Editing Split-brain (section)

== Hemispheric specialization ==

The two hemispheres of the [[cerebral cortex]] are linked by the corpus callosum, through which they communicate and coordinate actions and decisions. Communication and coordination between the two hemispheres is essential because each hemisphere has some separate functions.<ref>{{cite web|last=Adams|first=Juliet|title=The Neuroscience of Mindfulness|url=http://www.mindfulnet.org/page25.htm|publisher=Mindfulnet|access-date=28 April 2014|archive-date=February 10, 2011|archive-url=https://web.archive.org/web/20110210202730/http://www.mindfulnet.org/page25.htm|url-status=dead}}</ref> The right hemisphere of the cortex excels at nonverbal and spatial tasks, whereas the left hemisphere is more dominant in verbal tasks, such as speaking and writing. The right hemisphere controls the primary sensory functions of the left side of the body. In a cognitive sense the right hemisphere is responsible for recognizing objects and timing, and in an emotional sense it is responsible for empathy, humor and depression. On the other hand, the left hemisphere controls the primary sensory functions of the right side of the body and is responsible for scientific and math skills, and logic.<ref>{{cite journal|last=Witelson|first=Sandra F.|author2=Wazir Pallie|title=Left Hemisphere Specialization for Language in the Newborn|journal=Neuroanatomical Evidence of Asymmetry|year=1973|volume=96|issue=3|pages=641–646|doi=10.1093/brain/96.3.641|pmid=4795531}}</ref> The extent of specialized brain function by an area remains under investigation. It is claimed that the difference between the two hemispheres is that the left hemisphere is "analytic" or "logical" while the right hemisphere is "holistic" or "intuitive".<ref>{{Cite web |url=http://bama.ua.edu/~st497/pdf/rightorleftbrain.pdf |title=Archived copy |access-date=2012-03-15 |archive-date=2015-07-24 |archive-url=https://web.archive.org/web/20150724034044/http://bama.ua.edu/~st497/pdf/rightorleftbrain.pdf |url-status=dead }}</ref> Many simple tasks, especially comprehension of inputs, require functions that are specific to both the right and left hemispheres and together form a one-direction systematized way of creating an output{{clarify|date=June 2016}} through the communication and coordination that occurs between hemispheres.<ref>{{cite journal|last1=Borod|first1=Joan|first2=Fani |last2=Andelman |first3=Loraine K. |last3=Obler |first4=James |last4=Tweedy |first5=Joan |last5=Wilkowitz |title=Right Hemisphere Specialization for the Identification of Emotional Words and Sentences|journal=Neuropsychologia|year=1992|volume=30|issue=9|pages=827–844|doi=10.1016/0028-3932(92)90086-2|pmid=1407497|s2cid=31912202}}</ref>

