What does the corpus callosum connect?

I believe it connects the 2 halves of the brain

Conditions specific to the CC,

Agenesis of the corpus callosum (ACC) is an anomaly that may occur in isolation or in association with other CNS or systemic malformations. Because the corpus callosum may be partially or completely absent, the term dysgenesis has also been used to describe the spectrum of callosal anomalies.
Agenesis of the corpus callosum (ACC) is usually a sporadic occurrence, although the incidence is increased in patients with trisomy 18, trisomy 13, and trisomy 8. Several familial cases have been reported. Organ systems other than the CNS, particularly the musculoskeletal and genitourinary systems, may be affected as well.5,7,8

Fibers of the corpus callosum arise from the superficial layers of the cerebral cortex; they project to the homotypic region of the contralateral cortex by passing through the corpus callosum in crossing the midline. Disturbance of embryogenesis in the first trimester of gestation by some unknown insult leads to failure of the callosal axons to pass across the midline. These arrested axons form the longitudinally oriented bundles of Probst that are located medial to the lateral ventricles in patients with agenesis.

Spectrum of abnormalities

ACC may be complete, partial, or atypical.

With complete agenesis, the corpus callosum is totally absent.

With partial agenesis (hypoplasia), the anterior portion (posterior genu and anterior body) is formed, but the posterior portion (posterior body and splenium) is not formed. The rostrum and the anterior/inferior genu are also not formed.

An atypical appearance occurs when the anterior to posterior formation is not respected.

In holoprosencephaly, callosal anomalies are atypical; for example, the splenium may be present without a genu or body. In middle interhemispheric fusion, which is a variety of holoprosencephaly, the genu and splenium may be present without the callosal body.

With pseudo–corpus callosum, which involves conditions of complete or partial agenesis, the hippocampal commissure may become enlarged and appear like the posterior part of the corpus callosum.

Secondary destruction of corpus callosum occurs when the genu and anterior body are destroyed, leaving the posterior portion of the corpus callosum intact. This may occur secondary to porencephaly or schizencephaly; as a surgical complication in cases involving the transcallosal approach to the lateral and third ventricle; and with hemisection of the callosum for the treatment of seizures.

Other cerebral malformations may coexist with callosal dysgenesis. Examples of these include interhemispheric cysts; intracranial lipomas; and disorders of neuronal migration, such as schizencephaly, neuronal heterotopias, lissencephaly, and pachygyria.

Frequency of abnormalities

The frequency of occurrence of some of the more commonly associated anomalies are as follows:

* CNS anomalies (85%)
* Dandy-Walker cyst (11%)
* Interhemispheric cysts
* Hydrocephalus (30%)
* Midline lipoma of corpus callosum (10%)
* Arnold-Chiari malformation (7%)
* Midline encephalocele
* Porencephaly
* Holoprosencephaly
* Hypertelorism median cleft syndrome
* Polymicrogyria
* Gray-matter heterotopia
* Cardiovascular, GI, and GU anomalies (62%)

Frequency
United States

The reported frequency of agenesis of the corpus callosum in the US is 0.7-5.3%.
International

Internationally, the frequency of agenesis of the corpus callosum is not known but could be similar to that in the US.
Mortality/Morbidity

* Agenesis of the corpus callosum may occur as an isolated defect, but it is frequently associated with other malformations, chromosomal abnormalities, and genetic syndromes.
* Although ACC has been found in asymptomatic individuals, it is generally considered a potential marker for neurologic impairment.
* In children, the prognosis is frequently related to other associated abnormalities.

Sex

Agenesis of the corpus callosum is reported to be more common in males than in females.
Age

Agenesis of the corpus callosum is a congenital or a developmental anomaly and so is present at the time of birth. In many cases, agenesis is diagnosed later in infancy or in childhood because of its associated congenital malformations.
Anatomy

Development and anatomy

The corpus callosum develops from the lamina reuniens in the telencephalon; it begins to appear between the anterior and hippocampal commissures at about 10.5 weeks. The adult form of the corpus callosum is achieved by 17 weeks' gestational age. Initial formation of the corpus callosum occurs in the genu and the body, progressing posteriorly. The anterior genu and rostrum develop last, folding back under the genu. The callosum thickens with increasing myelination.

When the corpus callosum is absent, the third ventricle is often high riding, extending superiorly between the lateral ventricles. On coronal imaging, a candelabra appearance occurs, with the third ventricle forming the central vertical portion and the lateral ventricles the peripheral arms of the candelabra. On axial imaging, the lateral ventricles are parallel.

