Neuroanatomy of handedness

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An estimated 90% of the world's human population consider themselves to be right-handed. [1] The human brain's control of motor function is a mirror image in terms of connectivity; the left hemisphere controls the right hand and vice versa. This theoretically means that the hemisphere contralateral to the dominant hand tends to be more dominant than the ipsilateral hemisphere, however this is not always the case [2] and there are numerous other factors which contribute in complex ways to physical hand preference.

Contents

Language and speech areas

Language areas are represented unilaterally in the human brain. [3] In around 95% of right-handers, these brain areas are often located on the left hemisphere, however the proportion reduces in left handers down to around 70%. [2] Therefore 7 in every 100 individuals is right-hemisphered for language and left-hand dominant. [4] It is unclear as to whether or not left-hemisphered left handers suffer any language or writing deficits because of this. Broca's area has been found to have differing grey matter structures depending on handedness. The inferior frontal sulcus, which contains Broca's area, was found to be more continuous in the hemisphere ipsilateral to the dominant hand [5]

Corpus callosum

Because the left arm is controlled by the right hemisphere and vice versa, the corpus callosum has also been found to be larger in left-handers. This is theoretically so that language comprehension and production can more efficiently move from the primary language areas into the motor areas which control the contralateral arm. [6] [7] [8] No research has investigated the effect of being right-hemispheric for language whilst being left-handed, and whether or not the corpus callosum is still larger without the need to communicate across hemispheres, such would be the case in right-hemispheric left-handers.

Planum temporale

The planum temporale is a brain region within Broca's Area, and is thought to be the most asymmetric area of the human brain; with the left side having shown to be five times the size of the right in some individuals. [9] However in people who are left handed, this asymmetry has shown to be reduced [10]

Motor areas

Handedness correlates in motor areas have been found to be more subtle and less pronounced than language areas, [8] but are nevertheless still detectable.

Central sulcus

The surface area of the central sulcus has been found to be larger in the dominant hemisphere, as well as the 'hand knob', an area in the primary motor cortex which is responsible for hand movements, is located more dorsally in the left hemisphere of people who are right- compared to left-handed [11]

Right shift theory

Marian Annett devised the Right Shift Theory in 1972, which states that language areas and motor cortex development is preferential in the left hemisphere due to the theoretical gene RS+. [12] This theory also states that there is no particular gene which causes increased right-hemispheric development compared to left, instead without the RS+ gene the development is a gaussian curve which is centralised. The presence of the RS+ gene promotes left-hemispheric dominance, in turn introducing a right-handedness bias which shifts the curve towards the right. [13]

Corticospinal tract

The corticospinal tract is a bundle of white matter which connects the cerebral cortex with motor neurons in the spinal cord. Notably, humans show a natural asymmetry between left and right tracts, such that the left tract (and therefore connections to the right hand) is significantly larger. However this asymmetry has been found to be reduced in left-handers, suggesting a less biased connection to both hands. [8]

Forced handedness

In order to untangle causality, some research employs a 'forced handedness' group. Left-handers who were forced during childhood to use their right hand showed a larger surface area of the central sulcus in their left hemisphere, which is associated with right-handedness. Also, structures in the basal ganglia such as the putamen also mirrored developmental right-hand dominant individuals in the forced group. [8]

Face processing

The Fusiform Face area is an area typically unilaterally, much like the language areas, and localized on the right fusiform gyrus. [14] However, this brain region has been found to be more bilateral in left-handers; that is the left fusiform gyrus responds more to faces in left-handers than in right-handers. However the occipital face area [15] shows no such correlation, and so handedness is thought to impact face processing on a level in the hierarchy which does not involve the occipital face area, however does include the fusiform gyrus. [16]

Complications

Handedness inventory

Handedness in and of itself tends to be a grey area. The requirements for someone to be right- as opposed to left-handed have been debated, and because individuals who identify as left-handed may also use their right hand for a large number of tasks, identifying two clearcut groups of subjects is a challenging task. The Edinburgh Handedness Inventory is a common parametric test used to determine handedness, by comparing individuals to the population at large. However use of this inventory varies between researchers, and it has been criticized for its use in modern research. [17] This means that an individual which one research group may classify as a left-hander, may be classified as ambidextrous in another; leading to difficulties in comparison between the two.