=== Role of the corpus callosum ===
[[File:Corpus callosum.gif|thumb|The [[corpus callosum]], highlighted in red, is the main neural pathway between the two hemispheres.]]
The corpus callosum is a structure in the brain along the [[longitudinal fissure]] that facilitates much of the communication between the two hemispheres. This structure is composed of [[white matter]]: millions of [[axon]]s that have their [[dendrite]]s and [[terminal bouton]]s projecting in both the right and left hemisphere. However, there is evidence that the corpus callosum may also have some inhibitory functions.<ref>{{cite journal | last = O'Shea |first=R. P. | year = 2003 | title = Binocular rivalry in split-brain observers | journal = Journal of Vision | volume = 3 | issue = 10| pages = 610–615 | doi=10.1167/3.10.3| pmid = 14640884 | doi-access = free }}</ref> Post-mortem research on human and monkey brains shows that the corpus callosum is functionally organized.<ref>{{cite journal|last=Mooshagian|first=Eric|title=Anatomy of the Corpus Callosum reveals its Function|journal=Journal of Neuroscience|year=2008|volume=28|issue=7|pages=1535–1536| pmc=6671538|  doi=10.1523/JNEUROSCI.5426-07.2008|pmid=18272674|doi-access=free}}</ref> This organization results in modality-specific regions of the corpus callosum that are responsible for the transfer of different types of information. Research has revealed that the anterior midbody transfers motor information, the posterior midbody transfers somatosensory information, the isthmus transfers auditory information, and the splenium transfers visual information.<ref>{{cite journal |vauthors=Risse GL, Gates J, Lund G, Maxwell R, Rubens A | year = 1989 | title = Inter-hemispheric transfer in patients with incomplete section of the corpus callosum: anatomic verification with magnetic resonance imaging | journal = Archives of Neurology | volume = 46 | issue = 4| pages = 437–443 | doi=10.1001/archneur.1989.00520400097026| pmid = 2705905 }}</ref> Although much of the inter-hemispheric transfer occurs at the corpus callosum, there are trace amounts of transfer via commissural pathways, such as the anterior commissure, posterior commissure, habenular commissure, and the hippocampal commissure'''.'''<ref>{{Cite web |title=Commissural pathways |url=https://www.kenhub.com/en/library/anatomy/commissural-pathways |access-date=2023-11-16 |website=Kenhub |language=en}}</ref>

Studies of the effects on the visual pathway on split-brained patients has revealed that there is a redundancy gain (the ability of target detection to benefit from multiple copies of the target) in simple reaction time. In a simple response to visual stimuli, split-brained patients experience a faster reaction time to bilateral stimuli than predicted by model.<ref>{{cite journal |last1=Corballis |first1=M. C. |last2=Corballis |first2=P. M. |last3=Fabri |first3=M. | year = 2003 | title = Redundancy gain in simple reaction time following partial and complete callosotomy | journal = Neuropsychologia | volume = 42 |issue=1 | pages = 71–81 | doi=10.1016/s0028-3932(03)00152-0|pmid=14615077 |citeseerx=10.1.1.503.8952 |s2cid=45852555 }}</ref> A model proposed by Iacoboni et al.<ref>{{cite journal |last1=Iacoboni |first1=M. |last2=Fried |first2=I. |last3=Zaidel |first3=E. | year = 1994 | title = Callosal transmission time before and after partial commissurotomy | journal = NeuroReport | volume = 5 | issue = 18| pages = 2521–2524 | doi=10.1097/00001756-199412000-00029|pmid=7696594 }}</ref> suggests split-brained patients experience asynchronous activity that causes a stronger signal, and thus a decreased reaction time. Iacoboni also suggests there exists dual attention in split-brained patients, which implies that each cerebral hemisphere has its own attentional system.<ref>{{cite journal |last1=Arguin |first1=M. |last2=Lassonde |first2=M. |last3=Quattrini |first3=A. |last4=Del Pesce |first4=M. |last5=Foschi |first5=N. |last6=Papo |first6=I. | year = 2000 | title = Divided visuo-spatial attention systems with total and anterior callosotomy | journal = Neuropsychologia | volume = 38 | issue = 3| pages = 283–291 | doi=10.1016/s0028-3932(99)00077-9|pmid=10678694 |s2cid=23798895 }}</ref> An alternative approach taken by Reuter-Lorenz et al.<ref>{{cite journal |last1=Reuter-Lorenz |first1=P. A. |last2=Nozawa |first2=G. |last3=Gazzaniga |first3=M. S. |last4=Hughes |first4=H. C. | year = 1995 | title = Fate of neglected targets: a chronometric analysis of redundant target effects in the bisected brain | journal = Journal of Experimental Psychology: Human Perception and Performance | volume = 21 | issue = 2| pages = 211–230 | doi=10.1037/0096-1523.21.2.211|pmid=7714469 }}</ref> suggests that enhanced redundancy gain in the split brain is primarily due to a slowing of responses to unilateral stimuli, rather than a speeding of responses to bilateral ones.

