Lateralization of brain function
A longitudinal fissure separates the human brain into two, distinct cerebral hemispheres connected by the corpus callosum. The sides resemble each other, and, generally, each hemisphere's structure is mirrored by the other side, yet, despite the strong similarities, the functions of each cortical hemisphere are different.
Popular psychology often makes broad generalizations about certain functions (e.g. logic, creativity) being lateral, that is, located in either the right or the left side of the brain. These ideas must be treated carefully, because the popular lateralizations often are distributed across both hemispheres,  however, mental processing is divided between them.
Fundamental to brain process lateralization is the that the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere. The extent of specialised brain function by area remains under investigation. If a specific region of the brain is either injured or destroyed, its functions can, sometimes, be recovered by a neighboring region  even opposite hemisphere, depending upon the area damaged and the patient's age.
It must be noted that  while functions are lateralized  the lateralizations are functional trends, and are not applicable in every case. Short of having undergone a hemispherectomy (removal of a cerebral hemisphere) there are no "left-brain only" or "right-brain only" people.
Brain function laterization is evident in the phenomena of being right-handed or left-handedness, of right-ear or left-ear preference, and right-eye or left-eye preference, but a persons preferred hand is not a clear indication of the location of the brain function. Although 95 per cent of right-handed people have left-hemisphere language function, only 18.80 per cent of left-handed people have right-hemisphere language function. Additionally, 19.80 per cent of the left-handed have bilateral language functions.

# Which side?
Linear reasoning and language functions such as grammar and vocabulary often are lateralized to the left hemisphere of the brain. Dyscalculia is a neurological syndrome associated with damage to the left temporal-pariet junction . This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to either understand or apply mathematical concepts.
In contrast, holistic reasoning language functions, such as intonation and accentuation often are lateralized to the right hemisphere of the brain. Functions such as the transduction of visual and musical stimuli, such as spatial manipulation, facial perception, and artistic ability seem to be functions of the right hemisphere.
Other integrative functions, such as intuitive or heuristic arithmetic, binaural sound localization, emotions, et cetera, seem more bilaterally controlled.

# History

## Speech and language
Speech consists of the mechanical processes required for vocalization, such as articulation and phonation. Language is the set of arbitrary symbols used for communication, often in the form of words strung together per syntactical rules.

### Broca
One of the first indications of brain function lateralization resulted from the research of French physician Pierre Paul Broca, in 1861. His research involved the male patient nicknamed "Tan", who suffered a speech deficit (aphasia); "tan" was one of the few words he could articulate, hence his nickname. In Tan's autopsy, Broca determined he had a syphilitic lesion in the left cerebral hemisphere. This left frontal lobe brain area (Broca's Area) is an important speech production region. Speech production deficits caused by damage to Brocas Area are known as Broca's aphasia. In clinical assessment of this aphasia, it is noted that the patient cannot clearly articulate the language being employed.

### Wernicke
German physician Karl Wernicke continued in the vein of Broca's researched by studying language deficits unlike Broca aphasias. Dr Wernicke noted that not every deficit was in speech production; some were linguistic. He found that damage to the left posterior, superior temporal gyrus (Wernicke's area) caused language comprehension deficits rather than speech production deficits, a syndrome known as Wernicke's aphasia.

### Advance in imaging technique
These seminal works on hemispheric specialization were done on patients and/or postmortem brains, raising questions about the potential impact of pathology on the research findings.  New methods permit the in vivo comparison of the hemispheres in healthy subjects.  Particularly, magnetic resonance imaging (MRI) and positron emission tomography (PET) are important because of their high spatial resolution and ability to image subcortical brain structures.

## Handedness and language
Broca's Area and Wernickes Area are linked by a white matter fibre tract, the arcuate fasciculus. This axonal tract allows the neurons in the two areas to work together in creating vocal language. In more than 95 per cent of right-handed men, and more than 90 per cent of right-handed women, language and speech are subserved by the brain's left hemisphere. In left-handed people, the incidence of left-hemisphere language dominance is 73 per cent  or 61%, depending on the studies.
There are ways of determining hemispheric dominance in a person. The Wada Test introduces an anesthetic to one hemisphere of the brain via one of the two carotid arteries. Once the hemisphere is anesthetized, a neuropsychological examination is effected to determine dominance for language production, language comprehension, verbal memory, and visual memory functions. Less invasive (sometimes costlier) techniques, such as functional magnetic resonance imaging and trans-cranial magnetic stimulation, also are used to determine hemispheric dominance; usage remains controversial for being experimental.

## Movement and sensation
In the 1940s, Canadian neurosurgeon Wilder Penfield and his neurologist colleague Herbert Jasper developed a technique of brain mapping to help reduce side effects caused by surgery to treat epilepsy.  They stimulated motor and somatosensory  cortices of the brain with small electrical currents to activate discrete brain regions.  They found that stimulation of one hemisphere's motor cortex could produce muscle contraction on the opposite side of the body.  Furthermore, the functional map of the motor and sensory cortices is fairly consistent from person to person; Penfield and Jasper's famous pictures of the motor and sensory homunculi were the result.

## Split-brain patients
Research by Michael Gazzaniga and Roger Wolcott Sperry in the 1960s on split-brain patients led to an even greater understanding of functional laterality. Split-brain patients are patients who have undergone corpus callosotomy (usually as a treatment for severe epilepsy), a severing of a large part of the corpus callosum. The corpus callosum connects the two hemispheres of the brain and allows them to communicate. When these connections are cut, the two halves of the brain have a reduced capacity to communicate with each other. This led to many interesting behavioral phenomena that allowed Gazzaniga and Sperry to study the contributions of each hemisphere to various cognitive and perceptual processes. One of their main findings was that the right hemisphere was capable of rudimentary language processing, but often has no lexical or grammatical abilities.

# Pseudoscientific exaggeration of the research
Hines (1987) states that the research on brain lateralization is valid as a research program, though commercial promoters have applied it to promote subjects and products far out of the implications of the research. For example, the implications of the research have no bearing on psychological interventions such as EMDR, brain training equipment, or management training. One explanation for being so prone to exaggeration and false application is that the left-right brain dichotomy is an easy-to-understand notion, yet is often grossly oversimplified and misused for promotion in the guise of science.  The research on lateralization of brain functioning is ongoing, and its implications are always tightly delineated, whereas the pseudoscientific applications are exaggerated, and applied to an extremely wide range of situations.