Conflicting evidence

A number of asymmetrical findings have been disputed, with various studies stating null results in opposition to previously reported differences. [11] This is an issue in handedness neuroscience, as imaging methods are highly susceptible to type 1 errors due to the number of comparisons which they make. [18]

Complexity of causality

The relationship between handedness and its neuronal correlates is complex. Language areas themselves are not concretely correlated, and motor area show exceedingly subtle differences. [8] Because of this, the literature shows many differing opinions. Clearly, advances in research are still necessary to unveil true causal relationships between structural differences and their manifestation in the form of handedness.

Frontal Right/Left Areas and Psychopathology

It has been reported some cases of inmates, showing a larger Right-Prefrontal cortex, yet being controlled or dominant their Left-Prefrontal cortex. [19] [20] and it has been associated to criminal behaviour [21] and also to psychopathic traits. [22]

In a review, it was associated to the "impulsive behaviour", handedness, mostly left and/or crossed lateralities, and above of all, the eyedness or eye-laterality as a key to detect and to relate brain lateralization which that behavioural disorder when it is crossed-eye-hand laterality [23] which has also been related in a work, reporting a sample of 5% of crossed left-handedness into a population of 57 left-handedness (5-6% [24] ) and found possibly associated with emotions and limbic system, as well as to the emergence of need/lack of self-regulation. [25] )

Related Research Articles

<span class="mw-page-title-main">Language center</span> Speech processing areas of the brain

In neuroscience and psychology, the term language center refers collectively to the areas of the brain which serve a particular function for speech processing and production. Language is a core system which gives humans the capacity to solve difficult problems, and provides them with a unique type of social interaction. Language allows individuals to attribute symbols to specific concepts, and utilize them through sentences and phrases that follow proper grammatical rules. Finally, speech is the mechanism by which language is orally expressed.

<span class="mw-page-title-main">Broca's area</span> Speech production region in the dominant hemisphere of the hominid brain

Broca's area, or the Broca area, is a region in the frontal lobe of the dominant hemisphere, usually the left, of the brain with functions linked to speech production.

<span class="mw-page-title-main">Handedness</span> Better performance or individual preference for use of a hand

In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to it being stronger, faster or more dextrous. The other hand, comparatively often the weaker, less dextrous or simply less subjectively preferred, is called the non-dominant hand. In a study from 1975 on 7,688 children in US grades 1-6, left handers comprised 9.6% of the sample, with 10.5% of male children and 8.7% of female children being left-handed. Handedness is often defined by one's writing hand, as it is fairly common for people to prefer to do a particular task with a particular hand. There are people with true ambidexterity, but it is rare—most people prefer using one hand for most purposes.

<span class="mw-page-title-main">Corpus callosum</span> White matter tract connecting the two cerebral hemispheres

The corpus callosum, also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental mammals. It spans part of the longitudinal fissure, connecting the left and right cerebral hemispheres, enabling communication between them. It is the largest white matter structure in the human brain, about 10 in (250 mm) in length and consisting of 200–300 million axonal projections.

<span class="mw-page-title-main">Cerebral hemisphere</span> Left and right cerebral hemispheres of the brain

The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter, the cerebral cortex, that is supported by an inner layer of white matter. In eutherian (placental) mammals, the hemispheres are linked by the corpus callosum, a very large bundle of nerve fibers. Smaller commissures, including the anterior commissure, the posterior commissure and the fornix, also join the hemispheres and these are also present in other vertebrates. These commissures transfer information between the two hemispheres to coordinate localized functions.

<span class="mw-page-title-main">Cingulate cortex</span> Part of the brain within the cerebral cortex

The cingulate cortex is a part of the brain situated in the medial aspect of the cerebral cortex. The cingulate cortex includes the entire cingulate gyrus, which lies immediately above the corpus callosum, and the continuation of this in the cingulate sulcus. The cingulate cortex is usually considered part of the limbic lobe.

<span class="mw-page-title-main">Central sulcus</span> Crevice in the brain separating the frontal and parietal lobes

In neuroanatomy, the central sulcus is a sulcus, or groove, in the cerebral cortex in the brains of vertebrates. It is sometimes confused with the longitudinal fissure.

