obvious prevalence of right-handed people in the world. Aside from Broca's idea that left-handers might be right-brain dominant owing to principle of symmetry, the right-hemisphere languished until... Things changed around 1960 when Nobel Prize winning Roger Sperry and his colleagues made startling discoveries based on their method of helping patients with intractable epileptic seizures. The method involved the radical process of severing the corpus callosum and related neural pathways -- the communication link between the two hemispheres (commissurotomy). The operation successfully controlled the severity of the patient's epilepsy, with almost no noticeable side effects. However, the side effects were peculiar in that the disconnected hemispheres now functioned as two independent brains in one body (Sperry 1982; Levy 1985; Coren 1992). Because of the way our eyes are wired to the brain, it is feasible to communicate exclusively with one hemisphere or the other by stimulating the patient's left or right visual fields: things in the left field of view are seen and interpreted by the right hemisphere and vice versa (Coren 1992). Through various experiments, cognitive functions were able to be assigned to each hemisphere. Further research revealed the startling fact that each brain was capable of reasoning, remembering, communicating, and problem solving (Sperry 1982). Moreover, the formerly dull right hemisphere was shown to be conscious with a sense of self, of time and future. In addition, it can exhibit a sense of humor, and could even communicate with the left hemisphere through emotional pathways in the, still interconnected, limbic system (Sperry 1982). The dominant left-brain had been overthrown; long live both brains! So it is true, that the brain's hemispheres tend to be specialized in function. True too, the hemispheres can operate as independent brains under unusual circumstances. For the rest of us without split-brains, it has been shown that the two hemispheres integrate their contributions so as to form a unified mind (Sperry 1982). For example, the reciting of a poem, would involve the right side in the visual preprocessing of the pages of words, sending the results to the left side's reading association, Wernicke, area for recoding into words, sending the information back to the right side for the processing of the poem's emotional and visual imagery, going back again to the left side in combination with the left's Wernicke processed information to the left's Broca's area for recoding into speech motor movements (Levy 1985). With these, and surely other processes, the poem comes alive! What has become obvious, if more mysterious, is that both hemispheres are constantly cooperating as a seamless whole. Such inter working is vividly seen in PET brain-scan-sequences of minds engaged in problem solving or interpreting environmental stimuli (Tortora 1986; Myers 1998). Even the underlying assumption of functional assignment has been shown to be a general rule rather than an absolute imperative (Joynt 1985); not everyone processes speech in the left hemisphere, for example (Lake & Bryden 1976; Geschwind & Galaburda 1985).
Lateralization refers to the asymmetrical nature of the vertegrate brains. There are many ways in which are brains are lateralized including: anatomically, functionally, and chemically, among others. Evolutionarily speaking, lateralization is thought to increase the number of different ways the brain can respond to its environment in order to facilitate survival. (By contrast, a symmetrical brain would simply duplicate existing functionality, which while useful, is not seen to be as useful to survival.) In addition, as larger functional area increases, the number of possible environmental responses available to the entire species grows, hopefully enabling at least some members to survive most any environmental calamity. (Geschwind & Galaburda 1985). Recently S. H. Woodward proposed what I believe is another possible reason for lateralization, which involves the general difference in style of neural connectivity between hemispheres: the left hemisphere tends to form highly coupled, non-overlapping connections between adjacent groups, whereas the right side forms interconnections with widely spaced overlapping groups. Thus the hemispheres may be optimized for different types of signal processing (Springer & Deutsch 1997). (3) Research continues to reveal new classes and examples of lateralization asymmetries, both in humans and in animals. While the reasons for some of these differences seem apparent in light of existing understanding, other examples defy explanation at this time. The following is a brief set of asymmetrical classes, along with some examples of each:
Reproducible anatomic asymmetries include size differences among homologically similar areas of the cerebral cortex such as the temporal speech region (known as the Wernicke area), various measures (width/petalia {shape asymmetry}, by CT scan) of the frontal, temporal, occipital, and parietal lobes, interior frontal gyrus, sylvian fissure, thalamus, and bulbar pyramids. Some of these measured differences have come from fossilized hominids, adding an evolutionary dimension (Geschwind & Galaburda 1985). More recently, Volkmann et al., demonstrated "for the first time, a biological correlate of handedness in the human motor cortex." They conjecture that the enlarged motor cortex area of the dominant hand provides greater space for the encoding of additional motor skills (Volkmann & Schnitzler & Witte & Frenund 1997). In fact, it is generally surmised that larger cortical areas serve as a space for increased encoded information, already related to that region (Geschwind & Galaburda 1985). There are other anatomical differences which may occur through normal development and pathological cause, which are discussed below.
