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Beyond superstitions and mysticism, in its earliest history an individuals favoritism of the left or right hand proved significant mainly to those clinicians who utilized this physical clue of handedness as an indicator of brain lateralization (2). Lateralization in general "refers to the concept that a given function is controlled preferentially by one side of the brain or body (5). Brain lateralization further implicates the functional specialization of the two cerebral hemispheres - the left and right - which may be distinct in the localization of neural mechanisms for particular functions (3).
The corpus callosum serves as the physical and functional connection between these two cerebral hemispheres. Each hemisphere receives sensory information and controls movement on the side of the body opposite its location. An intact corpus callosum insures communication between the two hemispheres to build a full understanding of actions and perception. In certain extreme cases of severe epileptic seizures a "split-brain operation" is necessary, severing the corpus callosum, which serves to greatly reduce seizure frequency but leaves the cerebral hemispheres to function virtually independently (6).
"Handedness" then, has come to be considered - not merely the hand you favor for writing - but rather a possible indication of hemispheric language specialization. It was the French neurosurgeon, Paul Broca, who in the mid-1800s "identified a particular area of the left hemisphere that plays a primary role in speech production" (4). Shortly there after a German neurologist, Carl Wernicke, further identified a left hemispheric region significant to language comprehension (3).
The majority of the human population, 70% to 95%, exhibits "left-hemisphere language specialization", and are for the most part what one popularly considers "right-handed". The debate over the 5% to 30% who are not "right-handed" finds that they might be classified as: ambidextrous, left-handed, or non-right handed, depending on the individuals particular traits and by whom they are scrutinized. In relation to brain lateralization those individuals not characterized as "right-handed" tend to have, as a group, a less predictable pattern of specialization (3). A contrasting approach to addressing the "left-handed" and ambidextrous persons dispersed processes would be to conceive of the "right-handed" as dextrally bias (1).
In popular culture individuals have often been classified as "right or left -brained". This classification has to do with those functions generally specialized in each hemisphere. "Left-brained" individuals are deemed strong in those functions localized in the left hemisphere, which include: number skills, written language, reasoning, scientific, and the spoken language. While a "right-brained" individual proves characterized by those functions localized in the right hemisphere, which include: insight, 3D spatial reasoning, art awareness, imagination, and music awareness (5). These classifications do seem intriguing, but their relevance clearly demands scrutiny.
Not surprisingly then, much research has been conducted to assess right and left brained characteristics in relation to one's handedness. Research conducted by Anne Kilshaw in 1982 found that more dextrally biased, the "right-handed", individuals tended to perform comparatively poorly on measures of mathematical ability. She reasoned that the group of individuals utilizing both hemispheres, mixed-handers, were better able to convert "spatial constructs into linguistic terms" (1). In analyzing other handedness differences it was necessary to address men and women separately as results showed significant variation dependent up gender. For instance, Lester (1987) found that left-handed female undergraduates tended to be less extraverted than their right-handed counterparts. In 1993 Hassler and Gupta detected a higher level of musical talent in left-handed individuals verses right-handed. While overall, musicians were found to "exhibit less hemispheric specialization for linguistic processing than do non-musicians" (1). For all such correlative findings there also exists quite a bit of contradictory research as well as inconclusive correlates. Additionally, general exploration found that handicapped individuals tended to be significantly more likely to be left handed (Pipe 1990). In such cases it was hypothesized that these individuals were non-natural left handers, and that the mental disability had damaged the original dominance of the left hemisphere (1) .
Certainly then one might question when in development cerebral hemispheric dominance first initiates. Noted on ultrasound an arm preference can be seen within the first several months. However, the earliest stages of development are characterized by embryonic symmetry. Thus provokes the question as to whether brain asymmetries result in the preferential use or whether preference leads to asymmetries in brain organization. Beddington (1996) found that there is some asymmetric gene expression prior to the manifestation of morphological differences in the embryo. In addition, McCartney and Hepper's (1999) observation of in utero arm movements in early gestation led to the suggestion that such movements trigger the later asymmetries, which were likely to be of a genetic origin (2).
The possibility that a self-initiated preference may in fact influence genetic expression and brain development does not seem far fetched in the earliest embryonic stages. However, might one consider altering their current hand preference in hopes of further developing the underutilized hemisphere? The "Ambidexterity Exercise" claims that those individuals who are left-handed and ambidextrous have an 11% larger corpus callosum than right handed individuals. This masked advertisement further contends that one needs only "exercise" her brain to promote this physical growth (6). While clearly the possibility of actual brain growth remains absurd, the idea that uses might alter the way in which are brain functions is not an impossibility. Neuronal plasticity should allow for the establishment of new and additional synaptic connections that may very well result from the initiation of new behavioral patterns.
Such possibilities only further cloud our conception of hemispheric dominance as well as the question of true "handedness". Nonetheless, the possibility of a concurrent alteration of behavior and brain structure would seem an elegant illustration of the brain=behavior premise.
1) Handedness, Individual Differences, and Human-Computer Interaction by John Gregory Hibbard
3)Handedness and Brain Lateralization
4)Scientific American: Ask the Experts: Biology
5) Evolution of Lateralization and a Role in Language
6)Splitting the Human Brain by Paul Pietsch
7)Ambidexterity: Memory Power and Mindfulness - Ambidexterity Develops
TRY IT! Hemispheric Specialization
Left brain / Right Brain
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