Lateralization of brain function

This article is about specialization of function between the left and right hemispheres of the brain. For specialization of brain function generally, see Localization of brain function.

The human brain is divided into two hemispheres–left and right. Scientists continue to explore how some cognitive functions tend to be dominated by one side or the other, that is, how they are lateralized.

A longitudinal fissure separates the human brain into two distinct cerebral hemispheres, 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 anatomical similarities, the functions of each cortical hemisphere are managed differently. For example, the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere.

Broad generalizations are often made in popular psychology about one side or the other having characteristic labels such as "logical" or "creative". These labels need to be treated carefully; although a lateral dominance is measurable, these characteristics are in fact existent in both sides,^[1] and experimental evidence provides little support for correlating the structural differences between the sides with functional differences.^[2]

The extent of any modularity, or specialization of brain function by area, remains under investigation. If a specific region of the brain or even an entire hemisphere^[3] is either injured or destroyed, its functions can sometimes be assumed by a neighboring region, even in the opposite hemisphere, depending upon the area damaged and the patient's age. When injury interferes with pathways from one area to another, alternative (indirect) connections may come to exist to communicate information with detached areas, despite the inefficiencies.

While functions are lateralized, these are only a tendency. They trend across the many individuals may also vary significantly as to how any specific function is implemented. The areas of exploration of this causal or effectual difference of a particular brain function includes its gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variance of a particular, brain function, between the two hemispheres of one brain or between the same hemisphere of two different brains is still being studied. Short of having undergone a hemispherectomy (removal of a cerebral hemisphere), no one is a "left-brain only" or "right-brain only" person.^[4]

Brain function lateralization 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-handed people have left-hemisphere dominance for language, only 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of the left-handed have bilateral language functions.^[5] Even within various language functions (e.g., semantics, syntax, prosody), degree (and even hemisphere) of dominance may differ.^[6]

Left vs. right

Linear reasoning^[7] and language functions such as grammar and vocabulary^[8] often are lateralized to the left hemisphere of the brain. Dyscalculia is a neurological syndrome associated with damage to the left temporo-parietal junction.^[9] This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to either understand or apply mathematical concepts.^[10]

In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.^[11][12] The processing of visual and audiological stimuli, spatial manipulation, facial perception, and artistic ability seem to be functions of the right hemisphere. Depression is linked with a hyperactive right hemisphere, with evidence of selective involvement in "processing negative emotions, pessimistic thoughts and unconstructive thinking styles", as well as vigilance, arousal and self-reflection, and a relatively hypoactive left hemisphere, "specifically involved in processing pleasurable experiences" and "relatively more involved in decision-making processes".^[13] Additionally, "left hemisphere lesions result in an omissive response bias or error pattern whereas right hemisphere lesions result in a commissive response bias or error pattern." ^[14] The delusional misidentification syndromes reduplicative paramnesia and Capgras delusion are also often the result of right hemisphere lesions.^[15][16] There is evidence^[17] that the right hemisphere is more involved in processing novel situations, while the left hemisphere is most involved when routine or well rehearsed processing is called for.

Other integrative functions, including arithmetic,^[18][19] binaural sound localization, and emotions (lateralization of emotion), seem more bilaterally controlled.

Left hemisphere functions	Right hemisphere functions
numerical computation (exact calculation, numerical comparison, estimation) left hemisphere only: direct fact retrieval[18][19]	numerical computation (approximate calculation, numerical comparison, estimation)[18][19]
language: grammar/vocabulary, literal[20]	language: intonation/accentuation, prosody, pragmatic, contextual[20]

History

Speech and language

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

German physician Karl Wernicke continued in the vein of Broca's research by studying language deficits unlike Broca's aphasia. 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 Wernicke’s Area are linked by a white matter fiber tract, the arcuate fasciculus^{[dubious – discuss]}. This axonal tract allows the neurons in the two areas to work together in creating vocal language. In more than 95% of right-handed men, and more than 90% 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 has been reported as 73%^[21] and 61%.^[5]

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 Transcranial magnetic stimulation, also are used to determine hemispheric dominance; usage remains controversial for being experimental.

Movement and sensation

In the 1940s, American born, Montreal based 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 produces 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.^[22] Eran Zaidel, however, also studied such patients and found some evidence for the right hemisphere having at least some syntactic ability.

For example: Patients with brain damage from surgery, stroke or infection sometimes develop a syndrome in which they can feel sensations in their hand, but they don't feel responsible for nor able to control its movements. In patients with a corpus callostomy, alien hand syndrome most often manifests as uncontrolled but purposeful movements of the nondominant hand.^{[citation needed]}

Exaggeration

Terence Hines 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 outside the implications of the research.^[23] For example, the implications of the research have no bearing on psychological interventions such as EMDR and neurolinguistic programming,^[24] brain training equipment, or management training.^[25]

Nonhuman brain lateralization

Specialization of the two hemispheres is general in vertebrates including fish, frogs, reptiles, birds and mammals with the left hemisphere being specialized to categorize information and control everyday, routine behavior, with the right hemisphere responsible for responses to novel events and behavior in emergencies including the expression of intense emotions. An example of a routine left hemisphere behavior is feeding behavior whereas as a right hemisphere is escape from predators and attacks from conspecifics.^[26]

