Split-brain is a lay term to describe the result when the corpus callosum connecting the two hemispheres of the brain is severed to some degree. It is an association of symptoms produced by disruption of or interference with the connection between the hemispheres of the brain. The surgical operation to produce this condition results from transection of the corpus callosotomy, and is usually a last resort to treat refractory epilepsy. Initially, partial callosotomies are performed; if this operation does not succeed, a complete callosotomy is performed to mitigate the risk of accidental physical injury by reducing the severity and violence of epileptic seizures. Before using callosotomies, epilepsy is instead treated through pharmaceutical means. After surgery, neuropsychological assessments are often performed.
When split-brain patients are shown an image only in their left visual field (the left half of what both eyes take in (see optic tract)), they cannot vocally name what they have seen. This can be explained in three steps: (1) The image seen in the left visual field is sent only to the right side of the brain; (2) For most people, the speech-control center is on the left side of the brain; and (3) Communication between the two sides of the brain is inhibited. Thus, the patient cannot say out loud the name of that which the right side of the brain is seeing. In the case that the speech-control center is on the right side of the brain, the image must now be presented to only the right visual field to achieve the same effect.
If a split-brain patient is touching a mysterious object with only the left hand, while also receiving no visual cues in the right visual field, the patient cannot say out loud the name of that which the right side of the brain is touching. This can be explained in three steps: (1) Each cerebral hemisphere of the primary somatosensory cortex only contains a tactile representation of the opposite (contralateral) side of the body; (2) For most humans, the speech-control center is on the left side of the brain; and (3) Communication between the two sides of the brain is inhibited. In the case that the speech-control center is on the right side of the brain, the object must now be touched only with the right hand to achieve the same effect.
The same effect occurs for visual pairs and reasoning. For example, a patient with split brain is shown a picture of a chicken and a snowy field in separate visual fields and asked to choose from a list of words the best association with the pictures. The patient would choose a chicken foot to associate with the chicken and a shovel to associate with the snow; however, when asked to reason why the patient chose the shovel, the response would relate to the chicken (e.g. "the shovel is for cleaning out the chicken coop").
"Scientists have often wondered whether split-brain patients, who have had the two hemispheres of their brain surgically disconnected, are 'of two minds'" (Zilmer, 2001).
The modern era of split-brain research began in the late 1950s. The pioneers of split-brain research, Michael Gazzaniga and Roger Sperry, worked together at Caltech testing the functioning of each hemisphere independently of the other in split-brain patients. The results revealed an overall pattern among patients that severing the entire corpus callosum blocks the interhemispheric transfer of perceptual, sensory, motor, gnostic and other forms of information in a dramatic way. This allowed Gazzaniga and Sperry to gain insights into hemispheric differences as well as the mechanisms through which the two hemispheres interact.
One notable experiment performed by Roger Sperry involved having patients stare at a spot on the center of a screen, while Sperry and colleagues projected a stimulus on only one side of a screen. Through this projection, they were able to isolate the stimulus to only one hemisphere of the brain, and test the role of the corpus callosum in communication of information between the two hemispheres, i.e., whether the opposite hemisphere was able to respond to the stimulus being presented without the presence of the corpus callosum. 
The two hemispheres of the cerebral cortex are linked by the corpus callosum, through which they communicate and coordinate. Communication between the two hemispheres is essential because they have some separate functions. The right hemisphere of the cortex excels at nonverbal and spatial tasks, whereas the left hemisphere is usually more dominant in verbal tasks such as speaking and writing. 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." The right hemisphere controls (and receives sensory input from) the left side of the body and the left hemisphere controls (and receives sensory input from) the right side. Many simple tasks, especially comprehension of inputs, require functions specific to both hemispheres and thus require communication between hemispheres.
Role of the corpus callosum
The corpus callosum is a structure in the brain along the longitudinal fissure that facilitates much of the communication between the two hemispheres and its main function is in allowing for communication between the brain's right and left hemispheres; however, there is evidence that the corpus callosum may also have some inhibitory functions. Post-mortem research on human and monkey brains show that the corpus callosum is functionally organized. This organization results in modality-specific regions of the corpus callosum; that is, the corpus callosum has specific regions for 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. Although much of the interhemispheric transfer occurs at the corpus callosum, there are trace amounts of transfer via subcortical pathways.
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. A model proposed by Iacoboni et al. 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, implying that each cerebral hemisphere has its own attentional system. An alternative approach taken by Reuter-Lorenz et al. 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. It is important to note that the simple reaction time in split-brained patients, even with enhanced redundancy gain, is slower than the reaction time of normal adults.
