Corpus callosum disconnection syndrome: what is it?

The corpus callosum is a structure composed of nerve fibers that acts as a connecting bridge between the two cerebral hemispheres.

When damaged by injury or disease, it can cause corpus callosum disconnection syndrome, a severe neurological disorder that causes numerous sensory and motor disturbances.

In this article we explain what the corpus callosum disconnection syndrome consists of, where the term arises, what are its causes and the main signs and symptoms caused by this disorder.

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Corpus callosum disconnection syndrome: what is it?

Corpus callosum disconnection syndrome, also called Sperry syndrome, is a disorder that is produced by damage to the corpus callosum, a structure of nerve fibers that connects both cerebral hemispheres. The disconnection that occurs between one side of the brain and the other causes a series of clinical signs and symptoms in the patient that constitute the “disconnection syndrome”.

The effects of interhemispheric disconnection were initially investigated by the German neurologist, Carl Wernicke, who predicted the existence of the syndrome. disconnection aphasic, which occurred as a result of severing the connections between the posterior and anterior zones of the cortical structures responsible for language.

At the beginning of the last century, the German psychiatrist Hugo Liepmann also extensively studied the disconnection principle with respect to the idea that some Apraxias (neurological disorders characterized by the inability to carry out purposeful, learned or familiar movements) could be caused by this freak.

Beginning in the 1950s, Roger W. Sperry and his colleagues investigated the effect of disconnection of the corpus callosum in animals, specifically cats, showing that these could appear indistinguishable from those who were healthy in terms of their behavior, in most tests and training conditions.

However, Sperry's studies revealed that animals, under certain training procedures, presented significant deficiencies; so that if sensory information were allowed to have separate access to each hemisphere brain, it would be shown that each one of them has processes of perception, learning and memory separated.

Following animal studies, Norman Geschwind, Mishkin, and other behavioral scientists began to investigate the effect of disconnection (intra and interhemispheric) in humans, building models of disconnection syndromes that demonstrated the existence of a functional and structural interdependence between different neocortical regions.

Physiology and functions of the corpus callosum

The cerebral cortex is interconnected by three types of nerve fibers: those of projection, which include ascending and descending fibers that go to and from the neocortex to other regions such as the trunk and spinal cord spinal; association fibers, which include short and long fibers that connect different areas of the neocortex far apart; and the commissural fibers, which connect the cerebral hemispheres, such as those of the corpus callosum, among others.

The neurons of the corpus callosum exert their action, both inhibitory and excitatory, through interneurons.. Through the corpus callosum, the information generated in one hemisphere flows to the other and the different responses that occur when activated simultaneously are coordinated. Each cerebral hemisphere is capable of locally and selectively modulating the functioning of the homologous regions on the opposite side.

The corpus callosum participates in numerous functions; for example, in the transfer of sensorimotor information; in the transfer of semantic information by the verbalization (left hemisphere) of an object that has been touched using the left hand (right hemisphere); or in the transfer of learning.

In this regard, various studies have shown that each hemisphere can learn to discriminate visually from the information that reaches the contralateral hemisphere. However, this learning disappears after the injury or commissurotomy occurs.

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Causes

The disconnection syndrome of the corpus callosum It can be due to multiple causes that include injuries to this brain structure, infarcts (in the posterior or anterior cerebral artery or involving the transcallosal fibers), head trauma (edema, contusions, hematomas or axonal damage), tumors, malformations and neurological disorders such as Marchiafava disease Bignami.

Symptoms and signs

Regarding the clinical signs and symptoms that a disconnection syndrome can cause, these can be divided into two groups: the acute ones, which are observed within the first days or weeks after the disconnection; and the chronic ones, which appear after 6 months of the lesion and can be detected through methods such as dichotic listening and tachistoscopy. These are the most common:

Acute signs and symptoms

The most common signs and symptoms of corpus callosum disconnection syndrome include:

• Transient mutism.
• Indifference to external stimulation.
• Ideomotor apraxia left unilateral.
• Underutilization of the left hand.
• “Aberrant” behavior of the left hand (diagonistic dyspraxia)

Chronic signs and symptoms

These are the most common chronic signs and symptoms that appear after 6 months have passed after disconnection of the corpus callosum:

• Altered processing of somesthetic data (eg. eg inability to point with the hand to the contralateral part of the body that the examiner has touched)
• Impaired processing of visual information (inability to recognize an image projected on the right side after it has been projected several times on the left side)
• Alterations in naming: tactile anomia of the left hand, naming problems in bilateral condition simultaneous left visual anomia, left alexia, hemialexia, left auditory anomia, right olfactory anomia, and alexithymia.
• Disturbances of gestural actions: bimanual coordination problems, left unilateral apraxia, difficulties in imitation of gestural sequences, foreign hand sign, diagnostic apraxia, crossed optic ataxia and constructive apraxia right.
• Memory disorders (secondary to the affectation of the callous fibers that join both hippocampuses).

split brain

Research by Roger Sperry and Michael Gazzaniga with patients who present interhemispheric disconnection due to lesions in their corpus callosum, have shown that these people seem to experience a split consciousness: the left and right sides of your brain appear to be "aware" and capable of responding independently to stimuli.

Since this phenomenon began to be studied, it has been assumed that people with "split brain" can report seeing a stimulus in his left visual field, but only using his left hand, not his right. And they can report a stimulus in their right visual field only with their right hand, but also verbally. This phenomenon occurs because language is usually found in the left hemisphere of the brain, and this hemisphere receives visual input from the right visual field.

However, a recent study carried out by Pinto et al. has revealed evidence that split-brain patients are capable of reporting the presence of stimuli in the left or right visual field using either hand, and they can also do so verbal. These new findings suggest that the cortical disconnect between the hemispheres does not appear to lead to two "independent conscious agents" within the brain.

However, the patients presented certain deficits, such as the inability to state whether two stimuli presented on opposite sides of the visual field were the same or different, suggesting that, although the interhemispheric disconnection did not appear to affect their awareness, the transfer of information between cerebral hemispheres was altered.

Bibliographic references:

• Lassonde, M. (1994). Disconnection syndrome in callosal agenesis. In Callosal Agenesis (pp. 275-284). Springer, Boston, MA.
• Pinto, Y., Neville, D. A., Otten, M., Corballis, P. M., Lame, V. A., DeHaan, E. H.,... & Fabri, M. (2017). Split brain: divided perception but undivided consciousness. Brain, 140(5), 1231-1237.
• Thomson, P. M., Narr, K. L., Blanton, R. E., & Toga, A. W. (2003). Mapping Structural Alterations of the Corpus Callosum During Brain. The parallel brain: The cognitive neuroscience of the corpus callosum 93.