51Laterality.DOI: http://dx.doi.org/10.1016/B978-0-12-801239-0.00004-1 © 2014 Elsevier Inc. All rights reserved. 2016

Left-Handers and the Right Mind

4C H A P T E R

A pervasive notion in the history of brain science is that the two hemispheres of the human brain evolved for different functions. In the mid-nineteenth century, Paul Broca’s description of the loss of language in a patient with left-hemisphere damage provided strong initial evidence for functional asymmetry in the brain. The human population bias toward right-handedness was linked to the evolution of language localization in the left hemisphere as the left hemisphere controls the movements of the right side of the body. The idea that language function is lateralized to the hemisphere opposite to the side of handedness is called Broca’s rule. The language functions of left-handers should be lateralized to the right hemisphere to be consistent with this rule. There is an appealing simplic-ity to this reasoning but researchers soon discovered that the relation-ship between language lateralization and the side of handedness is not straightforward, especially for left-handers [1].

In the 1950s, Juhn Wada pioneered the technique of injecting sodium amobarbital into the carotid artery of patients who are about to undergo brain surgery. This procedure produces a brief anesthesia of the hemi-sphere on the same side as the injection. If left-hemisphere anesthesia dis-rupts verbal tasks, this demonstrates that speech functions are localized in the left hemisphere. If right hemisphere anesthesia disrupts verbal tasks or if speech interruptions occur when either hemisphere is anesthetized, then the right hemisphere or both hemispheres are involved in language control. Using the Wada test, researchers found that 96% of right-handers and 70% of left-handers had speech lateralized to the left hemisphere. Right hemisphere speech was found in 4% of right-handers and 15% of left-handers with an additional 15% of left-handers showing evidence of speech processing in both hemispheres.

Contemporary research methods using imaging techniques that measure neural activation in various regions of the brain confirm that

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language functions are left-lateralized for most right- and left-handers. The incidence of atypical right-hemisphere language lateralization varies with the side and strength of handedness. The lowest rate (4%) occurs in consistent right-handers, while the highest percent (27%) is found among consistent left-handers. The brain activation observed during language production is similar for left-handers with typical left-hemisphere speech and left-handers with atypical right-hemisphere speech. In individuals with bilateral language organization, each hemisphere seems to control specific components of speech functions. Naming items may be under the control of one hemisphere, whereas placing items in a sequence is a task performed by the other hemisphere [2].

Most right- and left-handers have language lateralized to the left hemi-sphere although a larger percentage of left-handers deviate from this pattern. However, the popular view persists that the cognitive processes of left-handers are dominated by the right hemisphere that controls the movements of the left hand. It is assumed that the hemisphere controlling the writing hand is dominant over the other and because of this dominance there are styles of thinking that are either left- or right-brained. The right and left hemispheres have both anatomical and functional differences. The question is whether or not these differences vary with handedness and, if so, whether they impact cognitive processing leading left-handers to think and behave differently than right-handers.

RIGHT BRAIN, LEFT BRAIN, AND HANDEDNESS

Scans of living brains indicate eight major anatomical differences between the hemispheres. These range from differences in the amount of neural matter to size differences of various brain regions. There is a corre-lation between anatomical asymmetries in the brain and the side of hand-edness but there is uncertainty as to what these differences may mean behaviorally. Some left- and right-handers show anatomical asymmetries in the expected direction of right or left, some show a reversed asymmetry, and some show no differences between the two sides of the brain. Also, there may be a dissociation between structural and functional asymmetry. For example, a Wada test could indicate a left-hemisphere speech while neurological examination suggests a right-hemisphere speech zone [3].