Geschwind & Galaburda (1985) relate the findings of several other researches pointing out asymmetrical differences in the number of pyramidal fibers crisscrossing the medullary decussations which are thought to imply greater control over the upper right arm by the left hemisphere. The lower medulla appears to posses such an asymmetry which is thought to relate to nerves going to speech muscles from the dominant hemisphere (Ibid.).
Cytoarchitectonics refers to the way neurons are grouped within various structures, and the characteristics of the neurons themselves, including their size and type. L. Sheldon found evidence of greater Dendritic contacts in the right auditory temporal association area (Ibid.). Increased size of certain speech areas, once again, are thought to provide a basis upon which more associations may be encoded (Ibid.).
Research has found asymmetries in the distribution of various neurotransmitters. For example, norepinephrine volume is greater in the left pulvinar of the thalamus than the right. In the Brodmann area of the left cerebral cortex there is a chemical asymmetry related to cholineacetyltransferase. Furthermore, animal studies are beginning to reveal other chemical asymmetries (Ibid.).
It has been noted that the right hemisphere of a fetus develops more quickly than the left. Furthermore, those neurons which develop and migrate to their assigned place first, are able to establish synaptic connections with other neurons, thus ensuring their survival, when the developmental purge of unconnected neural cells begins. As a result, the right hemisphere will tend to have more and better connected neurons than the left. The delayed development of the left hemisphere invites more accidental injury as well, because the window of critical exposure to teratogens is wider due to its slower development (Ibid.). Geschwind & Galaburda (1985) hypothesize that levels of the sex hormone testosterone may have also have a significant impact on neuron development, as well as other tissues, and the immune system. (In fact imbalances in this hormone may induce the correlation between immune problems and pathological left-handedness.) These researchers believe that many problems correlated with left-handedness (including learning disabilities), may be the result of minor developmental anomalies during gestation. Furthermore, anomalies which would tend to slow the growth of the right-hemisphere, allow the left hemisphere to catch-up, and perhaps share in some of the functions normally assigned to the right by virtue of the fact that its neurons (the left's) developed and connected first. This theory seems to fit the observation that left-handedness may or may not signal a shift in speech processing from the left to the right. This theory also fits the observation that speech processing may be shared in both hemispheres for a significant number of lefties (Geschwind & Galaburda 1985).
Finally, both the development and maintenance of neural and cortical health can be affected by various pathologies. There are pathologies which could fit into any of the categories above. There would also seem to be less of a causal relationship between handedness and various disorders that well-known correlates might lead to believe: more simply, it may be that left-handedness is merely a co-symptom of the difficulty with which it is associated. (Geschwind & Galburda 1985) Another very intriguing possibility is that which Geschwind & Galaburda (1985) refer to as the "Pathology of Superiority." Noting elevated prevalence of lefthanders and various developmental difficulties, and various stressful birth conditions (4) (Coren 1992), these researchers consider the paradoxical possibility that the same processes could cause a developmental problem which results in a superior ability. For example, it has been noted that dyslexics are known to have superior abilities among the functions of the right hemisphere (i.e. artistic, geometrical reasoning) (Geschwind & Galaburda 1985). This idea seems to be supported by Hicks & Dusek's (1980) study which suggested that as a group, gifted children tend to be less right-handed than their non-gifted peers (which contradicts Bakan's theory that left-handedness is caused by brain damage). Hicks & Dusek (1980) further speculate that the relationship between right-handedness and intelligence be represented by a curve the shape of an inverted-u, which implies that left/mixed handers would tend to cluster at both intelligence extremes. Ample evidence exists today to assign very different, complementary and supportive functions to each emisphere. What is has been recognized in recent decades, is that each hemisphere seems to also have its own cognitive domain with its own perceptual, learning, and memory experiences. Furthermore, each hemisphere has an essentially equal power to behave sentiently, feel emotions, be creative, and intuitive (Sperry 1982; Levy 1985). Finally, when we speak of specializations, we should not address them in binary terms of "this hemisphere's got it, but this one doesn't" You see, you really can draw on the left side of the brain -- just not very well! (And the right side can read and add a little too.)
The left hemisphere is specialized for linguistic processes which include speech, read and written language, and syntactic & semantic analysis. In addition, it is very good with symbolic relationships including logic and higher-order arithmetic reasoning. The left is good at sequential ordered processing, and keeping the basic beat of a melody too (Sperry 1982; Levy 1985; Springer Deutsch, 1997; Coren 1992; Buzond 1984).
