But the two halves of the brain are not exactly alike, and each hemisphere tends to have some functional specializations, where the neural mechanisms of a particular brain function are localized primarily in one half of the brain. A good example of this is the two areas of the brain where speech production and language comprehension are processed (known as Broca’s area and Wernicke’s area respectively), both of which are usually located in the left hemisphere of the brain. Mathematical, analytical and logical processing are also usually carried out in the left hemisphere, while spatial recognition, face recognition, sense perception, emotion processing and artistic functions usually occur in the right. This lateralization and specialization of different areas of the brain is much more marked in humans than in animals, and becomes increasingly marked as we progress for early childhood to adulthood.
It was the French physiologist Paul Broca in the 1860s (as well as his less well-known countryman and near contemporary, Marc Dax, almost 30 years earlier) who noted that, at least in general terms, a person’s handedness tends to indicate a specialized hemisphere on the brain's opposite side, so that a right-handed person probably has a left-hemisphere language specialization, and vice versa. Indeed, for almost a century, until the Wada test (a technique involving the anaesthetizing of one side of the brain using a drug such as sodium amytal or sodium amobarbital) was introduced in the 1960s, a person’s handedness was just about the only clue an operating neurosurgeon had about which hemisphere of a patient’s brain was probably the one specialized for language.
Following Broca's findings, it was initially assumed that handedness and the hemispheric dominance of speech processing were inextricably and intimately connected. However, it soon became apparent, even to Broca, that exceptions and mismatches existed, and that perhaps the association was not as fixed as he had initially thought. Although the incidence of right-hemisphere language dominance does increase more or less linearly with the degree of left-handedness, it turns out to be not quite as simple as that.
In fact, after the work of Springer & Deutsch, Damasio & Damasio, and others in the 1990s, we now know that, although about 95% of right-handers do have left-hemisphere dominance for language functions, only around 19% of left-handers have right-hemisphere language dominance, with another 20% or so processing language functions in both hemispheres (the incidence of language distribution in ambidextrous people is broadly similiar to that found in left-handed people). Other studies report percentages for left-handers of 70%, 15% and 15% (rather than 61%, 19% and 20%), but the finding all suggest that, perhaps unexpectedly, some 60% - 70% of left-handers process language in the left hemisphere, just like right-handers! Indeed, around 93% of all people have left hemisphere language dominance.
In the “standard” right-hander’s brain, the dominant left hemisphere is physically larger and more developed. In particular, Broca’s area and Wernicke’s area are significantly larger (up to three times larger) on the left side. The primary motor cortex - the brain region most directly involved with movement control - is also bigger, denser and more sensitive in the left hemisphere of the standard brain. The more developed motor cortex in the left hemisphere of right-handers is also marked by a deeper and more dramatically folded central sulcus (the deep groove between major folds of the brain). In fact, the left hemisphere is typically altogether more complex, with a greater cell-packing density, and its neural links more tightly connected, leading to a faster response time and a greater facility to process rapid stimuli.
Interestingly, though, the brain of a left-hander is not just a mirror image of a right-hander’s brain. With left-handers, the motor cortex does tend to be larger, and the central sulcus deeper, in the opposite right hemisphere than the left to some extent, as might be expected, but the differentiation is not as marked as with right-handers. In fact, the brain hemispheres of left-handers tend to be much more symmetrical and balanced than those of right-handers, and the differences between the hemispheres less pronounced.
With improvements in neurological techniques in the 1950s, and particularly with Roger Sperry’s ground-breaking “split-brain” experiments of the 1960s and the practice of anaesthetizing one side of the brain using the Wada test, it became possible to pinpoint with ever increasing accuracy the various functions of specific parts of the brain. Many of the generalizations, simplifications and myths about handedness (e.g. that left-handers are inventive, artistic and emotional, while right-handers are typically logical, analytical and cool) had their genesis during this period.
The advent of functional Magnetic Resonance Imaging (fMRI) scanning in the 1990s has enabled our picture of the way the brain functions to be sharpened still further. But, rather than clarifying the situation, recent research has if anything muddied the waters still further, and it seems that the more we find out the less clear-cut brain lateralization appears to be. For instance, Dutch research in 2009 showed that face recognition, normally a right hemisphere function, usually occurs in the left hemisphere of left-handers. However, the research also showed that the ability to order or sequence a list of manual activities appears to be an exclusive specialty of the left hemisphere, regardless of whether a person is left- or right-handed. Other findings appear to have little or no logic to them. For example, there is some evidence that women in general tend to process language more evenly between the two hemispheres (which also seems to fly in the face of the finding that more men than women are left-handed).
It is also becoming clear that there is a certain amount of redundancy built in to the brain’s systems. For instance, one eye is able to perceive both sides of a view if necessary; most but not all of one side of the body may be paralyzed after a one-sided brain injury; etc. In fact, if a whole brain hemisphere is removed at a young age, this redundancy and the brain's innate plasticity can mean that higher mental functions can develop almost completely unimpaired.
Thus, it appears that, while there may be some general rules about hemispheric function specialization, the actual situation is much more complex than we ever thought. Rather than firm rules, they should be seen more as indicators, and the plasticity and complexity of the brain appears to allow for significant variation from these indicators.
