Lateralization of brain function
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A [[longitudinal fissure]] separates the [[human brain]] into two distinct [[cerebral hemisphere]]s, connected by the [[corpus callosum]]. The sides resemble each other and each hemisphere's structure is generally mirrored by the other side. Yet despite the strong similarities, the functions of each [[cerebral cortex|cortical]] hemisphere are different.
[[Popular psychology]] tends to make broad and sometimes pseudoscientific generalizations about certain functions (e.g. logic, creativity) being lateral, that is, located in either the right or the left side of the brain. Researchers often criticize popular psychology for this, because the popular lateralizations often are distributed across both hemispheres, <ref name="Western 2006">Western et al. 2006 "Psychology: Austraian and New Zealand edition" John Wiley p.107</ref> although mental processing is divided between them.
Fundamental to brain process lateralization is the fact that the [[lateral sulcus]] generally is longer in the left hemisphere than in the right hemisphere. The extent of specialized 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 in the opposite hemisphere, depending upon the area damaged and the patient's age.
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), no one is a "left-brain only" or "right-brain only" person.
Brain function laterization is evident in the phenomena of right- or left-handedness and of right or left ear preference, but a person's preferred hand is not a clear indication of the location of brain function. Although 95% of [[Right-handedness|right-handed]] people have left-hemisphere language function, only 18.8% of [[Left-handedness|left-handed]] people have right-hemisphere language function. Additionally, 19.8% of the left-handed have bilateral language functions.<ref name="psycho2">Taylor, Insep and Taylor, M. Martin (1990) "Psycholinguistics: Learning and using Language". page 362</ref>
==Which side? ==
[[Linear]] [[reason]]ing and [[language]] functions such as grammar and vocabulary often are lateralized to the left hemisphere of the brain. [[Dyscalculia]] is a [[neurology|neurological]] syndrome associated with damage to the left [[temporal lobe|temporo]]-[[parietal lobe|parietal]] junction.<ref name="Levy_1">Levy LM, Reis IL, Grafman J. Metabolic abnormalities detected by 1H-MRS in dyscalculia and dysgraphia. ''Neurology''. 1999 Aug 11;53(3):639-41. PMID 10449137</ref> This syndrome is associated with poor numeric manipulation, poor mental [[arithmetic]] skill, and the inability to either understand or apply mathematical concepts.<ref name="Dyscalculia">[http://www.dyscalculia.org/calc.html Dyscalculia Symptoms]</ref>
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, spatial manipulation, [[face perception|facial perception]], and artistic ability seem to be functions of the right hemisphere.
Other integrative functions, including arithmetic,<ref name=Dehaene2>Dehaene S, Spelke E, Pinel P, Stanescu R, Tsivkin S. Sources of mathematical thinking: behavioral and brain-imaging evidence. Science. 1999 May 7;284(5416):970-4. PMID 10320379.</ref><ref name = Dehaene2003>Stanislas Dehaene, Manuela Piazza, Philippe Pinel, and Laurent Cohen. Three parietal circuits for number processing. Cognitive Neuropsychology, 20:487-506</ref> binaural sound localization, and emotions, seem more bilaterally controlled.
{| class="wikitable"
|'''Left hemisphere functions'''
|'''Right hemisphere functions'''
|----
| sequential
| simultaneous
|----
| analytical
| holistic
|----
| verbal
| imagistic
|----
| logical
| intuitive
|----
| linear algorithmic processing
| holistic algorithmic processing
|----
| mathematics (approximate calculation, numerical comparison, estimation) <br>left hemisphere only: direct fact retrieval<ref name=Dehaene2/><ref name = Dehaene2003/>
| mathematics (approximate calculation, numerical comparison, estimation)<ref name=Dehaene2/><ref name = Dehaene2003/>
|----
| present and past
| present and future {{Fact|date=June 2007}}
|----
| language: grammar/vocabulary, literal<ref name = Talyor>Taylor, Insep, and Taylor, M. Martin (1990) "Psycholinguistics: Learning and using Language". p.367</ref>
| language: intonation/accentuation, prosody, pragmatic, contextual<ref name = Talyor/>
|}
==History==
===Speech and language===
[[Manner of articulation|Speech]] consists of the mechanical processes required for vocalization, such as [[Manner of articulation|articulation]] and [[phonation]]. Language is the set of arbitrary symbols used for communication, often in the form of words strung together per [[syntax|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. The motor aspects of speech production deficits caused by damage to Broca’s 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====
[[Germany|German]] [[physician]] [[Karl Wernicke]] continued in the vein of Broca's researche by studying language deficits unlike Broca aphasias. Wernicke noted that not every deficit was in speech production; some were linguistic. He found that damage to the left [[Anatomical terms of location|posterior]], superior [[temporal lobe|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 Wernicke’s Area are linked by a [[white matter]] fiber tract, the [[arcuate fasciculus]]. This [[axon]]al 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 <ref name="Knecht_1">Knecht S, Dräger B, Deppe M, Bobe L, Lohmann H, Flöel A, Ringelstein EB, Henningsen H. Handedness and hemispheric language dominance in healthy humans. ''Brain''. 2000;123(12):2512-2518. http://brain.oxfordjournals.org/cgi/content/full/123/12/2512</ref> or 61%<ref name="psycho2" />, depending on the studies.
