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Lateralization of brain function

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Diagram of the human brain.
The human brain is divided into two hemispheres–left and right. Scientists explore how some functions tend to be dominated by one side or the other (how they are 'lateralized')

The longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The structure and function of both hemispheres is very similar. Large parts are bilaterally symmetrical, but there are some important differences.

An example of this is the lateral sulcus which usually longer in the left hemisphere than in the right one. Functionally, Broca's area and Wernicke's area are in the left cerebral hemisphere for about 95% of right-handers, but only about 70% of left-handers.[1] Both areas seem to be involved with speech production.

Popular psychology often makes broad generalizations about one side or the other, such as "logical thinking" being in the left side, or "creativity" being in the right one. However, lateralization does not add special uses in either hemisphere.[2] Both hemispheres contribute to both kinds of processes,[3] Experimental evidence does not support the idea that structural differences between the sides cause such clear functional differences.[4]

The issue is still being investigated. A part of the cerebral cortex, or even an entire hemisphere, may get injured or destroyed. Sometimes its function can be done by a neighboring region in the same hemisphere or the corresponding region in the other hemisphere. This depends on the area damaged and the patient's age.[5] When injury interferes with pathways from one area to another, other (indirect) connections may develop to communicate information with separated areas, but less efficiently.

Brain function lateralization is in right- or left-handedness,[6] and of right or left ear preference.[7] However, 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. Also, 19.8% of the left-handed have bilateral language functions.[8] Even within various language functions (e.g., semantics, syntax, prosody), degree of dominance (and even hemisphere) may differ.[9]

Even though some functions are lateralized, the examples above only show a general tendency. The areas which are being explored to explain these differences include the gross anatomy of the brain, its dendritic structure, and neurotransmitter distribution. The variation between structure and function between the two hemispheres of one brain (or between the same hemisphere of two different brains) is still being studied. Short of a removal of a cerebral hemisphere, no one is a "left-brain only" or "right-brain only" person.[10]

References

  1. ↑ Griggs, Richard A. Psychology: a concise introduction. p. 69.
  2. ↑ Nielsen, Jared A. et al. An evaluation of the left-brain vs. right-brain hypothesis with resting state functional connectivity magnetic resonance imaging. PLOS ONE 2013.
  3. ↑ Westen et al 2006 Psychology: Australian and New Zealand edition. John Wiley p.107
  4. ↑ Toga A.W. & Thompson P.M (2003). "Mapping brain asymmetry". Nature Reviews Neuroscience. 4 (1): 37–48. doi:10.1038/nrn1009. PMID 12511860.
  5. ↑ Pulsifer M.B.; et al. (2004). "The cognitive outcome of hemispherectomy in 71 children". Epilepsia. 45 (3): 243–254. doi:10.1111/j.0013-9580.2004.15303.x. PMID 15009226. {{cite journal}}: Explicit use of et al. in: |author= (help)
  6. ↑ Knecht S; et al. (2000). "Handedness and hemispheric language dominance in healthy humans". Brain : a journal of neurology. 123 (12): 2512–2518. doi:10.1093/brain/123.12.2512. PMID 11099452. {{cite journal}}: Explicit use of et al. in: |author= (help)
  7. ↑ Schönwiesner M; Rübsamen R. & von Cramon D.Y (2005). "Hemispheric asymmetry for spectral and temporal processing in the human antero-lateral auditory belt cortex". European Journal of Neuroscience. 22 (6): 1521–1528. doi:10.1111/j.1460-9568.2005.04315.x. PMID 16190905.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ↑ Taylor, Insep & Taylor, M. Martin 1990. Psycholinguistics: learning and using language. page 362
  9. ↑ BBC News: Regarding different languages. http://www.bbc.co.uk/news/health-11181457
  10. ↑ Goswami U (2006). "Neuroscience and education: from research to practice?". Nat Rev Neurosci. 7 (5): 406–11. doi:10.1038/nrn1907. PMID 16607400. {{cite journal}}: Cite has empty unknown parameter: |1= (help)
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