The right and left halves of the brain are connected by a bridge known as the corpus callosum. When people have their corpus callosum surgically removed, it can lead to disconnection syndrome, where one half of the brain might not know what the other half is doing. However, people who are born without the corpus callosum typically do not suffer from such disconnection syndrome. Now scientists find the brains of people born without a corpus callosum can form alternative bridges in the brain to link the hemispheres. The findings are detailed in the Proceedings of the National Academy of Sciences.
The surgical removal of the corpus callosum, a procedure known as a corpus callosotomy, is performed as a last resort to stop epileptic seizures, keeping them from spreading from one hemisphere of the brain to the other. However, the disconnection syndrome that results underscores how the different halves of the brain typically become specialized, and that decoupling them can have dramatic consequences.
For example, the left side of the brain specializes in speech and language, while the right specializes in visual-spatial processing and facial recognition. When the left side of the brain is presented with an image of half a man’s face and the right is shown a picture of half a woman’s face, if a split-brain patient is asked to say whether the picture is of a man or a woman, the patient will say the image was of a man, but if asked to point to a normal, whole picture of that face, the patient will pick out a woman.
On the other hand, people born without the corpus callosum — a condition known as callosal dysgenesis — lack disconnection syndrome. Some people with callosal dysgenesis may even be mentally normal, with regular lives and jobs, although others may be severely handicapped, with mental retardation, epilepsy and other symptoms.
To solve the paradox of why people born without the corpus callosum might not experience disconnection syndrome, which has defied researchers for decades, neuroscientist Roberto Lent, director of the Institute of Biomedical Sciences at Brazil’s Federal University of Rio de Janeiro, and his colleagues tested patients with callosal dysgenesis with neuropsychological exams and scanned their brains with a variety of magnetic resonance imaging (MRI) techniques, including fiber tracking techniques.
They discovered the brains of patients could rewire themselves, forming alternative white-matter tracts in the midbrain and ventral forebrain that connected the brain’s hemispheres. These anomalous bundles of nerve fibers led to partial compensatory effects, such as helping these patients name objects palpated out of view with the left hand. This simple task cannot be performed by patients with the corpus callosum cut apart.
“The findings solve a longstanding paradox of brain research,” Lent says. These findings show that when the brain suffers from early damage, “it may construct entirely new avenues of communication. Some may be compensatory, as those we showed in the paper, others may be deleterious, still unknown.”