Disconnection syndrome of the corpus callosum: main features

Also known as Sperry's syndrome, this disorder arises when the corpus callosum is injured.

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

When it is damaged by injury or disease it can lead to corpus callosum disconnection syndromeThis is a serious neurological disorder that causes numerous sensory and motor disturbances.

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

Corpus callosum disconnection syndrome: what is it?

The syndrome of disconnection of the corpus callosum, also known as Sperry's syndromeis a disorder caused by a lesion of 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 the interhemispheric disconnection were initially investigated by the German neurologist Carl Wernicke, who predicted the existence of the aphasic disconnection syndrome, which was produced as a result of severing the connections between the posterior and anterior areas 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 phenomenon.

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 cats could appear indistinguishable from healthy ones in terms of their behavior, in most tests and in most of the tests they underwent.The effect of the disconnection of the corpus callosum in cats, specifically in cats, showed that they could appear indistinguishable in behavior from healthy ones, in most tests and in training conditions.

However, Sperry's studies revealed that animals, under certain training procedures, had significant deficits; thus, if sensory information were allowed separate access to each brain hemisphere, each hemisphere would be shown to have separate perception, learning and memory processes.

Following the animal studies, Norman Geschwind, Mishkin and other behavioral scientists began to investigate the effect of disconnection (intra- and interhemispheric) in humans, constructing models of disconnection syndromes which demonstrated the existence of 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: projection fibers, which include ascending and descending fibers that go to and from the neocortex to other regions such as the trunk and spinal cord; association fibers, which include short and long fibers that connect different areas of the neocortex far from each other; and 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, by means of interneurons.. Through the corpus callosum, the information generated in one hemisphere flows to the other and the different responses that occur when they are activated simultaneously are coordinated. Each cerebral hemisphere is able to locally and selectively modulate the functioning of homologous regions on the opposite side.

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

In this sense, several studies have shown that each hemisphere can learn to discriminate visually from information reaching the contralateral hemisphere.. However, this learning disappears after the lesion or commissurotomy occurs.

Causes

Corpus callosum disconnection syndrome can be due to multiple causes that include lesions in this brain structure.In addition, infarcts (in the posterior cerebral artery, anterior cerebral artery or affecting the transcallosal fibers), craniocerebral trauma (edema, contusions, hematomas or axonal damage), tumors, malformations and neurological disorders such as Marchiafava Bignami disease.

Signs and symptoms

With regard to the clinical signs and symptoms that a disconnection syndrome can cause, these can be divided into two groups: acute, which are observed within the first few days or weeks after the disconnection; and chronic, which arise 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.

Common signs and symptoms in corpus callosum disconnection syndrome include the following:

• Transient mutism.
• Indifference to external stimulation.
• Left unilateral ideomotor apraxia.
• 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 elapsed after disconnection of the corpus callosum:

• Impaired processing of somesthetic data (e.g., inability to point with the hand to the contralateral location on 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 simultaneous bilateral condition, 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 the imitation of gestural sequences, foreign hand sign, diagonistic apraxia, crossed optic ataxia and right constructional apraxia.
• Memory disorders (secondary to involvement of the callosal fibers linking both hippocampi).

Split brain

Roger Sperry and Michael Gazzaniga's research with patients who present with an interhemispheric disconnection due to lesions in their corpus callosum has demonstrated that these people seem to experience a split consciousness: the left and right sides of theirThe left and right sides of their brain appear to be "conscious" and able to respond independently to stimuli.

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

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

However, the patients had certain deficits, such as the inability to manifest whether two stimuli presented on opposite sides of the visual field were the same or different, suggesting that, although the interhemispheric disconnection does not appear to have affected their consciousness, the transfer of information between brain hemispheres was altered, the transfer of information between brain hemispheres was altered..

Bibliographical 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., Lamme, V. A., De Haan, E. H., ... & Fabri, M. (2017). Split brain: divided perception but undivided consciousness. Brain, 140(5), 1231-1237.
• Thompson, 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.