CALTECH (US) — People who are born without a link between the brain’s left and right hemispheres still show surprisingly normal communication across the gap.
The findings are especially valuable in light of current theories that link impaired brain connections with clinical conditions including autism and schizophrenia, researchers say.
The brain is never truly at rest. Even when daydreaming, there is a tremendous amount of communication happening between different areas in the brain. Many areas of the brain display slowly varying patterns of activity that are similar to one another. The fact that these areas are synchronized has led many scientists to presume that they are all part of an interconnected network called a resting-state network.
Magnetic resonance images comparing a healthy subject (left) with an AgCC patient (right). The corpus callosum is the thick, ‘c’-shaped structure outlined in the healthy brain and missing from the AgCC brain. (Credit: Caltech)
Researchers at California Institute of Technology (Caltech) found that these resting-state networks look essentially normal in people missing the corpus callosum link, a condition called agenesis of the corpus callosum (AgCC), despite the lack of connectivity.
“This was a real surprise,” says J. Michael Tyszka, associate director of Caltech’s Brain Imaging Center. “We expected to see a lot less coupling between the left and right brain in this group—after all, they are missing about 200 million connections that would normally be there. How do they manage to have normal communication between the left and right sides of the brain without the corpus callosum?”
The work, reported in the Journal of Neuroscience, used functional magnetic resonance imaging (fMRI) to demonstrate that synchronized activity between the left and right brain survives even this sort of radical rewiring of the nerve connections between the two hemispheres.
The presence of symmetric patterns of activity in individuals born without a corpus callosum highlights the brain’s remarkable plasticity and ability to compensate, says coauthor Lynn Paul, research staff member and lecturer in psychology at Caltech. “It develops these fundamental networks even when the left and right hemispheres are structurally disconnected.”
The study is part of an ongoing research program led by Paul, who has been studying AgCC for several decades. The condition occurs in approximately one of every 4,000 live births. The typical corpus callosum comprises almost 200 million axons—the connections between brain cells—and is the largest fiber bundle in the human brain.
In AgCC, those fibers fail to cross the gap between the hemispheres during fetal development, forcing the two halves of the brain to communicate using more indirect and currently unknown means.
“In the 1960s and 1970s, Roger Sperry at Caltech studied ‘split-brain’ patients in whom the corpus callosum was surgically severed as a treatment for epilepsy,” Paul says.
“Our research on AgCC has moved in a different direction and focuses on a naturally occurring brain malformation that occurs before birth. This allows us to examine how, and to what extent, the brain can compensate for the loss of the corpus callosum as a person grows to adulthood.”
The researchers say the findings are significant when considering the link between brain connections and autism or schizophrenia.
“We are now examining AgCC subjects who are also on the autism spectrum, in order to gain insights about the role of brain connectivity in autism, as well as in healthy social interactions,” says Tyszka.
“About a third of people with AgCC also have autism, and altered connectivity in the corpus callosum has been found in autism. The remarkable compensation in brain functional networks that we found here may thus have important implications also for understanding the function of the brains of people with autism.”
The work was carried out in the laboratory of Ralph Adolphs, professor of psychology and neuroscience at Caltech, with the help of postdoctoral scholar Daniel Kennedy. It was supported by the Gustavus and Louise Pfeiffer Research Foundation, the Simons Foundation, the National Institute of Mental Health, and the National Alliance for Research on Schizophrenia and Depression.
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