The right hemisphere on the other hand, is specialized for non linguistic processes such as visual and spatial judgment. It is a high-order geometric processor, mentally able to fold, unfold, and rotate two and three dimensional object images, and solve visual riddles. It is very good with faces, and can read and draw maps. It is a musical listener, able to follow complex rhythms, discriminate simultaneous chords, and even sing. (Sperry 1982; Levy 1985; Springer Deutsch, 1997; Coren 1992; Buzond 1984)
Interest in handedness goes back at least as far as Paul Broca. Why all the fuss? Handedness has been seen as (and hoped to be) an inexpensive, noninvasive way for physicians and scientists to be able to locate the hemisphere controlling a person's speech ability; this information was used both for treatment and basic research into the brain (Springer & Deutsch, 1997; Level 1985). Handedness is also important to one degree or another in most cultures; albeit in a somewhat negative way. In some cultures it is "gauche," or "sinister" to be left handed (the French and Latin words for left handedness); many people have been pushed to conform to the right world, ostracized, or even worse. The right hand is often associated with eating while the left is assigned to the hygiene of elimination. Dextral, or right-handed, is where we get the word "dexterous:" a word with warm connotations. But the other handedness, sinistral, conveys evil and Satanic emotions. In North America today, left handedness is generally accepted, yet the stereotype of ineptitude and underhandedness persists (Coren 1992; Holder 1996). Still, some such as myself, find their left-handedness to be a sign of uniqueness and treasure this different way of being and doing things. Simple as it may seem, handedness does not have an easy definition. Most people casually think of handedness as being determined by their writing hand, but this is usually too imprecise for scientific purposes. So, as you might have guessed, science defines handedness in several (sometimes conflicting) ways: right-handed, ambidextrous, left-handed (or discrete gradations along the line between them), right-handed and everything else, right-handed and mixed/left-handed, or an infinite continuum stretching from extreme right-handed, through ambidextrous, to extreme left-handed. Approximately ninety people in a hundred are strongly right-handed, the remaining ten however, are not likely to be strongly left-handed -- that is why the term mixed handedness has come into use. Measuring handedness is not any easier. Generally there are two ways to do this: one is through the administration of preference tests such as the Oldfield handedness battery (5), Annett's handedness scale (6) , or a survey I recently took by M. K. Holder (1996). The other method is to objectively test dexterity. Unfortunately, both methods are vulnerable to bias. The survey asks for preferences that may not be connect well with the respondent (who might not answer all the questions, yielding less data than expected). Worse, an inappropriate or poorly worded question could inadvertently distort all the data collected in the survey, (yielding misleading results.) At first glance, survey results tend to correlate well with dexterity tests, unfortunately, some of the things researchers are looking for occur less times than the smallest margin for error! This makes collecting reliable data on handedness across disciplines (and even researchers) a difficult task, and may be a factor in the less than plentiful and useful data associating various things with handedness. M. K. Holder of the Center for the Integrative Study of Animal Behavior (Indiana University) is seeking to design a standardized handedness definition and measure to address these problems. (7) The following list of correlates has been assembled curtesy of Lorin Elias, on her Left-handedness web site (which includes an APA bibliography of references to these correlates):
A glance at the above list certainly lends plausibility to the conjecture of Hicks et al. about the possibility of a nonlinear relationship between right-handedness and intelligence (Hicks et al. 1980). In also resonates with Geschwind & Galaburda (1985) who add the following caution: that although left-handedness is associated with many unpleasant things, it has its advantages. That is, left-handedness may not be good or bad in the final analysis, rather simply different. (8)
Left-brain/right-brain proponents in popular culture will find substantial support for the idea that brain hemispheres are indeed functionally specialized. Moreover, there is significant, repeatable lateralization of the human brain along numerous dimensions. The left and right hemispheres can even perform as independent minds when their intercommunication is severed as in the rare commissurotomy procedure. However, there is significant empirical evidence to refute the idea of hemispheric dominance in the "on again, off again" sense which these proponents presume. Instead researchers continue to affirm the constant, subtle, and profound interplay of the two hemispheres; we do not posses two, limited brains that vie for control, rather we posses an almost transparently unified mind of much greater ability then merely the sum of the two parts. These are exciting times for all those interesting in the brain and how it works. Still, there are so many questions, seemingly contradictory experimental results, and hyperbole. The truth is, contrary to pop-psychology, we simply don't understand enough about cerebral dominance, brain lateralization, the causes and effects of left/mixed handedness, and how these things interact, to be able to make short, quick, easy-to-digest, blanket statements. It's even a little sad, because the truth is so much more profound, exciting, and interesting.
