A Major Malformation Illustrates the Incredible Plasticity of the Human Brain

Neuronal fibers in a healthy brain (top) and a brain with agenesis of the corpus callosum (bottom). In the healthy brain, the two hemispheres are connected by the corpus callosum fibers, shown in red. These fibers are absent in the brain with corpus callosum agenesis.
Credit: Unige/Siffredi

People born without a corpus callosum do not have a bridge between the two cerebral hemispheres. Neuroscientists from UNIGE have shown how the brain manages to adapt.

One in 4,000 people is born without a corpus callosum, a brain structure consisting of neural fibers that are used to transfer information from one hemisphere to the other. A quarter of these individuals do not have any symptoms, while the remainder either have low intelligence quotients or suffer from severe cognitive disorders. In a study published in the journal Cerebral Cortex, neuroscientists from the University of Geneva (UNIGE) discovered that when the neuronal fibers that act as a bridge between the hemispheres are missing, the brain reorganizes itself and creates an impressive number of connections inside each hemisphere. These create more intra-hemispheric connections than in a healthy brain, indicating that plasticity mechanisms are involved. It is thought that these mechanisms enable the brain to compensate for the losses by recreating connections to other brain regions using alternative neural pathways.

The corpus callosum develops in utero between the tenth and twentieth week of gestation. Agenesis of the corpus callosum is a congenital brain malformation in which this brain structure fails to develop, resulting in one out of 4,000 babies born without a corpus callosum. When it is missing, nothing replaces this structure measuring about ten centimeters, with the exception of cerebrospinal fluid. This means that the information transmitted from one hemisphere to the other can no longer be conveyed by the neuronal projections from the corpus callosum. “Their role in a healthy brain,” begins Vanessa Siffredi, a researcher in UNIGE’s Faculty of Medicine, “is to ensure the functioning of various cognitive and sensorimotor functions.” Surprisingly, 25% of people with this malformation have no visible signs; 50% have average intelligence quotients and learning difficulties; and the remaining 25% suffer from severe cognitive disorders.

The scientific literature shows that, in the absence of the corpus callosum, certain fibers designed to serve as a bridge between the hemispheres, known as Probst bundles, bypass the absent brain area and curl up inside each hemisphere. “The back-up zones vary from one individual to another. And we don’t understand their functions,” explains the neuroscientist. The UNIGE scientists – working in collaboration with their colleagues at the University of Melbourne – set out to understand this variability and to examine the role of the fibers. Using MRI brain imaging, they studied the anatomical and functional links between different brain regions of approximately 20 Australian children aged 8 to 17 suffering from agenesis of the corpus callosum.

A salutary role

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