Lefties, Language, and Lateralization | The Scientist Magazine®
brain lateralization for language functions that has to do with the question of which on handedness usually focused on the relationship between hande dness. What does Brain Lateralization have to do with Handedness (and who cares)? This association between hand and brain captured the imaginations of. Also well-known is that the brain is “cross-wired”, with the left hemisphere This lateralization and specialization of different areas of the brain is much more and that perhaps the association was not as fixed as he had initially thought.
Convergent models of handedness and brain lateralization
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.
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.
Another promising model for hemispheric simplification was put forward by the British-German team of John Marshall and Gereon Fink in the s.
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-handeron 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: However, inexplicably, if the navon is object-based e. It appears that little about the brain is ever straight-forward.
Handedness and Brain Lateralization
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.
The Function of the Corpus Callosum Given that, as we have seen, many if not most activities require both hemispheres, at least to some extent e. 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.Which Side of Your Brain Is More Dominant?
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.
- Lateralization of brain function
- Lefties, Language, and Lateralization
Stereotypes and Simplifications The corollary of all this complexity and inconsistency is that many of the common claims and stereotypes regarding handedness and left-handedness in particular arise from a simplified, outdated and unrigorous view of the left-handed brain which is just not borne out by the findings of modern science. The brain organization of left-handers may or may not manifest itself in the kinds of advanced intuitions and outside-the-box thinking that are often claimed for them, although such leaps of imagination may of course occur as indeed they may in right-handers.
The psychologist Stanley Coren was particularly influential in establishing some of these stereotypes. He described two modes of thinking: It is an example of a parsimonious principle that can account for a large range of observable behaviors, a foundation of the scientific process Brody, Rogers further hypothesized that separating neural circuits across the hemispheres might reduce interference between potentially competing processes, thus allowing more efficient behavior.
In a test of this hypothesis, Rogers and colleagues compared visual processing behaviors in groups of chicks with and without lateralized visual systems, controlled by exposing the embryo to different light regimes Rogers et al.
After hatching, the two groups of chicks were tested on a dual task, which required a normally right hemisphere process, scanning for predators, and a normally left hemisphere process, sorting food grains from pebbles. The results indicated that both groups performed each isolated task well, but only the lateralized chicks could effectively carry out the two tasks simultaneously. Thus, a single integrated behavior involving sorting food and scanning the environment is accomplished by recruiting two neural processes, across the two hemispheres.
This both supports the hypothesis that neural lateralization imparts behavioral efficiency through separation of parallel neural processes, and suggests how lateralization might have contributed to natural selection in the evolutionary process.
Recent research examining motor control differences between the dominant and non-dominant arms suggests that Roger's hypothesis might also explain handedness. That is, the left hemisphere in right handers might be specialized for controlling movements through predictive mechanisms that are most effective under consistent and stable mechanical conditions, while the right hemisphere might be specialized for impedance control, which imparts stability when mechanical conditions are unpredictable, or when stabilizing steady state position at the end of a movement.
The dynamic dominance hypothesis provides a framework for understanding handedness within roger's hypothesis Over the past decade, our laboratory has developed a model of motor lateralization Sainburg,; Mutha et al.
This model is based on fundamental principles of control theory that account for a range of experimental findings in different tasks and task conditions. The dynamic dominance hypothesis of motor lateralization proposes that the left hemsiphere in right-handers is specialized for processes that account for predictable dynamic conditions, in order to specify movements that are mechanically efficient, and have precise trajectories.
In contrast, the right hemisphere in right-handers is specialized for impedance control mechanisms that ensure positional and velocity stabilization in the face of unpredictable mechanical events and conditions, and accuracy and stability of steady state postures. The former process assures mechanical efficiency and trajectory specificity under predictable conditions, while the latter imparts robustness under unpredictable conditions, as well as postural stability.
Through studies in stroke patients with specific unilateral brain lesions, we have provided evidence that both processes contribute to control of each arm. However, the hemisphere contralateral to a given arm imparts the greatest influence to that arm's performance. In terms of Roger's hypothesis, the right hemisphere is specialized for a system that ensures stability and rapid online responses to unexpected stimuli in the internal and external environments, while the left hemisphere exploits predictive processes to assure trajectory precision and mechanical efficiency when conditions are consistent and predictable.
Hybridization of predictive and impedance mechanisms allows efficient and robust control of movements Energy conservation has clearly played a significant role in the process of human evolution, contributing to our tendency to exploit coordination patterns that are energy efficient Alexander, ; Nishii and Taniai, Predictive mechanisms can be used in order to minimize costs, such as energy and smoothness, when environmental conditions are predictable.
Thus, optimality is an important principle for predictive control Todorov, However, because environmental conditions are often unpredictable, impedance control through modulation of feedback gains is also an important component of biological movements Scott, ; Mutha et al.
Indeed, from a mechanical perspective, the world can be very unstable and unpredictable. For example, inertial interactions while riding in a vehicle and holding or reaching for a cup of coffee can be quite large when changes in acceleration are not anticipated. Similarly, slicing an irregular shaped piece of fruit or vegetable can be unstable because it can slip or rock with force components applied by a knife.
It should also be stressed that one's own motor commands can introduce unanticipated errors in intended movements, due to errors in prediction, and noise in central processes that might include erroneous sensory estimates Faisal and Wolpert, Thus, in addition to predictive mechanisms that can produce smooth and efficient coordination patterns, impedance mechanisms can assure stability in the face of unexpected external and internal conditions, and can assure steady state positions at the end of motion.
Predictive control mechanisms can be used to optimize a combination of kinematic and dynamic costs of movement Hogan and Sternad, ; Yadav and Sainburg, However, predictive control based on such optimization principles, whether implemented through open loop or optimal feedback control schemes Todorov,is not robust to unanticipated changes in task conditions.
In addition, achieving stable final positions through such mechanisms can be sensitive to internally generated prediction errors and neural noise. In fact, in a recent series of experiments, Scheidt and Ghez Ghez et al.