Editing Split-brain (section)

== History ==

Early anatomists, such as [[Galen]] (129{{Snd}}{{Circa|216&nbsp;[[Common Era|CE]]}}) and [[Vesalius]] (1514–1564&nbsp;CE), identified the [[corpus callosum]]. They generally described its function as a structure holding together the two halves of the [[cerebral cortex|brain]].<ref name="Vaddipartiet2021">{{cite journal |last1=Vaddiparti, A., Huang, R., Blihar, D., DuPlessis, M., Montalbano, M. J., Tubbs, R. S., & Loukas, M. |title=The evolution of corpus callosotomy for epilepsy management |journal=World Neurosurgery |date=2021 |volume=145 |pages=455–461 |doi=10.1016/j.wneu.2020.08.178 |pmid=32889189 |s2cid=221502280 |url=https://doi.org/10.1016/j.wneu.2020.08.178 |access-date=2 November 2022|url-access=subscription }}</ref> In 1784, [[Félix Vicq-d'Azyr]] described the corpus callosum as allowing communication between the two halves of the brain. He proposed that eliminating the corpus callosum would divide the brain into two independent parts.<ref name="Vaddipartiet2021" /> In 1892, [[Joseph Jules Dejerine]] reported symptoms in a person who had destruction of part of the corpus callosum (along with damage to the [[visual cortex]]: inability to read while retaining the ability to write, now referred to as [[pure alexia]] or as ''Dejerine syndrome''.<ref name="Vaddipartiet2021" /> In 1908, [[Hugo Liepmann]] observed left-sided [[apraxia]] (a motor disorder of motor planning to perform tasks or movements) and [[agraphia]] (loss of the ability to communicate through writing) following a lesion in the corpus callosum.<ref name="Vaddipartiet2021" />

According to Vaddiparti et al. (2021), the first surgical cuts to the corpus callosum, partial [[corpus callosotomy]], were made by [[neurosurgeon]] [[Walter Dandy]] in order to access and to remove [[pinealoma|tumors]] in the [[pineal gland]].<ref name="Vaddipartiet2021" /> In 1936, Dandy described three cases in which he cut the corpus callosum from its [[Posterior (anatomy)|posterior]] (towards the back of the head) across about two thirds of its width. He described these cuts as "bloodless" and that "no symptoms follow[ed] [the] ... division" of the corpus callosum. He concluded that his operations "dispose ... of the extravagant claims to function of the corpus callosum".<ref name="Dandy1936">{{cite journal |last1=Dandy |first1=Walter E. |title=Operative experience in cases of pineal tumor |journal=Archives of Surgery |date=1936 |volume=33 |issue=1 |pages=19–46 |doi=10.1001/archsurg.1936.01190010022002 |url=https://doi.org/10.1001/archsurg.1936.01190010022002 |access-date=3 November 2022|url-access=subscription }}</ref>{{Rp|page=40}}

Prior to the 1960s, research on people with certain brain injuries led to the notion that there is a "language center" only in the left hemisphere of the brain. For example, people with lesions in two specific areas of the left hemisphere lost their ability to talk, to read, and to understand speech. [[Roger Sperry]] and his colleagues pioneered research showing that creating another [[corpus callosotomy|lesion]] (done to relieve otherwise untreatable [[epilepsy]]), in the [[corpus callosum|connections between the left and right hemispheres]], revealed that the right hemisphere can allow people to read, to understand speech, and to say some simple words. Research over the next twenty years showed that the disconnected right hemisphere is superior to the disconnected left hemisphere in allowing people to understand spatial information (such as maps), music, and emotions, whereas the disconnected left hemisphere is superior in allowing analytical thinking, talking, reading, and understanding speech. This research led to a [[Nobel Prize in Physiology or Medicine]] for Sperry in 1981.<ref>{{cite web|title=The Split Brain Experiments|url=https://www.nobelprize.org/educational/medicine/split-brain/background.html|publisher=Nobel Media|access-date=27 April 2014}}</ref>

Sperry's initial colleagues included his [[Caltech]] PhD students, [[Michael Gazzaniga]] and [[Jerre Levy]]. Even though Sperry is considered the founder of split-brain research, Gazzaniga's clear summaries of their collaborative work are consistently cited in psychology texts. In Sperry and Gazzaniga's "The Split Brain in Man" experiment published in ''[[Scientific American]]'' in 1967 they attempted to explore the extent to which two halves of the human brain were able to function independently and whether or not they had separate and unique abilities. They wanted to examine how perceptual and intellectual skills were affected in someone with a split brain. At Caltech, Gazzaniga worked with Sperry on the effects of split-brain surgery on perception, vision and other brain functions. The surgery, which was a treatment for severe epilepsy, involved severing the corpus callosum, which carries signals between the left-brain hemisphere, the seat of speech and analytical capacity, and the right-brain hemisphere, which helps recognize visual patterns. At the time this article was written, only ten patients had undergone the surgery to sever their corpus callosum ([[corpus callosotomy]]). Four of these patients had consented to participate in Sperry and Gazzaniga's research. After the corpus callosum severing, all four participants' personality, intelligence, and emotions appeared to be unaffected. However, the testing done by Sperry and Gazzaniga showed the subjects demonstrated unusual mental abilities. The researchers created different types of tests to analyze the range of cognitive capabilities of the split-brain subjects. These included tests of their visual stimulation abilities, a tactile stimulation situation, and a test that involved both visual and tactile information.

