An estimated 90% of the world's human population consider themselves to be

right-handed 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 subjecti ...

. 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 Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

to the dominant hand tends to be more dominant than the

ipsilateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

hemisphere, however this is not always the case and there are numerous other factors which contribute in complex ways to physical hand preference.

Language and speech areas

Language Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of met ...

areas Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an op ...

are represented unilaterally in the human brain. 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%. Therefore 7 in every 100 individuals is right-hemisphered for language and left-hand dominant. It is unclear as to whether or not left-hemisphered left handers suffer any language or writing deficits because of this.

Broca's area Broca's area, or the Broca area (, also , ), is a region in the frontal lobe of the dominant Cerebral hemisphere, hemisphere, usually the left, of the Human brain, brain with functions linked to speech production. Language processing in the brai ...

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

Corpus callosum

Because the left arm is controlled by the right hemisphere and vice versa, the

corpus callosum The corpus callosum (Latin for "tough body"), 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 mam ...

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

arm. 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 The planum temporale is the cortical area just posterior to the auditory cortex (Heschl's gyrus) 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 languag ...

is a brain region within

Broca's Area Broca's area, or the Broca area (, also , ), is a region in the frontal lobe of the dominant Cerebral hemisphere, hemisphere, usually the left, of the Human brain, brain with functions linked to speech production. Language processing in the brai ...

, 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. However in people who are left handed, this asymmetry has shown to be reduced

Motor areas

Handedness correlates in motor areas have been found to be more subtle and less pronounced than language areas, 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

Right shift theory

Marian Annett devised the Right Shift Theory in 1972, which states that language areas and

motor cortex The motor cortex is the region of the cerebral cortex believed to be involved in the planning, control, and execution of voluntary movements. The motor cortex is an area of the frontal lobe located in the posterior precentral gyrus immediately a ...

development is preferential in the left hemisphere due to the theoretical gene RS+. 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 In mathematics, a Gaussian function, often simply referred to as a Gaussian, is a function of the base form f(x) = \exp (-x^2) and with parametric extension f(x) = a \exp\left( -\frac \right) for arbitrary real constants , and non-zero . It is ...

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.

Corticospinal tract

The

corticospinal tract The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neur ...

is a bundle of

white matter White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribution ...

which connects the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of ...

with

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...

s in the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

. 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.

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 The putamen (; from Latin, meaning "nutshell") is a round structure located at the base of the forebrain (telencephalon). The putamen and caudate nucleus together form the dorsal striatum. It is also one of the structures that compose the basal n ...

also mirrored developmental right-hand dominant individuals in the forced group.

Face processing

The

Fusiform Face area The fusiform face area (FFA, meaning spindle-shaped face area) is a part of the human visual system (while also activated in people blind from birth) that is specialized for facial recognition. It is located in the inferior temporal cortex (I ...

is an area typically unilaterally, much like the language areas, and localized on the right

fusiform gyrus 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 inf ...

. 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 The occipital face area (OFA) is a region of the human cerebral cortex which is specialised for face perception. The OFA is located on the lateral surface of the occipital lobe adjacent to the inferior occipital gyrus. The OFA comprises a network o ...

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.

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 The Edinburgh Handedness Inventory is a measurement scale used to assess the dominance of a person's right or Left-handedness, left hand in everyday activities, sometimes referred to as laterality. The inventory can be used by an observer assessing ...

is a common

parametric test Parametric statistics is a branch of statistics which assumes that sample data comes from a population that can be adequately modeled by a probability distribution that has a fixed set of parameters. Conversely a non-parametric model does not ass ...

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. 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. 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.

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. 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. and it has been associated to criminal behaviour and also to psychopathic traits. 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 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%) and found possibly associated with emotions and limbic system, as well as to the emergence of need/lack of self-regulation.{{Cite journal, last1=Sapolsky, first1=R.M., date=2004, title=Colateralization of Broca's area and the visual word form area in left-handers: fMRI evidence, journal=Phil. Trans. R. Soc. Lond., language=en, volume=359, pages=1787–1796)

References

Handedness Neuroanatomy Neuropsychology Motor skills