There are ways of determining hemispheric dominance in a person. The [[wada test|Wada Test]] introduces an anesthetic to one hemisphere of the brain via one of the two [[carotid artery|carotid arteries]]. Once the hemisphere is anesthetized, a [[neuropsychology|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.
[[Image:Sensory and motor homunculi.jpg|thumb|Sensory and motor [[Homunculus|homunculi]] at the London Natural History Museum]]
===Movement and sensation===
In the 1940s, [[Canada|Canadian]] [[neurosurgery|neurosurgeon]] [[Wilder Penfield]] and his [[neurologist]] colleague [[Herbert Jasper]] developed a technique of brain mapping to help reduce [[Adverse effect (medicine)|side effect]]s caused by [[surgery]] to treat [[epilepsy]]. They stimulated [[motor cortex|motor]] and [[somatosensory cortex|somatosensory cortices]] of the brain with small electrical currents to activate discrete brain regions. They found that stimulation of one hemisphere's motor cortex produces [[muscle]] contraction on the opposite side of the body. Furthermore, the functional map of the motor and [[Somatosensory system|sensory]] cortices is fairly consistent from person to person; Penfield and Jasper's famous pictures of the motor and sensory [[cortical homunculus|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 [[behavior]]al 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<ref name="Kandel_1">Kandel E, Schwartz J, Jessel T. ''Principles of Neural Science''. 4th ed. p1182. New York: McGraw-Hill; 2000. ISBN 0-8385-7701-6</ref>.
==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]] and [[neurolinguistic programming]] (Drenth 2003:53), brain training equipment, or management training. One explanation for why research on lateralization is so prone to exaggeration and false application is that the left-right brain dichotomy is an easy-to-understand notion, which can be oversimplified and misused for promotion in the guise of science.<ref>Sala, (1999). ''Mind Myths: Exploring Popular Assumptions about the Mind and Brain.'' New York; Wiley</ref> 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.
==See also==
*[[Ambidexterity]]
*[[Cross-dominance]]
*[[Dual brain theory]]
*[[Handedness]]
*[[Laterality]]
*[[Left-handed]]
*[[Right-handed]]
==References==
{{reflist}}
==Sources==
* Goulven Josse, Nathalie Tzourio-Mazoyer (2003) Review: Hemispheric specialization for language. Brain Research Reviews 44 1–12.
*[[Terence Hines|Hines, Terence]] (1987). ''Left Brain/Right Brain Mythology and Implications for Management and Training.'' The Academy of Management Review, 12:4, 600–606.
*Drenth,P (2003) Walks in the Garden of Science: Conference Allea. [www.allea.org/pdf/59.pdf]
==Further reading==
*[[A.R. Luria]], ''Higher cortical functions in man'', [[Basic Books]], 1966.
*[[Eric R. Kandel|Kandel E]], Schwartz J, Jessel T. ''[[Principles of Neural Science]]''. 4th ed. New York: McGraw-Hill; 2000. ISBN 0-8385-7701-6
*[[Michael Gazzaniga|Gazzaniga, M.S.]], Ivry, R., & Mangun, G.R. ''Fundamentals of Cognitive Neuroscience.'' 2nd ed. W.W. Norton, 2002. ISBN 0-393-97777-3
*[[Betty Edwards]], ''The New Drawing on the Right Side of the Brain'', revised & expanded edition. New York: Tarcher; August 30, 1999. ISBN 0-87477-424-1
[[Category:Neuropsychology]]
[[Category:Cerebrum]]
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