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While some functions ''are'' lateralized, these are only a tendency. The trend across 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 include 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.<ref>Goswami U (2006), "Neuroscience and education: from research to practice?" Nat Rev Neurosci 7(5):406&ndash;11 {{doi|10.1038/nrn1907}} pmid: http://www.ncbi.nlm.nih.gov/pubmed/16607400</ref>
While some functions ''are'' lateralized, these are only a tendency. The trend across 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 include 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.<ref>Goswami U (2006), "Neuroscience and education: from research to practice?" Nat Rev Neurosci 7(5):406&ndash;11 {{doi|10.1038/nrn1907}} pmid: http://www.ncbi.nlm.nih.gov/pubmed/16607400</ref>


Brain function lateralization is evident in the phenomena of right- or left-handedness and of right or left ear preference{{Citation needed|date=March 2012}}, 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 dominance for language, 18.8% of [[Left-handedness|left-handed]] people have right-hemisphere dominance for 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> Even within various language functions (e.g., semantics, syntax, [[prosody]]), degree (and even hemisphere) of dominance may differ.<ref>Regarding different languages: http://www.bbc.co.uk/news/health-11181457</ref>
Brain function lateralization is evident in the phenomena of right- or left-handedness and of right or left ear preference,{{Citation needed|date=March 2012}} 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 dominance for language, 18.8% of [[Left-handedness|left-handed]] people have right-hemisphere dominance for 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> Even within various language functions (e.g., semantics, syntax, [[prosody]]), degree (and even hemisphere) of dominance may differ.<ref>Regarding different languages: http://www.bbc.co.uk/news/health-11181457</ref>


==History of research on lateralization==
==History of research on lateralization==

Revision as of 18:29, 9 May 2013

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

Template:Image The longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The hemispheres exhibit strong, but not complete, bilateral symmetry in both structure and function. For example, structurally, the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere, and functionally, Broca's area and Wernicke's area are present only in the left hemisphere in greater than 95% of the population.

Broad generalizations are often made in popular psychology about one side or the other having characteristic labels, such as "logical" for the left side or "creative" for the right. These labels need to be treated carefully; although a lateral dominance is measurable, both hemispheres contribute to both kinds of processes,[1] and experimental evidence provides little support for correlating the structural differences between the sides with such broadly-defined 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, is either injured or destroyed, its functions can sometimes be assumed by a neighboring region in the ipsilateral hemisphere or a corresponding region in the contralateral hemisphere, depending upon the area damaged and the patient's age.[3] When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.

While some functions are lateralized, these are only a tendency. The trend across 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 include 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,[citation needed] 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, 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]

History of research on lateralization

Speech and language

Verbal speech is one of the strongest examples of lateralization in the brain. Nearly 90% of speech comes from the left hemisphere, as is evident after damage to the left side of the brain. Damage in that hemisphere typically leads to Broca's or Wernicke's aphasia, regardless of one's predisposition to hand dominance. [7]

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 Expressive 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 expressive 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 Receptive 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.

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.[8] 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 callosotomy, alien hand syndrome most often manifests as uncontrolled but purposeful movements of the nondominant hand.[citation needed]

Gender Differences

General

Sex and gender differences http://en.wikipedia.org/wiki/Sex_differences_in_human_psychology are apparent in almost every aspect of neural anatomy and physiological psychology. This is definitely true with regards to lateralization differences between men and women. It is generally accepted that male brains are typically much more lateralized than [9] female brains.

The major advantage to a greater lateralized brain is the ability for an individual to focus specific areas of the brain on distinct tasks. However, less lateralized brains, like the majority of women's, are better suited to "spread the work" and therefore engage many parts of the brain at once. Also, when multiple areas of the brain are engaged for similar tasks, the likelihood of recovering from Traumatic brain injury is much higher.

Language and Speech

Strokes and Recovery

Lateralized cognitive processes

Language functions such as grammar, vocabulary and literal meaning[10][11] are typically lateralized to the left hemisphere, especially in right handed individuals.[11] While language production is left-lateralized in up to 90% of right-handed subjects, it is more bilateral, or even right lateralized in approximately 50% of left-handers.[12] In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.[13][14]

The processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally, but may show a right hemisphere superiority.[12] Numerical estimation, comparison and online calculation depend on bilateral parietal regions[15][16] while exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing.[15][16] Dyscalculia is a neurological syndrome associated with damage to the left temporo-parietal junction.[17] This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to either understand or apply mathematical concepts.[18]

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".[19] 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."[20] The delusional misidentification syndromes, reduplicative paramnesia and Capgras delusion are also often the result of right hemisphere lesions.[21][22] There is evidence[23] 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.

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%[24] and 61%,[5] suggesting left handed people tend to be less lateralized than right-handed people.

Methods of Study

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.