The simple reaction time in split-brained patients, even with enhanced redundancy gain, is slower than the reaction time of normal adults.{{citation needed|date=December 2020}}

=== Functional plasticity ===

Following a stroke or other injury to the brain, functional deficiencies are common. The deficits are expected to be in areas related to the part of the brain that has been damaged; if a stroke has occurred in the motor cortex, deficits may include paralysis, abnormal posture, or abnormal movement synergies.<ref>{{cite journal |last1=Nudo |first1=R. J. |last2=Plautz |first2=E. J. |last3=Frost |first3=S. B. | year = 2001 | title = Role of adaptive plasticity in recovery of function after damage to motor cortex. [Review] | journal = Muscle & Nerve | volume = 24 | issue = 8| pages = 1000–1019 | doi=10.1002/mus.1104|pmid=11439375 |s2cid=32457492 }}</ref> Significant recovery occurs during the first several weeks after the injury. However, recovery is generally thought not to continue past six months. If a specific region of the brain is injured or destroyed, its functions can sometimes be transferred and taken over by a neighboring region. There is little functional plasticity observed in partial and complete callosotomies; however, much more plasticity can be seen in infant patients receiving a [[hemispherectomy]], which suggests that the opposite hemisphere can adapt some functions typically performed by its opposite pair. A study performed by Anderson et al. (2005) proved a correlation between the severity of the injury, the age of the individual, and their cognitive performance. It was evident that there was more neuroplasticity in older children—even if their injury was extremely severe—than in infants who suffered moderate brain injury.<ref name="Anderson 2005 1374–1382">{{cite journal|last1=Anderson|first1=Vicki|first2=Cathy |last2=Catroppa |first3=Sue |last3=Morse |first4=Flora |last4=Haritou |first5=Jeffrey |last5=Rosenfeld |author-link5=Jeffrey Rosenfeld |title=Functional Plasticity or Vulnerability after Early Brain Injury|journal=Pediatrics|year=2005|volume=11|issue=6|pages=1374–1382|doi=10.1542/peds.2004-1728|pmid=16322161|s2cid=25849302}}</ref> In some incidents of any moderate to severe brain injury, it mostly causes developmental impairments and in some of the most severe injuries it can cause a profound impact on their development that can lead to long-term cognitive effects. In the aging brain, it is extremely uncommon for neuroplasticity to occur; "olfactory bulb and hippocampus are two regions of the mammalian brain in which mutations preventing adult neurogenesis were never beneficial, or simply never occurred".<ref name="Anderson 2005 1374–1382"/>
[[File:WIKI PICTURE.png|left|thumb|291x291px|The picture is about a normal brain and a person with a split brain]]

=== Corpus callosotomy ===

[[Corpus callosotomy]] is a surgical procedure that sections the corpus callosum, resulting in either the partial or complete disconnection between the two hemispheres. It is typically used as a last-resort measure in treatment of intractable [[epilepsy]]. The modern procedure typically involves only the anterior third of the corpus callosum; however, if the epileptic seizures continue, the following third is lesioned prior to the remaining third if the seizures persist. This results in a complete callosotomy in which most of the information transfer between hemispheres is lost.<ref>{{Cite web |title=Corpus Callosotomy: How it's Done, Risks & Benefits, Recovery, Outlook |url=https://my.clevelandclinic.org/health/treatments/11546-corpus-callosotomy |access-date=2023-11-16 |website=Cleveland Clinic |language=en}}</ref>

Due to the functional mapping of the corpus callosum, a partial callosotomy has fewer detrimental effects because it leaves parts of the corpus callosum intact. There is little functional plasticity observed in partial and complete callosotomies on adults; the most neuroplasticity is seen in young children but not in infants.<ref name="Concha 2006 1090–1099">{{cite journal |last1=Concha |first1=Luis |first2=Donald |last2=Gross |first3=Matt |last3=Wheatley |first4=Christian |last4=Beaulieu |title=Diffusion Tensor Imaging of Time-Dependent Axonal and Myelin Degradation after Corpus Callosotomy in Epilepsy Patients|journal=NeuroImage |year=2006 |volume=32 |issue=3 |pages=1090–1099 |doi=10.1016/j.neuroimage.2006.04.187 |pmid=16765064 |s2cid=36514894}}</ref>

