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Disconnection syndrome is a general term for a collection of neurological symptoms caused – via lesions to
associational or
commissural
A commissure () is the location at which two objects abut or are joined. The term is used especially in the fields of anatomy and biology.
* The most common usage of the term refers to the brain's commissures, of which there are five. Such a commi ...
nerve fibre
An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
s – by damage to the
white matter
White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribution ...
axons of communication pathways in the
cerebrum
The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In ...
(not to be confused with the cerebellum), independent of any lesions to the
cortex
Cortex or cortical may refer to:
Biology
* Cortex (anatomy), the outermost layer of an organ
** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain''
*** Motor cortex, the regions of the cerebral cortex i ...
.
The behavioral effects of such disconnections are relatively predictable in adults.
Disconnection syndromes usually reflect circumstances where regions A and B still have their functional specializations except in domains that depend on the interconnections between the two regions.
Callosal syndrome, or
split-brain
Split-brain or callosal syndrome is a type of disconnection syndrome 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, ...
, is an example of a disconnection syndrome from damage to the
corpus callosum
The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental mam ...
between the two hemispheres of the brain. Disconnection syndrome can also lead to
aphasia
Aphasia is an inability to comprehend or formulate language because of damage to specific brain regions. The major causes are stroke and head trauma; prevalence is hard to determine but aphasia due to stroke is estimated to be 0.1–0.4% in th ...
, left-sided
apraxia
Apraxia is a motor disorder caused by damage to the brain (specifically the posterior parietal cortex or corpus callosum), which causes difficulty with motor planning to perform tasks or movements. The nature of the damage determines the disorder' ...
, and tactile aphasia, among other symptoms. Other types of disconnection syndrome include conduction aphasia (lesion of the association tract connecting Broca’s area and Wernicke’s), agnosia, apraxia, pure alexia, etc.
Anatomy of cerebral connections
Theodore Meynert, a neuroanatomist of the late 1800s, developed a detailed anatomy of white matter pathways. He classified the white matter fibers that connect the neocortex into three important categories –
projection fiber
The projection fibers consist of efferent and afferent fibers uniting the cortex with the lower parts of the brain and with the spinal cord. In human neuroanatomy, bundles of axons (nerve fibers) called tracts, within the brain, can be catego ...
s, commissural fibers and
association fiber
Association fibers are axons that connect cortical areas within the same cerebral hemisphere.
In human neuroanatomy, axons (nerve fibers) within the brain, can be categorized on the basis of their course and connections as association fibers, ...
s. Projection fibers are the ascending and descending pathways to and from the neocortex. Commissural fibers are responsible for connecting the two hemispheres while the association fibers connect cortical regions within a hemisphere. These fibers make up the interhemispheric connections in the cortex.
Callosal disconnection syndrome is characterized by left ideomotor apraxia and left-hand agraphia and/or tactile
anomia, and is relatively rare.
Hemispheric disconnection
Many studies have shown that disconnection syndromes such as
aphasia
Aphasia is an inability to comprehend or formulate language because of damage to specific brain regions. The major causes are stroke and head trauma; prevalence is hard to determine but aphasia due to stroke is estimated to be 0.1–0.4% in th ...
,
agnosia
Agnosia is the inability to process sensory information. Often there is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is not defective nor is there any significant memory loss. It is usually ...
,
apraxia
Apraxia is a motor disorder caused by damage to the brain (specifically the posterior parietal cortex or corpus callosum), which causes difficulty with motor planning to perform tasks or movements. The nature of the damage determines the disorder' ...
, pure
alexia and many others are not caused by direct damage to functional neocortical regions. They can also be present on only one side of the body which is why these are categorized as hemispheric disconnections. The cause for hemispheric disconnection is if the interhemispheric fibers, as mentioned earlier, are cut or reduced.
An example is commissural disconnect in adults which usually results from surgical intervention, tumor, or interruption of the blood supply to the corpus callosum or the immediately adjacent structures. Callosal disconnection syndrome is characterized by left ideomotor apraxia and left-hand agraphia and/or tactile
anomia, and is relatively rare.
Other examples include
commissurotomy
A commissurotomy () is a surgical incision of a commissure in the body, as one made in the heart at the edges of the commissure formed by cardiac valves, or one made in the brain to treat certain psychiatric disorders.
Patients with scleroderma, a ...
, the surgical cutting of cerebral commissures to treat epilepsy and callosal agenesis which is when individuals are born without a
corpus callosum
The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental mam ...
. Those with callosal agenesis can still perform interhemispheric comparisons of visual and tactile information but with deficits in processing complex information when performing the respective tasks.
Sensorimotor disconnection
Hemispheric disconnection has impacted behaviors relating to the sensory and motor systems. The different systems affected are listed below:
*
Olfaction
The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.
In humans, it ...
– The olfactory system is not crossed across hemispheres like the other senses, which means that left input goes to the left hemisphere and right input goes to the right hemisphere. Fibers in the
anterior commissure
The anterior commissure (also known as the precommissure) is a white matter tract (a bundle of axons) connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. In most exist ...
control the olfactory regions in each hemisphere. A patient who lacks an
anterior commissure
The anterior commissure (also known as the precommissure) is a white matter tract (a bundle of axons) connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. In most exist ...
cannot name odors entering the right nostril or use the right hand to pick up the object corresponding to the odor because the left hemisphere, responsible for language and controlling the right hand, is disconnected from the sensory information.
*
Vision
Vision, Visions, or The Vision may refer to:
Perception Optical perception
* Visual perception, the sense of sight
* Visual system, the physical mechanism of eyesight
* Computer vision, a field dealing with how computers can be made to gain un ...