Exaggeration of the lateralization concept

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.[25] For example, the implications of the research have no bearing on psychological interventions such as EMDR and neurolinguistic programming,[26] brain training equipment, or management training.[27]

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.[28]

Advantages of brain lateralization

The widespread lateralization of many vertebrate animals indicates an evolutionary advantage associated with the specialization of each hemisphere.[29] In one experiment, baby chicks were lateralized before hatching by exposing their eggs to light.[30] These chicks were set to a task of picking out food from a bed of pebbles. Neither the lateralized, nor the non-lateralized chicks had a problem with this task, but the lateralized chicks only used the eye on the side of which they were lateralized to pick up the pebbles. When presented with a second task of watching for a cutout of a predatory hawk, the discrepancy between lateralized and non-lateralized chicks became evident. Lateralized chicks could pick food out of the pebbles with one eye and one half of the brain[31] while using the other eye and other half of their brain to monitor the skies for predators.[32] Not only could non-lateralized chicks not complete the two tasks simultaneously, but their performance of the single task deteriorated. This suggests that the evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain.[29]

See also

3

References

  1. ^ Westen et al. 2006 "Psychology: Australian and New Zealand edition" John Wiley p.107
  2. ^ Template:Cite PMID
  3. ^ Template:Cite PMID
  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. ^ Carlson, Neil (2013). Physiology of Behavior. Upper Saddle River, New Jersy: Pearson Education, Inc. p. 480-483. ISBN 0205239390.
  8. ^ Kandel E, Schwartz J, Jessel T. Principles of Neural Science. 4th ed. p1182. New York: McGraw–Hill; 2000. ISBN 0-8385-7701-6
  9. ^ Maccoby, Eleanor (1974). The Psychology of Sex Differences. Stanford, California: Stanford University Press. ISBN 0804709742.
  10. ^ Boeree, C.G. (2004). "Speech and the Brain". Retrieved February 17, 2012.
  11. ^ a b Taylor, I. & Taylor, M. M. (1990). Psycholinguistics: Learning and using Language. Pearson. ISBN 978-0-13-733817-7.{{cite book}}: CS1 maint: multiple names: authors list (link) p. 367
  12. ^ a b Beaumont, J.G. (2008). Introduction to Neuropsychology, Second Edition. The Guilford Press. ISBN 978-1-59385-068-5. Chapter 7
  13. ^ Ross ED, Monnot M (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. {{cite journal}}: Unknown parameter |month= ignored (help)
  14. ^ George MS, Parekh PI, Rosinsky N, Ketter TA, Kimbrell TA, Heilman KM, Herscovitch P, Post RM (1996). "Understanding Emotional Prosody Activates Right Hemisphere Regions". Arch Neurol. 53 (7): 665–670. doi:10.1001/archneur.1996.00550070103017. PMID 8929174. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  15. ^ a b Dehaene S, Spelke E, Pinel P, Stanescu R, Tsivkin S (1999). "Sources of mathematical thinking: behavioral and brain-imaging evidence" (PDF). Science. 284 (5416): 970–4. doi:10.1126/science.284.5416.970. PMID 10320379. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  16. ^ a b Dehaene, S.., Piazza, M., Pinel, P. & Cohen, L. (2003). "Three parietal circuits for number processing" (PDF). Cognitive Neuropsychology. 20 (3–6): 487–506. doi:10.1080/02643290244000239. PMID 20957581.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ^ Levy LM, Reis IL, Grafman J (1999). "Metabolic abnormalities detected by 1H-MRS in dyscalculia and dysgraphia". Neurology. 53 (3): 639–41. doi:10.1212/WNL.53.3.639. PMID 10449137. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  18. ^ Dyscalculia Symptoms
  19. ^ Hecht D (2010). "Depression and the hyperactive right-hemisphere". Neurosci. Res. 68 (2): 77–87. doi:10.1016/j.neures.2010.06.013. PMID 20603163. {{cite journal}}: Unknown parameter |month= ignored (help)
  20. ^ Braun CM, Delisle J, Guimond A, Daigneault R (2009). "Post unilateral lesion response biases modulate memory: crossed double dissociation of hemispheric specialisations". Laterality. 14 (2): 122–64. doi:10.1080/13576500802328613. PMID 18991140. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  21. ^ Devinsky O (2009). "Delusional misidentifications and duplications: right brain lesions, left brain delusions". Neurology. 72 (1): 80–7. doi:10.1212/01.wnl.0000338625.47892.74. PMID 19122035. {{cite journal}}: Unknown parameter |month= ignored (help)
  22. ^ Madoz-Gúrpide A, Hillers-Rodríguez R (2010). "[Capgras delusion: a review of aetiological theories]". Rev Neurol. 50 (7): 420–30. PMID 20387212. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  23. ^ Goldberg, E. (2009). The New Executive Brain: Frontal Lobes in a Complex World. New York, NY: Oxford University Press. ISBN 978-0-19-532940-7.
  24. ^ Knecht, S.; Dräger, B.; Deppe, M.; Bobe, L.; Lohmann, H.; Flöel, A.; Ringelstein, E. B.; Henningsen, H. (2000). "Handedness and hemispheric language dominance in healthy humans". Brain. 123 (12): 2512–2518. doi:10.1093/brain/123.12.2512.
  25. ^ 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.
  26. ^ Drenth, J. D. (2003). "Growing anti-intellectualism in Europe; a menace to science". Studia Psychologica.
  27. ^ Sala, Sergio Della (1999). Mind Myths: Exploring Popular Assumptions about the Mind and Brain. New York: Wiley. ISBN 0-471-98303-9.
  28. ^ Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 16209828, please use {{cite journal}} with |pmid=16209828 instead.
  29. ^ a b Halpern et al. 2005 "Lateralization of the Vertebrate Brain: Taking the Side of Model Systems" The Journal of Neuroscience.25(45): 10351-10357
  30. ^ Rogers. 1990 "Light Input and the Reversal of Functional Lateralization in the Chicken Brain" Behav Brain Res 38(3): 211-21
  31. ^ Deng and Rogers. 1997 "Differential Contributions of the Two Visual Pathways to Functional Lateralization in Chicks" Behav Brain Res 87(2): 173-82
  32. ^ Rogers. 2000 "Evolution of Hemispheric Specialization: Advantages and Disadvantages" Brain Lang 73(2): 236-53

Further reading

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