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. Significant recovery occurs during the first several weeks after the injury; however, recovery is generally thought not to continue past 6 months. If a specific region of the brain is injured or destroyed, its functions can sometimes be assumed 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.
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 1/3 of the corpus callosum; however, if the epileptic seizures continue, the following 1/3 is lesioned prior to the remaining 1/3 if the seizures persist. This results in a complete callosotomy in which most of the information transfer between hemispheres is lost.
Due to the functional mapping of the corpus callosum, a partial callosotomy has less detrimental effects because it leaves parts of the corpus callosum intact. There is little functional plasticity observed in partial and complete callosotomies on adults; however, much more plasticity can be seen in infant patients.
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.
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. 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.
Experiments on covert orienting of spatial attention using the Posner paradigm confirm the existence of two different attentional systems in the two hemispheres. The right hemisphere was found superior to the left hemisphere on modified versions of spatial relations tests. The components of mental imagery are differentially specialized: the right hemisphere was found superior for mental rotation, the left hemisphere superior for image generation.
Case studies of split-brain patients
Patient JW is a right-handed male who was 47 years old at the time of testing. He successfully completed high school and has no reported learning disabilities. He had his first seizure at the age of 16, and at the age of 25, he underwent a two-stage resection of the corpus callosum for relief of intractable epilepsy. Complete sectioning of the corpus callosum has been confirmed by MRI. Post-surgical MRI also revealed no evidence of other neurological damage.
One of the experiments involving JW attempted to determine each hemisphere's ability to perform simple addition, subtraction, multiplication and division. On each trial, an arithmetic problem was presented in the center of the screen followed by a central cross hair. After studying the problem, a number posing as the answer was presented to each hemisphere exclusively by JW's vision fixated on a central cross hair. Probes (stimuli) were then presented for 150 ms to either the left visual field/right hemisphere (LVF/RH) or to the right visual field/left hemisphere (RVF/LH). The position of the probe fell outside any zone of naso-temporal overlap (binocular vision) to ensure that stimuli were perceived only by the hemisphere contralateral to the visual field of the stimuli. JW was instructed to press a certain key if the probe was the correct solution and another key if the probe was the incorrect solution. Results showed that the effects of visual field was significant with performance of the left hemisphere being better than that of the right hemisphere; the left hemisphere correctly chose the correct answer on all four arithmetic operations approximately 90% of the time while the right hemisphere was at chance. These results suggest left hemisphere specialization for calculation.
Patient VP is a woman who underwent a two-stage callosotomy in 1979 at the age of 27. Although the callosotomy was reported to be complete, follow-up MRI in 1984 revealed spared fibers in the rostrum and splenium. The spared rostral fibers comprised ~1.8% of the total cross-sectional area of the corpus callosum and the spared splenial fibers comprised ~1% of the area. VP's postsurgery intelligence and memory quotients were within normal limits.
One of the experiments involving VP attempted to investigate systematically the types of visual information that could be transferred via VP's spared splenial fibers. The first experiment was designed to assess VP's ability to make between-field perceptual judgements about simultaneously presented pairs of stimuli. The stimuli were presented in varying positions with respect to the horizontal and vertical midline with VP's vision fixated on a central crosshair. The judgements were based on differences in color, shape or size. The testing procedure was the same for all three types of stimuli; after presentation of each pair, VP verbally responded 'yes' if the two items in the pair were identical and 'no' if they were not. The results show that there was no perceptual transfer for color, size or shape with binomial tests showing that VP's accuracy was not greater than chance.
A second experiment involving VP attempted to investigate what aspects of words transferred between the two hemispheres. The set up was similar to the previous experiment, with VP's vision fixated on a central cross hair. A word pair was presented with one word on each side of the cross-hair for 150 ms. The words presented were in one of four categories: words that looked and sounded like rhymes (e.g. tire and fire), words that looked as if they should rhyme but did not (e.g. cough and dough), words that did not look as if they should rhyme but did (e.g. bake and ache), and words that neither looked nor sounded like rhymes (e.g. keys and fort). After presentation of each word pair, VP responded 'yes' if the two words rhymed and 'no' if they did not. VP's performance was above chance and she was able to distinguish among the different conditions. When the word pairs did not sound like rhymes, VP was able to say accurately that the words did not rhyme, regardless of whether or not they looked as if they should rhyme. When the words did rhyme, VP was more likely to say they rhymed, particularly if the words also looked as if they should rhyme.
Although VP showed no evidence for transfer of color, shape or size, there was evidence for transfer of word information.
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