It is known that when the corpus callosum is severed during an experimental procedure, the experimenter can ask each side of the brain the same question and receive two different answers. When the experimenter asks the right visual field / left hemisphere what they see the participant will respond verbally, whereas if the experimenter asks the left visual field / right hemisphere what they see the participant will not be able to respond verbally but will pick up the appropriate object with their left hand.<ref>{{cite book |last=Raven |first=Peter |title=Biology |year=2014 |publisher=McGraw-Hill |location=New York |isbn=9780073383071}}</ref>

=== Memory ===
It is known that the right and the left hemisphere have different functions when it comes to memory.<ref>{{Cite web |title=Anatomy of the Brain |url=https://mayfieldclinic.com/pe-anatbrain.htm |website=Mayfield Clinic}}</ref> The right hemisphere is better at recognizing objects and faces, recalling knowledge that the individual has already learned, or recalling images already seen. The left hemisphere is better at mental manipulation, language production, and semantic priming but was more susceptible to memory confusion than the right hemisphere.<ref name="Metcalfe 1995 157–164">{{cite journal |last1=Metcalfe |first1=Janet |first2=Margaret |last2=Funnell |first3=Michael |last3=Gazzanuga |title=Right-Hemisphere Memory Superiority:Studies of Split-brain Patient|journal=Psychological Science|year=1995|volume=6|issue=3|pages=157–164|doi=10.1111/j.1467-9280.1995.tb00325.x|jstor=40063008|s2cid=3187008}}</ref> The main issue for individuals that have undergone a callosotomy is that because the function of memory is split into two major systems, the individual is more likely to become confused between knowledge they already know and information that they have only inferred.<ref name="Metcalfe 1995 157–164"/>

In tests, memory in either hemisphere of split-brained patients is generally lower than normal, though better than in patients with amnesia, suggesting that the forebrain commissures are important for the formation of some kinds of memory. This suggests that posterior callosal sections that include the hippocampal commissures cause a mild memory deficit (in standardized free-field testing) involving recognition.<ref>{{cite book |vauthors=Tramo MJ, Baynes K, Fendrich R, Mangun GR, Phelps EA, Reuter-Lorenz PA, Gazzaniga MS |year=1995 |contribution=Hemispheric specialization and interhemispheric integration: Insights from experiments with commissurotomy patients |title=Epilepsy and the Corpus Callosum |volume=2 |veditors=Reeves AG, Roberts DW |location=New York |publisher=Plenum |pages=263–295}}</ref> This makes first-person accounts hard to consider and to assess as scientists don't know if the consciousness was split as well.<ref name=":02">{{Cite web |title=APA PsycNet |url=https://psycnet.apa.org/record/2020-32923-001 |access-date=2023-10-10 |website=psycnet.apa.org |language=en}}</ref>

=== Control ===

In general, split-brained patients behave in a coordinated, purposeful, and consistent manner, despite the independent, parallel, usually different, and occasionally conflicting processing of the same information from the environment by the two disconnected hemispheres. When two hemispheres receive competing stimuli at the same time, the response mode tends to determine which hemisphere controls behavior.<ref>{{cite journal |vauthors=Levy J, Trevarthen C |year=1976 |title=Metacontrol of hemispheric function in human split-brain patients |journal=Journal of Experimental Psychology: Human Perception and Performance |volume=2 |issue=3 |pages=299–312 <!-- The following details seem to be bogus – this article isn't present in the book's TOC and the date is much too late, rather they seem to be copied from the preceding citation: |veditors=Reeves AG, Roberts DW |year=1995 |title=Epilepsy and the Corpus Callosum |volume=2 |location=New York |publisher=Plenum-->|doi=10.1037/0096-1523.2.3.299 |pmid=993737 }}</ref>{{verify inline|date=September 2023}}