Because language lateralization is more variable among left-handers, researchers have theorized that left-hander brains function more sym-metrically than right-hander brains. In other words, left-handers are more whole-brained than right-handers. This idea has focused research attention on the corpus callosum, the large bundle of nerve fibers con-necting the right and left hemispheres. Theoretically, left-hander brains should show greater degrees of neural communication between the two

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hemispheres with more neural fibers in the corpus callosum. Sandra Witelson performed postmortem analyses of the corpus callosa in brains of individuals who were tested for handedness before death. She found that non-right-handed males had a larger callosal area especially in the regions of the callosum toward the back of the head. However, other studies found no variation related to handedness. Also, there are reports that it is handedness consistency not handedness side that correlates with the size of the corpus callosum. Individuals with mixed-handedness have larger corpus callosa than individuals with consistent-handedness. Symmetric or weakly lateralized brains have larger corpus callosa and more interhemispheric connectivity than asymmetric or strongly lateral-ized brains. The symmetry or asymmetry of brain functioning may be only partially related to handedness side [4].

Another brain region studied in regard to differences between right- and left-handers is the speech area in the left hemisphere. This brain region is known as Broca’s area in honor of Paul Broca. Its location in the frontal lobe of the left hemisphere is shown in Figure 4.1. Neural imag-ing data indicate that portions of Broca’s area are leftward asymmetric in right-handers and rightward asymmetric in left-handers. The implica-tion of these asymmetries for both language production and handedness is unclear. Other imaging studies of the speech center reveal that left-handers tend to show lesser degrees of asymmetry or weaker cerebral lateralization in these areas. Individuals with left-handed relatives (FS+ as discussed in Chapter 3), regardless of their handedness, display asym-metries favoring the right hemisphere and show a more variable pattern of asymmetries than individuals without left-handed relatives (FS−) [5].

A third brain region studied in connection with handedness is the motor cortex, a strip of tissue across the top of the brain, as shown in Figure 4.1. The right hemisphere motor cortex controls left hand and body movements, while the motor cortex of the left hemisphere controls move-ments of the right hand and body. This opposite side (contralateral) neu-ral control mechanism seems complex but one theory contends that this arrangement is advantageous. If a person suffers an accidental injury, it is most likely to affect the same side of the brain and the body, for example, the left side. If the left hemisphere controlled the left side of the body, both the brain and the body on one side would be debilitated. Contralateral control means that an undamaged right brain can compensate for the injuries to the left side of the body, while the damaged left brain can still exert some control over an undamaged right side. This two-sided distri-bution of resources in the case of injury can offer a survival benefit [6].

Studies of the motor cortex use neural imaging techniques and have participants perform either simple hand movements, like finger flexion, or more complex movements, such as pantomiming gestures used in tool manipulation. In the latter case, the brains are scanned while individuals

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pantomime actions such as using a computer mouse, hairbrush, or wire cutters. Scans show that the hemisphere controlling the preferred hand has larger areas of movement representation and greater interconnectivity between these areas. Also, the magnitude of the differences between acti-vation of the motor cortex in the control versus the noncontrol hemisphere varies with the strength of handedness. Consistent-handers show larger activation asymmetries between the two hemispheres when performing hand actions than mixed-handers [7].

Research on the motor cortex has examined the link between brain areas activated by well-practiced pantomimed hand gestures and brain areas activated by language tasks such as word generation. Scans indi-cate that these two functions reside in the same hemisphere even in individuals who have atypical right hemisphere language lateralization. Hemispheric specialization for a verbal task, for example, predicts the side of lateralization for a hand gesture task. One hemisphere is special-ized to perform both functions with the side of handedness influencing only the strength of this colateralization. Left-handers are colateralized but display more symmetrical neural activity in the two hemispheres. This finding has led to the speculation that the evolutionary association is not language in the left hemisphere and right-handedness as proposed

FIGURE 4.1 The left hemisphere of the human brain showing the four lobes—frontal, parietal, temporal, and occipital—and the locations of Broca’s area and the motor, auditory, and visual cortex.

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by genetic theories. Rather the evolutionary link is between language and hand-gesture control with both being located in one hemisphere of the brain. The colateralization of speech and gesture supports the theory that human speech evolved from the gestural communication used by our pre-verbal hominid ancestors. As brain size increased over millions of years of evolution, vocal communication began to dominate and communicating by means of gestures alone took on a secondary role [8].