Split-brain or callosal syndrome is a type of disconnection syndrome when the corpus callosum connecting the two hemispheres of the brain is severed to some degree. It is an association of symptoms produced by disruption of, or interference with, the connection between the hemispheres of the brain. The surgical operation to produce this condition involves transection of the corpus callosum, and is usually a last resort to treat refractory epilepsy. Initially, partial callosotomies are performed; if this operation does not succeed, a complete callosotomy is performed to mitigate the risk of accidental physical injury by reducing the severity and violence of epileptic seizures. Before using callosotomies, epilepsy is instead treated through pharmaceutical means. After surgery, neuropsychological assessments are often performed.

<span class="mw-page-title-main">Brodmann area 44</span> Brain area

Brodmann area 44, or BA44, is part of the frontal cortex in the human brain. Situated just anterior to premotor cortex (BA6) and on the lateral surface, inferior to BA9.

<span class="mw-page-title-main">Brodmann area 45</span> Brain area

Brodmann area 45 (BA45), is part of the frontal cortex in the human brain. It is situated on the lateral surface, inferior to BA9 and adjacent to BA46.

<span class="mw-page-title-main">Wernicke's area</span> Speech comprehension region in the dominant hemisphere of the hominid brain

Wernicke's area, also called Wernicke's speech area, is one of the two parts of the cerebral cortex that are linked to speech, the other being Broca's area. It is involved in the comprehension of written and spoken language, in contrast to Broca's area, which is primarily involved in the production of language. It is traditionally thought to reside in Brodmann area 22, which is located in the superior temporal gyrus in the dominant cerebral hemisphere, which is the left hemisphere in about 95% of right-handed individuals and 70% of left-handed individuals.

The term laterality refers to the preference most humans show for one side of their body over the other. Examples include left-handedness/right-handedness and left/right-footedness; it may also refer to the primary use of the left or right hemisphere in the brain. It may also apply to animals or plants. The majority of tests have been conducted on humans, specifically to determine the effects on language.

<span class="mw-page-title-main">Fusiform gyrus</span> Gyrus of the temporal and occipital lobes of the brain

The fusiform gyrus, also known as the lateral occipitotemporal gyrus,is part of the temporal lobe and occipital lobe in Brodmann area 37. The fusiform gyrus is located between the lingual gyrus and parahippocampal gyrus above, and the inferior temporal gyrus below. Though the functionality of the fusiform gyrus is not fully understood, it has been linked with various neural pathways related to recognition. Additionally, it has been linked to various neurological phenomena such as synesthesia, dyslexia, and prosopagnosia.

Expressive language disorder is one of the "specific developmental disorders of speech and language" recognised by the tenth edition of the International Classification of Diseases (ICD-10). As of the eleventh edition, it is considered to be covered by the various categories of developmental language disorder. Transition to the ICD-11 will take place at a different time in different countries.

<span class="mw-page-title-main">Inferior frontal gyrus</span> Part of the brains prefrontal cortex

The inferior frontal gyrus(IFG), (gyrus frontalis inferior), is the lowest positioned gyrus of the frontal gyri, of the frontal lobe, and is part of the prefrontal cortex.

<span class="mw-page-title-main">Planum temporale</span>

The planum temporale is the cortical area just posterior to the auditory cortex within the Sylvian fissure. It is a triangular region which forms the heart of Wernicke's area, one of the most important functional areas for language. Original studies on this area found that the planum temporale was one of the most asymmetric regions in the brain, with this area being up to ten times larger in the left cerebral hemisphere than the right.

<span class="mw-page-title-main">Lateralization of brain function</span> Specialization of some cognitive functions in one side of the brain

The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. The median longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. Although the macrostructure of the two hemispheres appears to be almost identical, different composition of neuronal networks allows for specialized function that is different in each hemisphere.

<span class="mw-page-title-main">Brain asymmetry</span> Term in human neuroanatomy referring to several things

In human neuroanatomy, brain asymmetry can refer to at least two quite distinct findings:

Emotional lateralization is the asymmetrical representation of emotional control and processing in the brain. There is evidence for the lateralization of other brain functions as well.

Sign language refers to any natural language which uses visual gestures produced by the hands and body language to express meaning. The brain's left side is the dominant side utilized for producing and understanding sign language, just as it is for speech. In 1861, Paul Broca studied patients with the ability to understand spoken languages but the inability to produce them. The damaged area was named Broca's area, and located in the left hemisphere’s inferior frontal gyrus. Soon after, in 1874, Carl Wernicke studied patients with the reverse deficits: patients could produce spoken language, but could not comprehend it. The damaged area was named Wernicke's area, and is located in the left hemisphere’s posterior superior temporal gyrus.

References

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