Medial to the lateral ventricles, longitudinal bundles of white matter are present in patients with agenesis of the corpus callosum (ACC). These are known as Probst bundles and presumably would have formed a normal corpus callosum. Probst bundles are best seen on coronal or axial T1-weighted MRIs. The occipital horns of the lateral ventricles are dilated in patients with ACC, probably because of a deficiency of peritrigonal white-matter fibers. This anatomic finding is known as colpocephaly. When the corpus callosum is absent, the cingulate gyrus is inverted, the normal cingulate sulcus is absent, and the medial cerebral sulci radiate toward the midline in a radial configuration. This finding is especially helpful in evaluating newborns in whom the corpus callosum is normally thin.

The hippocampal formations are frequently hypoplastic in patients with ACC, with resulting mild dilatation of the temporal horns. In partial callosal agenesis, the posterior body, splenium, and rostrum are usually absent. Absence of the posterior body and splenium is especially common in patients with a Chiari II malformation. Barkovich has described the unusual absence of the genu or the midbody of the corpus callosum in patients with atypical or mild forms of holoprosencephaly.9

Associated midline cysts are noted in some cases. The exact origin and nature of these cysts are controversial. Whereas some of these cysts represent a dilated superiorly migrated third ventricle, others represent true midline cysts that may be lined by ependymal cells or by arachnoid membranes.

Types of midline cyst formation

Raybaud and Girard suggest that there are 3 types of midline cyst formation in association with agenesis or hypogenesis of the corpus callosum.10

Type 1 is a large midline cyst that communicates with the third ventricle and the lateral ventricles.

Type 2 is similar to type 1; associated cortical anomalies (eg, polymicrogyria, gray-matter heterotopia, schizencephaly) are present.

Type 3 involves complex, multilocular cysts that are asymmetric and independent of the ventricles. Cortical malformations are uncommon. With large cysts, the ipsilateral lateral ventricle may be compressed, and the contralateral ventricle may be obstructed and enlarged (hydrocephalus). A CT cystogram may be helpful in identifying the communications between the loculations of the cysts and the ventricles and in guiding the placement of a ventriculostomy shunt.

Associated anatomic abnormalities

Other anatomic abnormalities in patients with ACC include hydrocephalus; cephaloceles; and neuronal migration disorders such as lissencephaly, schizencephaly, gray-matter heterotopias, pachygria, and polymicrogyria.
Presentation

The white-matter fibers forming the corpus callosum predominantly connect symmetrical regions in the frontal, parietal, temporal, and occipital lobes. Experimental observations indicate that the corpus callosum allows the sharing of learning and memory between the 2 cerebral hemispheres.

The clinical manifestations of callosal agenesis may be described under 2 headings: nonsyndromic and syndromic.11

Nonsyndromic forms are the most common. An unknown, though probably small, proportion of patients are completely asymptomatic; commonly, their condition is incidentally discovered during neuroimaging. Patients may present with mental retardation or delayed development; seizures; and cerebral palsy. Macrocephaly may occur as a result of hydrocephalus; it is sometimes associated with interhemispheric cysts.

A number of syndromes may be associated with ACC. Some of the more common ones include Dandy-Walker syndrome, Aicardi syndrome, fetal alcohol syndrome, and several of the trisomies.
Preferred Examination

The diagnosis of callosal agenesis depends on neuroimaging. In the newborn, before closure of the anterior fontanelle occurs, screening ultrasonography (US) may clearly show the absence of the corpus callosum; it may also show parallel lateral ventricles, interhemispheric cysts, hydrocephalus, and other related anomalies. US was the first imaging modality to allow direct sagittal imaging of callosal dysgenesis.8,12,13,14,15

Antenatal diagnosis of agenesis of the corpus callosum (ACC) is possible from about 20 weeks' gestation. Characteristic intrauterine US findings include colpocephaly and parallel ventricular walls. CT findings are also diagnostic of ACC. Parallel lateral ventricles, colpocephaly, and extension of the third ventricle into the interhemispheric fissure are particularly pertinent findings. In patients with ACC who have an interhemispheric cyst, the preoperative injection of nonionic water-soluble contrast material into the cystic loculations for CT evaluation enables assessment of the ventricular system or of the communication of the cystic components with one another.

MRI is currently the imaging procedure of choice in infants and children with ACC, even in patients who have previously undergone CT and US examinations. The multiplanar capability and high soft tissue contrast that are possible with MRI permit confident diagnosis of ACC and its associated anomalies, especially neuronal migration anomalies or atypical forms of holoprosencephaly. These entities may be extremely subtle or indiscernible on CT or US images.
Limitations of Techniques

Agenesis of the corpus callosum may be depicted on both CT and US, but MRI is the preferred imaging modality because of its greater sensitivity for depicting associated cerebral anomalies.

The corpus callosum (CC) is the major white matter tract connecting the right and left cerebral hemispheres of the brain.

As the largest fiber tract in the brain, it is composed of approximately 200 million interhemispheric nerve fibers (in the average human adult) representing approximately 2-3% of all cortical fibers.