Words on the Left, Images on the Right

The question of whether or not there are right- versus left-hemisphere styles of thinking cannot be answered by analyzing brain anatomy. The overall cognitive specialties, if any, of the two hemispheres must be examined to address this question. Over the last half of the twentieth cen-tury, a general picture of hemispheric specialization and its relationship to cognitive processing emerged. This view was based on the results of two influential research paradigms. The first was the study of right- versus left-hemisphere function in individuals who had undergone surgery to sever the corpus callosum, the communication bridge between the two hemispheres. The surgery was an attempt to help people with intractable epilepsy and, after surgery, they were called split brain patients. Roger Sperry and his associates conducted extensive postsurgery studies of these individuals using a research methodology called visual field testing. The entire visual field is experienced as one unit, but anatomically, the brain processing of visual information is partitioned between the right and left hemispheres. Images presented to the left visual field (LVF) are processed by the visual cortex in the occipital lobe of the right hemisphere. Images in the right visual field (RVF) are analyzed in the same region in the left hemisphere. If a person is instructed to stare at the center of the visual field and images are presented to the left and right of center, these images are processed differentially by either the right or the left hemisphere.

Figure 4.2 shows the experimental design of a visual field study. A research participant sits in front of a computer screen and stares at a spot in the center of the screen. An image of a simple object, such as a line, is flashed briefly to either the RVF or the LVF and the research participant is asked to identify the object. Individuals with an intact brain verbally name objects presented to either visual field because the corpus callo-sum allows contact with the left-hemisphere speech center, regardless of the visual field of presentation. However, as Figure 4.2 shows, when the corpus callosum is severed, split brain participants can only name objects presented to the RVF where the left-hemisphere processing of the image is consistent with the location of the speech center. Images in the LVF go to the silent right hemisphere. Split brain individuals can

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identify them only by nonverbal means such as touching the appropriate object with the left hand. The fact that LVF objects can be identified by touch indicates that the right hemisphere processes the information from the LVF but the outcome cannot be expressed verbally without contact with the left hemisphere [9].

Doreen Kimura’s work with dichotic listening was the second approach to examining the functional roles of the two hemispheres. This experimen-tal paradigm involves presenting different information simultaneously to the two ears. The majority of neural connections in the auditory system are from the ear of input to the hemisphere on the opposite side of the brain. As the left hemisphere is lateralized for language, the right ear of participants should show superior performance when processing verbal information. Participants should be able to report more of the words or syl-lables presented to the right ear when compared to those presented to the left. In general, this is what Kimura found, a right ear advantage (REA) for verbal input. The dichotic listening technique was also used to study what happens when nonverbal information, such as musical tones, is presented to the right and left ears. A left ear advantage was discovered for melodies, musical chords, and other environmental sounds, indicating superior per-formance of the right hemisphere when processing nonverbal input [10].

The integration of data from years of visual half field and dichotic listen-ing research using both clinical and nonclinical samples and presentations of both verbal and nonverbal material formed what M.P. Bryden called the Modal Model of the functions of the two hemispheres. This model sees the left hemisphere as the language, analytic, and serial processing part of the brain. The right hemisphere is holistic, integrative, and a parallel processor that deals with visual–spatial information, emotions, and music.

FIGURE 4.2 The visual field testing procedure used to study split brain individuals. Right visual field (RVF) images are sent to the occipital lobe of the left hemisphere for visual processing. Left visual field presentation (LVF) results in right hemisphere processing. Split brain individuals can verbally identify images of objects presented to the RVF but can only identify images of objects presented to the LVF by nonverbal methods such as touch. Source: Adapted from Ref. [9, figure 11.8, p. 293].

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The functional asymmetry of the human brain can be summarized along a language (left hemisphere) versus visual–spatial (right hemisphere) dimen-sion. The Modal Model inspired the notion of left- versus right-hemisphere styles of thinking, an idea that captured the imagination of the public. Many popular book titles encourage readers to tap into the power of either their right or their left brain to tackle a variety of life challenges [11].