The topic I have just discussed is of great personal interest to be because I am left/mixed handed. Actually, until recently, I would have said that I was left handed. I've always taken note of other lefties and spoken to them a few words of encouragement and affirmation to let them know that I too am part of this "exclusive club," "and isn't it great?" For me, part of my uniqueness is in my handedness. (I was blessed in that I was not discouraged from using my left hand.) Every few years, I touch up my penmanship skills so as to write to be more easily read, but that's my sole inconvenience. Now I have a more ambitious project, after having researched this topic: It is that of teaching myself to write: right-handed. Crazy you say? Maybe not. The results of recent hand-preference surveys I have taken seem to indicate that I am actually slightly right-handed. Since then, I have more carefully noted the activities that I perform with my hands, and have come to the conclusion that I am nearly ambidextrous. Though there is little support for this (Coren, 1997), I figure it can't hurt my mind to cross-train its hands (you never know when you'll get a stroke). I have also begun to perform tasks with opposite hands than the way I unusually do them; its fun! (I can now use chopsticks with both hands, for example.) I think I now know how I became this way: I was born breech during an excruciatingly long labor and delivery, and forceps were necessary to complete the job -- but at the cost of my left eye. I theorize that the left side of my face/head were traumatized at birth, and that may have retarded the development of my left hemisphere, causing my present handedness. Now, I fit the stereotype of a leftie pretty well: I'm very nearsighted, artistic, musical, visiospatial, with a very good geometric sense, and I'm also somewhat gifted in intelligence. My very good linguistic ability is the only thing I can think of that rubs against the stereotype (yes, I have some of the lefties problems too). Anyway, I'm excited to learn more about the way I work, and gain insights that I can use to solve the problems that get sent my way. Besides, its nice to know that I do "use more than 10% of my brain." (I need all I can get!)
Buzand, Tonv (1984). Make the Most of Your Mind. New York: Simon & Schuster, ISBN 0-671-49519-4. Coren, Stanley (1992). The Left-Hander Syndrome: The Causes and Consequences of Left-Handedness. New York: The Free Press, (ISBN 0-02-906682-4) ISBN 0-029-06682-4. Elias, Loren. (1996). Groups with Elevated Prevalence of Left-handedness. [On-line]. Available: URL http://duke.usask.ca/~elias/left/groups.htm. Geschwind, Norma, & Galaburda, Albert M. (1985). Cerebral Lateralization: Biological Mechanisms, Associations, and Pathology: I. A Hypothesis and a Program for Research. Archives of Neurology, 42(5), 428-459. Hicks, Robert A., & Dusek, Christine, & Larsen, Ferol, & Williams, Susan, & Pellegrini, Robert J. (1980). Birth Complications And The Distribution of Handedness. Cortex, 16, 483-486. Hicks, Robert A., & Dusek, Christine M. (1980). The Handedness Distributions of Gifted And Non-Gifted Children. Cortex, 16, 479-481. Holder, M. K. (1996). The World of Sinistral Subterfuge. Internet Underground, 13(December 1996), 67. Holder, M. K. (1996). Hand Preference Questionnaire: One Gets What One Asks For. [On-line]. Available: URL http://www.indiana.edu/~primate/forms/hand.html. Joynt, Robert J. (1985). Editorial: Cerebral Dominance. Archives of Neurology, 42(5), 427. Lake, Deborah A., & Bryden, M. P. (1976). Handedness and Sex Differences in Hemispheric Asymmetry. Brain And Language, 3, 266-282. Levy, Jerre (1985). Right Brain, Left Brain: Fact and Fiction. Psychology Today, (May 1985), 38-44. Myers, David G. (1998). Psychology, 5th Edition. New York: Worth Publishers, ISBN 1-57259-590-6 (1-57259-207-9 comp.). Sperry, Roger (1982). Some Effects of Disconnecting the Cerebral Hemispheres. Science, 217(24 September 1984), 1223-1226. Springer, Sally P. & Deutsch, Georg (1997 5th ed.). Left Brain, Right Brain, 3rd Edition, Left Brain, Right Brain: Perspective from Cognitive Neuroscience 5th Edition, New York: W. H. Freeman and Company, (ISBN 0-7167-1999-1) ISBN 0-716-73110-X, 5th ed. Tortora, Gerard J. (1986). Principles of Human Anatomy, 4th Edition. New York: Harper & Row, ISBN 0-06-046623-5. Volkmann, J. & Schnitzler, A. & Witte, O. W. & Freund, H. J. (1997). Handedness and Asymmetry of Hand Representation in Human Motor Cortex. The American Physiological Society, Article publication pending, ISSN 1080-4757. (From ABStracts, 4:351N, 1997, December 1997).
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