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
Latin Hemisphaerium cerebri
Identifiers
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 hippocampal commissure 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 types of cells, types of neurotransmitters 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.

Function

Hemisphere lateralization

Broad generalizations are often made in popular psychology about certain functions (e.g. logic, creativity) being lateralized, that is, located in the right or left side of the brain. These claims are often inaccurate, as most brain functions are actually distributed across both hemispheres. Most scientific evidence for asymmetry relates to low-level perceptual functions rather than the higher-level functions popularly discussed (e.g. subconscious processing of grammar, not "logical thinking" in general).[3][4]

Cerebral hemispheres of a human embryo at 8 weeks.

The best evidence of lateralization for one specific ability is language. Both of the major areas involved in language skills, Broca's area and Wernicke's area, are in the left hemisphere.

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.[5]

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, emotions, etc. seem to be more bilaterally controlled.[6]

Left hemisphere functions Right hemisphere functions
numerical computation (exact calculation, numerical comparison, estimation)
left hemisphere only: direct fact retrieval[6][7]		 numerical computation (approximate calculation, numerical comparison, estimation)[6][7]
language: grammar/vocabulary, literal[8] language: intonation/accentuation, prosody, pragmatic, contextual[8]

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. ↑ Western et al. 2006 "Psychology: Australian and New Zealand edition" John Wiley p.107
  4. ↑ "Neuromyth 6" http://www.oecd.org/document/63/0,3746,en_2649_35845581_34555007_1_1_1_1,00.html Retrieved October 15, 2011.
  5. ↑ R. Carter, Mapping the Mind, Phoenix, London, 2004, Originally Weidenfeld and Nicolson, 1998.
  6. ↑ 6.0 6.1 6.2 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.
  7. ↑ 7.0 7.1 Dehaene, Stanislas; Piazza, Manuela; Pinel, Philippe; Cohen, Laurent (2003). "Three parietal circuits for number processing". Cognitive Neuropsychology 20 (3): 487–506. doi:10.1080/02643290244000239. PMID 20957581.
  8. ↑ 8.0 8.1 Taylor, Insup & Taylor, M. Martin (1990). Psycholinguistics: learning and using language. Englewood Cliffs, N.J: Prentice Hall. p. 367. ISBN 0-13-733817-1.