THINKING LEFT AND RIGHT

Understanding the different functions of the two hemispheres is impor-tant in clinical situations. Impairments resulting from brain trauma, such as the loss of language abilities after a left-hemisphere stroke, are best understood with the knowledge of left-hemisphere specialization for speech functions. But is this functional difference important during the performance of everyday cognitive tasks? If so, how do these processes relate to handedness? Neurological evidence indicates that the brains of right- and left-handers are more alike than different. Virtually, all right-handers and most left-handers display the typical left-hemisphere lan-guage lateralization pattern. When language functions are lateralized to the right hemisphere, neural activation observed for verbal (right hemi-sphere) and nonverbal (left hemisphere) tasks is similar to that observed for the more typical left-lateralized language pattern. The colateralization of gestures and speech in the same hemisphere is found in both right- and left-handers with typical (left hemisphere) and atypical (right hemi-sphere) language function. Left-handers are more variable and symmetric in their brain lateralization patterns than consistent right-handers but does that mean that their thinking is dominated by the right hemisphere that controls the movements of their left hands?

Creativity: Left, Right, or Both?

One heavily researched area concerns the role of the right hemisphere in creative thinking. Creativity, the ability to generate new and useful ideas free from the constraints of established habits, is a valuable human cogni-tive skill. Does the right hemisphere hold the key to creative thought? If so, left-handers whose preferred hand is controlled by the creative right hemi-sphere should have a creativity advantage over right-handers. Left-handers have direct access to the creative representations thought to be processed in the right hemisphere. An alternate view argues that associating creativ-ity with only one side of the brain is simplistic. Creativity is a whole brain process where both hemispheres play critical roles. It is the interaction between the hemispheres that generates creative thought. This approach also predicts a creative advantage for left-handers because the cerebral

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functions of the two hemispheres tend to be more symmetrical in the brains of left-handers. Left-handers are more whole-brained than right-handers.

Creativity is often defined as divergent or lateral thinking where indi-viduals are asked either to enumerate many examples or to make novel connections between items. Researchers might ask participants to gener-ate stories or come up with lists of innovative uses for common objects, such as shoes or paper clips. Mental improvizations of dance and music have also been used as creativity tasks. The evidence is mixed, and there are some discrepant findings but higher than expected percentages of left-handers are found among individuals in creative professions, such as art, music, and architecture. Some studies find that left-handers obtain higher scores on laboratory tests of creativity, while other studies find no differences between the scores of right- and left-handers [12].

Creative thinking has been linked to another type of cognitive process called magical ideation. Magical ideation is the tendency to believe in conventionally invalid forms of causality. It is measured by question-naire items such as Some people can make me aware of them just by think-ing about me or I think I could learn to read other’s minds if I wanted to. Individuals who agree with these types of statements are given high magical ideation scores. Research finds that magical ideation and creativ-ity are related. Individuals with high scores on creativity tests also have high scores on measures of magical ideation. However, the relationship among handedness, magical ideation, and creativity is one of consistency rather than side. Mixed-handers have higher scores on test of both cre-ative and magical thinking. There is disagreement about the underlying cause of the association between these forms of cognition and mixed-handedness. One view states that reduced hemispheric asymmetry, such as that might be found in mixed-handers, forms the cerebral basis of the relationship. Other researchers argue that the cause of the association among mixed-handedness, creativity, and magical ideation is unknown. It may be related to individual differences in responding to questionnaire items and may have no neuropsychological basis at all [13].

Creative thought is a whole-brained activity, and both hemispheres contribute to the process. Are left-handers more creative? The answer might lie in the tendency of left-handers to be mixed in their hand use and symmetrical in their hemispheric activity rather than in the side of their handedness. Right- and left-brain thinking go together leaving left-handers without an advantage related to the right hemisphere control of the movements of their preferred hand. Right- and left-hander differences in the fluidity of access to the resources of both hemispheres may generate the creative cognition that leads left-handers to excel in certain profes-sions. However, the mixed results from laboratory studies comparing the creativity test scores of right- and left-handers do not find a left-hander advantage in creative thought [14].