=== Visual test ===

The first test started with a board that had a horizontal row of lights. The subject was told to sit in front of the board and stare at a point in the middle of the lights, then the bulbs would flash across both the right and left visual fields. When the patients were asked to describe afterward what they saw, they said that only the lights on the right side of the board had lit up. Next, when Sperry and Gazzaniga flashed the lights on the right side of the board on the subjects left side of their visual field, they claimed not to have seen any lights at all. When the experimenters conducted the test again, they asked the subjects to point to the lights that lit up. Although subjects had only reported seeing the lights flash on the right, they actually pointed to all the lights in both visual fields. This showed that both brain hemispheres had seen the lights and were equally competent in visual perception. The subjects did not say they saw the lights when they flashed in the left visual field even though they did see them because the center for speech is located in the brain's left hemisphere. This test supports the idea that in order to say one has seen something, the region of the brain associated with speech must be able to communicate with areas of the brain that process the visual information.{{citation needed|date=December 2020}}

=== Tactile test ===

In a second experiment, Sperry and Gazzaniga placed a small object in the subject's right or left hand, without the subject being able to see (or hear) it. When the object was placed in the right hand, the isolated left hemisphere perceived the object and could easily describe and name it. However, when the object was placed in the left hand, the isolated right hemisphere could not name or describe the object. Questioning this result, the researchers found that the subjects ''could'' later match it from several similar objects; tactile sensations limited to the right hemisphere were accurately perceived but could not be verbalized. This further demonstrated the apparent location (or lateralization) of language functions in the left hemisphere.{{citation needed|date=December 2020}}

=== Combination of both tests ===

In the last test the experimenters combined both the tactile and visual test. They presented subjects with a picture of an object to only their right hemisphere, and subjects were unable to name it or describe it. There were no verbal responses to the picture at all. If the subject was able to reach under the screen with their left hand to touch various objects, however, they were able to pick the one that had been shown in the picture. The subjects were also reported to be able to pick out objects that were related to the picture presented, if that object was not under the screen.<ref>{{cite journal|last=Gazzaniga|first=Michael|title=The Split Brain in Man|journal=Scientific American|year=1967|volume=217|issue=2|pages=24–29|doi=10.1038/scientificamerican0867-24|bibcode=1967SciAm.217b..24G}}</ref>

Sperry and Gazzaniga went on to conduct other tests to shed light on the [[language processing in the brain|language processing]] abilities of the right hemisphere as well as auditory and emotional reactions as well. The significance of the findings of these tests by Sperry and Gazzaniga was extremely telling and important to the psychology world. Their findings showed that the two halves of the brain have numerous functions and specialized skills. They concluded that each hemisphere really has its own functions. One's left hemisphere of the brain is thought to be better at writing, speaking, mathematical calculation, reading, and is the primary area for language. The right hemisphere is seen to possess capabilities for problem solving, recognizing faces, symbolic reasoning, art, and spatial relationships.{{citation needed|date=December 2020}}

Roger Sperry continued this line of research up until his death in 1994. Michael Gazzaniga continues to research the split brain. Their findings have been rarely critiqued and disputed; however, a popular belief that some people are more "right-brained" or "left-brained" has developed. In the mid-1980s [[Jerre Levy|Jarre Levy]], a psychobiologist at the University of Chicago, was at the forefront of scientists who wanted to dispel the notion we have two functioning brains. She believes that because each hemisphere has separate functions that they must integrate their abilities instead of separating them. Levy also claims that no human activity uses only one side of the brain. In 1998 a French study by Hommet and Billiard was published that questioned Sperry and Gazzaniga's study that severing the corpus callosum actually divides the hemispheres of the brain. They found that children born without a corpus callosum demonstrated that information was being transmitted between hemispheres, and concluded that subcortical connections must be present in these children with this rare brain malformation. They are unclear about whether these connections are present in split-brain patients though. Another study by Parsons, Gabrieli, Phelps, and Gazzaniga in 1998 demonstrated that split-brain patients may commonly perceive the world differently from the rest of us. Their study suggested that communication between brain hemispheres is necessary for imaging or simulating in one's mind the movements of others. Morin's research on inner speech in 2001 suggested an alternative for interpretation of [[commissurotomy]] according to which split-brain patients exhibit two uneven streams of self-awareness: a "complete" one in the left hemisphere and a "primitive" one in the right hemisphere.<ref>{{cite book|last=Hock|first=Roger R.|title=Forty Studies that Changed Psychology: Explorations into the History of Research|publisher=Upper Saddle River|isbn=978-0-13-114729-4|year=2005|url-access=registration|url=https://archive.org/details/fortystudiesthat00hock_1}}</ref>