– Information from one visual field travels to the contralateral hemisphere. Therefore, with a commissurotomy patient, visual information presented in the left visual field travelling to the right hemisphere would be disconnected from verbal output since the left hemisphere is responsible for speech.
*
Somatosensory
In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...
– If the two hemispheres are disconnected, the somatosensory functions of the left and right parts of the body become independent. For example, when something is placed on the left hand of a blindfolded patient with the two hemispheres disconnected, the left hand can pick the correct object within a set of objects but the right hand cannot.
*
Audition
An audition is a sample performance by an actor, singer, musician, dancer or other performer. It typically involves the performer displaying their talent through a previously memorized and rehearsed solo piece or by performing a work or piece giv ...
– Though most of the input from one ear would go through the same ear, the opposite ear also receives some input. Therefore, the disconnection effects seems to be reduced in audition compared to the other systems. However, studies have shown that when the hemispheres are disconnected, the individual does not hear anything from the left and only hears from the right.
*
Movement
Movement may refer to:
Common uses
* Movement (clockwork), the internal mechanism of a timepiece
* Motion, commonly referred to as movement
Arts, entertainment, and media
Literature
* "Movement" (short story), a short story by Nancy Fu ...
–
Apraxia
Apraxia is a motor disorder caused by damage to the brain (specifically the posterior parietal cortex or corpus callosum), which causes difficulty with motor planning to perform tasks or movements. The nature of the damage determines the disorder' ...
and
agraphia may occur where responding to any verbal instructions by movement or writing in the left hand is inhibited because the left hand cannot receive these instructions from the right hemisphere,
History
The concept of disconnection syndrome emerged in the late nineteenth century when scientists became aware that certain neurological disorders result from communication problems among brain areas.
In 1874,
Carl Wernicke
Carl (or Karl) Wernicke (; ; 15 May 1848 – 15 June 1905) was a German physician, anatomist, psychiatrist and neuropathologist. He is known for his influential research into the pathological effects of specific forms of encephalopathy and also t ...
introduced this concept in his dissertation when he suggested that
conduction aphasia
Conduction aphasia, also called associative aphasia, is an uncommon form of difficulty in speaking (aphasia). It is caused by damage to the parietal lobe of the brain. An acquired language disorder, it is characterised by intact auditory comprehen ...
could result from the disconnection of the sensory speech zone from the motor speech area by a single lesion in the left hemisphere
to the
arcuate fasciculus
The arcuate fasciculus (AF) is a bundle of axons that generally connects the Broca's area and the Wernicke's area in the brain. It is an association fiber tract connecting caudal temporal cortex and inferior frontal lobe. ''Fasciculus arcuatus'' ...
. As the father of the disconnection theory, Wernicke believed that instead of being localized in specific regions of the brain, higher functions resulted from associative connections between the motor and sensory memory areas.
Lissauer, a pupil of Wernicke, described a case of
visual agnosia Visual agnosia is an impairment in recognition of visually presented objects. It is not due to a deficit in vision (acuity, visual field, and scanning), language, memory, or intellect. While cortical blindness results from lesions to primary visual ...
as a disconnection between the visual and language areas.
Dejerine in 1892 described specific symptoms resulting from a lesion to the corpus callosum that caused
alexia without
agraphia. The patient had a lesion in the left occipital lobe, blocking sight in the right visual field (
hemianopia
Hemianopsia, or hemianopia, is a loss of vision or blindness (anopsia) in half the visual field, usually on one side of the vertical midline. The most common causes of this damage are stroke, brain tumor, and trauma.
This article deals only wi ...
), and in the splenium of the corpus callosum. Dejerine interpreted this case as a disconnection of the speech area in the left hemisphere from the right visual cortex.
In 1965,
Norman Geschwind
Norman Geschwind (January 8, 1926 – November 4, 1984) was a pioneering American Behavioral neurology, behavioral neurologist, best known for his exploration of behavioral neurology through disconnection models based on lesion analysis.
Earl ...
, an American neurologist, wrote ‘Disconnexion syndromes in animals and man’ where he described a disconnectionist framework that revolutionized neurosciences and clinical neurology. Studies of the monkey brain led to his theory that disconnection syndromes were higher function deficits. Building on Wernicke and previously mentioned psychologists’ idea that disconnection syndromes involved
white matter
White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribution ...
lesion to association tracts connecting two regions of the brain, Geschwind was more detailed in explaining some disconnection syndromes as lesions of the association cortex itself, specifically in the parietal lobe. He described the callosal syndrome, an example of a disconnection syndrome, which is a lesion in the corpus callosum that leads to tactile anomia in just the patient’s left hand.
Though Geschwind made significant advances in describing disconnection syndromes, he was not completely accurate. He didn’t think the association cortex had any specialized role of its own besides acting as a relay station between the primary sensory and motor areas. However, in the 1960s and 1970s, Mesulam and Damasio incorporated specific functional roles for the association cortex. With Mesulam and Damasio’s contributions, Geschwind’s model has evolved over the past 50 years to include connections between brain regions as well as specializations of association cortices.
More recently, neurologists have been using imaging techniques such as
diffusion tensor imaging
Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data that uses the diffusion of water molecules to generate contrast in MR images. It ...
(DTI) and
functional magnetic resonance imaging
Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area o ...
(fMRI) to visualize association pathways in the human brain to advance the future of this disconnection theme.
See also
*
Functional disconnection Lingesha
Functional disconnection is the disintegrated function in the brain in the absence of anatomical damage, in distinction to physical disconnection of the cerebral hemispheres by surgical resection, trauma or lesion. Applications have incl ...
References
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Neuropsychology
Cerebral white matter
Syndromes affecting the nervous system