These fibers establish primarily homotopic connections along an anterior-posterior gradient which parallels that of the cerebral cortex. Heterotopic connections also exist.

The majority of callosal fibers connect cortical association areas rather than primary cortical regions or subcortical structures.

Callosal fibers contains both unmyelinated and myelinated axons.

Most CC fibers are physiologically excitatory in nature however, inhibitory functions are possible.

Generally speaking, the CC can be conceptualized as a set of overlapping channels responsible for the control of interhemispheric communication.

In fact, the CC has been noted as being the most important structure for interhemispheric communication of sensory, motor and higher-order information between the hemispheres.

In addition to shuttling information between the hemispheres, the CC may allow information in one hemisphere to be shielded from the other. For further discussion on models of callosal functioning, please go to What does the Corpus Callosum do for a Living?

Researchers have sub-divided the CC to examine the structure and function of these sub-regions. Some researchers sub-divide the CC into 4, 5, 6 or more regions however, a very well-known and commonly utilized method divides the CC into 7 sub-regions.
Callosal Sub-region


Cortical Region

rostrum


caudal/orbital prefrontal, inferior premotor

genu


prefrontal, caudal/orbital, inferior premotor

rostral body


premotor, supplementary motor

anterior midbody


motor

posterior midbody


somaesthetic, posterior parietal

isthmus


superior temporal, posterior parietal

splenium


inferior temporal, occipital, superior temporal, posterior parietal

Historical chronology of events (cited in Hoptman & Davidson, 1994)

ª Akelaitis (1941) suggested that there were few long-term behavioral effects from cutting the corpus callosum.

ª McCulloch (1949) proposed that the corpus callosum was merely involved in the spread of epileptic seizures.

ª Lashley (1951) commented that the role of the corpus callosum was to prevent the two cerebral hemispheres from sagging!!

ª A better understanding of corpus callosum functioning emerged in 1953 from ‘split-brain’ studies .

ª For a complete historical account of corpus callosum functioning, see Harris (1995).



Today (Hellige, 1993)

ª The corpus callosum allows information to be shared between the two hemispheres– this role is undisputed, how this process occurs is a matter of some debate.

Excitatory model:

ª The role of the corpus callosum is based predominantly on excitatory effects.

ª Processing that involves a particular region in one hemisphere serves to activate similar regions in the corresponding hemisphere.

Inhibitory model:

ª The role of the corpus callosum is based predominantly on inhibitory effects.

ª Contradictory to the excitatory model, processing that involves a particular region in one hemisphere serves to suppress similar regions in the corresponding hemisphere.

OR

ª The corpus callosum acts as an ‘inhibitory barrier’ to block the passage of some types of information allowing each hemisphere to work autonomously.

ª This hypothesis can explain cerebral laterlization and specialization.



Conclusions (Hellige, 1993)

ª It is impossible to reduce corpus callosum functioning to a single biological/functional purpose.

ª The corpus callosum is probably involved in numerous aspects of normal interhemispheric functioning.

ª Regardless of which theory or combination of theories is correct, the corpus callosum plays a crucial role in the transfer of at least some kinds of information from one hemisphere to another.

Ground-breaking studies discover brain differences in autism.
PITTSBURGH, Pennsylvania, USA: In a pair of ground-breaking studies, brain scientists at Carnegie Mellon University and the University of Pittsburgh have discovered that the anatomical differences which characterise the brains of people with autism are related to the way those brains process information.

Previous studies have demonstrated a lower degree of synchronisation among activated brain areas in people with autism, as well as smaller size of the corpus callosum, the white matter that acts as cables to wire the parts of the brain together. This latest research shows for the first time that the abnormality in synchronisation is related to the abnormality in the cabling. The results suggest that the connectivity among brain areas is among the central problems in autism. The researchers have also found that people with autism rely heavily on the parts of the brain that deal with imagery, even when completing tasks that would not normally call for visualisation.

"Human thought is a network property. You think not with one brain area at a time, but with a network of collaborating brain areas, with emphasis on collaborating. In autism, the network connectivity (the bandwidth) through which the areas communicate with each other may be limited, particularly in the connections to the frontal cortex, limiting what types of networks can be used," said Dr Marcel Just, co-author of the studies and director of Carnegie Mellon's Center for Cognitive Brain Imaging.

Both studies focused on people with autism who have normal IQs. In one study, the researchers used functional magnetic resonance imaging (fMRI) to view which parts of the brain were activated in people with autism compared to a control group of normal participants while completing the Tower of London task. In a Tower of London task, participants must - in a set number of moves - rearrange the positions of three distinctive balls in three suspended pool pockets to match a specified pattern. This requires a person to strategise and plan several moves ahead.