Often, split-brained patients are indistinguishable from normal adults. This is due to the compensatory phenomena; split-brained patients progressively acquire a variety of strategies to get around their interhemispheric transfer deficits.<ref>{{cite web |last=Manning |first=Mark L. |title=Other Multiplicity |url=http://www.legiontheory.com/split-brain.html |archive-url=https://web.archive.org/web/20170301144134/http://www.legiontheory.com/split-brain.html |archive-date=1 March 2017 |access-date=28 April 2014 |publisher=Dr. Mark and Rana Manning}}</ref>{{sps|reason=Publisher is the same as author. Surely a reputable author can get a reputable publisher?|date=September 2023}} One issue that can happen with their body control is that one side of the body is doing the opposite of the other side, called the intermanual effect.{{Citation needed|date=September 2017|reason=Behavioural Brain Research Volume 56, Issue 1, 30 July 1993, pages 43–57|The bilateral reach to grasp movement=}}

=== Attention ===

Experiments on covert orienting of spatial attention using the [[Posner cueing task|Posner paradigm]] confirm the existence of two different attentional systems in the two hemispheres.<ref>{{cite book |last=Zaidel |first=E. |year=1994 |contribution=Interhemispheric transfer in the split brain: Long term status following complete cerebral commissurotomy |title=Human Laterality |veditors=Davidson RH, Hugdahl K |location=Cambridge, Massachusetts |publisher=MIT Press |pages=491–532}}</ref> The right hemisphere was found superior to the left hemisphere on modified versions of spatial relations tests and in locations testing, whereas the left hemisphere was more object based.<ref>{{cite book |vauthors=Nebes RD |year=1990 |section=The commissurotomized brain (Section 7) |title=Handbook of Neuropsychology |volume=4 |veditors=Boiler F, Grafman J |location=Amsterdam |publisher=Elsevier |pages=3–168}}</ref> The components of mental imagery are differentially specialized: the right hemisphere was found superior for mental rotation,<ref>{{cite journal |last1=Sergent |first1=Justine |last2=Corballis |first2=Michael C. | year = 1989 | title =  Categorization of disoriented faces in the cerebral hemispheres of normal and commissurotomized subjects | journal = Journal of Experimental Psychology: Human Perception and Performance | volume = 15 | issue = 4| pages = 701–710 | doi=10.1037/0096-1523.15.4.701|pmid=2531205 |s2cid=30870644 }}</ref> the left hemisphere superior for image generation.<ref>{{cite journal | last = Farah |first=Martha J. | year = 1986 | title = The laterality of mental image generation: A test with normal subjects | journal = Neuropsychologia | volume = 24 | issue = 4| pages = 541–551 | doi = 10.1016/0028-3932(86)90098-9 | pmid = 3774139 | s2cid = 3262591 }}</ref> It was also found that the right hemisphere paid more attention to landmarks and scenes whereas the left hemisphere paid more attention to exemplars of categories.<ref name="Zaidel">{{cite web|last=Zaidel|first=Eran|title=The Split Brain|url=http://www.its.caltech.edu/~jbogen/text/ref130.htm|access-date=28 April 2014|archive-url=https://web.archive.org/web/20140729185505/http://www.its.caltech.edu/~jbogen/text/ref130.htm|archive-date=29 July 2014|url-status=dead}}</ref>

=== Surgery procedure ===
The surgical operation to produce this condition (corpucallosotomy) involves transection of the corpus callosum, and is usually a last resort to treat refractory [[epilepsy]]. To lower the degree and ferocity of epileptic convulsions, partial callosotomies are first done; if these are unsuccessful, a callosotomy is next carried out to reduce the risk of unintentional bodily harm. Epilepsy is first managed with medications rather than callosotomies. Neuropsychological evaluations following surgery are frequently carried out.