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Handedness and Intelligence

The traditional approach to this issue assumed that right- and left-handers think differently because their cognitive abilities are dominated by different hemispheres specialized for different functions. This assump-tion prompted researchers to examine possible overall intelligence (IQ) differences between right- and left-handers. The interest in the association between handedness and IQ generated a large number of studies over many years, and all forms of relationships between the two are reported in the published literature. For example, there is research evidence that points to both an IQ advantage and an IQ disadvantage for left-handers relative to right-handers. Researchers report that IQ scores are lower for individuals with extremely strong right-handedness when compared to those with more mixed handedness patterns. Alternatively, others argue that mixed-handers show lower scores on tests of general intellectual ability when compared to other handedness types. Finally, there is an abundance of research that indicates no differences between right- and left-handers when it comes to intellectual ability [15].

In 2003, Daniel Nettle tried to clarify the nature of the relationship between handedness and IQ with an analysis of data from the National Child Development Study (NCDS). These data come from an ongoing longitudinal study of over 17,000 children born in Great Britain in March 1958. Nettle analyzed data from verbal and nonverbal IQ tests taken by the children at 11 years of age. He compared these scores to handedness data derived from a test of hand skill (the number of boxes ticked by the right or left hand in 1 min). He found that average test scores increase among individuals who show strong laterality effects between the two hands regardless of the direction of the laterality. Individuals who per-form equally well with the right and left hand have lower IQ scores than more strongly lateralized right- and left-handers. The effect is weak, how-ever, and all participants in the study are well within the normal range of abilities [16].

Since 2003, other researchers have collected data from large samples of both children and adults in the United Kingdom, Australia, and New Zealand. These studies generally confirm that mixed-handers have lower cognitive ability scores when compared to other handedness types. The effect may be confined to particular cognitive tasks, such as reading or mathematics, and, in some instances, the reduction is stronger for males than for females. Two studies also found lower IQ scores among left-handed children although the deficits are not as large as for those of equal hand performance. The majority of studies report that individuals who are mixed-handed show lower scores on tests of cognitive ability. The hypothesis of hemispheric indecision is one explanation for this find-ing. Individuals who fail to establish firm dominance in one or the other

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of the hemispheres, as demonstrated by the equality of their hand skill or their mixed handedness patterns, are cognitively delayed relative to their peers [17].

It seems odd that weak hemispheric lateralization is associated simulta-neously with heightened creativity and lower IQ test scores. The paradox is resolved when one considers that IQ tests involve convergent thinking skills where a person must pinpoint one correct solution as when solving a math problem. Creativity involves divergent thinking where a variety of solutions are emphasized when addressing an issue. A certain level of IQ is required to be creative but the two are not completely overlapping types of thinking. The case of Albert Einstein is offered as one example of a highly creative individual who was also a slow learner as a child. The two hemispheres of Einstein’s brain are unusually symmetrical on post-mortem examination but whether or not Einstein’s brain has any unique characteristics remains a controversial topic [18].

Another issue to consider is the effect of aging on the right and left hemispheres. There is a general pattern of decline in intellectual per-formance with advancing age although there are large individual dif-ferences in the magnitude and rate of the decline. The HAROLD model (Hemispheric Asymmetry Reduction in Older Adults) attempts to explain changes in older adult cognition as they relate to the operation of the two hemispheres. According to this model, older adults rely on both hemi-spheres to complete cognitive tasks done with only one hemisphere by young adults. The age-related shift toward greater cooperation between the hemispheres is thought to be a compensatory strategy that buffers an individual against possible decline in intellectual performance. Data from neuroimaging studies support this view. The older adults who recruited resources from both hemispheres when performing a cognitive task per-formed better than older adults who did not display hemispheric coopera-tion. Whether one hemisphere dominates over the other (asymmetry) or whether the two hemispheres cooperate (symmetry) changes throughout life. High-performing older adults access resources in both hemispheres to maintain their levels of cognitive performance. In regard to handed-ness, left- and right-handers show similar age-related changes in intellec-tual tasks such as the learning and remembering of verbal material and information processing speed [19].