The language functions of left-handers in particular are more diffuse and less restricted to one hemisphere than those of right-handers. In fact, Dr. Dan Geschwind, who pioneered brain comparison studies between twins, suggests that this distribution of language functions across the hemispheres in left-handers may actually put them at some risk from neurological disorders such as dyslexia, although it may also afford them some advantages. For example, left-handed war veterans recovering from brain injuries are more likely to regain speech and movement quicker than right-handers, perhaps due to the left-handed brain’s greater "plasticity” (its ability to reconfigure itself and its neural connections).
Interestingly, in the brain of “converted right-handers” (natural left-handers who switched to the right, usually at a young age), most of the activity in the motor and sensory integration input occurs in the left hemisphere of the brain, the same as for most innate right-handers. But there also appears to be significant activity in “hot spots” in the motor, sensory-somatic and audio-visual perception areas of the right hemisphere, which some scientists have interpreted as the brain’s attempts to suppress unwanted left-hand movement, suggesting that the brain is perhaps more hard-wired for handedness than previously thought.
Another promising model for hemispheric simplification was put forward by the British-German team of John Marshall and Gereon Fink in the 1990s. They posited that the left hemisphere of the brain focuses on detail while the right hemisphere is more concerned with the broad background picture. Thus, the left brain (and, by extension, it may be argued, a right-handed person) is better adapted to mental skills requiring a series of discrete steps or to focus on a small fragment of what we perceive. The right hemisphere (and a left-hander), on the other hand, is better able to represent the relative position of objects in space and to handle the emotional and metaphorical aspects of speech. Initially, the model had strong experimental support: when looking at a "navon" (for example, a large letter "R" composed of many small letter "L"s), focusing on the small "L"s triggers activity in the left hemisphere, while focusing on the large "R" predominantly creates mental activity in the right hemisphere. However, inexplicably, if the navon is object-based (e.g. a large anchor shape made up of many small cup shapes), the exact opposite brain activation pattern occurs. It appears that little about the brain is ever straight-forward.
Whatever a person’s handedness, the non-dominant hand also needs to have a fairly active motor-output centre in the brain, especially as many two-handed activities actually require quite complex manipulation of both hands (think of playing a violin, for example, or even peeling a potato). Modern fMRI imaging has shown the extent to which both hemispheres are involved in almost all activities, often in different ways but usually working together simultaneously. One possible explanation for the more balanced hemispheres in the brains of left-handers, then, is that they are more likely to use their non-dominant hand than right-handers in order to cope with right-handed tools, appliances, etc.
Given that, as we have seen, many (if not most) activities require both hemispheres, at least to some extent (e.g. the right hemisphere remembers musical melodies, but rhythm and absolute pitch comes from the left hemisphere; sexual arousal activates the right hemisphere, but the involuntary sexual response occurs in the left; etc), the corpus callosum becomes of paramount importance.
Some studies suggest that left-handers and mixed-handers have a physically thicker and more developed corpus callosum connecting the two hemispheres, in order to facilitate the additional inter-hemispheric communication or “cross-talk” their brain organization requires.This has been dramatically illustrated in patients whose corpus callosum has been severed for medical reasons, and whose right hand can make neat well-formed drawings but with no concept of three-dimensional space or perspective, while their left hand can only make messy malformed figures that neverthless exhibit a complete understanding of three-dimensional space and form.
It should perhaps come as no surprise, then, that musicians who play two-handed wind and string instruments are much more likely to be mixed-handed or left-handed than the average citizen. Piano players, on the other hand, need their two hands to act independently, and so accomplished pianists are more likely to have one dominant hand, whether it be left or right. As an interesting aside, there is at least anecdotal evidence that left-handed pianists have more trouble keeping rhythm (rhythm is typically controlled by the left-side of the brain).
As psychologist Stephen Christman points out, though, there are potential drawbacks to having a highly developed corpus callosum. Part of the job of the corpus callosum is to filter out cross-talk in order to prevent stimuli overload, so the elevated levels of hemispheric interaction in left-handers and mixed-handers can lead to more trouble with tasks requiring multi-tasking and independent processing by the hemispheres (for example, the childhood game of simultaneously patting and rubbing the tummy, interpreting the word “blue” written in green ink, etc).
The psychologist Stanley Coren was particularly influential in establishing some of these stereotypes. He described two modes of thinking: one he called convergent (“a fairly focused application of existing knowledge and rules to the task of isolating a single correct answer”), which he associated with the right-brain dominant left-hander; and the other he called divergent (“moves outward from conventional knowledge into unexplored association”), which he associated with the left-brain dominant right-hander.
Ed Wright, in his popular but rather suspect book A Left-Handed History of the World, goes further in listing among the dominant traits of left-handers: intuition, empathy, visual-spatial ability, lateral thinking, hot-temperedness, solitariness, iconoclasm, risk-taking, experimentalism and fantasy. All this is presented in a pseudo-scientific manner but, given that Wright also incorrectly identifies several famous left-handers in the book, all of this should be taken with a pinch of salt, and merely indicates Wright's apparent ability to find “typically” left-handed traits in individuals who are actually right-handers.
Any kind of left brain-right brain dichotomy of this type is at best a gross simplification. But, more importantly, as we have seen above, linking handedness with hemispheric brain dominance is even more suspect in the light of recent research which shows that, contrary to earlier assumptions, only about 20% of left-handers have right-hemisphere language dominance. Thus, while left-handers may be largely right-brain dominant in terms of motor control, this may or may not be associated with any of the commonly-claimed right-brain attributes (such as intuition, creativity, imagination, etc), which themselves may not be as rigorously associated with the right hemisphere of the brain in the first place.