Cerebral hemisphere

Cerebral hemisphere

Human brain seen from front.

The sheep brain seen from the back. Opening longitudinal fissure, the fissure which separates left and right cerebral hemispheres.
Details
Identifiers
Latin Hemisphaerium cerebri
NeuroLex ID Hemisphere of cerebral cortex
TA A14.1.09.002
FMA 61817

Anatomical terms of neuroanatomy

The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the medial longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter, the cerebral cortex, that is supported by an inner layer of white matter. In eutherian (placental) mammals, the hemispheres are linked by the corpus callosum, a very large bundle of nerve fibers. Smaller commissures, including the anterior commissure, the posterior commissure and the fornix, also join the hemispheres and these are also present in other vertebrates. These commissures transfer information between the two hemispheres to coordinate localized functions.

The central sulcus is a prominent fissure which separates the parietal lobe from the frontal lobe and the primary motor cortex from the primary somatosensory cortex.

Macroscopically the hemispheres are roughly mirror images of each other, with only subtle differences, such as the Yakovlevian torque seen in the human brain, which is a slight warping of the right side, bringing it just forward of the left side. On a microscopic level, the cytoarchitecture of the cerebral cortex, shows the functions of cells, quantities of neurotransmitter levels and receptor subtypes to be markedly asymmetrical between the hemispheres.[1][2] However, while some of these hemispheric distribution differences are consistent across human beings, or even across some species, many observable distribution differences vary from individual to individual within a given species.

Structure

Development

The cerebral hemispheres are derived from the telencephalon. They arise five weeks after conception as bilateral invaginations of the walls. The hemispheres grow round in a C-shape and then back again, pulling all structures internal to the hemispheres (such as the ventricles) with them. The intraventricular foramina (also called the foramina of Monro) allows communication with the lateral ventricles. The choroid plexus is formed from ependymal cells and vascular mesenchyme.

Anthropology

The human brain massively varies from its evolutionary predecessors. Humans are the only animal to possess a distinct right hemisphere in their cerebrum.[3] This hemisphere's development can, however, be tracked biologically to its beginnings roughly 100,000 years ago. In a study of lionesses rearing cubs, disciplinary actions, such as swatting for misbehavior, occurred exclusively using the left paw. Due to decussation this action is found to be initiated by the rightmost part of the lioness’ brain. Each type of animal has a unique number of genes; reflecting on evolutionary science, it is inferred that the coding for the creation of human’s right hemisphere is found in the connection of the ape’s 2a and 2b genes. It is so inferred as these two gene lengths connected are identical to human’s 2nd and no other genetic difference between man and great ape exists.[4][5][6]

Function

Hemisphere lateralization

Lateralization of brain structures is based on general trends expressed in healthy patients, however, there are numerous counterexamples to each generalization. Each human’s brain develops differently leading to unique lateralization in individuals. This is different than specialization as lateralization refers only to the function of one structure divided between two hemispheres. Specialization is much easier to observe as a trend since it has a stronger anthropological history.[4]

The best example of an established lateralization is that of Broca's and Wernicke's Areas (language) where both are often found exclusively on the left hemisphere. These areas frequently correspond to handedness however, meaning the localization of these areas is regularly found on the hemisphere opposite to the dominant hand. Function lateralization such as semantics, prosodic, intonation, accentuation, prosody, etc has since been called into question and largely been found to have a neuronal basis in both hemispheres.[7][8]

Cerebral hemispheres of a human embryo at 8 weeks.

Perceptual information is processed in both hemispheres, but is laterally partitioned: information from each side of the body is sent to the opposite hemisphere (visual information is partitioned somewhat differently, but still lateralized). Similarly, motor control signals sent out to the body also come from the hemisphere on the opposite side. Thus, hand preference (which hand someone prefers to use) is also related to hemisphere lateralization.

Neuropsychologists, including Roger Sperry and Michael Gazzaniga, have studied split-brain patients to better understand lateralization. Sperry pioneered the use of lateralized tachistoscopes to present visual information to one hemisphere or the other. Scientists have also studied people born without a corpus callosum to determine specialization of brain hemispheres.

The magnocellular pathway of the visual system sends more information to the right hemisphere, while the parvocellular pathway sends more information to the left hemisphere.

In some aspects, the hemispheres are asymmetrical; one side is slightly bigger. There are higher levels of the neurotransmitter norepinephrine on the right and higher levels of dopamine on the left. There is more white matter (longer axons) on right and more grey matter (cell bodies) on the left.[9]

Linear reasoning functions of language such as grammar and word production are often lateralized to the left hemisphere of the brain. In contrast, holistic reasoning functions of language such as intonation and emphasis are often lateralized to the right hemisphere of the brain. Other integrative functions such as intuitive or heuristic arithmetic, binaural sound localization, etc. seem to be more bilaterally controlled.[10]

Popular psychology[11] considers each hemisphere a unique brain that is called upon to serve its lateralized functions. This belief has led to ideas such as ‘right-brain and left-brain’ dominating popular belief. This stance, however, is rooted in the misinterpretation of studies done in the early 2000s and has since been fully disproven. These ideas held denote LHS as the 'logical brain' and RHS as the 'creative brain' which, even in the most generalized sense, is opposite the truth.[12][13]

Lifestyle and environment's biological effect

Lifestyle choices and environment shape the way humans' hemispheres develop. This process is ongoing meaning the shape and distinctions present in one’s brain may change throughout life. Those who overindulge, live in excess or take numerous uncalculated risks cultivate distinct LHS with underdeveloped RHS. Alternatively, those who never take risks, are overprotective or excessively seclude themselves grow distinct RHS with underdeveloped LHS. There are two middle grounds. First, those who live average, safe, and unthreatening lives such as those in North American cities do not develop either of their hemispheres.[14] As these brains are not required to take risks nor make calculations to survive the hemispheres remain bland and rather indistinguishable. As expected, individuals facing fatal environments develop both hemispheres along with a greater ability to judge risk versus reward.[15][16][17]

Similarly, those who live in warmer and thus less demanding climates have less distinct hemispheres than those in cold.