HANDWRITING POSTURES AND THE HEMISPHERES

Today, researchers identify the language hemisphere by scanning the brains of individuals while they perform language tasks. They exam-ine the relative levels of neural activation in the right versus left hemi-sphere to assess which hemisphere is playing the major role in performing

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these functions. In the past, the most reliable ways of determining the speech hemisphere were more invasive. Researchers either examined language deficits in individuals with brain damage or performed the Wada test to anesthetize one of the hemispheres before surgery. In the 1970s, Jerre Levy and colleagues proposed a way of determining language lateralization in individuals whose brains were intact and functioning normally. Levy theorized that the posture an individual assumes when writing with the preferred hand indicates the direction of hemispheric lateralization of language. Figure 4.3 shows the two types of writing postures used by both right- and left-handers. The straight or normal pos-ture indicates opposite or contralateral language lateralization, while the inverted posture shows same-side or ipsilateral language function. Levy used a visual field methodology to test this theory and found that the handedness and writing posture of most of her participants predicted the direction of cerebral lateralization for language. Straight left-hand writers showed a LVF (right hemisphere) advantage for verbal material, while inverted left-hand writers showed a RVF (left hemisphere) advantage for verbal presentations [20].

Levy’s theory drew a lot of scientific attention. Some researchers col-lected data on the overall population prevalence of the two writing pos-tures in right- and left-handers. They wanted to assess the match between Levy’s estimates of right- versus left-hemisphere language based on handwriting posture with estimates derived from other methods such as Wada testing. Other researchers concentrated on studying writing posture effects on visual field or dichotic listening performance.

I conducted a study with a colleague on the first issue. We studied a large sample of individuals ranging in age from 10 to 75 years and asked them to match their writing posture to pictures like those in Figure 4.3. Table 4.1 shows data from this study. Our sample had the usual percent-ages of right- and left-handers (89% vs. 11%, respectively). However, the 8% incidence of inverted writing among right-handers, indicative

FIGURE 4.3 Straight and inverted handwriting postures for left- and right-handers. Source: Adapted from Ref. [21, figure 1, p. 679].

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of right hemisphere language lateralization according to Levy, is higher than the 4% estimate of right hand/right hemisphere language deter-mined through Wada testing. Of the 11% of the left-handers in the study, approximately 50% of them show each writing posture, straight versus inverted. These percentages also disagree with the 70% estimate of left hand/left-hemisphere language found with the Wada method. We found the incidence of inverted writing to be more common among young adults and higher in both male right- and left-handers. The sex difference in the rates of the inverted handwriting posture favoring males and the age differences in the incidence of inverted writing have been replicated by other researchers. In another study I did with colleagues, we found that the incidence of inverted writing was higher in children who had at least one parent who wrote with the inverted posture [21].

Photos of Presidents of the United States illustrate what straight and inverted handwriting postures look like in real life, at least for left-handers. Figure 4.4 shows Presidents Gerald Ford, Bill Clinton, and Barack Obama signing documents with their left hands. Presidents Obama and Ford use an inverted posture, while President Clinton’s handwriting posture is straight. President George H.W. Bush is also a left-hander. Left-handers explain the prominence of their peers in US politics by arguing that left-handers have unique cognitive abilities particularly suited for navigating the complexities of political life. Although this is an unlikely explanation for the impressive achievements of left-handed politicians, the frequency with which they reside in the White House is a point of pride for left-handers [22].

As Table 4.1 shows, the estimates of the side of language lateralization using handwriting posture do not match the estimates obtained from Wada testing. Attempts to verify the Levy model using visual field and dichotic listening paradigms failed to find performance differences con-sistent with the predictions based on handwriting posture. Straight and inverted hand writers do not show the expected directions of visual field or ear advantage predicted by the theory. Ten years after the model was

TABLE 4.1 The side of the writing Hand, straight and inverted Handwriting Posture, and the Proposed Relationship to Hemispheric Lateralization for Language

Side of writing hand

% Observed writing hand posture

% Predicted from Wada test

Right 89% Straight (left-hemisphere speech) 81% Left-hemisphere speech 96%

Inverted (right-hemisphere speech) 8% Right-hemisphere speech 4%

Left 11% Straight (right-hemisphere speech) 6% Right/both hemisphere speech 30%

Inverted (left-hemisphere speech) 5% Left-hemisphere speech 70%

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proposed most investigators dismissed the idea that handwriting posture is linked to language lateralization based on the lack of research evidence to support the theory [23].

