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
NeuroNames	241
NeuroLex ID	birnlex_1796
TA98	A14.1.09.002
TA2	5418
FMA	61817
Anatomical terms of neuroanatomy [edit on Wikidata]

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 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.

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.

The human brain massively varies from its evolutionary predecessors. Humans are the only animal to possess a distinct right hemisphere in their cerebrum.^[3][4] This hemisphere’s development can, however, be tracked biologically to its beginnings roughly 100,000 years ago.^[4] 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]

Recent studies have since discredited the paradigm that no major lateralization exists between cerebral hemispheres.^[5][6][7]

The following chart highlights the overarching differences without making specific reference to neuron clusters.

The right hemisphere’s main function is to inhibit the left hemisphere.The LHS “is all go”, it is the cause of all action.^[4] LHS has no ability to consider consequence, longevity or risk it merely acts. RHS is the opposite of LHS; it calculates risk versus reward and inhibits LHS based off these calculations among other things. An overdeveloped LHS leads to the inability to control one’s actions and emotions as well as volatility. An overdeveloped RHS leads to inaction, reclusive behavior and phobias.^[5][6][7]

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).^[8]

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.^[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, emotions, etc. seem to be more bilaterally controlled.^[10]

Lifestyle choices and environment shape the way human’s 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. 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.^{[13][5][6][7]}

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.^[4]

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.^[4]

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/into intersections.^[4][14]

It has been inferred, due to decussation, that those who are left-handed or footed have naturally more distinct RHS. Similarly, those who are ambidextrous or are left-handed and right-footed (or vice versa) have naturally distinct LHS and RHS. This topic is yet to be adequately studied, however.^[4]

Wikimedia Commons has media related to Cerebral hemispheres.

• Hemispherectomy
• Corpus callosotomy