Spatial attention and interhemispheric visuomotor integration in the absence of the corpus callosum
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Commissurotomy
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The appearance of the corpus callosum is one of the most distinct modifications seen in placental mammals and is a diagnostic character just like the placenta itself. It signified a major leap in evolution and its presence maybe linked to the binding mechanisms related to midline fusion and bimanual coordination [1]. Prior to the corpus callosum interhemispheric transfer amongst vertebrates was via the anterior commissure and the hippocampal commissure. It is believed that the corpus callosum originated in placental mammals as a short-cut to reduce interhemispheric transmission times for fibers connecting the motor and sensory areas which otherwise would have to take the longer route through the anterior commissure [2, 3]. In present day placental mammals however only a small contingent of callosal fibers are seen to connect the primary sensory and motor cortices and most callosal fibers are seen to connect higher order centers as has been demonstrated in this study. Callosal axons have been found to terminate in diverse regions of the brain apart from the topographically equivalent areas thus participating in spreading corticocortical synchronous activation to multiple homotropic and heterotopic regions in the hemispheres. This synchronous activity can explain the callosal propagation of an epileptic focus and the effectiveness of a callosotomy to prevent expansion of epileptic activity [1]. This chapter discusses the course and connections of the corpus callosum.
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Our recent target article on the allocation of attention to action (herein called the AAA model; Franz, 2012) considered implicated subcortical processes and networks in people with intact corpus callosum and people without a corpus callosum due to commissurotomy or callosotomy. However, a small error in print— namely that the term 'commissurotomy' was printed in place of 'callosotomy' in some instances—led us to further explore whether any key functional roles have been attributed to the two primary cortical commissures (the anterior and posterior commissures) which remain intact in people with callosotomy, and if so, whether those would be relevant to our current AAA framework. Although existing evidence is sparse, here we consider the hypothesis that the anterior commissure is a remnant fiber tract which has been largely replaced with evolution of the corpus callosum (and we do not herein discuss the posterior commissure further). Indeed, a dearth of studies is available on the anterior commissure, calling the need for further research. Herein, we briefly review literature on the anterior commissure (AC) in humans and then propose a method that might be worthwhile to pursue in future studies.
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Cases of absence or defect of the corpus callosum are of interest not only because of their great rarity, but because of the light which they throw on the distribution and functions of this commissure, and on the development of the mesial aspects of the cerebral hemispheres.
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Cerebral commissurotomy, the "split-brain" procedure, has been employed for the control of intractable seizures, in conjunction with moderate doses of anticonvulsant drugs. The results have been encouraging in several small series. The use of microsurgical techniques and the restriction of surgery to one commissure, the corpus callosum, has reduced morbidity without apparent change in result. The eight patients in our first series who underwent the prescribed division of several forebrain commissures are compared to the four patients in our second series who underwent division of the corpus callosum alone. The technique of callosotomy is described.
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THE SIZE and the location of the corpus callosum and anterior commissure suggest an important role in propagation of seizures from one side of the brain to the other. In 1940, Van Wagenen and Herren1cut the corpus callosum to various extent in two dozen patients; only one had complete section of both corpus callosum and anterior commissure. The subsequent report by Akelaitis2on these patients indicates that a number of them were improved. Complete section of corpus callosum and anterior commissure was therefore offered to an apparently hopeless epileptic having one to three generalized convulsions per week, with best medical control.3He has not had a generalized convulsion since that operation (Feb 6, 1962). Furthermore, although deficits in interhemispheric integration could be demonstrated under specialized testing conditions,4the patient's well-being and general behavior were improved. Because of his excellent response, five more patients underwent this
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The corpus callosum is the largest and phylogenetically one of the oldest of the commissures, yet its function is an enigma. In primitive mammals (edentata) it is formed by a thin membrane. In phylogenesis the corpus callosum becomes larger and larger. It is proportional to the growing size of the telencephalon and to the number of cerebral convolutions. Congenital absence of the corpus callosum in man is a rare occurrence. However, we have had the opportunity to observe 24 cases of agenesis of this commissure. The main purpose of this chapter is to demonstrate some typical variations of this pathological condition.
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The corpus callosum is the major axonal commissure of the brain, connecting the two cerebral hemispheres and providing communication between the cortical and subcortical neurons. With MR imaging in the sagittal plane, the corpus callosum can be depicted in great detail. We review the normal anatomy, development, and process of myelination of the corpus callosum. The MR features of various pathologic conditions involving the corpus callosum are described. Finally, we discuss the evolving role of MR imaging in neuropsychiatric diseases with respect to the corpus callosum.
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Abstract The corpus callosum is the largest white matter pathway connecting homologous structures of the two cerebral hemispheres. Remarkably, children and adults with developmental absence of the corpus callosum (callosal dysgenesis, CD) show typical interhemispheric integration, which is classically impaired in adult split‐brain patients, for whom the corpus callosum is surgically severed. Tovar‐Moll and colleagues (2014) proposed alternative neural pathways involved in the preservation of interhemispheric transfer. In a sample of six adults with CD, they revealed two homotopic bundles crossing the midline via the anterior and posterior commissures and connecting parietal cortices, and the microstructural properties of these aberrant bundles were associated with functional connectivity of these regions. The aberrant bundles were specific to CD and not visualised in healthy brains. We extended this study in a developmental cohort of 20 children with CD and 29 typically developing controls (TDC). The two anomalous white‐matter bundles were visualised using tractography. Associations between structural properties of these bundles and their regional functional connectivity were explored. The proposed atypical bundles were observed in 30% of our CD cohort crossing via the anterior commissure, and in 30% crossing via the posterior commissure (also observed in 6.9% of TDC). However, the structural property measures of these bundles were not associated with parietal functional connectivity, bringing into question their role and implication for interhemispheric functional connectivity in CD. It is possible that very early disruption of embryological callosal development enhances neuroplasticity and facilitates the formation of these proposed alternative neural pathways, but further evidence is needed.
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