In 2015 a study on veteran and high ranking gang members concluded that this group possesses some of the most distinct hemispheres observable. Their ability to take great risk in life-threatening situations and also inhibit themselves well enough to survive is largely unparalleled.[18]

In 2015 a study was conducted on chickadees in two locations, Yukon, Canada, and Texas, USA. Those found in the Yukon had distinct and well developed brains, whereas those in Texas had bland-molded brains. When Yukon chickadees were introduced to various Texan chickadee populations they quickly became an effective alpha among the group. The Texan chickadees began mimicking and following the Yukon chickadee. When Texan chickadees were released in the Yukon they all soon after died.[18]

Pop culture

In 2015 a study conducted on deaths while taking selfies concluded that since the danger was not visible to the individuals they were not inhibited. While taking the photo and attempting to get the best shot many focused their field of vision solely in front of them while backing off cliffs or into intersections.[18][19]

See also

This article uses anatomical terminology; for an overview, see Anatomical terminology.
Wikimedia Commons has media related to Cerebral hemispheres.

References

  1. ↑ Anderson, B.; Rutledge, V. (1996). "Age and hemisphere effects on dendritic structure". Brain 119: 1983–1990. doi:10.1093/brain/119.6.1983.
  2. ↑ Hutsler, J.; Galuske, R.A.W. (2003). "Hemispheric asymmetries in cerebral cortical networks". Trends in Neurosciences 26 (8): 429–435. doi:10.1016/S0166-2236(03)00198-X.
  3. ↑ Garavan, H., T. J. Ross, and E. A. Stein. "Right Hemispheric Dominance of Inhibitory Control: An Event-related Functional MRI Study." Proceedings of the National Academy of Sciences (1999): 8301-306. US National Library of Medicine.
  4. 1 2 Boughner, Julie, and Campbell Rolian. "Developmental Approaches to Human Evolution." Google Books. 22 Jan. 2016. Web. 31 Mar. 2016.
  5. ↑ Sherwood, Chet C., Adam D. Gordon, John S. Allen, Kimberley A. Phillips, Joseph M. Erwin, Patrick R. Hof, and William D. Hopkins. "Aging of the Cerebral Cortex Differs between Humans and Chimpanzees." Proceedings of the National Academy of Sciences of the United States of America. National Academy of Sciences, 25 July 2011. Web. 31 Mar. 2016
  6. ↑ Brown, John N.A. "The Evolution of Humans and Technology Part 1: Humans."Springer. 17 Mar. 2016. Web. 31 Mar. 2016.
  7. ↑ Weiss, Peter H., and Simon D. Ubben. "Where Language Meets Meaningful Action: A Combined Behavior and Lesion." Springer. 29 Oct. 2014. Web. 31 Mar. 2016.
  8. ↑ Riès, Stephanie K., and Nina F. Dronkers. "Choosing Words: Left Hemisphere, Right Hemisphere, or Both? Perspective on the Lateralization of Word Retrieval."Wiley Online Library. 14 Jan. 2016. Web. 31 Mar. 2016.
  9. ↑ R. Carter, Mapping the Mind, Phoenix, London, 2004, Originally Weidenfeld and Nicolson, 1998.
  10. ↑ Dehaene S, Spelke E, Pinel P, Stanescu R, Tsivkin S (1999). "Sources of mathematical thinking: behavioral and brain-imaging evidence". Science 284 (5416): 970–4. doi:10.1126/science.284.5416.970. PMID 10320379.
  11. ↑ Nielsen, Jared A., Brandon A. Zielinski, Michael A. Ferguson, Janet E. Lainhart, and Jeffrey S. Anderson. "An Evaluation of the Left-Brain vs. Right-Brain Hypothesis with Resting State Functional Connectivity Magnetic Resonance Imaging." PLOS ONE, 14 August 2013. Web. 30 August 2013.
  12. ↑ Westen et al. 2006 Psychology: Australian and New Zealand edition. John Wiley p.107
  13. ↑ Toga AW, Thompson PM (2003). "Mapping brain asymmetry". Nature Reviews Neuroscience 4 (1): 37–48. doi:10.1038/nrn1009.PMID 12511860.
  14. ↑ Lindwall, Harry. Knowing Yourself: A Narrative of Accessing the Right Brain Hemisphere. Friesen, 2015. Print.
  15. ↑ Ananthaswamy, Anil, Harald Baayen, Colin Barras, Tim Bayne, and Kirsten Weir. "The Human Brain." New Scientist: The Collection Sept. 2015. Print.
  16. ↑ R. J. Morris (2006) Left Brain, Right Brain, Whole Brain? An examination into the theory of brain lateralization, learning styles and the implications for education. PGCE Thesis, Cornwall College St Austell, http://singsurf.org/brain/rightbrain.html
  17. ↑ Harrison, David W. Brain Asymmetry and Neural Systems: Foundations in Clinical Neuroscience and Neuropsychology. Springer International, 2015. Print.
  18. 1 2 3 Block, J. D. "Hemispheric Differences." Personal interview. 26 September 2015. http://www.iagim.org/Glimer%20III%20study.php
  19. ↑ Newton, Jennifer. "More People Have Died While Taking Selfies than Have Been Killed by Sharks so Far This Year." Mail Online. Associated Newspapers, 23 September 2015. Web.
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