Split-brain syndrome, also known as callosal disconnection syndrome, occurs when the corpus callosum, which connects the right and left hemispheres of the brain, is severed or lacerated.
Though it is unclear whether each hemisphere of the brain has some control over specific tasks, it is known that each hemisphere influences some of them. For example, the left hemisphere is typically used for analytical activities such as reading and calculating. Language processing is typically dominated by the left hemisphere, albeit the right hemisphere is involved to some extent.
It is generally believed that the right hemisphere is better at handling spatial tasks — dealing with maze navigation or reading a map, for example — compared to the left hemisphere. However, the corpus callosum facilitates long-term communication between the two hemispheres. The right hemisphere controls the left side of the body, and the left hemisphere controls the right. It’s a connection that transmits certain sensory signals from one side of the body to the other side of the brain.
Roger Wolcott Sperry, an American neurobiologist who studied split-brain subjects in the 1960s, described the split-brain syndrome by suggesting that the left and right hemispheres of the brain function differently. For this work, Sperry received a share of the 1981 Nobel Prize for Physiology or Medicine.
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Causes of split-brain syndrome
Surgical removal of the corpus callosum, either partially or completely, is the primary cause of split-brain syndrome. This operation is performed rarely in the 21st century (mostly replaced by drug treatments and other treatments) and is reserved as the last treatment option for refractory epilepsy with violent attacks affecting both sides of the brain.
It can significantly improve the quality of life of patients with epilepsy by preventing the propagation of seizure activity across hemispheres. However, patients often experience chronic and sometimes acute hemispheric disconnection symptoms for days or weeks following the operation.
Split-brain syndrome may be caused by stroke, infectious lesion, tumor, or ruptured artery, among others. The corpus callosum is damaged in varying degrees by many of these events. There may also be a genetic component involved, such as through multiple sclerosis or incomplete development of the corpus callosum.
Split-brain syndrome may also result in damage to the corpus callosum, but the more global damage caused by this disease leads to stupor, seizures, and coma, rather than the symptoms usually associated with the condition.
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Symptoms of split-brain syndrome
Memory and social skills are often intact in patients with split-brain syndrome. Patient with split-brain also maintains both-side motor skills that they learned before they were diagnosed, such as walking, swimming, and biking. Additionally, hand and finger movements involving parallel or mirrored movement can also be learned.
But, they are unable to learn new tasks that require interdependent movements from both hands, for example, playing the piano, where both hands are necessary to produce music. They also have coordinated eye movements.
Patients with split brains display unusual behaviors, particularly related to speech and object recognition, since information cannot be directly shared between the two hemispheres. A patient with split brain may be unable to name an object when blindfolded since touch information is relayed from the left hemisphere to the right hemisphere, which tends to have a weak language center.
While the object is in the left hand, a person cannot access verbal information on the left side of the brain without a functioning corpus callosum. Apraxia can also be caused by the patient’s difficulty in using the left hand to execute verbal commands. If the patient is unable to respond to verbal commands using the left hand, then this is a form of apraxia.
When a patient is blindfolded, they may ask for the name of the object using the right hand, which sends information to the left hand, compensating for the left hand’s shortcomings in touch recognition and left-hand apraxia. This provides access to the patient’s dominant verbal bank.
The patient may use his left hand in addition to the right hand when hearing the name of an object. This could be due to the fact that auditory information is processed by both hemispheres. Split-brain patients seem to experience other complications as a result of the diffuse way sounds and smells are processed across the brain. It is not uncommon for patients to be unable to identify odors in their right nostril, but they can identify the source with their left hand. Patients with chronic disconnection may be able to improve their symptoms over time.
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Memory
Memory relates to the right hemisphere and the left hemisphere differently. It is the right hemisphere that makes it easier for someone to recognize objects and faces, recall information that has been previously learned, or recall visual images that have already been viewed.
However, the left hemisphere was more prone to memory confusion than the right hemisphere, even though that hemisphere is better at language production, language manipulation, and semantic priming. Individuals with callosotomies tend to be more confused about information they already know and information that they have only inferred because memory is divided between two major systems.
Memory performance in split-brained patients is generally low, but better than that of amnesic patients, indicating that forebrain commissures play an important role in the formation of some types of memory. Based on standardized free-field tests, posterior callosal sections containing hippocampal commissures can result in mild memory deficits.
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Control
Although separated hemispheres are processing the same information in two independent streams, often differently and occasionally in conflict with one another, split-brained patients generally behave coherently, purposefully, and consistently. Response modes determine which hemisphere controls behavior when two hemispheres are exposed simultaneously to competing stimuli.
Split-brained patients often look like healthy adults. A compensatory phenomenon occurs; split-brained patients learn to overcome their interhemispheric transfer deficits through a variety of strategies.
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Attention
Researchers have confirmed the existence of two separate attentional systems in each hemisphere by examining covert orienting of attention using the Posner paradigm. According to modified versions of spatial relations and location tests, the right hemisphere performed better than the left hemisphere, while the left hemisphere performed better with object-based tasks.
Researchers found that the right and left hemispheres are differentially specialized for different aspects of mental imagery: the right hemisphere is better at mental rotation, while the left hemisphere is better at generating imagery. Interestingly, the right hemisphere showed a greater focus on landmarks and scenes while the left hemisphere was more preoccupied with examples of categories.