See also

• Ambidexterity
• Bicameralism
• Brain asymmetry
• Cross-dominance
• Dual brain theory
• Handedness
• Laterality
• Left brain interpreter

References

1. ^ Westen et al. 2006 "Psychology: Australian and New Zealand edition" John Wiley p.107
2. ^ Toga AW, Thompson PM. (2003). Mapping brain asymmetry. Nat Rev Neurosci. 4(1):37–48.PubMed
3. ^ Girl recovers after half of brain removed. 9 News. March 26, 2010. http://news.ninemsn.com.au/world/1032397/us-girl-recovers-after-risky-brain-surgery 
4. ^ Goswami U (2006), “Neuroscience and education: from research to practice?” Nat Rev Neurosci 7(5):406–11 doi: 10.1038/nrn1907 pmid: http://www.ncbi.nlm.nih.gov/pubmed/16607400
5. ^ ^{a b} Taylor, Insep and Taylor, M. Martin (1990) "Psycholinguistics: Learning and using Language". page 362
6. ^ Regarding different languages: http://www.bbc.co.uk/news/health-11181457
7. ^ Left/Right Processing.
8. ^ Dr. C. George Boeree. Speech and the Brain.
9. ^ 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
10. ^ Dyscalculia Symptoms
11. ^ Ross ED, Monnot M (January 2008). "Neurology of affective prosody and its functional-anatomic organization in right hemisphere". Brain Lang. 104 (1): 51–74. doi:10.1016/j.bandl.2007.04.007. PMID 17537499. 
12. ^ George MS, Parekh PI, Rosinsky N, Ketter TA, Kimbrell TA, Heilman KM, Herscovitch P, Post RM (July 1996). "Understanding Emotional Prosody Activates Right Hemisphere Regions". Arch Neurol. 53 (7): 665–670. PMID 8929174. 
13. ^ PMID 20603163
14. ^ PMID 18991140
15. ^ PMID 19122035
16. ^ PMID 20387212
17. ^ Elkhonon Goldberg.(2009) The New Executive Brain: Frontal Lobes in a Complex World, NY: Oxford University Press; paperback. ISBN 978-0195329407
18. ^ ^{a b c} 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.
19. ^ ^{a b c} Stanislas Dehaene, Manuela Piazza, Philippe Pinel, and Laurent Cohen. Three parietal circuits for number processing. Cognitive Neuropsychology, 20:487–506
20. ^ ^{a b} Taylor, Insep, and Taylor, M. Martin (1990) "Psycholinguistics: Learning and using Language". p. 367
21. ^ 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
22. ^ Kandel E, Schwartz J, Jessel T. Principles of Neural Science. 4th ed. p1182. New York: McGraw–Hill; 2000. ISBN 0-8385-7701-6
23. ^ Hines, Terence (1987) Left Brain/Right Brain Mythology and Implications for Management and Training, The Academy of Management Review, Vol. 12, No. 4, October 1987
24. ^ Drenth, J. D. (2003) Growing anti-intellectualism in Europe; a menace to science, Studia Psychologica
25. ^ Sala, (1999). Mind Myths: Exploring Popular Assumptions about the Mind and Brain. New York; Wiley
26. ^ Vallortigara G, Rogers LJ. (2005). Survival with an asymmetrical brain: advantages and disadvantages of cerebral lateralization. Behav Brain Sci.28(4):575–89. PMID 16209828

Sources

• Drenth, Pieter (2006). Walks in the Garden of Science: Selected Papers and Lectures. http://www.allea.org/pdf/59.pdf.  Conference allea.
• Josse, Goulven; Nathalie Tzourio-Mazoyer (2003). "Review: Hemispheric specialization for language". Brain Research Reviews 44 (1): 1–12. doi:10.1016/j.brainresrev.2003.10.001. PMID 14739000. 
• Hines, Terence (1987). "Left Brain/Right Brain Mythology and Implications for Management and Training". The Academy of Management Review 12 (4): 600–606. doi:10.2307/258066. JSTOR 258066. 

Further reading

• Edwards, Betty (1999-08-30). The New Drawing on the Right Side of the Brain. New York: Tarcher. ISBN 0-87477-424-1. 
• Gazzaniga, Michael S.; R. Ivry; G.R. Mangun (2002). Fundamentals of Cognitive Neuroscience (2 ed.). W. W. Norton. ISBN 0-393-97777-3. 
• Harnad, Stevan; Doty, R.W., Goldstein, L., Jaynes, J. & Krauthamer, G. (1977). Lateralization in the nervous system. Academic Press. ISBN 978-0123257505. 
• Kandel, Eric R.; T. Jessel; J. Schwartz (2000). Principles of Neural Science (4 ed.). New York: McGraw–Hill. ISBN 0-8385-7701-6. 
• Luria, A. R. (1966). Higher cortical functions in man. Basic Books. 
• Ornstein, Robert (1998). The Right Mind: Making Sense of the Hemispheres. Harcourt Brace International. ISBN 9780156006279. 

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