One is still left with the question as to why individuals write with dif-ferent hand positions. An inverted handwriting posture is more frequent among left-hand writers, so research into this issue has often focused on left-handers. There are technical considerations in producing legible cur-sive writing that may make it beneficial for left-handers to write with an inverted posture. The left-handed inverter can pull rather than push the pen across the paper, can avoid smudging ink and can see what is written immediately. Another suggested advantage for left-handed writing inver-sion is as an adaptation to right-slanted writing. Requiring individuals to write at slant angles either to the right or to the left can cause a change from a straight to an inverted posture or the reverse. For example, straight right-hand writers switch to an inverted posture when required to write with a leftward slant. The most common cursive handwriting slants to the right making an inverted posture adaptive for a left-hander who writes with a rightward slant.

The fact that the frequency of inverted writing posture varies with sex and age has been attributed to social factors. More recent generations and young boys may not be subject to the pressure to switch from the awkwardness of the inverted writing position in the same way as older generations and young girls. Rates of inverted hand writing increase among children who have a parent who writes in this way. This suggests a modeling effect whereby children adopt the handwriting position they

FIGURE 4.4 Left-handed Presidents of the United States displaying straight and inverted left-hand writing. Presidents Obama and Ford are inverted left-hand writers while President Clinton writes with a straight left-hand posture. Source: Photos were retrieved on July 9, 2015. (A) https://commons.wikimedia.org/wiki/File:President_Ford_signs_his_Crime_Message_to_Congress_-_NARA_-_7140642.jpg; Ricardo Thomas [Public domain], via Wikimedia Commons. (B) https://commons.wikimedia.org/wiki/File:Clinton_signing_document.jpg; by the original uploader was Textorus at English Wikipedia [Public domain], via Wikimedia Commons. (C) https://commons.wikimedia.org/wiki/File:Barack_Obama_signs_HR_3630.jpg; by Pete Souza (White House Flickr Account) [Public domain], via Wikimedia Commons.

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observe being used by a parent. The use of a particular writing posture seems to be caused by a number of factors related to technical aspects of cursive writing and/or social influence rather than by any underlying neurological organization [24].

RIGHT BRAIN, LEFT BRAIN

This chapter tackled a number of fictions about left-handers and the brain. First, language lateralization to the left hemisphere is the most com-mon brain organization for both right- and left-handers. If the language hemisphere dominates thinking, which is a questionable assumption, then most right- and left-handers are logical and analytic left-hemisphere thinkers. If the hemisphere controlling the movements of the preferred hand dominates thinking, another questionable assumption, then right-handers are logical and analytic left-hemisphere thinkers while left- handers are holistic and integrative right-hemisphere thinkers. In reality, there is little evidence to support the idea of either systematic creativity or cognitive style differences between right- and left-handers.

However, several facts have emerged. Language lateralization is more variable among left-handers who are more likely than right-handers to have language lateralized to the right hemisphere (15% vs. 4%, respec-tively). Researchers report that the two hemispheres of the brain func-tion more symmetrically in left-handers when compared to right-handers. Consistency of handedness rather than side, right or left, predicts individ-ual differences in cognitive ability. Left-handers tend to be more mixed in their handedness behaviors when compared to the consistent hand use of right-handers. Past research may have failed to separate the two issues, side versus consistency, leading to the conclusion that left-handedness rather than mixed-handedness is associated with variation in cognitive abilities.

How important is brain lateralization? Neuroscientists point out that laterality is relative and not absolute, and both hemispheres play a role in most behaviors. For example, the left hemisphere is important for language production but the right hemisphere also has language capa-bilities. Laterality also changes. As one ages tasks typically performed by one hemisphere come to depend on both hemispheres. Reductions in hemispheric asymmetry with age are associated with better cognitive performance. Site is as important in understanding brain function as side. The frontal lobe of the right hemisphere, for example, is more similar to the frontal lobe of the left hemisphere than it is to other areas of the right hemisphere. Although there is a general distinction of language in the left hemisphere and visual–spatial processing in the right hemisphere, there is still a lot to learn about how the lateralized processing occurs and/or how the two hemispheres cooperate to complete a language or visual–spatial task [9].

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65REfEREnCEs

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