The Tower of London task is used to gauge the functioning of the pre-frontal cortex. This brain area, located in the front, upper part of the brain, deals with strategic planning and problem-solving. The pre-frontal cortex is the executive area of the brain, in which decision making, judgment, and impulse control reside.

A little further back is the parietal cortex, which controls high-level visual thinking and visual imagery, supporting the visual aspects of the problem-solving. Both the pre-frontal and parietal cortex play a critical part in performing the Tower of London test.

In the normal participants, the pre-frontal cortex and the parietal cortex tended to function in synchrony (increasing and decreasing their activity at the same time) while solving the Tower of London task. This suggests that the two brain areas were working together to solve the problem. In the participants with autism, however, the two brain areas, pre-frontal and parietal, were less likely to function in synchrony while working on the task.

The researchers made another discovery, for the first time finding a relationship between this lower level of synchrony and the properties of some of the neurological "cables" or white matter fibre tracts that connect brain areas. White matter consists of fibres that, like cabling, connect brain areas. The largest of the white matter tracts is known as the corpus callosum, which allows communication between the two hemispheres (halves) of the brain.

"The size of the corpus callosum was smaller in the group with autism, suggesting that inter-regional brain cabling is disrupted in autism," Dr Just said.

In essence, the extent to which the two key brain areas (pre-frontal and parietal) of the autistic participants worked in synchrony was correlated with the size of the corpus callosum. The smaller the corpus callosum, the less likely the two areas were to function in synchrony. In the normal participants, however, the size of the corpus callosum did not appear to be correlated with the ability of the two areas to work in synchrony.

"This finding provides strong evidence that autism is a disorder involving the biological connections and the co-ordination of processing between brain areas," Dr Just said.

He added, however, that the thickness, or extent, of connections between brain areas may not be the basis for the disorder. Although the neurological connections between the pre-frontal cortex appear to be reduced in autism, the brains of people with autism have thicker connections between certain brain regions within each hemisphere.

"At this point, we can say that autism appears to be a disorder of abnormal neurological and informational connections of the brain, but we can't yet explain the nature of that abnormality," Dr Just said.

The experiment confirmed the authors' previous findings that people with autism suffer from a lack of synchronisation among brain regions, which helps to explain why some people with autism have normal or even superior skills in some areas, while many other types of thinking are disordered.

The study will be published in the journal, Cerebral Cortex.

The second study, to be published in the journal, Brain, examined a long-standing belief, supported through scientific research as well as anecdotal accounts, that people with autism rely heavily on visualisation to process information. Dr Temple Grandin, a professor at Colorado State University who has autism, says in her autobiography,

"Thinking in Pictures," that "Words are like a second language to me ... When someone speaks to me, his words are instantly translated into pictures."

To test this relationship between the language and visuospatial systems of the brain, the team used fMRI scans to view the patterns of activation in the brains of autistic and normal participants while they read a series of sentences to determine whether each one was true or false. The researchers used fMRI to examine brain functioning in participants with autism and in normal participants during a true-false test involving reading sentences with low imagery content and high imagery content. A typical low imagery sentence consisted of constructions like "Addition, subtraction, and multiplication are all math skills." A high imagery sentence, "The number eight, when rotated 90 degrees,looks like a pair of eyeglasses," would first activate left pre-frontal brain areas involved with language, and then would involve parietal areas dealing with vision and imagery as the study participant mentally manipulated the number eight.

As the researchers expected, the visual brain areas of the normal participants were active only when evaluating sentences with imagery content. In contrast, the visual centres in the brains of participants with autism were active when evaluating both high imagery and low imagery sentences.

"The heavy reliance on visualisation in people with autism may be an adaptation to compensate for a diminished ability to call on pre-frontal regions of the brain," Dr Just said.

The results also replicated the researchers' findings in the Cerebral Cortex study, in that functional connectivity was lower among participants with autism, and that structural connectivity was positively correlated with functional connectivity. The authors believe that the heavy reliance on visualisation by people with autism may be an adaptation to compensate for their lower ability to call on frontal regions of the brain.

"Thinking in autism is an adaption to the brain that Mother Nature provided. We now have evidence of a systematic relation between the properties of the brain and the properties of the thinking in autism," said Dr Just, the D.O. Hebb Professor of Psychology at Carnegie Mellon.

Dr Just and his colleagues are conducting additional studies to ascertain the nature of the abnormality of the connections in the brains of people with autism.

"These findings provide support to a new theory that views autism as a failure of brain regions to communicate with each other," said Dr Duane Alexander, director of the National Institute of Health's National Institute of Child Health and Human Development.

"The findings may one day provide the basis for improved treatments for autism that stimulate communication between brain areas."

The research was led by Professor Just and Dr Nancy Minshew, professor of psychiatry and neurology at the University of Pittsburgh School of Medicine.