How does the corpus callosum mediate interhemispheric transfer? A review

Abstract

The corpus callosum is the largest white matter structure in the human brain, connecting cortical regions of both hemispheres. Complete and partial callosotomies or callosal lesion studies have granted more insight into the function of the corpus callosum, namely the facilitation of communication between the cerebral hemispheres. How the corpus callosum mediates this information transfer is still a topic of debate. Some pose that the corpus callosum maintains independent processing between the two hemispheres, whereas others say that the corpus callosum shares information between hemispheres. These theories of inhibition and excitation are further explored by reviewing recent behavioural studies and morphological findings to gain more information about callosal function. Additional information regarding callosal function in relation to altered morphology and dysfunction in disorders is reviewed to add to the discussion of callosal involvement in interhemispheric transfer. Both the excitatory and inhibitory theories seem likely candidates to describe callosal function, however evidence also exists for both functions within the same corpus callosum. For future research it would be beneficial to investigate the functional role of the callosal sub regions to get a better understanding of function and use more appropriate experimental methods to determine functional connectivity when looking at interhemispheric transfer.

Introduction

The corpus callosum is a brain structure in placental mammals that connects the left and right cerebral hemispheres. Containing numerous intra- and interhemispheric myelinated axonal projections it is considered to be the largest white matter structure in the brain. Patients undergoing complete or partial corpus callosotomies and callosal lesion studies have provided more insight into its function over the years. These callosotomies served as a treatment for intractable epilepsy, preventing seizures from spreading over the entire brain. The first callosotomy was performed by van Wagenen and Herren [1], and on first sight did not appear to induce any large cognitive or functional deficits. However, more elaborate behavioural studies have shown symptoms specific to callosotomies, now known as the callosal disconnection syndrome. Complete sectioning of the corpus callosum blocked the transfer of information to the opposing hemisphere, resulting in dissociation between left and right and difficulties in transferring learned information.

The unique cognitive state of callosotomized patients has led to more elaborate research regarding hemispheric transfer and communication between different cortical areas and the functional specialization of the corpus callosum. This review will address and investigate recent literature concerning callosal morphology, function and dysfunction in order to investigate the involvement and function of the corpus callosum in brain hemisphere communication.