Editing Human brain (section)

===Lateralisation===
{{Main |Lateralization of brain function}}
{{Further |Functional specialization (brain)}}
{{See also|Contralateral brain}}
The cerebrum has a [[contralateral brain|contralateral organisation]] with each hemisphere of the brain interacting primarily with one half of the body: the left side of the brain interacts with the right side of the body, and vice versa. This is theorized to be caused by a developmental [[Axial Twist theory|axial twist]].<ref name="Lussanet2012">{{cite journal | first1=M.H.E. | last1=de Lussanet | first2=J.W.M. | last2=Osse | year=2012 | title=An ancestral axial twist explains the contralateral forebain and the optic chiasm in vertebrates | journal=Animal Biology | volume=62 | issue=2 | pages=193–216 | doi=10.1163/157075611X617102 | arxiv=1003.1872 | s2cid=7399128}}</ref> Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both [[decussation|cross sides]] in the brainstem. Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called the [[optic chiasm]], and half of the fibres from each nerve split off to join the other.<ref>{{cite book |author=Hellier, J. |title=The Brain, the Nervous System, and Their Diseases [3 volumes] |isbn=978-1-61069-338-7 |publisher=[[ABC-CLIO]] |year=2014 |page=1135 |url=https://books.google.com/books?id=SDi2BQAAQBAJ&pg=PA1135}}</ref> The result is that connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain.<ref name="Kolb 2">{{cite book |last1=Kolb |first1=B. |last2=Whishaw |first2=I.Q. |title=Introduction to Brain and Behavior |isbn=978-1-4641-3960-4 |publisher=[[Macmillan Higher Education]] |year=2013 |page=296 |url=https://books.google.com/books?id=teUkAAAAQBAJ}}</ref> Because each half of the retina receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa.<ref name="Berntson">{{cite book |last1=Berntson |first1=G. |last2=Cacioppo |first2=J. |title=Handbook of Neuroscience for the Behavioral Sciences, Volume 1 |publisher=[[John Wiley & Sons]] |year=2009 |page=145 |isbn=978-0-470-08355-0 |url=https://books.google.com/books?id=LwdJhh8bOvwC&pg=PA145}}</ref> Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field.<ref name="Sherwood">{{cite book |last1=Sherwood |first1=L. |title=Human Physiology: From Cells to Systems |isbn=978-1-133-70853-7 |publisher=[[Cengage Learning]] |year=2012 |page=181 |url=https://books.google.com/books?id=CZkJAAAAQBAJ&pg=PT181}}</ref><ref name="Kalat">{{cite book |author=Kalat, J |title=Biological Psychology |isbn=978-1-305-46529-9 |publisher=[[Cengage Learning]] |year=2015 |page=425 |url=https://books.google.com/books?id=EzZBBAAAQBAJ&pg=PA425}}</ref>

The left and right sides of the brain appear symmetrical, but they function asymmetrically.<ref name="Cowin">{{cite book |last1=Cowin |first1=S.C. |last2=Doty |first2=S.B. |title=Tissue Mechanics |isbn=978-0-387-49985-7 |publisher=[[Springer Science & Business Media]] |year=2007 |page=4 |url=https://books.google.com/books?id=8BJhRkat--YC&pg=PA4}}</ref> For example, the counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling the left hand. There are, however, several important exceptions, involving language and spatial cognition. The left frontal lobe is dominant for language. If a key language area in the left hemisphere is damaged, it can leave the victim unable to speak or understand,<ref name="Cowin"/> whereas equivalent damage to the right hemisphere would cause only minor impairment to language skills.

A substantial part of current understanding of the interactions between the two hemispheres has come from the study of "[[split-brain]] patients"—people who underwent surgical transection of the corpus callosum in an attempt to reduce the severity of epileptic seizures.<ref name="Myers">{{cite book |last1=Morris |first1=C.G. |last2=Maisto |first2=A.A. |title=Understanding Psychology |isbn=978-0-205-76906-3 |publisher=[[Prentice Hall]] |year=2011 |page=56 |url=https://books.google.com/books?id=hoVWAAAAYAAJ}}</ref> These patients do not show unusual behaviour that is immediately obvious, but in some cases can behave almost like two different people in the same body, with the right hand taking an action and then the left hand undoing it.<ref name="Myers"/><ref name="Kolb 3">{{cite book |last1=Kolb |first1=B. |last2=Whishaw |first2=I.Q. |title=Introduction to Brain and Behavior (Loose-Leaf) |isbn=978-1-4641-3960-4 |publisher=[[Macmillan Higher Education]] |year=2013 |pages=524–549 |url=https://books.google.com/books?id=teUkAAAAQBAJ}}</ref> These patients, when briefly shown a picture on the right side of the point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable to describe it, but may be able to give an indication with the left hand of the nature of the object shown.<ref name="Kolb 3"/><ref name="Schacter">{{cite book |last1=Schacter |first1=D.L. |last2=Gilbert |first2=D.T. |last3=Wegner |first3=D.M. |title=Introducing Psychology |isbn=978-1-4292-1821-4 |publisher=[[Macmillan Publishers|Macmillan]] |year=2009 |page=80 |url=https://books.google.com/books?id=gt8lpZylVmkC&pg=PA80}}</ref>